Michael John Nigel Priestley
memoria nostra durabit, si vita meruimus
Michael John Nigel Priestley
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ISBN: 978-88-85701-00-7
Table of Contents 1.
Photographs and Memories
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Often you and I had tired the sun with talking G. Michele Calvi
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Remembrance will endure, if the life shall have merited it Michael P. Collins His efforts will be significant for a long time into the future Athol J. Carr
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An implacable search for truth André Filiatrault
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Never able to touch the same water twice Rob Chai
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He let out a subtle smile and said: “damn it” Mervyn Kowalsky
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The Gracefield connection Sri Sritharan
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“My supervisor taught me that”…and this is the end of the discussion Jason Ingham
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His influence on engineering practice here (in the US) has been enormous Joe Maffei
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You know, Nigel would say… Katrin Beyer
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2.
Practical Lessons from Nigel Joe Maffei, Carlos Blandón, Sri Sritharan and Katrin Beyer
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3.
The World’s Greatest Structural Systems Experimentalist José I. Restrepo, Christopher Latham, Sri Sritharan and Nihal Vitharana
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4.
The Development of Direct Displacement-Based Design Mervyn Kowalsky, Tim Sullivan and Greg MacRae
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5.
Seismic Design and Retrofit of Bridges Sri Sritharan, Jason Ingham, Rob Chai, Mervyn Kowalsky,Jay Holombo and Mark Yashinsky
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6.
Masonry and Earthquakes: Not a Matter for Blockheads Katrin Beyer, Jason Ingham, Guido Magenes and Rob Chai
7.
From the PioneeringWork on Presss-Technologyto the New Paradigmof Low-DamageDesign 131 Stefano Pampanin, James Conley, Suzanne Dow Nakaki, Sri Sritharan, Christopher Latham and John F. Stanton
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Preface Gian Michele Calvi and José I. Restrepo
Michael John Nigel Priestley was born in 1943 and passed away in 2014. He has been a scientist, a designer, a carpenter, a gourmet, a poet and a free man. His influence on the growth of many people and on the evolution of several areas of structural engineering has been enormous. For this reasons, his name has been borrowed to entitle a number of initiatives in earthquake engineering, including an international seminar, a scientific prize, a museum. For this same reason a number of friends, colleagues, fellows and alumni have devised a volume of memories, in which his scientific and human heritage will be presented and discussed from several points of view, in the hope of further disseminating seeds of sapience. Michael Collins was one of Nigel schoolmates in the early sixties at the University of Canterbury. The closure of his contribution may possibly best describe our feelings when remembering Nigel:
Nigel Priestley changed my life in a number of positive ways. First in 1963 he showed me how to lift my academic performance by devoting some hours each day to studying the fundamentals of engineering. Then in 1997 he demonstrated that you could be a successful North American academic and still spend two or even three months a year in New Zealand where the spectacular scenery that we both loved is inspiring and helps the creative spirit. Finally in 2001 he made me part of the ROSE School at Pavia with the concept of teaching intensive graduate courses on seismic design of structures in a whole new way. As the Roman statesman, consul, governor of Britain and keeper of the aqueducts, Julius Frontinus stated when writing to Pliny about the death of a mutual friend: “Remembrance will endure, if the life shall have merited it. 1” For Nigel the life has merited it.
The following friends of Nigel have contributed to this volume: Katrin Beyer, Carlos Blandón, Gian Michele Calvi, Athol J. Carr, Rob Chai, Michael P. Collins, James Conley, André Filiatrault, Jay Holombo, Jason Ingham, Mervyn Kowalsky, Christopher Latham, Greg MacRae, Joe Maffei, Guido Magenes, Suzanne Nakaki, Stefano Pampanin, José I. Restrepo, Sri Sritharan, John F. Stanton, Tim Sullivan, Nihal Vitharana and Mark Yashinsky.
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Gaius Plinius Caecilius Secundus (Pliny the Younger) Epist. IX.XIX.1, 6: Vetuit exstrui monumentum; sed quibus verbis? Impensa monumenti supervacua est; memoria nostra durabit, si vita meruimus.
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1. Photographs and Memories
Often you and I had tired the sun with talking G. Michele Calvi
Nigel Priestley studies and academic appointments
July 21st, 1943 Born in Wellington, New Zealand. 1956 – 1959 High School at Wellington Technical College. 1959 – 1966 University studies at the University of Canterbury, at Christchurch: Bachelor of Engineering with first-class honours and PhD (with a thesis on Moment redistribution in prestressed concrete continuous beams). 1968 – 1969 Post-Doc at the Laboratorio Nacional de Engenharia Civil , Lisbon, Portugal. 1969 – 1975 Head of the structures laboratory at the Ministry of Works and Development central laboratories in Lower Hutt, New Zealand. 1976 – 1986 Senior Lecturer and then Reader, University of Canterbury. 1986 – 2000 Professor, University of California, San Diego. 2000 – 2014 Founder, Co-Director and later Emeritus Director, ROSE School, IUSS Pavia, Italy. Dec. 23rd, 2014 Decease.
Nigel Priestley main Awards and recognitions
1973 1983, 1989 2003 2006 2008 2010 2014 2014
Fulton Gold Medal (Institution of Professional Engineers New Zealand (IPENZ)). Raymond C. Reese Award (American Concrete Institute (ACI)). HonoraryDoctorate(EidgenössischeTechnischeHochschule(ETH), Zürich, Switzerland). Honorary Doctorate (Universidad Nacional de Cuyo, Mendoza Argentina). ROSE School Prize (IUSS Pavia, Italy). Freyssinet Medal (International Federation for Structural Concrete (fib)). Officer of the New Zealand Order of Merit(the Queen in Right of New Zealand). Housner Gold Medal (Earthquake Engineering Research Institute (EERI)).
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Figure 1.1 - Nigel Priestley in 2007.
Early days
I met Nigel for the first time in 1987, in La Jolla, at a meeting of the TECCMAR project, which aimed to reduce the seismic risk related to masonry buildings in Southern California. He had recently moved to the University of California San Diego from New Zealand and instead of agent sitting silently and humbly listening to more established and older colleagues, he was playing the provocateur , asking questions and expressing statements that in literature would sound like the king has no clothes. One of the reasons why his name was well known to me was that a few years earlier, when I was sitting in Vitelmo Bertero’s class at Berkeley, the number one reference he was recommending was a book edited by Emilio Rosenblueth, titledDesign of Earthquake Resistant Structures, published in 1980 (Rosenblueth, 1980). Nigel was the author of the chapter on masonry structures and considering the eminent authors listed in the table of contents, I was expecting to meet a senior and mature professor. On the contrary, he looked like a young actor and had the scathing irony of a GB Shaw. It is a matter of fact that Nigel in his thirties was already a recognized star in earthquake engineering, namely on masonry, one of his first engineering passions. I soon learnt that his area of expertise was indeed much wider (including prestressed concrete, bridges, shear wall buildings, tanks and silos, seismic response, etc.) and that he had gone through an incredibly fast and varied education, entering university at the age of 16, obtaining a PhD when he was 23, going
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for a Post-Doc in Lisbon and then heading a public laboratory for seven years before joining back the university of Canterbury at the age of 33. One of many examples of his encyclopedic knowledge stood out a few years later, while we were working together on the repair and strengthening of the Anatolian Viaduct, which had been shortened of about 1.5 m by the 1999 Duzce earthquake, with a similar permanent lateral displacement. We decided to transform 119 simply supported spans into six continuous decks of about 800 m each (Priestely and Calvi, 2002). A problem to be checked and solved was srcinated by the stresses induced by differential temperature variations in the upper and lower parts of the deck. I confess I was surprised to learn that a viable simple approach to face this problem had been developed and published by Nigel some fifteen years earlier. I quote here from our design report: “Differential thermal loading, with top surface hotter than soffit will tend to cause the spans to hog upwards. In the current configuration this is unrestrained, but making full continuity over supports will mean that hogging is restrained, inducing tension stress on the soffit. The method of analysis is that developed by Priestley (see chapter 5, The Thermal Response of Concrete Bridges in (Cope, 1987))”. Another enlightening example of his influence on engineering practice can be derived by the observation that most codes used worldwide to design tanks against seismic action are based on similar approaches. The reason is simple: they are all essentially derived from the same document: “Seismic Design of Storage Tanks” (Priestley et al., 1986).
US times
In Christchurch, he was third in line of a breed of giants, spaced ten years apart by birth: Tom Paulay, Bob Park and himself. One of the reasons to move to San Diego might have been some need of more independency, but he always kept strict and friendly relations with his mentors, particularly with Tom Seismic design of Paulay. Actually, in 1992 they published together the first of Nigel’s bestsellers: reinforced concrete and masonry buildings (Paulay and Priestley, 1992). Though published when Nigel had been living in Solana Beach for about six years, I believe that this book (the drafting of which actually started eight years earlier) could be considered the summary of his work in Christchurch, only marginally influenced by his American experience. Once in California, he was the most relevant player in launching UCSD in the arena of structure and earthquake engineering and in constructing and extensively using the new experimental laboratory. The timing was perfect, since the earthquakes of Loma Prieta (1989) and Northridge (1994), followed one year later by the event in Kobe (1995), demonstrated the inadequacy of the design approaches applied worldwide and required extensive experimental testing and revisiting of equations and detailing used in practice, as well as the development of dependable though immediately applicable strengthening measures. In those years Nigel proposed novel equations to estimate the shear strength of columns and walls (known as the UCSD model), equations to estimate the flexural capacity in presence of inappropr iate bar termination or insufficient overlapping, techniques to enhance shear strength and flexural deformation capacities based on steel or composite encasing. Seismic Most of this enormous amount of work is distilled in another bestseller, published in 1996: design and retrofit of bridges (Priestley et al., 1996). This book constituted the answer waited for worldwide to change the logic of bridge design, construction, assessment and strengthening. It was immediately translated into Japanese and Chinese and it is still the internationally recognized reference on the subject. A very fortunate book, still selling. The book is permeated with Nigel’s way of approaching problems and life, of his spirit, thoughts and
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work on earthquake engineering. A book that undoubtedly had an influence on engineering practice and on the intellectual growth of younger generations of engineers. Similarly to what I stated with reference to the book published with Tom Paulay, I believe that this book can be considered the summary of Nigel’s work and developments while living in the US.
ROSE School times
While still mainly living in California, Nigel was extensively travelling all over the world, for consulting on major structural projects, such as the Anatolian Viaduct mentioned earlier, but as well because of his unceasing curiosity for places and humanity. We had many opportunities to discuss about art and philosophy, often deviating to art and philosophy of structural and earthquake engineering. One of the outcome of these symposia was the inception of a new graduate school focusing on seismic design, based on the association of a group of well known professors who accepted to teach intensive courses in series, with students fully immersed in a single course at a time. The school had an enormous success and is now recognized as one of the world leading places to study earthquake engineering. In his last public appearance, Nigel stated: “ professor Calvi and I spent a lot of time discussing the format and mechanics of this rather unusual school. This association with the ROSE school has been the most enjoyable part of my technical career. I feel cheated that this association has to terminate”. The representative legacy of these years’ work is undoubtedly another book, published in 2007: Displacement Based Seismic Design of Structures (Priestley et al., 2007). To express the relevance and impact of this book, I like to borrow some words from a review published on a major international journal by Graham Powell, Emeritus at the University of California, Berkeley: “It is rare for a book on structural engineering design to be revolutionary. I believe that this is such a book. If you are involved in any way with seismic resistant structural design, this should be on your bookshelf, and you should read at least the first three chapters”. Displacement based design is actually today the subject of studies of hundreds of researchers and students, who are working on refinements and extensions, thus confirming Powell’s statement.
The end
Nigel Priestley was a lover of poetry and a poet himself all through his life. For this reason I believe it is appropriate to close this memories quoting a message he sent me some time before passing away. “Caro Michele, I came across this translation of a poem by Callimachus (310/305-240 BC) on the death of his friend Heraclitus. I think it is rather beautiful, and thought you might appreciate it:
They told me, Heraclitus: they told me you were dead. They brought me bitter news to hear and bitter tears to shed. I wept when I remembered how often you and I Had tired the sun with talking, and sent him down the sky” I feel that Nigel’s final legacy is his intimate pleasure in tiring the sun with talking, provided that the arguments have adequate substance and intensity to send him down the sky.
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Figure 1.2 - The essence of displacement-based design in one figure: · The real structure is represented by a single degree of freedom model. · The actual non-linear response is simulated considering a secant stiffness to the design displacement and a damping equivalent to the energy dissipated in the response loops. · The equivalent damping ratio is estimated on the base of the ductility demand. · A displacement spectrum reduced according to the equivalent damping is entered with the design displacement, to obtain a period of vibration, and consequently a value for the equivalent stiffness and for the required design strength.
References Callimachus (310/305-240 BC). Epigrams, II. (the Greek srcinal text follows).
Cope R.J. (1987) - Concrete Bridge Engineering: Performance + Advances, Elsevier Applied Science. Paulay T., Priestley M.J.N. (1992) - Seismic design of reinforced concrete and masonry buildings. John Wiley. Priestley M.J.N, Calvi G.M., Kowalsky M.J. (2007) - Displacement Based Seismic Design of Str uctures, IUSS Press. Priestley M.J.N, Seible F., Calvi G.M. (1996) - Seismic design and retrofit of bridges. John Wiley. Priestley M.J.N., G.M. Calvi (2002) - Strategies for Repair and Seismic Upgrading of Bolu Viaduct 1, Turkey, Journal of Earthquake Engineering, 6:SP1, 157-184. Priestley M.J.N., Wood J.H., Davidson B.J. (1986) - Seismic Design of Storage Tanks, Bulletin of the New Zealand National Society for Earthquake Engineering. 19:4, 272-284. Rosenblueth E. (1980) - Design of Earthquake Resistant Structures. Pentech Press.
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Remembrance will endure, if the life shall have merited it Michael P. Collins
Nigel and I met in 1961 as fellow undergraduate students at the University of Canterbury. The 62 of us in that Civil Engineering class were privileged to be there at the start of what was to be a golden era for structural engineering at Canterbury. The School of Engineering had just moved to a new campus where, under the long-term leadership of Professor Harry Hopkins (a structural engineer, a Rhodes Scholar at Oxford and the winner of the Distinguished Flying Cross), Civil Engineering now had a large, modern Structures Laboratory and excellent technical support staff. Most importantly, Hopkins had hired some very talented academic staff with the newest addition being Tom Paulay, a graduate of Canterbury who after eight years as a structural engineer in Wellington had been hired to teach Structural Design. There was a sense that this was a special place and a special time with serious problems to be solved but like nearly all engineering students our belief was that we should work hard but play even harder. Nigel and I became friends because we enjoyed the same courses, went to the same parties, and both loved the mountains and skiing. It also helped that our girlfriends were cousins. For our final year of undergraduate studies Nigel and I moved out of student residences and with another student, Dave Murray, rented a house some miles from the campus. We had many memorable parties at the house attended not only by our fellow students and their partners but also by a number of the faculty including Harry Hopkins, Frank Henderson and Tom Paulay. Dave and I learnt that Nigel was a wonderful cook and we happily acted as dish washers and kitchen help to this chef. I also observed that while Nigel always seemed to pick up difficult technical material effortlessly, this academic brilliance was kept polished by long hours of intense study late at night. I decided to try out this novel idea of studying for hours each night and as a result, like Nigel, earned my BE with First Class Honours and went on to win a scholarship to pursue graduate studies. In fact seven of us from this class of 1963 went on to earn PhD degrees. Both Nigel and I returned to the University of Canterbury in 1975-76, he as a new faculty member in Structural Design and me as a visiting lecturer on sabbatical from the University of Toronto. It was a very productive time for us all with daily exchanges of ideas over morning and afternoon cups of tea or coffee and then later a few beers at the Faculty Club. The staff-student cricket match was particularly memorable with Harry Hopkins again demonstrating the
Figure 1.3 - With Nigel in Mount Hutt, Canterbury.
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sporting mastery that had earned him his Rhodes Scholarship. The three P’s, Tom Paulay, Bob Park and Nigel Priestley, all hired by Harry Hopkins were now in place and the golden years at Canterbury were well under way.
Figure 1.4 - John Ber rill, Nigel, Harry Hopkins and Russell Poole... Christchurch 1976.
Nigel had a sabbatical at Toronto in 1981. At that time he was completing his research on “Thermal Stresses in Concrete Structures” which had demonstrated that just the sun shining on a concrete structure could, in some cases, result in significant cracking of the structure. A key insight was that these thermal events imposed not loads on the structure but rather caused deformations of the structure. Thus we need to design for the deformations not the loads. Nigel joined the University of California San Diego in 1986 and began an ambitious research program aimed at upgrading the seismic resistance of California’s bridges. After the Loma Prieta earthquake of October 1989 the program was accelerated with significant funding increases from Caltrans, the agency responsible for the highway bridges in California. In 1995, 1998 and 1999 we spent time in San Diego often when Nigel was in New Zealand. One of the Caltrans engineers who came on a site visit while Nigel was away explained to me why they liked funding Nigel’s research. He said that if Nigel asks for two million dollars to solve a given problem in two years, at the end of the two years he has spent the money and the final report gives you the solution to the problem. Most other professors, given the same opportunity, will spend the money and the final report will typically explain why, although progress has been made, another two years and another two million dollars are needed to solve the problem. Probably this is why at the 2008 Lake Tahoe symposium honouring Nigel, the former Caltrans Chief Earthquake Engineer Ray Zelinski, stated that in his opinion no one had done more for the safety of the people of California than Nigel Priestley.
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Figure 1.5 - Nigel is “hands on” both in the experiment and at the barbecue afterwards, California, San Diego 1995.
Figure 1.6 - Tom, Nigel, Jan and Judy at Pegasus Bay Vineyard, Canterbury.
Figure 1.7 - Five members of the class of 1963 at Waipara Vineyard, Canterbury, February 2014.
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Figure 1.8 - Nigel, Jan, grand-daughter and Judy, Diamond Harbour, Canterbury, March 2014.
Nigel Priestley changed my life in a number of positive ways. First in 1963 he showed me how to lift my academic performance by devoting some hours each day to studying the fundamentals of engineering. Then in 1997 he demonstrated that you could be a successful North American academic and still spend two or even three months a year in New Zealand where the spectacular scenery that we both loved is inspiring and helps the creative spirit. Finally in 2001 he made me par t of the ROSE School at Pavia with the concept of teaching intensive graduate courses on seismic design of structures in a whole new way. As the Roman statesman, consul, governor of Britain and keeper of the aqueducts, Julius Frontinus stated when writing to Pliny about the death of a mutual friend:
“Remembrance will endure, if the life has merited it.” For Nigel the life has merited it.
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His efforts will be significant for a long time into the future Athol J. Carr
Student Days
My contacts with Nigel go back to our First Professional Year Civil Engineering studies in 1961 at the University of Canterbury where Mike Collins (now University of Toronto). John Berrill (Canterbury Seismic Instruments) and I, together with many others, were class-mates. Through the undergraduate course Nigel quickly became regarded as one of the out-standing members of our class. After completing our BE (Civil) degrees Nigel started on a Ph.D programme at the University of Canterbury. Nigel later went to Portugal on a post-doctoral study and then came back to join the New Zealand Ministry of Works. I worked in a consulting engineering practice for 10 months before going to the University of California to complete an M.S.Eng. and a Ph.D with Professor Ray Clough as my graduate advisor as well as the supervisor for both degrees. On my return to New Zealand I joined the Department of Civil Engineering at the University of Canterbury as a lecturer in Civil Engineering. My major interest at that time was finite element analyses and structural dynamics, and we lost contact over this period.
Ministry of Works
In the early 1970s one of the major engineering concerns in New Zealand was the behavior of boxgirder bridges subjected to thermal loadings, with severe distress showing in some of the bridges. The National Roads Board had set up a study group to investigate the problem and a group of researchers was established to carry out appropriate research. Nigel Priestley was then in charge of the Central Laboratories of the Ministry of Works who were carrying out measurements of the effects on the bridges as well as determining a means of calculating the thermal profiles in the bridges under the effects of solar radiation (Priestley 1976; Priestley & Buckle, 1978). At the University of Canterbury, I was working with a Ph.D student, T.A. Moore, (Moore, 1975) to develop a finite element analysis for non-prismatic box-girder bridges which included the thermal effects using the data Nigel was gathering. We later showed that for some multi-span reinforced concrete box-girder bridges the thermal loading case was the critical design case, see Figure 1.9. The team from the Civil Engineering Department at the University of Auckland was working on finite strip analysis techniques for the same types of reinforced concrete bridges. These three research groups met three to four times a year to report progress, compare results and work out strategies for the next stage of the research. Nigel was also instrumental in developing a code of practice for the seismic design of reinforced concrete tanks and reservoirs in New Zealand.
University of Canterbury
In 1976 Nigel left the Ministry of Works and joined the Department of Civil Engineering as a Senior Lecturer. He brought with him his analytical and experimental skills and immediately fitted into the concrete laboratory with Professors Park and Paulay, forming the team known locally as the Three Ps (Park, Paulay and Priestley). Nigel carried out research with Professor Park on concrete structures and as a large part of the funding for the research came from the National Roads Board bridge structures and bridge substructures
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featured strongly in the research (Priestley and Park, 1984). Nigel worked with Professor Park to get funding for the Dartec test machine that was later installed in the Concrete laboratory in the Civil Engineering Department. The installation was not that easy with the very high water-table at the Ilam Campus. Nigel was given charge of the laboratory and of the Dartec test machine. This test machine, see Figure 1.10, has enabled many research projects for both Master of Engineering and Doctor of Philosophy students.
Figure 1.9 - Cumberland Overpass, Dunedin. This box-girder bridge divides into three on-ramps at the near end and two on-ramps at the far end.
Besides his interests in reinforced concrete structures and tanks Nigel was also interested in the seismic performance of masonry structures (Priestley, 1977). Nigel also led the development of a shake-table for the Civil Engineering Department. This simple single acting shake-table uses the same hydraulic pumps as those for the Dartec test machine but the table has been in very heavy demand ever since its installation for both ME and Ph.D students (Priestley, 1978), (Kao, 1998) as well as for demonstration of inelastic behavior to both final year and post-graduate students. See Figures 1.11 and 1.12. With the shake table installed Nigel had students studying rocking systems. While the experimental results were very good Nigel was concerned with getting a good computational model that would replicate the experimental results. Nigel, as with Professor Paulay, realized that with good computational models further parametric studies could be obtained analytically much faster and more easily than by repeated experimental work. Nigel worked with me in getting the appropriate computational models that could be input into the non-linear dynamic analysis program Ruaumoko (Carr, 1982, 2016). Nigel and I collaborated with several research projects where nonlinear computational models were involved. This shake-table is now over 30 years old and although it has a number of problems it is likely to remain in use for many years to come.
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University of California, San Diego
In 1987 Nigel moved to a professorship in California at UCSD. He was in his element with large scale experimental work in the PRESSS programme and also did considerable work with non-linear dynamic analyses. He obtained a license for Ruaumoko for UCSD, the second overseas university license and the first in the USA.
Figure 1.10 - University of Canterbury Dartec Test Machine.
Figure 1.11 - Shake Table with Rocking Joint Low Damage Frame (Murahidy, 2004).
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Figure 1.12 - Frame with Replaceable Plastic Hinges with Semi-Active Control System Attached (Franco-Anaya, 2008).
There seemed to be a feeling that for computer software that if it was not written in the USA it did not exist. I still have a feeling that that belief still exists even today as I note that things that we incorporated into our software in 1979 have only been put into software in the US within the last year. This was the start of the spread of Ruaumoko in the USA. As Nigel’s graduate students finished their degrees and moved to other universities they also wanted the computational facilities of Ruaumoko and obtained further academic licenses. I have much to thank Nigel for. During his time in San Diego Nigel also maintained his strong contacts in New Zealand and he and Professor Tom Paulay produced their book on the design of reinforced concrete walls and masonry structures in 1992 (Paulay, 1992). During this time Nigel also pursued his push to change the ways engineers designed structures to resist earthquake excitation, from designing for so-called earthquake forces to designing for seismic displacements.
Pavia and the Rose Program
In the late 1990s, Nigel and Professor G.M. Calvi established the Rose School at the University of Pavia, Italy. This is a post-graduate program, taught in English, using top academic researchers and teachers from around the world to give the best students the best post-graduate education and research in earthquake engineering. Since the turn of the century this program has produced a large number of exceptional engineers who have found academic positions throughout the world. The students have had the best of laboratory and computational opportunities as well as a range of very good graduate courses to give them the tools to carry out their studies. In 2004 Nigel asked me if I would offer my particular non-linear dynamic analysis course as part of the Rose program. I was honoured to be asked and the course has now been given four times during the past twelve years. I have been most impressed by the calibre of the students in the Rose Program. With the Rose School Nigel made great contributions to research and teaching as well as furthering his efforts in displacement-based design. A large part of Nigel’s research involved non-linear analyses and many of the features in Ruaumoko are there
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because researchers needed those features. Nigel has made a major change in the way a large part of the world designs structures. His design concepts tested by many research students led eventually with Professors Calvi and Kowalsky, to the creation of their book on Displacement-Based Design in 2007 (Priestley, 2007). In this book Nigel wanted the readers to be able to follow the non-linear analyses aspects covered in the examples in the book, and he obtained a license for a restricted version of Ruaumoko to be made available with the book.
The Canterbury Earthquakes nd of February 2011 Christchurch Following the 4th of September 2010 Darfield earthquake and the 22 earthquake the New Zealand Government established an engineering advisory group for the recovery and a Royal Commission to investigate the collapse of some of the buildings during the Christchurch earthquakes. Nigel was one the eminent engineers chosen to help guide the engineering profession through the assessment and evaluation of consequences of the strong shaking that Christchurch buildings had been subjected to.
Summary
Nigel has had a major influence on structural design, on research and the teaching of structural engineering in New Zealand and throughout the world. The results of his efforts will be significant for a long time into the future.
References Carr A.J. (1982) - Ruaumoko manual. Report, Department of Civil Engineering, University of Canterbur y, Christchurch, New Zealand. Carr A.J. (2016) - Ruaumoko manuals. Volumes 1-5, Carr Research Ltd., Christchurch, New Zealand. Franco-Anaya. R. (2008) - Use of Semi-Active Devices to Control Deformation of Structures subjected to Seismic Excitation., PH.D Thesis, Department of Civil Engineering, University of Canterbur y, 2008. Kao G.C. (1998) - Design and Shaking Table Tests of a Four Storey Miniature Structure Built With Replaceable PlasticHinges. ME Report. Department of Civil Engineering, University of Canterbur y, February 1998, 224p. Moore T.A. (1975) - Finite element analysis of box-girder bridges. Ph.D Thesis. Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand, 1975. 288p. Murahidy A.G. (2004) - Design, Construction, Dynamic Testing and Computer Modelling of a Precast Prestressed Reinforced Concrete Frame Building with Rocking Beam-Column Connections and ADAS Elements. ME Report, Department of Civil Engineering, University of Canterbury, February 2004. 154p. Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concreteand Masonry Buildings. J, Wiley and Sons Inc. Nework, Y 1992, 744p. Priestley M.J,N., Buckle I.G. (1978) - Ambient thermal response of concrete bridges. RRU Bulletin 42, Bridge Seminar 1978, Volume 2. Road Research Unit, National Roads Board, Wellington, New Zealand. Priestley M.J,N., Park R. (1984) - Strength and ductility of bridge substructures. RRU Bulletin 71, Bridge Design and Research Seminar, Auckland 1984. Road Research Unit, National Roads Board, Wellington, New Zealand. Priestley M.J.N, Calvi G.M., Kow alsky M.J. (2007) - Displacement -Based Seismic Design of Structures. IUSS Press,Pavia, Italy. 2007, 719p. Priestley M.J.N. (1976) - Design thermal gardients for concrete bridges. New Zealand Engineering, 15 September 1976, pp 213-219. Priestley M.J.N., Crosbie R.L., Carr A.J. (1977) - Seismic Forces in Base-Isolated Masonry Structures. Bulletin of N.Z. National Society for Earthquake Engineering. 10 (2), June 1977: 55-68. Priestley M.J.N., Evison R.J., Carr A.J. (1978) - Seismic Response of Str uctures Free to Rock on Their Foundations. Bulletin of the N.Z. National Society for Earthquake Engineering, 11 (3), September 1978: 141-50.
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An implacable search for truth 1 (We must find time to stop and thank the people who make a difference in our lives )
André Filiatrault
Unlike most contributors to this volume, I have not had direct technical interactions with Professor Nigel Priestley. I have not been one of his students. I have not had the privilege to co-author technical papers with him. I have not participated in consulting projects with him. Yet, I felt compelled to contribute to this volume to stop and thank Professor Nigel Priestley for making a difference in my life. I first met Professor Nigel Priestley in 1987 when I was a PhD student in Civil Engineering at the University of British Columbia (UBC). Professor Don Anderson, one of my mentors at UBC, had spent a sabbatical year at the University of Canterbury a few years back and became a good friend with Professor Priestley. Don invited Professor Priestley to visit the UBC campus after he had moved to the University of California at San Diego (UCSD) earlier that year. Among the various seminars and activities scheduled during the visit, I had the opportunity to participate in a dinner with Professor Priestley and a few senior PhD Students. This meal, like many others to come, provided an opportunity to meet up close an upcoming giant in our field. I was impressed by his interest with the various su bjects that the students were working on and immediately felt that Professor Priestley had a genuine thirst for the truth. Professor Priestley was never blinded nor limited by politics. He lived his professional career in search of true innovations that ultimately would save lives during earthquakes. Although this purist approach may have perhaps prevented him from being recognized by some peers and organizations to the highest level that he deserves, it also allowed him to focus his multiple talents to truly making a difference in the field of earthquake engineering. It is during that dinner that he asked me to call him Nigel. Despite the short time that we had met, I felt that he was my friend. My next significant personal contact with Nigel was during a sabbatical leave that I spend at the University of Canterbury from the fall of1994 to the spring of 1995. Nigel was on a three-month visit at Canterbury during that time and we interacted often during the “sacrosanct” morning teatimes under the strict supervision of the Department’s Bishop (a.k.a. Professor Thomas Paulay). I have fond memories of Nigel bursting out laughing at Tom’s “daily jokes”. Beyond everything else, Nigel loved and enjoyed life. The proudest period of my professional career was the short time that I was able to call myself a colleague of Nigel, when we shortly overlapped from 1998 to 2000 at UCSD. At that time, Nigel was at the tail end of directing the US-PRESSS Program and his research directly inspired the development of selfcentering steel structures that Constantin Christopoulos and I initiated at UCSD. Through interactions with his graduate students and while serving on some of his students PhD defense committees, I could witness first-hand Nigel’s genius and his passion for the truth. Nigel amazed me on how he could take a complicated problem and turn it into a simple applicable solution. To me Nigel embodied the famous quote by Albert Einstein: “ Everything should be made as simple as possible, but not simpler”. It was during that period that Nigel changed my life. I will never forget the day that I walked in front of the old army barracks at UCSD (the 409 Building) and met Nigel coming down the short stairs. While we walked together towards the Price Center, Nigel told me that he was starting a new earthquake engineering graduate school (the so-called Rose School) in Pavia in Italy with a peculiar academic program. He told me that they were looking for someone to teach a course on seismic isolation and
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John F. Kennedy
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supplemental damping and casually asked me if I was interested in teaching it. This short exchange with Nigel was the starting point of my long-time association with Pavia that has steadily intensified over the last 15 years. In fact, it is from Nigel’s old apartment in Pavia that I write these lines today. Through Nigel’s casual request, I was given the opportunity to grow through the rich professional and cultural experiences with the wonderful people of Pavia. As Nigel’s health was failing, I felt sad and helpless. I kept contact with Nigel through occasional messages but mostly through his long-time friend and confident Michele Calvi. In 2012, I decided to pose a symbolic gesture to celebrate the life of my friend. I have been running marathons for many years and I had registered to run the Goodlife Fitness Marathon in Toronto that year. Running a marathon for me represents the metaphor of life and I decided to dedicate my race to Nigel. I asked for Nigel’s permission to print his name on my running shirt (Figure 1.13) and I was thrilled when he accepted. I spent the nearly four hours that I required to make it to the finish line thinking positively about Nigel and the severe health challenges that he was facing. The several thousands of people that lined up the streets of Toronto during the race that day encouraged runners by their names when they see it on their running shirt. For 42.2 kilometers, I was Nigel to them. Although I am not a religious person, I believed that thousands of people shouting encouragements at Nigel’s name sent positive energy his way. I was happy to receive Nigel’s positive response after the race (Figure 1.14). The marathon concluded with a copious meal at Constantin Christopoulos’ house with Judy and Michael Collins (Figure 1.15), where we toasted several times to our friend Nigel.
Thank you Professor Priestley for pushing the boundaries of earthquake engineering through your implacable search for truth. Thank you Professor Priestley for being such an inspiration. Thank you Nigel for changing my life. Your friend forever.
From the Toronto Marathon “The several thousands of people that line up the streets of Toronto during the race always encourage runners by their names when they see it on their running shirt. Although i am not a religious person. I believe that thousands of people shouting encouragements at Nigel’s name can only send positive energy his way.”
Prof. André Filiatrault May 6th 2012
Figure 1.13 - Celebrating Nigel’s life during the 2012 Toronto Marathon.
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Figure 1.14 - Post-marathon dinner in honor of our friend Nigel.
Dear Andre, Your impressive T-shirt has been delivered, and is cur rently prominently displayed in our new apartment in Pavia. Regardi ng the positive thoughts you put into my health during your ru n: I am feeli ng rather better at present. Perhaps it is as a result of your thoughts, or possibility just bei ng in Pavia , or a combi nation of th e two. Let’s hopr th e improvem ent conti nues. Many thanks, and Cheers, Nigel
Figure 1.15 - Post-marathon message from Nigel.
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Never able to touch the same water twice (just like the passing of time, but warm memory from reflection of calm flowing water never fades)
Rob Chai
My days as a graduate student saw the dawn of Nigel’s era at UCSD. I still remember that incredible, reverent feeling when I first came to the Powell Lab, first among the series of great laboratories at UCSD. It was the spring of 1987 when Nigel offered me a research assistantship to work as a PhD student. It was still early in my career, but I knew then the opportunity to work with a great mentor while conducting cutting-edge research in a world-class facility was a once-in-a-lifetime opportunity. In those fledging days though, Powell Lab was a facility waiting to be discovered. When I first joined, the lab was filled by no more than two or three projects at any given time, and the unused floor area was spacious enough for a forklift to make a full 360 degree turn, but that all changed once Nigel arrived. A career transition from California to New Zealand would be challenging for most, but not for Nigel. He was in the company of some of the top bridge engineers in the world at the admirable young age of 43. The late 80s was an opportune time for bridge research, as the State of California was embarking on Phase II of their retrofit program for bridge substructures. Nigel was fully cognizant of the various structural deficiencies of old California bridges, and he immediately initiated a comprehensive and innovative retrofit program for these substructures. His efforts proved to be very successful, garnering respect and praise from the research community. Capitalizing on his earlier work with Rob J.T. Park Jr. in New Zealand on steel-encased piles, son of Professor Park, he immediately saw an almost natural, application of steel tubes as jackets to enhance the ductility capacity of deficient flexural columns, regain their flexural capacity from inadequate lap-splice length, or increase the force capacity of the shearcritical columns. The retrofit technique was later extended to fiber-reinforced polymer jackets with equally promising results when compared with steeljacketed columns. His leadership on the substructure test program brought California to the forefront of seismic retrofit research for highway bridges around the world. I have always enjoyed Nigel’s writing. He was, without a doubt, a prolific author. He taught me generously in
Figure 1.16 - Nigel in the 2008 dedicated to him.
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technical writing, lessons for which I am forever grateful. I still remember his frequent, patient reminders: “Don’t forget your punctuation!” I admit I still struggle with writing to this day, but thanks to his little prompts, I have since learnt to recognize the intricacy and importance of punctuation in the English language. In 1992, Nigel’s research on steel jacketing received the K. B. Woods Award, presented by the Transportation Research Board in recognition of a paper of outstanding merit. I felt immensely proud and honored to be associated with Nigel as a co-author on the paper, and I tagged along for the award ceremony on my first ever red-eye flight to Washington D.C. Little did I know that Nigel needed little or no sleep on the long flight, and his ability to focus during the exhausting journey was nothing short of impressive. On our return flight to San Diego, which took about 6 or 7 hours including a short layover, Nigel was able to finish a handwritten draft of a paper, probably 10 or 12 pages long, ready for final typeset the next Figure 1.17 - Nigel next to a test column with a steel jacket. day. His clear thinking, skillful writing, and the drive to get things done were an inspiration to all fortunate enough to witness it. The seismic retrofit program in California and around the world was given further impetus after the 1989 Loma Prieta Earthquake, despite the tragic backdrop. I made a reconnaissance trip to Oakland with Nigel and Dr. Seible immediately after the earthquake, primarily to learn about the collapsed double-deck Cypress Street Viaduct on Interstate 880. Prior to the trip, Nigel asked me, with a sincere and fatherly tone, if I was fully aware of the dangers of traveling to ground zero in an earthquakeravaged region. I knew he expressed concern due to the imminent birth of my son, but how could I stay behind when disaster had struck so close to the core topic that motivated our research? The slow walk along the 1.6 miles stretch of the collapsed freeway was poignant and emotionally draining. When I first approached the structure, I was speechless for what I felt was the longest time. I kept asking myself how such a highly-engineered structure could have collapsed the way it did when seemingly old warehouses, designed and built with much less scrutiny, remained unscathed no more than 200 or 300 feet away. I had driven on the lower deck of the Cypress structure several times while visiting the Bay Area, so as I walked, I could only count my blessings when I saw crashed cars between those decks. When I looked at Nigel, he remained calm and collected, taking occasional notes, despite the horrid images unfolding in front of us. The earthquake added nine months to complete my dissertation but it was the best extension I could have asked for. For in the process, Nigel taught me one lesson of utmost importance, which is that one must start by appreciating the complex seismic response of structures,
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Figure 1.18 - Nigel and Rob in Idyllwild in 2014.
for only after that one can seek simple solutions. His tutelage subconsciously planted my feet firmly on the ground, allowing me to learn to be an engineer. I often ask myself how I want to remember Nigel and what I would consider as his legacy. He has set examples that many of us have try to emulate. One of my guiding principles is that I would like to treat my students the way Nigel had so kindly treated me. When I talk to my students about concrete structures, whether it is confinement requirements or ductility estimation, or capacity design principles, or displacement-based design, I am invariably carrying on the Priestley legacy. He had always encouraged us to take the road less traveled, for challenges in life not only shape our characters, they also instill in us the belief that humanity is better served by our strong will to make things better. I am also reminded of a fitting quote I read some time ago: “ The first test of a truly great man is his humility”. Many of us wish we had spent more time with him, professionally in the lab or socially over a glass of wine, but on days like this, thinking of him and seeing his picture on the wall makes me feel better already. He will always remain an inspiration to me and I can never thank him enough.
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He let out a subtle smile and said: “damn it” Mervyn Kowalsky
I suspect just about everyone who has taken courses, or observed Nigel’s teaching will come away with similar observations to mine. I present the observations below from multiple perspectives, starting with my time as an undergraduate student at UCSD. UCSD utilizes a very transparent system with regards to course evaluations, which is referred to as ‘CAPE’ (Course and Professor Evaluations). At the end of the quarter, a CAPE representative, would come to the class and hand out the forms to be filled by each member of the class and then collected by the CAPE representative. During this time, the professor would not be present. Each year, the annual ‘CAPE’ book was published and sold at the bookstore for $1 or $2. Faculty at UCSD needed to have thick skin, as these books would not only provide quantitative results regarding the number of enrolled students who recommend the course and instructor, but also a summary of written comments, many of which would be rather honest, and often brutal in assessment. The system is still in place at UCSD, although it is now electronic, and participation voluntary (a quick review of the UCSD-CAPE site shoes 20 to 30% participation, in many cases). During the 1990’s, participation was 100% of people who were in class on the day of the evaluation. Each quarter, as students planned their courses, we would consult the CAPE book, and where possible, choose instructors that were highly rated. I recall suffering through some courses from instructors who received scores of 10 to 20% in prior quarters. Back then, one could assume that instructors receiving 60 to 70% would likely be very good teachers. I recall looking at all of the courses in the structures curriculum early in my undergraduate degree, and dreading the courses and instructors that received low scores, and delighting in the anticipation of an instructor who approached 80% recommended. I also recall each year, a professor by the name of Nigel Priestley earning scores that seemed impossible, always above 90%, and in many cases 100%, in courses with 40+ students, where nearly every student submitted an evaluation. It was so far outside the norm at the time, great anticipation built until finally, in the second quarter of my senior year in 1993, I enrolled in AMES 135 – Reinforced Concrete Design. It was immediately obvious how Nigel managed to earn such high scores. As faculty, we often debate the merits of student evaluations suggesting that the scores are proportional to the grade distribution, or ease of a course. In my experience the opposite is often thecase. Nigel’s courses were not ‘entertaining’, i.e., he didn’t tell jokes ortry to amuse us (at least no intentionally!). They most certainly were not easy . Instead, he earned his high evaluations via an uncanny ability to communicate concepts clearly, with no wasted words, in a manner that built your confidence in the subject matter (while also identifying when a particular code provision was ‘total rubbish!’). We did have one amusing instance during that first course I took with Nigel. As anyone who has taken AMES 135 from Nigel will know, he always brought a large three ring binder to class. It was several inches thick, with several well-worn pieces of paper sticking out from it. The brick colored notebook was placed (closed) each day on a table at the front of the class. Nigel would then proceed to give each day’s lecture purely from memory, including several detailed numerical examples. Even though he was not trying toentertain us, the manner in which he delivered the material left us feeling as if we were watching an incredible performance, all the while learning at the same time. His classes were not to be missed. One day, towards the end of the semester, Nigel was working a problem on the board when he paused. For several seconds, he was simply looking at the chalk board, and then he sheepishly glanced at his closed three ring binder on the table. At this time, the class was silent, and holding their breath, wondering if the lonely notebook was finally about
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to be opened. Nigel let out a subtle smile and said, “Damn it!”, as he walked over to his binder to open to a page to find a number in a calculation that he had apparently forgotten. In much the same manner as a baseball pitcher who carries a perfect game into the ninth inning only to give up a hit and receive a nice ovation from the crowd, our class erupted in applause for his incredible performance over the entire semester, not once having looked at hisnotebook. It would be the first of two such ovations Nigel would receive during that course, the second of which occurred after the las t lecture. In the 1990s (and perhaps still today), it was a tradition that spanned all classes at UCSD where instructors would be applauded after the last class was held, however, the cheering was especially robust in AMES 135. As far as that one instance during the course when Nigel consulted his notebook, I still wonder if it was intentional, aimed at determining if the class was paying attention! In several more classes I would take with Nigel over the coming years, I never again saw him consult his notebook. Nigel’s teaching technique was one of treating the student with respect while expecting you to pay attention, and to be engaged in the topic at hand. He could make difficult material very accessible, as long as you were paying attention. Nigel taught as he wrote, deliberately, with no wasted words. As a graduate student, we had the opportunity to observe Nigel’s teaching style in smaller settings during meetings about our respective research topics. Research discussions with Nigel during my days as a student would often last an hour or more, usually with me feeling mentally exhausted as a consequence of the depth of discussion that took place. However, Nigel had a way to make every student feel that their work was special – you couldn’t help but feelthat you were working on a problem of great importance, which of course motivated us even further. He would also give us great freedom to pursue our own vision of the research. I recall during my PhD work suggesting to Nigel that instead of doing static knee-joint tests on light weight concrete, that we explore a multi-column bent shake table test that would allow us to evaluate joint performance while also providing experimental data for verification of displacement based design. He endorsed the idea, which for a young student was a wonderful feeling. Nigel also appreciated when we were honest about what we didn’t know. In the immediate aftermath of the Northridge earthquake, Nigel and Frieder Seible visited Los Angeles to conduct reconnaissance. Upon their return, and after securing bridge plans for many of the collapsed bridge structures, it was all hands on deck. Each bridge was assigned to 2 or 3 students, reporting to either Nigel or Frieder. The goal was to conduct an assessment of all collapsed bridges to determine the mode of failure, while preparing a detailed technical report summarizing the findings, and to do so as quickly as possible. I was 22 years old and had only been in graduate school for 4 months at the time, and in our first meeting to discuss the analysis plans for the bridge, Nigel in rapid fire described what we were to do. Described quickly, and with no wasted words, he asked us: “Got it?”. Five to 10 seconds of silence passed – I most certainly did not “get it” and after pondering my options, I responded with a “No”. He immediately smiled and said, “Honesty! That is what I like.” I use this example with my own students now, always encouraging them to be honest about what they know, and what they don’t. Throughout graduate school each of us had many opportunities to present at conferences. Much of the work we were doing was considered ‘controversial’ at the time, in my case being displacementbased design and shear capacity models for reinforced concrete columns. I remember several instances early in my career as a student where Nigel would listen to my presentations from conspicuous (or up front!) seats in the audience, and providing welcomed feedback after. If we got in trouble answering a question, he wouldn’t hesitate to jump in and help out. For many of us, our graduate student time with Nigel not only included mentorship, but also friendship. This took the form of lunches at Round Table pizza, tennis matches, visits to his home in honor of a student finishing their degree, visits to the mountains on New Year’s Day, or discussions on some new wine he had discovered. Around 2001 or so, Nigel retired from UCSD – several of us who had moved away to start our own academic careers returned for his retirement party, and Rob Chai gave a
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speech that I think every former student of Nigel could relate to. Rob, who is one of the most eloquent speakers I know, stood up and told the crowd: “Whenever I am in a difficult situation, about research, teaching, or life in general, I ask, what would Nigel do?” I was fortunate to continue my friendship with Nigel beyon d graduate school, and had theopportunity to see how he mentored other students at UCSD aswell as the Rose School. When I firsttaught at the Rose school in 2004, he alerted me to a student auditing my course and warned me that she was exceedingly sharp, and that I would need to be on my toes at all times when she would ask a question. Needless to say, Katrin Beyer herself has become a successful professor, a talent that Nigel was immediately able to identify early on in her graduate career. One year earlier, upon my first extended visit to Pavia, he brought me to the grocery store and taught methe proper techniques for securing produce, while also looking out for me when I rented my first vehicle abroad. These are small things, but are remembered well for their kindness. Nigel would always look out for his students, even after they had ‘grown up’. I also had the wonderful opportunity to work with Nigel and Michele on our textbook on displacementbased design. When it was completed in 2007, we had numerous trips around the world – a sort of book tour, where we would give 1 to 2 day seminars on the textbook. The first seminar that I was involved with were held in December of 2007 in North Carolina. It was a wonderful experience to have Nigel visit the area, and although our turnout was modest, it was a great experience. We followed that with seminars in the Dominican Republic, Vancouver, San Francisco, and Los Angeles, amongst other locations. The last two seminars with Nigel were done in San Francisco in 2013 and Los Angeles in 2014, and were the only ones to include all three of us. We had seminars in Alaska in late summer of 2014, but at that time, Nigel was no longer able to travel. At the February , 2014 seminar in Los Angeles, Nigel had been ill for some time, yet gave an incredible performance. There were over one hundred people in attendance, and as was customary, Nigel was the ‘closer’, always teaching, and driving home the case for Displacement-Based Design. During that seminar, Michele and I sat in the back during Nigel’s lectures and were amazed at his performance. For me, it was as if I was transported back to AMES 135. While he wasn’t moving around as normal, the seminar was vintage Nigel. Fittingly, at the end of the lecture, the attendees rose and gave Nigel a standing ovation.
Nigel and Tom catching up at Pigeon Bay, 2005.
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The Gracefield connection Sri Sritharan
Most of you know by now that Nigel began his professional career in 1967 at Central Laboratories in Gracefield, a small suburb of Wellington in New Zealand. My professional career also began in Gracefield, about 200 meters from Central Laboratories in the Department of Scientific and Industrial Research (DSIR), in 1989-22 years later. I took this position after receiving my MS degree at the University of Auckland under the mentorship of Richard Fenwick. Richard, an incredible and talented researcher, was Tom Paulay’s first PhD student. Richard assigned a nonseismic project to me, thinking that I would not have much interest in seismic research as my home country of Sri Lanka is not a country that experiences frequent earthquakes. While at Auckland, I developed interest in seismic engineering and I further delved into it when I joined DSIR as an Engineering Seismologist. While I enjoyed this research, it quickly became clear that I wasn’t appreciating terms such as plastic hinge length, ductility and concrete spalling as I never had a feel for what these terms meant in real structures. To overcome this challenge, I decided to find the best experimental university to pursue my PhD. David Dowrick, my colleague at the time, suggested that I write to Nigel to see if I could work with him to pursue my PhD. Though he noted that Nigel’s interest was more in masonry, which was true at that time, I followed David’s advice and asked Nigel if I could study with him. I did, however, question if this plan would materialize, so I applied to a few other schools. Now and then, I would get a fax from Nigel informing me about the application process. I finally received a communication from Nigel indicating that the University of California at San Diego (UCSD) would accept my application, despite some restrictions by the UC system to limit admissions to international students. Nigel wrote happily that I was the only international student allowed to join his department that year. To this date, I have no idea why Nigel gave me a life-changing opportunity to study under him, but I think that it must be the Gracefield connection! I was accepted, with full financial support, by a few other universities, including Stanford University. Several of my friends and colleagues encouraged me to attend Stanford, as it’s a more prestigious university. I knew that Stanford was another excellent opportunity, that my seismology background would be well received at this institution. However, this opportunity was not in line with why I wanted to do a PhD at the first place. I informed Nigel of my dilemma and he didn’t hesitate to tell me that there was no better place to pursue my PhD than at UCSD! Thus, my decision was made. As I was preparing to move to San Diego, a couple of unexpected events happened. First, my applicat ion for New Zealand citizenship came through and I was advised to go through a special ceremony so that I could go to San Diego as a “Kiwi”. I happily went through the ceremony, though doing so created some immigration-related challenges. Second, a few days before my departure a young beautiful girl walked into my office and introduced herself as Rebecca Priestley. Apparently, Rebecca was hired as a journalist at DSIR. I asked her if she had any connection to my new boss in San Diego… with a smile, she blurted, “He is my dad …you like that concrete stuff? Condolences!” Later I found out Rebecca came to see me after she found out that I was leaving DSIR to join Nigel’s research group. I left for San Diego a couple of weeks early as Nigel advised me to attend the Tom Paulay symposium. I previously met Tom during my MS study at Auckland. When he found out that I was working with Richard, he fondly suggested that I call him a “grandpa”, given that I was his student’s student. When I arrived in San Diego and met Nigel at the Tom Paulay symposium, his first concern was about my well-
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being. Then with a smile, he reminded me that I made the right decision to come to UCSD. As the school quarter began, I startedtaking classes, including one taught by Nigel. The depth and breadth oftopics that he covered in his classes were incredible. He was ready to tackle any questions that the students posed. There were plenty of them partly because the class had some experienced engineers from California-including Jay Holombo and Robert Dowell. I recall Nigel enjoying the interactions he had with the students and often challenging them (on the spot) tomake sure they learned and retained what he taught. One day in class, I asked a silly question without thinking it through. He turned, looked at me with a cunning smile, paused until I realized whatever I said was “nonsense” (one of Nigel’s favorite words), and finally turned back without saying a word and continued to teach class. No matter how blunt Nigel might have been, he readily appreciated when there was good feedback from his students. He took a great deal of pride in his students’ success, whether it was in doing well on exams, oral defenses or conference presentations. Over the course of my PhD career, I took every class that Nigel offered; this included seismic design, bridge design, assessment and retrofit of structures, masonry, and prestressed concrete. The classes were useful, informative, and challenging. I also started making frequent visits to the Powell lab where there was a seismic test almost every other day. If Nigel happened to be in the lab, he made sure the students understood what was being tested and what could be learned from the test observations. The combination of Nigel’s classes and the opportunity to witness tests in the lab tremendously enhanced my fundamental seismic knowledge. I knew then that I was at the right place with the right professor. Knowing what I know now, “clearly” (another Nigel’s favorite word) seismic design knowledge grew significantly through the 80s and 90s and Nigel/ UCSD played integral roles in creating that knowledge. I now realize that some of us were incredibly fortunate to be students in the middle of such tremendous seismic advancements at UCSD, as well as being closely associated with Nigel. This unique opportunity, along with what I gained from Nigel’s classes, formed the foundation of my seismic knowledge and provided me with the confidence to teach seismic-related courses. Nigel recommended a few topics for my PhD research; then we narrowed it down to two: bridge joints and displacement-based design (DBD). Both were excellent choices, but the bridge joint project had a clearly defined experimental program. I chose to do research on a new design method for bridge joints. My first test went very well and I gained confidence in experimental research. Nigel left me in charge of the second test and that didn’t go well. We applied the first load step, expecting a 2.5 mm (0.1 in.) of displacement at the column top and the next thing I knew, the column was leaning with a displacement of about 115 mm (4.5 in)! There was noticeable damage to the column and cracking on the joint, though we hadn’t even started the test. The technician acknowledged this and indicated that he knew what went wrong. I ran back to Nigel’s office, thinking that this would be the end my PhD research. When he saw me, he knew something went wrong in the lab and that I was really stressed. As I anxiously explained the mishap, he was very calm and attentive. He put his arm around my shoulder, and said things happen and that we could still get useful information from the test. He walked back with me to the lab, assessed the situation and made sure we had a plan to move forward. This is just one example of how gently he treated his students. Looking back, he was absolutely right that I was still able to extract valuable information from that test. Interestingly, I have witnessed mishaps in the tests conducted by my own students. A big “thank you” to Nigel for teaching me to remain calm and help my students get through their tests. What a wonderful mentor! As I progressed through more tests in the lab at UCSD, I continued to gain confidence and was then ready to perform more complex tests. Instead of testing one joint at a time, I wanted to test multiple joints in a frame, knowing we could learn more from such tests. Nigel was at ease in providing such freedom to his students as they matured and strongly encouraged them to be as independent as poss ible. I’m not sure if he ever realized this, but when he supported a research idea or test plan, it gave his
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students a moral boost and the confidence that they were heading down the right path. At the end of my first quarter in 1993, Nigel checked with me to see what my plan for the Christmas holiday was. I was new to California and had no plans. He invited me to his Idyllwild house where he spent his Christmas holidays with his family. He made sure there was a means for me to get there. I joined him and his family and friends for Christmas Eve dinner and stayed at his house. The next morning more people joined. Nigel and Jan’s (his wife) New Year’s Day tradition included a walk up into the mountains and lunch—we all had great time! This then became my tradition during Christmas breaks. Just like we learned about concrete, we learned about other aspects of life from Nigel—like how to relax and enjoy life. I then realized Nigel wanted to make sure each of his students had a plan for the Christmas holidays. The domestic students went back to their families and he made sure the international students had some plans or that they were invited to join his family. After I earned my PhD, I stayed at UCSD and continued to work with Nigel for about 18 months as a post-doc. My main responsibility was to build and test the PRESSS building. This consisted of precast concrete and unbonded post-tensioning—the very first test structure designed to resist seismic loads using unbonded post-tensioning as the primary thesis. This building was also the largest and tallest to be tested inside a laboratory at that time! My research efforts related to the PRESSS building precipitated another unique opportunity for my academic growth and continued close interactions with Nigel. After completing this research project, I accepted a position at Iowa State University. Nigel wanted to know how I felt about accepting the
Figure 1.19 - Students and spouses following Nigel’s lead in 1994.
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position of assistant professor, since being a student and researcher is one thing, but being a teacher and building a research program of my own was a completely different challenge. I expressed my doubts and how I wasn’t sure if I could succeed. His response was—being over-confident is not good, but understand that a little bit of lack of confidence is good for you; “it helps you to be successful in what you do,” he said with a smile. That statement has resonated with me to this day and has helped me during successful times and challenging times. My purpose in sharing this tribute to Nigel is not just to impart what I think of Nigel and what my interactions with him were. I want you to get a sense of his personality and his life. If you have known Nigel and interacted with him, I’m sure you felt his presence just as I did. Nigel was “clearly” one-ofa-kind—brilliant, elegant, quick, and witty—with no patience for (technical) “nonsense”.
Figure 1.20 - Group photo taken during a hike in 1994.
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“My supervisor taught me that” …and this is the end of the discussion Jason Ingham
Nigel undoubtedly had by far the most significant influence on my professional career of any of my mentors or teachers. In 1990 I arrived in San Diego to study with Nigel, as a naive young 23 year old research student having never previously left New Zealand, looking forward to spending a few years in California before becoming a professional structural engineer. Instead I have become a career academic and barely a day goes by when I don’t share with my own research students some of Nigel’s wisdom. Sometime I find myself saying ‘My supervisor taught me that …’ and for my students this is the end of the discussion – case closed, because it remains unthinkable that I would be advocating an alternative approach to the one that Nigel would have recommended. When I arrived in San Diego I recall that there was some university paperwork for international students that required me to have a nominated guardian, and so of course the obvious solution was to ask Nigel if he would be my guardian. After a year in San Diego I had mastered ‘driving on the wrong side of the road’ and was ready to purchase a car. It transpired that Nigel wanted to sell his Volvo and so it was convenient for both of us that I bought his car. When I later opened the glovebox I found that Nigel had left behind some old ownership documents that recorded his date of birth and I realised that Nigel was very similar in age to my own father. So for these and many other reasons it remains natural for me to think of Nigel as more than just my doctoral supervisor, and instead as a father figure. Soon after my arrival at UCSD I was designing my first experiment in the lab and labelled it ‘Specimen 1’. Nigel’s reaction was that ‘specimen’ made it sound like a urine sample, so to this day that word is outlawed in my research group and we only ever build ‘test units’ instead. On more than one occasion I have repeated the urine sample anecdote to my students and it always has the same effect on them as it did on me. After about a year at UCSD I had an opportunity to make my first ever conference presentation at a Caltrans conference. The night before the conference Nigel asked several of us eager young researchers to drop by his room to discuss and practice our presentations, and I remember seeing a novel sitting on the bedside table in Nigel’s hotel room. It seems a bit comical now as I think back to more than 25 years ago, but at the time I remember it being quite a stunning revelation – Nigel seemed to always be so up-to-speed with any and every issue that I had somehow assumed that he spent all of his spare time absorbing vast amounts of technical literature. But instead, he was reading a novel! During my studies at UCSD I recall meeting Michele a few times when he visited San Diego, and I was aware that Nigel was working with Michele and others on some new things in Italy, but it was only years later when I eventually managed to visit the Rose School in Pavia that it really dawned on me just how significant this achievement was. Whilst myself and the other students at UCSD where busy tackling our studies, Nigel was already looking well ahead to a whole new set of challenges and an entirely new model for teaching earthquake engineering. Somehow this realisation made me think that it was ‘classic Nigel’, conceiving and implementing opportunities that people like myself cannot even see. As I approached the end of my doctoral studies Nigel made a comment, on several occasions actually, that it was completely to be expected that at the end of a doctoral study the student should know more about the topic than does the supervisor. It is a comment that I completely agree with, and that I tell my own students from time to time. The problem is – I’m pretty sure that I failed this test because at the end of my doctorate I still felt like Nigel knew far more about what I was studying than I did myself,
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perhaps because he ended up publishing his text book ‘Seismic Design and Retrofit of Bridges’ with Frieder and Michele soon after I graduated. My only consolation is that I expect that I was not alone, and that there were few people who knew more than Nigel about any subject where Nigel had invested energy into considering and solving the complex problems. In October 2000 I was 5 years into my academic career and I delivered a presentation at the annual concrete industry conference at Wairaki in New Zealand. At the end of the presentation Nigel got up to ask a question and I recall feeling as if all the blood had just drained out of my checks. Sure enough, Nigel had some insightful comments to make but I recall a ‘deer caught in the headlights’ feeling as I replied to Nigel by saying something like ‘I agree’ to avoid any further anxiety on my part. As I type this I can’t help but suspect that there will be others who recall a similar experience, as I have seen a number of young (and sometime not so young) researchers make what they no doubt thought was a very successful presentation, only to see Nigel raise his hand with a question and the presenter suddenly look a little panicked. In 2011 I was a member of the management committee of NZSEE and I had the honour of reading Nigel’s Life Member citation at the annual NZSEE conference dinner. It did not begin to ‘balance the books’ for all the wisdom and insights that Nigel had shared with me during my studies and beyond, but I was immensely pleased to have been able to speak publically about Nigel’s extensive achievements and to present the citation to him. Soon afterwards I bumped into Nigel and Jan while we were all waited in the frequent flier lounge at Auckland airport. Having known Nigel for more than 20 years at this point, it seemed that I was able to finally relax in his presence enough to simply chat. Somehow we returned to the subject of his achievements, and Nigel made a comment along the lines that perhaps if he could do it all over again he might have instead chosen medicine as his career choice. I was gobsmacked – to think that one of the most preeminent thinkers within the realm of earthquake engineering in our lifetime could be so casual about his own achievements. But on reflection it was rather typical of Nigel, that he was always looking way beyond my horizons. Sadly, that was my last conversation with Nigel. In closing I note that I regard Nigel as the most brilliant thinker that I have ever met, and that I consider myself to have been very lucky to have been one of his students. Thank you Nigel.
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His influence on engineering practice here (in the US) has been enormous Joe Maffei
I first had the chance to fall under the influence of Nigel when I went to Christchurch in 1992 to start what would become my Ph.D. research at the University of Canterbury. I had Bachelor and Masters degrees from the U.S., and five years of work experience in structural engineering in San Francisco. My topic was bridge retrofitting, and my supervisor Professor Bob Park encouraged me to work closely with Nigel, who at that time was based in San Diego, but was beginning a fellowship in which he spent three months per year back in New Zealand. I came to my PhD studies with a practical bent, and I was immediately drawn to Nigel’s way of thinking about problems. I leaned heavily on Nigel’s wisdom, and I studied the extensive and unparalleled body of research on reinforced concrete and bridge design and retrofitting that he was producing at UC San Diego. This was a thrilling time for me. Nigel’s book with Professor Tom Paulay was fresh off the presses, and, with my break from the world of consulting, I made a pointed effort at Canterbury to take time to just read and learn, exploring topics that sparked my curiosity. I appreciated the lack of immediate deadlines, and I treasured the chance to interact closely with Nigel, Tom, and Bob. Nigel was always gracious, generous, patient, and encouraging. I still remember his multi-page hand-written notes about fruitful PhD topics for me. I benefited from the international-minded outlook of Nigel, Tom, and Bob, and I had a chance to meet new professional colleagues from around the world. After completing my PhD in 1996, I spent 3 months doing post-doctoral work in Japan before returning to a practice in San Francisco that focused on seismic-structural design related to new and existing buildings, including design practice, applied research, and code and standard development. In these subsequent years of busy consulting practice, I would often refer to Nigel’s books and papers. The book on bridges (with Frieder Sieble and Michele Calvi) is surprisingly valuable to me even when I am working with buildings-there is great material there, for example, on lap splices. I had opportunities to meet with Nigel infrequently, typically at a conference or workshop, but after every meeting I would go back to my work and wish that Nigel could be stationed at a desk nearby, so that I could get his viewpoint on any question that came up for me. I have worked on a wide range of structures: new construction and seismic retrofitting, in concrete, steel, and other material. When I look back at almost any particular project, I can find the DNA of Nigel in some technical aspect of how I approached a solution. I have always found this about Nigel’s work: Not only do I immediately learn from it, but it catalyzes me to think more, and explore new ideas on my own. Although Nigel was never an insider in the world of U.S. codes and standards, his influence on structural engineering practice here has been enormous. To me this is the triumph (over inertia and bureaucracy) of the truth and practical value of Nigel’s work.
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You know, Nigel would say… Katrin Beyer
I first met Nigel when I was doing my Master’s project with Tom Paulay in Christchurch 2000/2001. He walked into Tom’s office and I – awestruck by the fact that I was just meeting the third of the three big P’s – did not say much. I am sure Nigel would not remember that encounter. I had decided to first work some years in industry, but already pounded on the idea of doing a PhD at a later stage. Jose Restrepo, then a young professor at the University of Christchurch and just about to leave to follow Nigel’s path to San Diego, told me to consider the Roseschool in Pavia that Michele Calvi just had founded, should I ever start a PhD. This idea never left my mind and so two years later, I decided to apply. I wrote Tom Paulay about my plans and asked him whether he could write me a reference letter. Tom agreed but also wrote that more importantly he would have “a little chat with Nigel”. I presume that this was the key for me entering the PhD program at the Roseschool. When I arrived in Pavia for the Roseschool seminar in 2003, which was followed by the PhD admission exam, it was very hot and humid and I did not know anybody. It made all the difference to me that Nigel Priestley knew my name and introduced me to various people. All but myself and one other student taking the exam were already Roseschool students and I fear that my answers to some questions might have been more of the unusual type. I remember Nigel asking me how it went after the written exam and saying some encouraging words. I appreciated his kindness even more during the oral exam, when he gave me some time to think about the difference between intensity and magnitude, which I had learnt during the exam, bevor having to answer the question. In 2004, Nigel was to teach the course “Fundamentals of Seismic Design”. To me this course was really a game-changer, because for the first time I saw the flaws of force-based design and started to think about earthquakes as a displacement load case. It was a unique course because Nigel was just preparing the DDBD-book with Michele Calvi and Mervyn Kowalsky. Our course documentation consisted largely of the first drafts of some of the chapters and many of us did course projects on problems Nigel wanted to have confirmed. Though at the time I was certainly not yet in the position to appreciate just how revolutionary DDBD is, I was absolutely excited at seeing for the first time what research could lead to. My task was to look into torsion and I carried out parametric studies on in-plan asymmetric structure, focusing on the displacement demand on the structural elements. In the discussions, I could see how Nigel distinguished quickly between factors that mattered and factors that a theoretical person would have considered but from an engineering point of view did not play any significant role. My PhD project, which I did under the supervision of Alessandro Dazio on the seismic behaviour of U-shaped walls, Nigel co-supervised but followed it at a certain distance as I moved to Zurich to conduct my experimental work and Nigel spent only some months a year in Europe. The meetings we had were true review meetings and I always tried to prepare them as well as I could and try to have clear arguments in place. His approval meant that what we had done was not too wrong and gave me the confidence to seek a career in research. Having had the opportunity to attend Nigel’s courses and to work with him was a true gift and I am extremely grateful for it. I miss his guidance and the possibility to share with him successes and failures. But Nigel has also left us with an amazing group of students and colleagues who are all on similar wavelengths. I met many of them for the first time during the Nigel Priestley Symposium at Lake Tahoe, which Mervyn Kowalsky and Sri Sritharan had organized for Nigel’s 70th birthday. With
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this group and the RoseSchool-Alumni one meets at every conference a unique group of friends and colleagues, with whom one can discuss technical matters as well as share memories. Mervyn and Sri became very good friends and took on the mentor role, Nigel can no longer fill. Often their advices start with “You know, Nigel would say…”.
2. Practical Lessons from Nigel By Joe Maffei, Carlos Blandón, Sri Sritharan and Katrin Beyer
2.1 Introduction
In August of 2008 many of Nigel’s collaborators and former students gathered at Lake Tahoe, California for a symposium in honor of Nigel’s 65th birthday. The presentations showed the wide range of Nigel’s influence, and the questions and discussion represented the atmosphere of curiosity and collaborative energy present at every meeting and institution that has absorbed Nigel’s inspiration. The proceedings are published by the IUSS Press (Kowalsky and Sritharan, 2008) and include a number of revealing short tributes in addition to the technical papers. It was clear that Nigel’s work had a meaningful objective: to serve the practicing structural engineering community worldwide. Many noted Nigel’s clear thinking, creativity, and personal connection to his students. The students recalled his ability to deliver lengthy lectures from memory. (Many of us recall the laboratory technicians at the University of Canterbury saying how Nigel could remember long strings of four-digit dial gage readings without ever making a mistake.) We suggest that you find and read this proceedings volume; here are a few excerpts from the tributes: Mervyn Kowalsky: The reluctant leader is in almost all cases, the most effective …such leaders possess brilliance and talent and are always the most humble and genuine… John Stanton, referring to the PRESSS project on precast seismic systems: He put together a terrific group of people, fed them a diet of intellectual challenges and culinary delights, and
expected them to give their best as he led them into uncharted territory. Michael Collins: It amazed me how quickly Nigel found and rectified my mistake. Rob Park: Nigel Priestley was the clearest thinking and most innovative engineering thinker I have ever met. José Restrepo: Complex work landing on his hands has always been distilled into beautifully simple solutions. John Mander: …his presented work, in whatever form, is clear, no-nonsense, concise, and revolutionary. Rob Chai: …a giant whose height can only be dwarfed by his modesty and humility. Scott Arnold: …he explained ‘I understand what you are referring to, but the structure has not read the code and the hinges will form as I’ve described.’ Greg MacRae: …and I also want to say that I am really sorry about the …shaking table test where we put the wrong scale factor into the controller… In his two “Myths and Fallacies” publications (Priestley, 1993, 2003), Nigel critically challenged assumptions about stiffness, elastic analysis, and detailing that are still commonly made in seismic design and are prescribed by building codes. He found that such assumptions are not only false, but they can also impede or distract the structural engineer from conceiving an appropriate design. The following section describes some of Nigel’s practical lessons.
2.2 Practical lessons from Nigel
Here is a sampling of the lessons from Nigel, many arising from projects and problems where the authors have done their best to think like Nigel.
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a. Love your work and dedicate to it
It is clear that Nigel had great passion for his work and followed his passion. His productivity was astounding. b. Work isn’t everything
For example, we should all admire Nigel’s expertise in the Italian distilled spirit grappa. c. Have conferences in beautiful places
The International Bridge Workshop held in 1994 in Queenstown, New Zealand attracted many of Nigel’s collaborators and friends the world over in a beautiful setting (Figure 2.1). Figure 2.2 shows presentation by Rui Pinho at the Rose School Seminar, May 2006 in Pavia Italy; Rui’s slides of fragility curves had to compete for the attention with the room’s frescoes. d. There is a “flexure-then-shear” failure mode in reinforced concrete
We are not sure if it was Tom Paulay or Nigel who first identified a mixed failure mode where nonlinear response first takes place in flexure and then switches to shear, resulting in failure soon after. This mode of failure is evidenced in Figure 2.3 on a wall tested by Mestyanek (1986). Nigel clearly identified it in bridge column failures in Loma Prieta, and subsequently developed the UCSD model for shear strength, applicable to columns, and extended to walls in FEMA 306 (ATC, 1999) and by Krolicki et al. (2011). The UCSD model was a major milestone in understanding and predicting the behavior of reinforced concrete elements in earthquakes. An interesting aspect of the model is that is shows that the expected behavior mode is not sensitive to the level of axial load.
Figure 2.1 - Bridge workshop in Queenstown New Zealand, 1994 with par ticipants from around the world. The program was equal parts skiing and technical discussions. Those pictured include Peter North, Joe Maffei, Des Bull, John Berrill, Athol Carr, Howard Chapman, Barry Davidson, Richard Fenwick, Kazuhiko Kawashima, Donald Kirkcaldie, Bob Park, Toru Terayama, Hajime Ohuchi, Ian Billings, Mick Pender, Dale Turkington, Brian Maroney, Po Lam, Greg Fenves, Eduardo Carvalho, Nigel, Michele Calvi, Frieder Seible, and Camillo Nuti.
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Figure 2.2 - The ROSE School Seminar 2002 at the Salone degli Figure 2.3 - Shear failure of a wall after flexural yielding Affreschi of the Collegio Borromeo. (Mestyanek, 1986).
e. Here’s how to design or evaluate pier structures
With Rutherford + Chekene, the first author contributed to the retrofit of a pier structure for the Exploratorium, a hands-on science museum in San Francisco (Figure 2.4). Nigel’s methodology for this type of structure is clear and to the point - in refreshing contrast to many other US codes and guidelines. Port facilities located in the west US coast required improvement in the design methodo logies that were used before the Loma Prieta (1989) and Northridge (1994) earthquakes. Nigel was actively involved in providing analytical and design approaches that incorporated key but basic principles of structural design such as limit states, plastic hinge length, and moment curvature of reinforced concrete sections, among others. A good example of his capacity tosimplify complicated problems is the designexample that he produced for the design of typical wharves existing in the west coast (Priestley, 2000). In a few pages, the design example addresses the effect of the soil, the nonlinear behavior of the pile and the connection, the axial load variation and even the torsional response to obtain the displacement capacity of the wharf. Nigel was always driven to test that the values he used for design were adequate, and he helped lead a team that carried out full scale testing of connections and piles of wharf structures (Figures 2.5 and 2.6). The design principles and experimental findings developed and applied by Nigel are now part of design guidelines for such structures (ASCE-COPRI, 2014).
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Figure 2.4 - The Exploratorium in San Francisco, built on a 1930s wharf with existing non-ductile concrete piers, seismically strengthened with new 6-ft (1.8m) diameter piers. Nigel’ s clear methodology for wharf structures was thetechnical basis for the design.
Figure 2.5 - Photo of the team that developed design regulations for the Port of Los Angeles and that now are the base of recent ASCE-COPRI design guidelines (2014). Participants include Po Lam, Arul Arulmoli, Max Weismair, José Restrepo, Peter Yin, Nigel, Geoff Martin and Omar Jaradat.
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Figure 2.6 - Full-scale tests carried out at the University of California, San Diego. The pictures were taken by Nigel during a short stop at the Engelkirk Research center. The appreciation and respect from the laboratory staff for Nigel was evident as everybody quickly mobilized to excavate the rock fill around the pile, so Nigel could take photo a of the damage.
f. Here’s how to design unbonded PT for frames
Another unique and influential contribution of Nigel was in convincing structural engineers about the use of unbonded prestressing in seismic-resistant design. Though the concept was previously investigated, it was the PREcast Seismic Structural Systems (PRESSS) program that pushed the boundaries and facilitated the use of unbonded post-tensioning broadly in seismic design practice (Priestley et al., 1999). This system and its impact in practice is described in more detail in Chapter 7 of this volume. As the leader of the PRESSS program, Nigel worked with academic and industry partners to design multiple precast frame and wall systems that used unbonded post-tensioning as a means to connect the precast members with each other or with the foundation. The unique benefits of this concept include minimal structural damage and re-centering capability for the seismic force-resisting system that is designed to experience a dependable yield mechanism while providing adequate lateral force resistance. The ductile response of four precast seismic frame systems and one jointed wall system were demonstrated successfully in the PRESSS five-story building-the largest structure to be tested inside the laboratory at that time in 1999. In addition to the innovative lateral-force-resisting systems, the test building included two realistic types of floor structures: pre-topped double-tee precast floors and hollow core slabs with in situ topping. The value and success of the PRESSS program is evident in that (a) several precast buildings have been built using the unbonded post-tensioning in high seismic
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Figure 2.7 - Parking garage forMills Peninsula hospital in Burlingame California, by Culpand Tanner structural engineers, usingthe PRESSS moment frame system. The owners of the new base-isolated hospital desired operational performance from their parking garage. The PRESSS system provided it ata cost lower than competing systems.
regions in the U.S., New Zealand and other countries, and (b) the unbonded post-tensioning concept has been used by other researchers and practitioners for other structural systems developed using materials such as steel, masonry, and timber. g. Here’s how to do it for structural walls
Based on the principles and design approach defined by Nigel and his team, vertical unbonded prestress for walls has become increasingly common. In the San Francisco Bay Area, such walls are commonly cast-in-place, with about ten projects having been constructed in the last several years, including new structures and walls used in retrofitting. h. Walls can rock and that can be good
Early on Nigel published work related to foundation rocking, following on from the approach by Housner (Priestley et al., 1978). At the time, few engineers in practice thought about such things. Recently, a major retrofit project in San Francisco -The War Memorial Veterans Building, by SGH Structural Engineers- used an innovative system of rocking walls. i. The C-column bridge bent and the problem of unbalanced moment
Nigel’s study of C-shaped bridge bents showed that having a large gravity moment at a column or wall plastic hinge zone can lead to undesirable ratcheting of displacement in one direction only. In a practical application of this concept, the design of the tower at the DeYoung museum in San Francisco used unbonded post-tensioning in the tilted walls to counteract the gravity moment, so that the plastic hinge region sees balanced cyclic moment demands under earthquakes (Figure 2.8).
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Figure 2.8 - The tower of the DeY oung museum in San Francisco, structural designby Rutherford + Chekene.
Figure 2.9 - Twelve-story concrete wall building retrofitted by Rutherford +Chekene, including horizontally oriented carbonfiber to change walls from shear governed to flexure governed. Nigel’s colleague Frieder Sieble acted as a Peer Reviewer.
j. Here’s how to retrofit using fiber composites
The recommendations from Nigel and his collaborators on how to use fiber for shear strength and (in elliptical configurations) for confinement have provided structural engineers with practical seismic retrofit methods, (Figure 2.9). k. Slab flanges contribute to coupling beam strength
Related to the project shown in Figure 2.9, Nigel’s publications provide clear recommendations on practical topics such as “how much slab width contributes to a coupling beams strength?” Nigel’s answer: ½ of clear span on each side of beam web. l. Distribute the flexural reinforcement
Tom Paulay and Nigel seemingly were the first to realize that concentrating flexural reinforcement near the extreme fibers of a wall or beam is neither necessary nor useful for seismic-dominated designs. This
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practice, shown in Figure 2.10a, is efficient for resisting static loads such as those coming from gravity or static earth pressure. This efficiency no longer holds for reversing seismic loads. The arrangement shown if Figure 2.10b provides nearly equivalent flexural strength while reducing reinforcement congestion, improving the performance of beam-column joints, better controlling of shear deformation in beam plastic hinge regions, and reducing the potential for sliding shear failure.
(a) Conventional reinforcement
(b) Distributed reinforcement
Figure 2.10 - Arrangements of flexural reinforcement inbeams (Priestley, 2003).
m. Here’s how masonry works
While the world was thinking in working stress seismic design for reinforced masonry buildings, Nigel had developed capacity design guidelines (Priestley, 1986) and had introduced the concept of the confinement plate to increase the compression strain capacity and limit crushing at the toe of masonry walls (Priestley and Bridgeman, 1974). His book with Tom Paulay (Paulay and Priestley, 1992) moved the seismic design of masonry buildings many notches up. n. Concrete is not as stiff as you think
Nigel’s method for estimating the stiffness of concrete elements, based on simple principles to estimate yield curvature, show how many other methods for estimating stiffness are off the mark (Priestley, 2003; Schotanus et al., 2007; Maffei et al., 2004). o. Stiffness and strength are not independent
This is of course a major contribution from Nigel, and a key underpinning of his work in displacementbased design. It is well highlighted in his Myths and Fallacies paper of 2003. What amazes us is how quickly Nigel realized all of the consequent implications of this fundamental truth. In contrast, the structural engineering community remains slow to incorporate this concept into design practice. p. Here’s how to design a coupled wall
In the US, most new tall buildings in high-seismic areas are using concrete core walls, typically with coupling beams. These are designed using non-prescriptive approaches with nonlinear response-history analysis. Despite the design freedom of such an approach, some engineers are uncomfortable deviating from elastic analysis results, even though those results are based on stiffness assumptions shown by Nigel to be false. In the displacement-based design textbook, Nigel emphasizes that the strength for coupling beams can be assigned almost arbitrarily and he gives useful rules for proportioning, considering the total coupling strength compared to the wall axial load and strength. q. Stand on the shoulders of others, even as you blaze new paths
Refreshingly, we have found that Nigel has always been scrupulous about acknowledging contributions that precede his efforts.
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r. Here’s how to know when it really is the vertical earthquake component
We have seen a number of presentations in which an instance of damage is attributed to the vertical earthquake component. Typically this has seemed speculative, as most types of damage that might be attributed to vertical motion (e.g. buckling of vertical wall reinforcement) could also be caused by lateral motion only. Nigel is the only one we know who found clear evidence of damage that could only have come from vertical motion. This was a case of damage to a column at a basement level where surrounding walls restrict any lateral motion. s. Cut thru vagaries and conventional wisdom; seek clear evidence
In the professional career of the first author there have been instances where adherence to conventional wisdom has led our profession into trouble. One case is steel moment frames. Their ductility capacity was questioned in the early 1990s after the tests by Mike Engelhardt, but conventional wisdom held that moment frames were an excellent seismic system and that the research must be flawed. The 1994 Northridge earthquake proved the research to be correct and conventional wisdom to be wrong. Similarly, in Chile after the 1985 earthquake, some researchers assured the Chileans (without clear technical evidence we see inretrospect) that since their buildings had numerous walls, tie reinforcement was not needed. The 2010 Maule Chile earthquake disproved this proposition. Nigel was great at dismantling of “conventional wisdom” because he valued clear truths over vague assurances. This was his calling. t. Earthquakes have no borders-explore the world
Like Bob Park (who took a sabbatical in China back when few westerners had been there) and Tom Paulay, Nigel travelled the world building professional and personal ties. He was a pivotal member of three major institutions on three different continents. 2.3 References ASCE/COPRI (2014) - 61-14 Seismic Design of Piers and Wharves (ASCE 61-14), American Society of Civil Engineers, Reston, VA. ATC (1999) - Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings, prepared by the Applied Technology Council (ATC-43 project) for the Partnership for Response and Recovery, published by the Federal Emergency Management Agency, Report No. FEMA 306, Washington D.C. Kowalsky M., Sritharan S. editors. (2008) - Proceedings of the M.J. Nigel Priestley Symposium, King’s Beach, CA, August, IUSS Press, Pavia, Italy. Krolicki J., Maffei J., Calvi G.M. (2011) - Shear strength of reinforced concrete walls subjected to cyclic loading. Journal of Earthquake Engineering, 15(S1), 30-71. Maffei J., Stanton J., Priestley M.J.N., Park R. (2004) - Design Approaches, in Seismic Design of Precast Building Structures, State of the Art Report [Robert Park editor], Commission 7, Federation International du Beton, Lausanne, Switzerland, Chapter 4, January. Mestyanek J.M. (1986) - The earthquake resistance of reinforced concrete structural walls of limited ductility. M.E. Thesis, Department of Civil Engineering, University of Canterbur y, Christchurch New Zealand. Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley & Sons, New York. Priestley M.J.N., Evison R.J., Carr A.J. (1978) - Seismic Response of Structures Free to Rock on their Foundations, Bulletin of the New Zealand National Society for Eart hquake Engineering, Vol. 11, No. 3, pp. 141-150, September. Priestley M.J.N. (1986) - Seismic Design of Concrete Masonry Shearwalls. ACI Journal Proceedings 83(1):58-68. Priestley M.J.N. (1993) - Myths and fallacies i n earthquake engineering-conflicts between design and reality. Bulletin of the New Zealand National Society forF., Earthquake Engineering, 26(3), Design 329-341. Priestley M.J.N., Seible Calvi G.M. (1996) - Seismic and Retrofit of Bridges, John Wiley & Sons Inc., New York. Priestley M.J.N., Sritharan S., Conley J.R., Pampanin S. (1999) - Preliminary results and conclusions from the PRESSS five-story precast concrete test building. PCI journal, 44(6), 42-67. Priestley M.J.N. (2000) - Seismic Criteria for California Marine Oil Terminals, volume 3: Design Example. TR-2103-SHR, Naval Facilities Engineering Service Center, Port Hueneme. Priestley M.J.N. (2003) - Myths and fallacies in earthquake engineering, revisited. IUSS press, Pavia, Italy. Priestley M.J.N., Bridgeman D.O. (1974) - Seismic Resistance of Brick Masonry Walls. Bulletin of the New Zealand Society for Earthquake Engineering 7(4):167-87. Schotanus M., Maffei J. (2007) - Computer modeling and effective stiffness of concrete wall buildings, Proceedings of the International FIB Symposium on Tailor Made Concrete Structures - New Solutions for Our Society, Amsterdam.
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3. The World’s Greatest Structural Systems Experimentalist By José I. Restrepo, Christopher Latham, Sri Sritharan and Nihal Vitharana
3.1 Introduction
Nigel was an outstanding engineer and human being with progressive and social views of the world very hiughly esteemed by his peers, friends and his family. Many of his post-graduate students were from various parts of the world and many from countries with developing eco nomies who had a meager income and he never forgot to consider their welfare, well beyond his role as a mentor. Nigel’s such qualities would be eternally carried forward because most of his students are scattered throughout the world contributing in various ways. Nigel was a simple and no show-off man. He could be seen interacting with technical staff day and night doing experiments first at the University of Canterbury then at the University of California San Diego, and lastly at the ROSE School, which he co-founded with Michele Calvi. Nigel’s clarity of thought, ability to understand very complex problems in very little time and ability to propose simple and practical and economical solutions, all backed up by a sound but simplified theoretical approach, are some of his most fascinating facets. Nigel’s theories created lively discussions and apparent disagreements among peers. Many researchers required “further research” just to find out, that after arduous, long and often sophisticated work, his simple theories were, in fact, quite accurate and well within the acceptedlex artis. One could say that Nigel did ninety percent of the work in ten percent of the time, whereas the more mundane of us spent ninety percent of the time working on the remaining ten percent. He was never afraid to look at a problem in a very different perspective from his younger age; whether it is confined concrete, bridges under thermal gradients, soil-structure interaction or water-retaining structures. As a structural experimentalist, Nigel almost immediately developed a comprehensive view of an issue in question. This proved useful when one consulted him, as he clearly (and sometimes bluntly) would let anyone know the limitations of a proposed test or the practic al value of its expected findings. There is little doubt that Nigel was a pioneer and the best structural system experimentalist of all times. He was incredibly talented, quick-witted, creative and prolific. He mastered not only the field of experimental mechanics, but also the mechanics of concrete, thermal and deformation-induced loadings, masonry and structural dynamics and soil-structure interaction, among a few. In short, Nigel was another giant in the field of Earthquake Engineering and the fourth of a prodigious New Zealand generation of earthquake engineers which also include: Tom Paulay, Bob Park and Ivan Skinner. His work has inspired many academics and professionals. This chapter summarizes the main accomplishments in the field of large-scale experimental structural engineering.
3.2 Nigel’s First Steps as an Experimentalist
Nigel’s first exposure to structural concrete experimental mechanics was during his PhD studies at the University of Canterbury, which he carried out with minimal supervision in a startling short period of two years between 1964 and 1966 (Priestley, 1966). Nigel completed his PhD when he was just 23 years old. In this work, he presented a comprehensive analytical and experimental investigation about moment redistribution in prestressed concrete beams, which up until then had been rather controversial. To support this work, Nigel tested seven simply supported and seven continuous beams and in each test he obtained a wealth of data that he shared by appending it in his dissertation.
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3.3 Accomplishments at Central Laboratories
Upon completion of his PhD, Nigel went on to become Head of the Structures Laboratory of the New Zealand Ministry of Works (Central Laboratories). The Structures Laboratory was involved in three different types of testing: small scale model testing insupport of design efforts by Head Office, large scale laboratory testing of concrete models, and in-situ testing ofstructures either built or under construction. Routine inspection of the prestressed concrete Newmarket viaduct in Auckland , see Section 5.2, revealed cracking in the soffit of the bridge, where conventional analysis would have indicated no tension should exist because at that time the temperature distribution was assumed to be uniformly distributed through the section in standards throughout the world. In Nigel’s own fashion of intuition, he knew it could be different and he proved this by carrying out a simple 1-dimensional finite difference analysis considering diurnal temperature and solar radiation. Few decades later, attempts by other researchers with sophisticated 3-dimensional finite element models showed that Nigel was correct with his simple approach. His model is now adopted throughout the world in bridge standards and is knownthas -power 5 parabolic temperature distribution. Not many engineers would, however, know that this was developed by Nigel in his hay days in New Zealand. The cracks were found to be thermally active: they opened during days of high sunshine, and closed at night. Nigel began to study this problem, combining finite differences and experimental work. He builta ¼ scale simply-supported prestressed box-gird er span and comprehensively instrumented it with thermocouples, strain-gauges and displacement transducers. The top surface of the bridge was enclosed within an environmental box which included 100 infrared light bulbs controlled through a variable output transformer to model diurnal variations of solar radiation, and propeller fans to control night-time cooling, see Figure 3.1. The design guidelines that stemmed from this research (Priestley, 1971; 1972; 1976; 1978a) form a major contribution to the design of bridges for thermal loading, bridges in many parts of the world. In 1968 Nigel took a one year leave from Central Laboratories and traveled to Lisbon, Portugal as a Post-doctoral Fellow at theLaboratorio Nacional de Engenharia Civil (LNEC), whose Structures group was being led by Prof. J. Ferry Borges. This period marked a turning point for Nigel, as it was at LNEC where he acquired a strong background in structural dynamics and earthquake engineering as he himself pointed it out (Priestley, 1996):
The writer had the great good fortune, and considerable pleasure, to spend a year in 1968/69 as a post-doctoral fellow at the Laboratorio Nacional de Engenharia Civil in Lisbon. At that time J. Ferry Borges was the head of the Structures group at LNEC, and it was a result of his personal decision that my application was approved. I came to LNEC with no real background in Earthquake Engineering, and spent the year attempting to get up to speed in the general areas of structural dynamics and earthquake engineering, while simultaneously trying to hide my ignorance. I do not think that Ferry Borges was fooled, but he was tolerant, and patient, and I ended up learning a great deal, to the extent that my future professional activities were to be completely dominated by a fascination for the seismic response of structures. Upon his return, Nigel conducted several in-situ tests on bridges and bridge components. Of these tests, the lateral load test of an extensively instrumented 1.8 m diameter steel-encased pile embedded on soft marine mud is a true landmark (Priestley, 1974). Nigel meticulously designed and conducted this test. Data logged in this test enabled the calculation of the lateral pressure profile acting on the pile and of the lateral subgrade material moduli. This was the world’s first full-scale lateral load test performed on a pile and today it is a point of reference for such types of tests. The calibrated p-y curves from this experiment form the basis of the recommendations made for analysis of deep foundations in geotechnical engineering.
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Figure 3.1 - One quarter scale box-girder bridge under thermal loading (Courtesy of Nigel Priestley).
3.4 The University of Canterbury Famous “Ps”: Paulay, Park and Priestley
In 1976 Nigel joined the Department of Civil Engineering at the University of Canterbury after conversations with Professor Bob Park while golfing in Wairakei, and when Professor Harry Hopkins was ending a shining career as Head of Department and was about to be succeeded by Professor Park. Professor Hopkins had a strong vision for the Department. He was also responsible for the hire of Professors Bob Park, Tom Paulay. The hiring of the three “Ps” in the field of structural concrete and the ability of all three to combine strengths, collaborate with industry and help solve complex problems in seismic design practice, propelled the University of Canterbury to world fame in the field. At Canterbury, Nigel carried out collaborative work mainly with Bob Park, Tom Paulay and also with Professor Athol Carr, who worked in structural dynamics and computational mechanics. In the early days in his new job at Canterbury, Nigel andAthol worked in rocking walls as a potential seismic system for use in buildings (Priestley et al., 1978b). With limited and small-scale testing, they could extend Professor Housner’s theory and propose a simple method for predicting maximum displacements of rocking systems using the displacement response spectra and linearized properties for the rocking system. This, in a way, was a very early precursor of the Direct Displacement Based Design method Nigel would go on to develop in the early 90s, and is also an early precursor of the work on lowdamage walls that he developed further in the PRESSS program, which will be briefly described in the following section and detail in Chapter 7. Nigel is the person who identified the significance of temperature loading on the behaviour and durability-performance of concrete cylindrical tanks which had shown various cracking patterns despite designed with adequate “factor of safety” with respect to crack width. In mid-late 1970’ s, in line with Nigel’s’ simple approach to any given problem, he instrumented wall panels near the Christchurch airport with rudimentary thermocouples. He borrowed a pyranometer to measure solar radiation from
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the then Lincoln Agricultural College. This simple test showed various temperature gradients resulting in significant thermal stresses. He showed that on a vertical face, the radiation from other buildings could be up to 20% of direct solar radiation. Nigel went to develop temperature gradients for winter and summer conditions which are believed to be the world’s first which are in AS 3735 (SA, 2001) and NZS 3106 (SNZ, 2009). Ironically, these are used in many designs/investigations throughout the world today probably without knowing that a young researcher developed these in New Zealand. Nigel worked together with Bob Park in the testing of reinforced concrete columns using a NZD 300,000 servo-controlled hydraulic 10 MN dynamic tension/compression capacity universal testing machine supplied by Dartec in the UK. He was responsible for design of the foundation, set-up and commissioning of this machine, see Figure 3.2. The Dartec machine became a workhorse for testing reasonably large size columns. Data acquired from column tests carried out with this machine was used to validate the concrete confinement models of Scott et al., (1982) and of Mander et al., (1988a, 1988b), and to propose prescriptive code design requirements. The Dartec machine was also used by Bob and Nigel on seismic pile-to-cap connections (Park et al., 1984) and was used by Nigel to conduct seismic tests on encased steel piles (Park et al., 1983). These encased steel piles were the forerunners of the steel jackets he would investigate at the University of California at San Diego and successfully recommend as a retrofit scheme for use in the bridge retrofit program carried out by the State of California after the damaging 1989 Loma Prieta earthquake (Chai et al., 1991). Shear transfer in reinforced concrete is an aspect of research that Nigel took special interest in at the University of Canterbury. He collaborated with Bob Park and Tom Paulay in relevant aspects of shear transfer mechanisms in beam-column joints, with support from large scale experimental work (Paulay et al., 1978). His work on shear extended to the shear capacity of circular columns, which involved extensive testing and collaboration with Tom (Ang et al., 1989; Wong et al., 1993). Nigel would culminate his work on shear in columns at the University of California at San Diego by presenting the UCSD shear model (Priestley et al., 1993), which could predict shear failure in elements after flexural yielding had occurred. The conceptual clarity and simplicity of this model has resulted in widespread acceptance in the community. Nigel also had his independent line of research on reinforced and unreinforced masonry as well as thermal and dynamic effects on liquid storage tanks. In reinforced masonry, Nigel moved the design away from Working Stress into Ultimate Strength, making the New Zealand Code for the Seismic Design of Masonry Structures a role model for codes aroun d the world (Priestley, 1985).This work was supported by extensive testing, of which the work leading to the seismic design of masonry walls for ductility (Priestley, 1981, 1982) is worth highlighting. Chapter 6 gives a detailed description on Nigel’s work on masonry. With Stuart Thurston, Nigel developed a methodology to predict thetemperature rise and thermalstresses in hardening concrete (Thurston et al., 1980). To prove its accuracy, Nigel monitored the concreting of the foundation of airport hangar at the Christchurch International Airport. Italso coupled heat-of-hydration and heat-transfer in a simple but smart way. This is in fact the first such prediction model which set the benchmark for predicting thermal stresses in early-age concrete including creep relaxation. At the University of Canterbury, Nigel maintained his line of research on thermal action on water-retaining structures, and with Nihal Vihtarana (Vitharana et al., 1998) he carried out a set of complex tests and proposed a methodology to incorporate thermal loading in the design of liquid storage tanks. This was the first reported testing on structural wall elements subjected to thermal gradients in conjunction with applied tensile axial loads. With its findings, they developed useful guidelines for evaluating thermal stresses in concrete structures, which are used in many modern standards on water-retaining structures, and developed thermal stress tables by smartly manipulating beam-on-elastic foundation formulations. On the structural dynamics side, not involving experimental work, Nigel led a group of the New Zealand Society for Earthquake Engineering on theSeismic Design of Liquid Storage Tanks (1986). This work is
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a major contribution in the field andis a point of reference for international guidelines. When the first author was a Senior Lecturer at the University of Canterbury in the late 90s, he was very privileged to work in two bridge seismic retrofit projects with Nigel. The Aotea Quay Overbridge, a 1930s bridge strategically located in Wellington, was retrofitted by enlarging the footings and by wrapping the columns with FRP jackets, a first outside the United States, see Figure 3.3. In this project Nigel provided the design guidelines written with support from testing he had performed at the University of California at San Diego where he had pioneered such well-known retrofit technique (Priestley et al., 1992). We also worked together in the seismic retrofit of the Thorndon Overbridge, a major lifeline crossed by the Wellington fault. Nigel was the primary external consultant for Beca, the structural engineering company responsible for the design of the retrofit. The Overbridge presented several design deficiencies such as short longitudinal bar cut-offs, small number of columns transverse reinforcement, short seating for the girders, foundation deficiencies and various others. Proof of concept testing was carried out on pile, pile cap column specimens, in the unretrofitted and retrofitted conditions at the University of Canterbury, see Figure 3.4. In 1986 Nigel looked for new challenges and moved to the University of California at San Diego. Nigel cared for his graduate students whom he would leave behind at Canterbury. Despite all hectic work associated with moving overseas with a young family, Nigel wanted to make sure that the students were coping well in difficult times in various ways. As an example, the fourth author’s wife (Padmini Vitharana, a student at Canterbury as well) was eight months pregnant. Nigel visited them at night with a cot for their expected new arrival. The cot was passed onto another student when Nihal and Padmini were leaving Canterbury hence passing Nigel’s legacy on kindness and care towards others.
3.5 Advancing the State-of-the Art in Structural Testing at the University of California at San Diego
The Charles Lee Powell Structures Laboratories of the University of California at San Diego, the largest of its kind in the world, were funded in 1986 under the direction of Professors Gil Hegemier and Frieder Seible. The University of California at San Diego looked at hiring a researcher of significant talent and world-class trajectory to ensure the new labs were successful. Nigel was found to be the most suited candidate for this position. He assumed this new challenge and joined the University of California at San Diego in early 1987 and collaborated with Frieder Seible in several research projects, including the testing of the full-scale 5-story masonry building. This work was part of the Technical Coordinating Committee for Masonry Research (TCCMAR) program for development of a limitstate design standard for masonry buildings in seismic zones. This program culminated in the testing of a full-scale five story reinforced masonry building (Seible et al., 1994). The building consisted of 150 mm wide concrete masonry blocks with concrete topped precast prestressed floor slabs. The building was tested using hybrid simulation, termed pseudo-dynamic testing then. There were several challenges in performing this testing. The building had a C-shape plan configuration. This necessitated two actuators per floor level. To correctly simulate the distributed loading present in an actual structure each actuator was attached to a long beam which had two elastomeric pads attached on the bottom of either end. These beams were then clamped to the floor slabs to transfer the forces by friction. Initial testing started during the day in the summer of 1992. It quickly became apparent that testing would need to be performed at night instead. The overall vertical temperature gradient in the lab during the day in summer was giving poor results. Also, the sun light streaming in through the labs windows was heating up individual transducers and giving false displacement readings. The testing continued over a period of about two months with increasing demand and damage to the structure. This showed the effectiveness of the design methodologies developed in the small-scale component testing. Chapter 6 provides an in-depth discussion about the impact of this test program in industry.
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Figure 3.3 - Seismic retrofit of the Aotea Quay Overbridge, Wellington in 1996. First seismic retrofit application using FRP jackets outside the United States.Nigel provided design guidelines for thisproject (Gray and White 1997).
Figure 3.4 - Seismic retrofit of the ThorndonOverbridge, Wellington in 2000. Nigel was anexternal consultant in thisproject. Proof of concept testing was carried out at the University of Canterbury (Presland et al., 2001). Inthe top left photo Bob Park(right above) and Nigel (extreme right) inspect the construction of a test unit.
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Figure 3.5 - Double-deck testcomponent at the Powell Laboratories of the University of California atSan Diego.
The 1987 Whittier earthquake and the more destructive 1989 Loma Prieta earthquake in the Bay Area called into question many of the seismic design procedures for bridges inCalifornia, and also called for urgent research to develop assessment and retrofit techniques forbuildings and bridges (Housner, 1990). The 1989 Loma Prieta earthquake that caused catastrophic collapses of bridges andbuildings around the San Francisco Bay area raised the state priority toseismically retrofit the potentially “at risk” bridge stock comprising about 4,000 bridges by 1994. Nigel’s unique ability to easily distill complex problems and propose simple and effective solutions was what the State of California needed in those difficult times. The most complex test carried out in the Powell Labs was a half-scale model of a double-deck viaduct in the San Francisco bay area, which was funded by Caltrans Priestley ( et al., 1992). The collapse of the Cypress Viaduct caused concern about the seismic response of other double-deck viaducts in the State. The test unit in the laboratory consisted of a column, half of the lower level cap beam and deck, and an added external edge girder parallel to the superstructure, see Figure 3.5. Nigel had come up with the idea of adding a maximum eight-foot-deep, four-foot-wide “edge beam” located on the outside of the bridge bent and a retrofit of the joints. Not only was this solution economical, but it could easily add significant lateral deformation capacity to these rather brittle bridge types. Jim Roberts, the then chief of Caltrans division of structures, commented on the impact of the solution proposed by Nigel (UCSD, 1991): The University of California at San Diego has produced a proven, safe and cost- effective design that we can use to provide additional strength for double-deck bridges. The retrofit techniques we are developing in California will be the model for the rest of the world. To appropriately simulate the boundary conditions in this test, a total of thirteen hydraulic actuators had to be used twelve of which were controlled simultaneously - a formidable task that advanced the state-of-the art of structural testing to new frontiers, and one that is very challenging even with today’s
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highly automatized controls. Add to this the fact that this test was large-scale and was also multidirectional, that no one in the team had any experience with multi-actuator testing of such nature, and that the control software had to be re-written on the fly to accommodate newly behavioral modes of response seen by Nigel as the test progressed. It was in the early 90s that Nigel tackled a myriad of structural problems, endemic to the building and bridge design practice, which were leading to poor seismic performance. To solve these problems, Nigel pioneered effective retrofit solutions and supported these with theory and with design guidelines. All this work wasvalidated with large-scale tests and included: (i) Assessing and addressing shear problems in columns, (ii) Assessing and retrofitting columns with longitudinal bar lap-splice deficiencies, (iii) Lack of anti-buckling reinforcement and confinement, (iv) Assessment and retrofit of footings and (v) Assessment and retrofit of various types of joints. The tests were performed by visiting professors, Post-doctoral Fellows and PhD students whom today have successful academic careers and contribute to this publication. Chapter 5 gives a detailed recount of Nigel’s research work in these areas. Of all the research work Nigel performed during the short period from 1990-1994, it is worth highlighting the implementation of steel and FRP jackets to retrofit bridge and building columns, which have become a standard seismic retrofit technique worldwide. Despite the huge pressure to deliver while keeping a lab full of research work, Nigel managed to do work on the development of his Direct Displacement Based Design method and to publish two highly successful books, one with Tom Paulay on the Seismic Design of Buildings and Masonry Structures (Paulay and Priestley, 1992), and another one with Professors Frieder Seible and Michele Calvi onSeismic Design and Retrofit of Bridges (Priestley et al., 1996). The catastrophic 1994 Northridge earthquake in Southern California put to test the bridge seismic retrofit schemes implemented because of the 1989 Loma Prieta earthquake. Most of these retrofit schemes resulted from Nigel’s The California Seismic Advisory Board reported that despite new collapses of oldrecommendations. bridges, retrofitted bridges performed rather well (Housner, 1994), thus strongly endorsing Nigel’s work:
All structures in the region of strong shaking that were retrofitted since 1989 performed adequately, thus demonstrating the validity of the Caltrans retrofit procedures; there were 24 retrofitted bridges in the region of very strong shaking and a total of 60 in the region having peak accelerations of 0.25g or greater. The retrofitted structures resisted the earthquake motions much better than the unretrofitted structures. The Board’s conclusion is that if the seven collapsed bridges had been retrofitted, they would have survived the earthquake with little damage. Nigel had a very close affinity with the Precast/prestressed Concrete Institute (PCI). PCI is a progressive organization in the United States whose research and development committee looks for ways to enhance the structural response of precast concrete systems. In the mid-90s, the branch of PCI in California (PCMAC) and Caltrans funded a project for him to investigate the continuity of precast concrete spliced-girder bridges when subjected to seismic loading (Holombo et al., 2000). The experimental component of this project required solutions to new challenges, but for Nigel and cohorts these challenges were welcomed. Two forty percent scale bridge structures were built in the Powell Labs. One had a superstructure comprised of precast bathtubs, see Figure 3.6, and the other had bulb tees. The end-to-end distance of the bridge units was 20 m, which, by the time the horizontal actuators were deployed, the test unit practically spanned the entire length of the labs. These tests were carried out with four vertical and two horizontal servo-controlled hydraulic actuators plus four passively controlled plunger jacks simulating gravity loading. The horizontal actuators simulated the inertial forces and had to be controlled considering the possible lengthening of the girder upon cracking. The
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Figure 3.6 - Seismic testing of a precast concretesplice-girder bridge (Holombo et al., 1999).
vertical actuators were placed to act as points of inflection, but had to be controlled also to ensure the lengthening of the column could occur without imposing an unnecessary constraint. In the late 80s the PCI and the National Science Foundation (NSF) jointly funded a large multiuniversity/industry program to develop a new generation of precast seismic structural systems (PRESSS), see Chapter 7. Brainstorming in this program led to several possible connections between the precast elements. Nigel proposed that the connection type with a high potential for superb seismic performance had to make use of unbonded post-tensioning tendons. As the leader of the PRESSS program, Nigel worked with academic and industry partners to design multiple precast frame and wall systems to demonstrate the benefits that precast systems had when incorporating unbonded posttensioning tendons as a means to connect the precast members with each other or with the foundation. The unique benefits of this concept include minimal structural damage and a recentering capability for the seismic load-resisting system. Proof of concept tests on frame and wall components were carried out by Nigel’s colleagues Professors John Stanton at the University of Washington, Richard Sause at Lehigh University, and Greg MacRae, then a Post-doctoral Fellow at UC San Diego, all with excellent seismic performance. The capstone test in the PRESSS program consisted of a five-story fully precast concrete building built at two-thirds scale, see Figure 3.7 (Priestley et al., 1999). The height of the PRESSS building was controlled by the useable height of the Powell Labs. The building was designed using the Direct Displacement-Based Design method, which had reached maturity by then, for specific performance objectives. The building was tested in two directions sequentially using hybrid simulation. This test method brought a new set of challenges, such as coupling between actuators and the control of the building’s higher modes of response. In total, ten actuators (two at each floor) were used in the test. Because of difficulties obtaining reliable lateral displacement measurements, Nigel decided to conduct the tests late at night when the daily temperature gradient was at the minimum. To date, the PRESSS building remains the largest structure ever tested using hybrid simulation.
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Figure 3.7 - View of the PRESSS building with Nigel on the roof top to the right of the middle column (Courtesy of Prof. Sri Sritharan).
3.6 The ROSE School
In 2000 Nigel opted for a new adventure and found, together with Professor Michele Calvi, the ROSE School at the University of Pavia in Italy. The School became an instant success and attracted students and visitors from around the world. Since 2000, Nigel spent most of his time carrying out analytical work, with occasional jaunts into the experimental field both at the ROSE School and the University of California at San Diego Labs, where, in the latter, the first two authors had pleasure in collaborating with him in structural testing and design methods of bridges and container wharves supported on piles (Restrepo et al., 2005, Blandon et al., 2007). Nigel’s final major contribution to the field of Earthquake Engineering was as leader of the expert panel put together by the Department of Building and Housing in the New Zealand Royal Commission of Enquire set out to determine the reasons leading to the catastrophic collapse and poor seismic performance of several buildings in Christchurch during the 2011 earthquakes for the future benefit of the community. We, first as his students and then as friends, undoubtedly believe that Nigel is the world’s greatest engineer/researcher in many facets of engineering with a unique approach to any problem he was given. With his contribution in the field of earthquake engineering, millions of lives would be saved in several hundreds of years to come. May you rest in peace and may you attain nibbana.
3.7 Acknowledgements
We sincerely thank the generous contributions received from Dr. John H. Wood, Principal of John Wood Consulting and former Head of Central Laboratories and of Dr. David Hopkins of David Hopkins Consulting Ltd. John and David are former Presidents of the New Zealand Society for Earthquake Engineering. Emeritus Professor Athol Carr of the University of Canterbury shared with me some
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of his personal experiences with Nigel as his colleague at the University of Canterbury. Dr. Edward Fyfe provided the background to the development of FRP for the seismic retrofit of columns. Finally, Michelle Chen, PhD student at the University of California at San Diego very generously reviewed and corrected this paper. Her numerous comments are graciously thanked.
3.8 References Blandon C.A. (2007) - Seismic Analysis and Design of Pile Supported Wharves. Ph.D. Thesis. Rose School, Pavia, Italy. Chai Y.H., Priestley M.J.N., Seible F. (1991) - Seismic Retrofit of Circular Bridge Columns for Enhanced Flexural Performance, ACI Journal, pp. 572-584. GrayStructural A.,White B. (1997)88(5), - Seismic Retrofitting of Aotea Quay Overbridge, Wellington. In IPENZ Annual Conference 1997, Proceedings of: Engineering our nation’s future; Volume 1, Wellington, New Zealand. Holombo J., Priestley M.J.N., Seible F. (2000) - Continuity of Precast Spliced-Girder Bridges under Longitudinal Seismic Loads, PCI Journal, 45(2), pp.40-63. Housner G.W. (1989) - Competing Against Time. Report to Governor George Deukmejian from t he Governor’s Board of Inquiry on the 1989 Loma Prieta Earthquake, Department of General Services, North Highlands, California, p. 264. Housner G.W. (1994) - The Continuing Challenge - The Northridge Earthquake of 17 January 1994, Seismic Advisory Board Report, Department of General Services, North Highlands, California, p. 77. Mander J.B., Priestley M.J.N., Park R. (1988a) - Theoretical Stress-Strain Model for Confined Concrete,” ASCE Journal Structural Division, 114(8), pp. 1804-1826. Mander J.B., Priestley M.J.N., Park R. (1988b) - Observed Stress-Strain Behavior of Confined Concrete, ASCE Journal Structural Division, 114(8), pp. 1827-1849. Park R., Priestley M.J.N., Falconer T.J., Pam H.J. (1984) - Detailing of Prestressed Concrete Piles for Ductility, Bulletin of the New Zealand National Society for Earthquake Engineering, 17(4), pp. 251-271. Park R.J.T., Priestley M.J.N., Walpole W.R. (1983) - The Seismic Performance of Steel Encased Reinforced Concrete Piles, Bulletin of the New Zealand National Society for Earthquake Engineering, 16(2), pp. 123-140. Paulay T., Park R., Priestley M.J.N. (1978) - Reinforced Concrete Beam-Column Joints Under Seismic Actions, ACI Structural Journal, 75(11), pp. 585-593. Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concrete and Masonry Buildings. New York: John Wiley & Sons, Inc. Potangaroa R.T., Priestley M.J.N., R. Park (1979) - Ductility of Spirally Reinforced Concrete Columns Under Seismic Loading, Report No. 79-8, Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand, 116 pp. Presland R. Restrepo J.I., Park R. (2001) - Seismic Performance of Retrofitted Reinforced Concrete Bridge Piers, Research Report 2001-3, Department of Civil Engineering, University of Canterbur y, Christchurch, New Zealand, 2001, 513 pp. Priestley M.J.N. (1981) - Ductility of Unconfined and Confined Concrete Masonry Shear Walls, The Masonry Society Journal, 1 (2), pp. T28-T39. Priestley M.J.N. (1966) - Moment Redistribution in Prestressed Concrete Continuous Beams. Ph.D. Thesis. University of Canterbury, Christchurch, New Zealand. 189 p. Priestley M.J.N. (1971) - Effects of Transverse Temperature Gradients on Bridges. Central Laboratories Report No. 394, Gracefield, New Zealand, 30 p. Priestley M.J.N. (1972) - Thermal Gradients in Bridges - Some Design Considerations. New Zealand Engineering, 27(7), p.228. Priestley M.J.N. (1974) - Mangere Bridge Foundations Cylinder Test. Ministry of Works and Development Central Laboratories, MWD-CL Report No. 488, 98 pp. Priestley M.J.N. (1976) - Design Thermal Gradients for Concrete Bridges. New Zealand Engineering, 31(9), p.213. Priestley M.J.N. (1978a) - Design of Concrete Bridges for Temperature Gradients, ACI Structural Journal, 75(5), pp. 209-217. Priestley M.J.N. (1982) - Ductility of Confined Concrete Masonry Shear Walls. Bulletin of the New Zealand National Society for Earthquake Engineering, 15(1), March 1982, pp. 22-26. Priestley M.J.N. (1985) - Seismic Design of Masonry Structures to the New Provisional New Zealand Standard, NZ 4230P, Bulletin of the New Zealand National Society for Earthquake Engineering, 18(1), pp. 1-20. Priestley M.J.N. (1996) - Simplified Limit States Seismic Design Philosophy, Paper No. 2018, 11th World Conference in Earthquake Engineering, Acapulco, México. Priestley M.J.N., Evison R.J., Carr A.J. (1978b) - Seismic Response of Str uctures Free to Rock on Their Foundations. Bulletin of the New Zealand National Society for Earthquake Engineering, 11(3), pp.141-150. Priestley M.J.N., Seible F., Anderson D.L. (1992) - Proof Test of a Retrofit Concept for the San Francisco Double-Deck Viaducts. Report SSRP 92-03, Dept. of Applied Mechanics & Engineering Sciences, University of California, San Diego. Priestley M.J.N., Seible F., Calvi G.M. (1996) - Seismic Design and Retrofit of Bridges. John Wiley & Sons, Inc. Priestley M.J.N., Seible F., Fyfe E. (1992) - Column Seismic Retrofit Using Fiberglass/Epoxy Jackets. SEQAD Report to Fyfe Associates. Priestley M.J.N., Seible F., Verma R., Xiao Y. (1993) - Seismic Shear Strength of Reinforced Concrete Columns, University of California, San Diego, Structural Systems Research Project, Report No. SSRP-93/06, 120 pp. Priestley M.J.N., Sritharan S., Conley J.R., Pampanin S. (1999) - Preliminary results and conclusions from the PRESSS five-story precast concrete test building. PCI journal, 44(6), pp.42-67. Priestley M.J.N., Wood J.H., Davidson B.J. (1986) - Seismic Design of Storage Tanks, Bulletin of the New Zealand National Society for Earthquake Engineering. 19(4), pp. 272-284.
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Restrepo J.I., Ha I., Priestley M.J.N. (2005) - Seismic Behavior of Four-CIDH Pile Supported Foundations. Proc. 21st US Japan Bridge Engineering Conference, Tsukuba, Japan. SA (2001) - Australian Standard: Concrete Structures for Retaining Liquids AS 3735-2001, Standards Australia, North Sydney. Scott R.D., Park R., Priestley M.J.N. (1982) - Stress-Strain Behaviour of Concrete Confined by Overlapping Hoops at Low and High Strain Rates, ACI Structural Journal, 79(1), pp. 13-27. Seible F., Priestley M.J.N., Kingsley G.R., Kürkchübasche A.G. (1994) - Seismic Response of Full-scale Five-story Reinforced-masonry Building. Journal of Structural Engineering, 120(3), pp. 925-946. SNZ. (2009) - Design of Concrete Structures for the Storage of Liquids NZS 3106:2009, Standards New Zealand, Wellington. Thurston S.J., Priestley M.J.N., Cooke N.(1980) - Thermal Analysis of Thick Concrete Sections. Int ernational Concrete, 77(55), pp. 347-357. UCSD, 1991, https://library.ucsd.edu/dc/object/bb1776013p/_2.pdf Accessed 26 November 2016. Vitharana N.D., Priestley M.J.N., Dean J.A. (1998) - Behavior of Reinforced Concrete Reservoir Wall Elements under Applied and Thermally-Induced Leadings, ACI Structural Journal, 95(3), pp. 238-248.
Nigel, Bob Park and Tom Paulay in the early days at UC.
Tom’s birthday. Celebrating the 200th collective birthday. Late 1990s.
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4. The Development of Direct Displacement-Based Design By Mervyn Kowalsky, Tim Sullivan, and Gregory MacRae
4.1 Introduction
Here in we hope to provide some insight into the early development of what is now known as “Direct Displacement-Based Design (DDBD)”, which was described in the 2007 textbook by Priestley, Calvi, and Kowalsky. In order to understand the significance of Nigel’s contributions to DDBD, it is essential to be transported back to 1993, which can be thought of as the genesis of the concept with Nigel’s seminal papers on the topic. However, before exploring that era, a brief discussion of the history of seismic design is warranted. In the early 1990’s, seismic design was decidedly force-based. The engineering community was firmly entrenched in a design approach that dated back to the early 1900’s where a corollary to wind design was drawn and the seismic lateral force estimated as 10% of the gravity load. Over the balance of the 20th century, numerous advances were made, first with regards to structural dynamics and the dependence of period on the dynamic response of systems (19401950), followed by a deep understanding of the concept of ductility and the capacity design approach (1960-1980). These advances led to what could be termed ‘force-based and displacement-check methods’ which recognized the importance of calculating structural deformations (to characterize damage), even if the design itself was conducted with traditional force based approaches. Along the way, researchers made numerous contributions that would eventually form the foundation of DDBD. In 1930, Lydik Jacobsen, then an assistant professor in mechanical engineering at Stanford University, proposed the concept of “equivalent viscous damping” for machine vibrations. Over the following thirty years, having served as the first president of the Earthquake Engineering Research Institute in 1948, his research transitioned to structural engineering where he revisited his work on equivalent viscous damping, now cast as a means for equivalent linearization of systems (1960). Jacobsen’s 1960 paper opened the floodgates of work on equivalent linearization methods throughout the 1960’s, with examples including Rosenblueth (1964), and numerous others as summarized in the 1968 paper by Jennings. In 1974, Gulkan and Sozen introduced the concept of the substitute structure analysis which was extended MDOF systems, specifically multi-story RC moment frames in 1976. It was during this time that the engineering community started to recognize the importance of ductility, drift, and deformation in representing damage potential. In 1975, Ed Teal, a California structural engineer, posited the importance of drift in controlling damage for moment frame buildings. Throughout the 1980s, extensive research on ductility and non-linear behavior of systems was conducted. During this time, Nigel led numerous efforts aimed at developing constitutive models for concrete (Mander et al., 1988), as well as the seminal paper on the plastic hinge method for member deformations (Priestley and Park, 1987). In 1986, Nigel moved to California to serve on the faculty at UCSD. A few years, later, the Loma Prieta earthquake resulted in an extensive research program at UCSD on the seismic design of bridge systems (details of which are discussed Chapter 5) that continued through several earthquakes in the 1990s, which brings our story to June of 1993.
4.2 Observations from UCSD – Early Development of Displacement Based Design
Note: Much of this section is written in the first person from Mervyn’s perspective when he was a student at UCSD. I graduated with a BS degree from UCSD in June of 1993, having had the opportunity to take classes
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from Nigel as an undergraduate, it was clear that I wanted to work with him, assuming he would be willing to take me on. Thankfully, he was willing to serve as my adviser, and I vividly recall our very first meeting after graduation. At his office, we talked about some of the work he was doing, and rather early on in the meeting we walked down the hall to the office of Greg MacRae, who was a post-doc at the time and who I knew as my instructor in the undergraduate steel design course at UCSD. With the three of us standing in the hall, Nigel told Greg I’d like to have Mervyn work with you on Displacement Based Design. At the time, I had no idea what he was talking about, of course, and to this day, I wonder if Nigel simply decided what I would be working on in the hallway on that June day.
Figure 4.1 - Early results from SDOF displacement based design (Kowalsky et al., 1995).
During that summer of 1993, Greg (an early mentor of mine and an integral part of the first displacementbased design efforts) and I worked with Nigel on his initial concept for displacement based design. That summer, we formalized the method that Nigel had laid out in his 1993 Myths and Fallacies paper published also in the proceedings of the Tom Paulay Symposium, which was held in September of 1993 in San Diego, as well as in Concrete International). During the first month or so, we struggled with aspects of the method that in retrospect seem obvious, which is typical when solving new problems, of course. I kept all of my notes from the very beginning of our work on displacement based design, and reviewed them for this manuscript. From the start, we wanted to develop a design approach that would not require any member cross section information. That is, our goal was to simply have as our initial input parameters the height of a column, the inertia mass, and the target displacement. We used a drift ratio of 3% and the Brawley record from the 1979 Imperial Valley earthquake as our input motion. We were a long way from even thinking about strain limits or design spectra as the input. Since we were adamant about not requiring any cross section information to conduct designs, yield displacement was not known at the start of the design process. This led to an iterative approach where we attempted to converge onto a yield displacement while maintaining a target displacement. We quickly found that in some cases, this resulted innegative yield displacements – shown inAppendix 1 is our very first attempt at displacement-based design where this result was observed. We eventually resolved our difficulty, which was a function of how we calculated the yield displacement and made some minor adjustments to our approach. Notes of the approach, as transcribed by Greg are shown in Appendix 2. After some time, this led to a series of successful designs, with the resulting outcome of the non-linear time history analysis shown in Figure 4.1. After some initial struggles, it was rather satisfying to see the procedure
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work! We expanded our studies to include designs to different drift ratios, including some which were designed to achieve prescribed plastic drift ratios. We met with Nigel, who was clearly happy with the progress, and we developed a plan going forward where we conducted designs to smooth spectra, and used spectrum compatible records for the non-linear time history analysis. Our first of several presentations on the results of the work was in the 1994 Queenstown Conference on the Seismic Design of Bridges (Kowalsky et al., 1994a). Nigel, Greg, and I prepareda paper on our SDOF work, while at the same time, Michele Calvi and Greg Kingsley were formulating a plan for the design of MDOF systems that was also presented at the conference. A more detailed discussionof the SDOF work followed in our first research report on the topic in 1994 (Kowalsky et al., 1994) and the Earthquake Engineering and Structural Dynamics paper in 1995 (Kowalsky et al., 1995). We eventually realized that if we knew the cross section dimensions,calculation of the yield displacement would be straightforward,thus removing iteration from the design approach. This was facilitated by Nigel’s observations on yield curvature and the simple expressions that related it to the section depth and steel yield strain, such as those shown in Priestley and Kowalsky (1998). Some of the salient features related to the early development of displacement based design included: (1) An emphasis on displacement as the key design parameter. While numerous researchers had focused on accumulation of energy as a possible response parameter, in displacement based design, a much simpler approach was taken, which related design of the system purely to the peak displacement response and corresponding level of hysteretic damping (which does not account for cyclic dissipation of energy) and (2) Direct use of the displacement spectra as the representation of the hazard. Other researchers at the time started to develop methods that were also termed “displacement based design”, however, they were largely still force-based, displacement-check methods. Nigel and I met to discuss how we could differentiate ourselves from other work in the area. Several ideas were discussed, and the initial decision from that meeting was to call our work ‘strain-based design’ since we felt that the performance limit states would eventually be based on strain, at least for reinforced concrete structural members. We drafted a conference talk with that title, however, Nigel quickly realized that such a name would be limiting and instead proposed the nameDirect Displacement-Based Design, since in essence, what we were doing was directly designing for a specified target displacement (irrespective of the source of that target displacement). Our last publication without direct in the title came in in 1995, and our first with it appeared in the 1996 Caltrans Seismic Research Workshop (Kowalsky et al., 1996). The fundamentals of the DDBD approach, so developed, are summarised in Figure 4.1. One first identifies the “target” or “design” displacement for the limit state (e.g. a serviceability or a collapse limit state) being examined. This requires consideration of potentially critical limit state criteria, which are typically material strain or storey drift limits. For MDOF systems this step also relies on the use of a design displaced shape expression and the substitute structure concept of Gulkan and Sozen (1974) and Shibata and Sozen (1976) in order to convert from the MDOF structure to an equivalent SDOF system, as per Figure 4.2a. The next step, shown in Figure 4.2c, is to compute the ductility demand at the limit state, as this is required for the estimation of EVD values. A scaled spectrum thentoconstructed at the design damping enter the spectrum and read off ∆d, isisused level, as shown in Figure 4.2d, and thedisplacement design displacement, a required effective period,Te, for the structure. Ke in Figure 4.2b, is obtained With the required effective period known, the effective (secant) stiffness, by rearranging the expression for the period of a SDOF system as follows: (1)
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Figure 4.2 - Fundamentals of Direct Displacement Based Design (Priestley et al., 2007).
where me is the effective mass of the equivalent SDOF system. Subsequently, the required design base shear, Vb, can be obtained as the product of the secant stiffness and the target design displacement: (2) Readers will appreciate that the design approach so formulated differs greatly from force-based design methods in numerous codes, placing the focus on displacements and identifying the required period, stiffness and then strength as outputs from the procedure. Also in 1996, we had the opportunity to conduct some shake table tests with the dual intention of verifying the DDBD approach experimentally, while also studying the seismic behavior of lightweight concrete. In Figure 4.3 below (Kowalsky et al., 2000), the time history response of a two-column bent shake table test is shown for four limit states, along with the target displacement consistent with the limit state. Following the work on SDOF systems, we turned our attention to MDOF systems and combined some of the ideas presented in the Queenstown papers towards the development of an approach for multi-span bridge structures. The primary issues addressed included the calculation of a target displaced shape for a multi-span bridge, as well as equivalent viscous damping for a system where multiple plastic hinges form at different levels of ductility. In 1997, we turned our attention to building structures for the first time, developing the approach for moment frames (Loeding et al., 1998; Priestley and Kowalsky, 2000), and structural walls (which I worked on with Nigel during 1997-1998 as part of my post-doc
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at UCSD). Some handwritten notes by Nigel, shown in Appendix 3, illustrate a clarity of thought with minimal edits. Even though word processing was commonplace by the late 1990s, Nigel often preferred to hand write material, which surely sharpened his focus even further. The notes on structural walls will be apparent to students of DDBD, as it highlights differences between force-based design and DDBD that were noted in future papers.
4.3 Further Development - DDBD Studies at the Rose School and Beyond
Starting around the year 2000, an explosion of research on the topic of DDBD took place with contributions from many students across multiple universities. This could be partly attributed to the attention brought to DDBD by Nigel during his keynote presentation at the world conference in earthquake engineering in Auckland, New Zealand in 2000. However, it should also be attributed to the active role Nigel took at that time with the ROSE School, a post-graduate school in Earthquake Engineering that started in 2001 in Pavia, northern Italy. Michele Calvi and Nigel Priestley were codirectors of the school, which is run in a non-conventional framework, inviting leading academics from around the world to teach in Pavia for a month at a time, such that students follow a series of courses on a range of topics in earthquake engineering. Nigel taught “The fundamentals of seismic design” there in the school’s first year (2001) and “Seismic design and retrofit of bridges” in 2003. He
Figure 4.3 - Experimental verification of displacement based design (Kowalsky et al., 2000).
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Figure 4.4 - Examples of mixed structural systems together with possible force-displacement relationships and corresponding system ductility capacities.
also taught courses with similar titles in years to come. These courses were particularly stimulating and inspirational for the students who took them, and many of the students from these courses are now successful academics or leading consulting engineers. In addition to courses, masters students in Pavia are also required to undertake research and to this extent, the subject of displacement-based design lent itself wonderfully as a masters research topic; students could be given the opportunity to go and develop means of applying DDBD to a specific structural system or develop simple means of dealing with complex phenomena within the DDBD process. Nigel was a key supervisor in the studies that were conducted of this nature, and is likely to have sparked the interest of other academics who became active in this field of research. One of the first masters projects assigned in Pavia related to the subject of DDBD was to the second author of this chapter, who was charged with comparing eight different displacement-based design methods
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available in the literature at the time to five different case study buildings. This work, co-supervised mainly by Mervyn and Michele, revealed (Sullivan et al., 2003) a numberof the strengths of the DDBD approach relative to other methods in the literature and helped further motivate its development. When developing the DDBD method for mixed structural systems, the benefits of explicit calculation of design ductility quickly became apparent. In this discussion, a mixed structural system will be defined as any system that possesses a number of lateral load resisting systems that are characterized by different force-displacement relationships. As such, mixed systems would include not only dual systems, such as the frame-wall structure shown in Figure 4.4c, but also those buildings that possess walls or frames of different length, such as those shown in Figure 4.4a and 4.4b. The right side of Figure 4.4 illustrates possible force-displacement relationships for the different mixed systems and includes the characteristics of the individual lateral load resisting sub-systems. Nigel recognized that the yield displacement and hence ductility capacity of a mixed structural system will be a function of its geometrical proportions and material properties. However, if a mixed structural system possesses components with different values of yield displacement, how can the system yield displacement be defined? In such cases, Priestley et al. (2007) recommended that the system yield displacement, ∆y,sys, be found by using flexural strain energy proportions (building on the proposal of Shibata and Sozen, 1976) as follows: Vi
y,i y, sys
=
(3)
Vb
where ∆y,i is the yield displacement of sub-systemi and Vi is the equivalent SDOF base shear resistance Vb. The system ductility offered by sub-systemi at the development of the system design base shear, capacity,µsys, can therefore be computed by dividing Eq.(3) into the system design displacement, to give: µ sys
=
d
Vb
(4)
V
y,i i
where all of the symbols have been defined above. Figure 4.4 illustrates the calculation of the system ductility capacity in this way for a few different types of mixed structural system. The values are presented for an arbitrarily selected maximum displacement of 5.0 (no units are required for this example but one could imagine 5.0 cm if desired), which could be dictated by the ductility capacity of one of the sub-systems (such as Wall B in Figure 4.4a) or a non-structural drift limit (of say 2%, as presumed in Figure 4.4b and 4.4c). The specific values shown in Figure 4.4, which range from 1.6 to 3. 6, are presented only to demonstrate that the system ductility capacity calculation is simple, once the force-displacement relationships of the sub-systems are known. Also note that for a mixed system, the ductility capacity is always less than the ductility capacity of the sub-systems but the actual system ductility capacity will depend greatly on the structural geometry, strength proportions and material properties, which becomes evident when one considers the yield displacement expressions presented previously. Reflecting on the implications of the points made above, current code guidelines tend to neglect the mixed nature of the frame and wall systems indicated in Figures 4.4a and b, recommending that such systems be assigned the same value of system ductility factor ir respective of structural geometry, strength proportions and material properties. For mixed systems of the type shown in Figure 4.2c, international codes provide differing recommendations with the Eurocode 8 (CEN, 2004) recommending that they be treated in a similar fashion to frame or wall systems (depending on the proportion of base shear resisted by the frames and walls in the dual system) and with the New Zealand Concrete Design
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Standard (Standards New Zealand, 2004) recommending that the design seismic action for mixed systems be determined by a rational analysis (without providing details for such a rational analysis). Such mechanics-based reasoning, as used for mixed structural systems, was ubiquitous in Nigel’s teaching at the time (and, one imagines, throughout his career). At the ROSE school Nigel would teach using an overhead projector (commonly referred to as OHP) in which his hand written reasoning would be clearly projected onto the wall as he talked. An example projection from his 2003 class on the seismic design of bridge structures is shown in Figure 4.5, where the type of reasoning just described for mixed systems is clearly presented and easy to follow. One can also see from Figure 4.5how Priestley et al. (2007) would recommend the evaluation of equivalent viscous damping for mixed systems. The case shown above is for the specific case where the displacement
Figure 4.5 - An illustration, from an OHP slide prepared by Nigel during his 2003 ROSE School course on Seismic Design and Retrofit of Bridges, of the clear reasoning Nigel would use to explain how ductility and equivalent viscous damping could be found in mixed systems – in this case a bridge with piers of different height.
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demand on all sub-systems isthe same and ingeneral, Nigel would argue that it should be found as follows: ξ sys
Vi ξ i
i =
(5)
Vi
i
where ξi is the damping and∆i the displacement of sub-system i andξsys is the total system damping. This expression could therefore be used to account for the possibility of different hysteretic characteristics being present within a mixed system when relating inelastic and elastic spectral displacement demands (Figure 4.2c and 4.2d). This provides an improved alternative to the use of expressions such as the equal displacement rule present in current codes. Early critics of DDBD identified the use of equivalent viscous damping as a limitation of the method. This was overcome through the use of equivalent viscous damping expressions that are calibrated to the results of non-linear response history analyses - thus giving them equivalent accuracy to empirical R-m-T relationships used in FBD. However, whereas in FBD the use of the equal-displacement rule appears to be prevalent for any type of structural system, in DDBD different EVD expressions were developed for a wide range of structural systems. This included studies by Blandón (2004) and Grant et al. (2005), Dwairi et al. (2007), and more recently Pennucci et al. (2011). By around 2006, guidelines for the DDBD of a number of different structural systems had been developed and verified, including various MSc and PhD studies at the ROSE School covering DDBD of bridges (Alvarez, 2004), moment resisting frame structures (Pettinga, 2004), RC wall structures (Beyer, 2005), and dual frame-wall systems (Sullivan et al. 2006). In addition, some means of dealing with issues such as torsion, P-delta, soil-structure interaction, and elastic damping had been formulated. Thus, in 2007, the first text on DDBD was published by IUSS Press (Priestley et al., 2007) representing a huge milestone for the approach.
4.4 Future of the DDBD
By developing the DDBD method, Nigel has provided the engineering community with an approach that addresses many of the shortcomings and issues with current code design practice. In addition, the methodology provides engineers with a better sense of the role that structural proportions, material properties, member detailing, and capacity design concepts all play in the seismic riskaof building or structure. Despite the significant developments made to the Direct DBD procedure and the extensive testing it has undergone, the approach is not yet widely used in practice. The reasons for this may well be that (i) it is not a codified procedure, (ii) is not implemented in commercial software and (iii) does not appear worthwhile financially to consultants, principally because it is not a codified procedure. Reflecting on these points, they are all non-technical issues and can be addressed with time. To this extent, a model code was included in Chapter 14 of the DDBD book, in an effort to illustrate how the approach might be transitioned into practice. This model code was developed further in the years that followed, and Nigel was co-editor of a more detailed version published in 2012 (Sullivan et al., 2012a). This model code incorporates the results of various developments that were made at the ROSE School and in other areas of Italy as part of a national effort by the RELUIS consortium (www.reluis.it). Software for DDBD has also undergone development (see, for example, Sullivan et al., 2012) but will require more development before being widely used in consulting. Developments of the DDBD method for specific structural and non-structural systems are expected to continue for years to come, widening its applicability. Of course, none of this would have happened had it not been for the foresight of Nigel in establishing a design methodology that at the time of its inception represented a complete inversion of traditional seismic design approaches.
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4.5 References Alvarez Botero J.C. (2004) - Displacement-based design of continuous concrete bridges under transverse seismic excitation. ROSE School MSc dissertation, Super vised by Nigel Priestley, IUSS Pavia, Pavia, Italy. Beyer K. (2005) - Design and analysis of walls coupled by floor diaphragms. ROSE School MSc dissertation, Supervised by M.J.N. Priestley, G.M. Calvi and R. Pinho, IUSS Pavia, Pavia, Italy. Blandon C. (2004) - Equivalent viscous damping equations for direct displacement-based design, ROSE School MSc dissertation, Supervised by Nigel Priestley, IUSS Pavia, Pavia, Italy. CEN. (2004) - Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings. EN 1998-1, Comité Européen de Normalisation, Brussels, Belgium. Dwairi H.M., Kowalsky M.J., Nau J.M. (2007) - Equivalent damping in support of direct displacement-based design. Journal of Earthquake Engineering, 11 (4), pp. 512-530. Grant D.N., Blandon C.A., Priestley M.J.N. (2005) - Modelling Inelastic Response in Direct Displacement-Based Design, Research Report Rose 2005/03. Gulkan P., Sozen M. (1974) - Inelastic response of reinforced concrete structures to earthquake motion, ACI Journal Vol. 71, pp. 604-610. Jacobsen L.S. (1930) - Steady forced vibrations as influenced by damping, ASME Transactions 52(1), pp. 169–181. Jacobsen L.S. (1960) - Damping in composite str uctures, Proceedings, 2nd World Conference on Earthquake Engineering, Vol. 2, Tokyo and Kyoto, Japan, pp. 1029-1044. Jennings P.C. (1968) - Equivalent viscous damping for yielding structures, ASCE Journal of Engineering Mechanics Division 94(1), pp. 103-116. Kowalsky M.J., Priestley M.J.N., MacRae G.A. (1994) - Displacement-Based Design: A Methodology for Seismic Design Applied to Single Degree of Freedom Reinforced Concrete Structures, University of California, San Diego, La Jolla, CA, October 1994, Structural Systems Research Project SSRP - 94/16. 131 p. Kowalsky M.J., Priestley M.J.N., MacRae G.A. (1994a) - Displacement-Based Design of RC Bridge Columns, Proceedings: 2nd International Workshop on t he Seismic Design of Bridges, Queenstown, New Zealand, pp. 138-163. Kowalsky M.J., Priestley M.J.N., MacRae G.A. (1995) - Displacement-Based Design of RC Bridge Columns in Seismic Regions, Earthquake Engineering and Structural Dynamics, Vol. 24, No.12, pp. 1623-1644, December. Kowalsky M.J., Priestley M.J.N. (1996) - A Direct Displacement-Based Design Approach for Reinforced Concrete Bridges, Proceedings: Fourth Caltrans Seismic Research Workshop, Sacramento, CA. Kowalsky M.J., Priestley M.J.N., Seible F. (2000) - Dynamic Behavior of Lightweight Concrete Bridges, ACI Structural Journal, Vol. 97, No. 4, pp. 602-618. Loeding S., Kowalsky M.J., Priestley M.J.N. (1998) - Direct displacement-Based Design of Reinforced Concrete Building Frames, University of California, San Diego, La Jolla, CA (1998) - Structural Systems Research Project SSRP - 98/08, 297 p. Mander J.B., Priestley M.J.N., Park R. (1988) - Theoretical stress-strain model for confined concrete. Journal of Structural Engineering, 114(8), pp.1804-1826. Pennucci D., Sullivan T.J., Calvi G.M. (2011) - Displacement Reduction Factors for the Design of Medium and Long Period Structures, Journal of Earthquake Engineering. 15(S1), pp. 1-29. Pettinga J.D. (2004) - Dynamic behaviour of reinforced concrete frames designed with direct displacement-based design, ROSE School MSc dissertation, Super vised by Nigel Priestley, IUSS Pavia, Pavia, Italy. Priestley M.J.N., Park R. (1987) - Strength of ductility of concrete bridge columns under seismic loading, ACI Structural Journal, 84 (1), pp. 61-76. Priestley M.J.N., Kowalsky M.J. (1998) - Aspects of Drift and Ductility Capacity of Rectangular Cantilever Structural Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 31, No. 2, pp. 73-85. Priestley M.J.N., Kowalsky M.J. (2000) - Direct Displacement-Based Seismic Design of Concrete Buildings, Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 33, No. 4 pp. 421-444. Priestley M.J.N., Calvi G.M., Kowalsky M.J. (2007) - Direct displacement-based seismic design of structures, IUSS Press, Pavia, Italy, 720 p. Rosenblueth E., Herrera I. (1964) - On a Kind of Hysteretic Damping. ASCE Journal of Engineering Mechanics 90(4), pp. 37-48. Shibata A., Sozen M. (1976) - Substitute structure method for seismic design in RC, J. Struct. Div. ASCE 102(ST1), pp. 1-18. Standards New Zealand. 2004. Structural Design Actions Part 5: Earthquake actions – New Zealand. NZS 1170.5:2004, Wellington, New Zealand. Sullivan T.J., Bono F., Magni F., Calvi G.M. (2012) - Development of a Computer Program for Direct Displacement-Based Design, Proceedings 15th World Conference on Earthquake Engineering, Lisbon, Portugal, paper 4135. Sullivan T.J., Calvi G.M., Priestley M.J.N, Kowalsky M.J. (2003) - The Limitations and Performances of Different Displacement-Based Design Methods, Journal of Earthquake Engineering, Vol. 7, No. 1, pp. 201-241. Sullivan T.J., Priestley M.J.N., Calvi G.M. (2006) - Seismic Design of Frame-Wall Structures, Research Report Rose 2006/02. Sullivan T.J., Priestley M.J.N., Calvi G.M. Editors (2012a) - A model code for the Displacement-Based Seismic Design of Structures, DBD12, IUSS Press, Pavia. ISBN 978-88-6198-072-3, 105 p. Teal E.J. (1975) - Seismic Drift Control Criteria, AISC Journal, pp. 56-67.
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Appendix One First Displacement Based Design Calculation (3m Tall Column designed to Brawley, 1979 EQ Record)
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Appendix Two Handwritten notes prepared by Greg MacRae based upon meetings with Nigel
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Appendix Three Notes on DDBD of Walls Prepared by Nigel
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5. Seismic Design and Retrofit of Bridges By Sri Sritharan, Jason Ingham, Rob Chai, Mervyn Kowalsky, Jay Holombo and Mark Yashinsky
5.1 Introduction
One can state with confidence that practicing engineers, professors, researchers and graduate students working on topics associated with the seismic design of bridges have come across multiple contributions from Nigel and that they have advanced their knowledge based on his contributions. This observation speaks volumes for Nigel’s natural talent for understanding technical challenges and producing impactful research in a manner that was simple, elegant and easily applicable. One can also make the same statement for several of Nigel’s other research areas, including masonry, structural walls, precast structural systems, and displacement based design. His bridge research work has significantly influenced seismic design practice in all earthquake-prone states in the U.S. (e.g., California, Alaska, South Carolina, and Washington) as well as by extension bridge practice across many other states. Similarly, several seismic countries and regions have also embraced Nigel’s recommendations (New Zealand, Costa Rica, Japan and Chile, to name a few). While there is little that this paper could do to draw more attention to Nigel’s bridge related work than it has already received, the purpose of this article is to provide a few highlights to show how he influenced the art and practice of the seismic design and retrofit of bridges during his incredible professional career. Because the authors were fortunate enough to participate in Nigel’s bridge related research and field implementaion, they are able to present these studies based on their personal interactions and experiences. Nigel published hundreds of technical articles and reports on bridge related studies, but his textbook published more than 20 years ago on Seismic Design and Retrofit of Bridges (Priestley et al., 1996) is still considered to be the definitive source on this topic. While most of his publications are from the 1980 and 1990 era when he was a professor at the University of California at San Diego (UCSD), Nigel’s interest in bridge related work started earlier in the 1970s. Following his PhD at the University of Canterbury, with the thesis focus on moment re-distribution-not directly related to earthquake engineering-he began his career at the Central Laboratories of the Ministry of Works in Gracefield, which is a suburb of Wellington in New Zealand, where he developed an interest in bridge structures. From this point onwards, Nigel’s research involved two key elements: large-scale structural testing (Priestley, 1971) and formulation of design recommendations (Priestley, 1972). These two aspects not only became the signature elements of Nigel’s research, but also made UCSD known for seismic related research. Nigel’s attention turned towards seismic design of bridges after he joined the University of Canterbury, with his first publication on the topic titled “Seismic Design of South Brighton Bridge - A decision against mechanical energy dissipaters” (Priestley and Stockwell, 1978), followed by a publication on soil-structure interaction in 1979 (Priestley et al., 1979) and reinforced concrete filled steel piles in 1983 (Priestley et al., 1983). It was a series of publications in the mid 1980’s that set the stage for an incredible run that would take Nigel and his family from Christchurch, New Zealand to San Diego, California. In the early 1980s, John Mander, worked on his PhD dissertation on seismic design of bridge piers under the guidance of Nigel and Professor Bob Park. Their work led to a unified concrete stress-strain model, frequently referred to as the Mander model, which is commonly used in seismic design and analyses worldwide. Built on the early work of Richart and others (dating back to the 1920’s), this constitutive model defines the confined concrete strength and ultimate concrete compression strain and establishes the stress-
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strain curve as a function of the amount of confinement provided by the transverse steel (Mander et al., 1998). Nigel’s work on the behavior of bridge columns continued, leading to his seminal work with Bob Park on the plastic hinge method for member deformations (Priestley and Park, 1987) and publications on the design of reinforced concrete bridge columns for strength and ductility (Zahn et al., 1986). Combined, these publications set the course for consideration of the nonlinear behavior of bridge sub-structure components in earthquakes that had far-reaching consequences not only for seismic design and detailing, but in a broader sense for performance-based seismic design. Nigel’s seismic bridge work continued in California after he joined UCSD in 1986. After examining the details adopted in bridge columns in California, he was convinced that bridge columns and cap beams were being designed with several deficiencies and that they would need immediate upgrading to prevent them from experiencing catastrophic failure when exposed to earthquake action. Using hand calculations, he showed that not only were flexure and shear deficiencies prevalent in these members, but that the longitudinal bars were being terminated at incorrect locations with insufficient development length. He also argued that the longitudinal steel in the cap beams were placed in the wrong location, and yet gave credit to the working stress method used for designing these bridges as that procedure provided some beneficial consequences (Priestley et al., 1996). In collaboration with the California Department of Transportation (Caltrans), Nigel began to evaluate the expected performance of as-built columns and develop retrofit strategies for them, which of course included large-scale testing. This effort led to the use of steel jackets for column retrofitting. This technique, lovingly referred to by one of his daughters as ‘Priestley’s full metal jackets’ in her piece honoring his lifetime of achievements towards seismic safety, was very effective in overcoming both flexural and shear deficiencies. Then PhD students Rob Chai, and Ravindra Verma, worked on these projects as part of their dissertation work. The 1989 Loma Prieta Earthquake struck northern California before Nigel’s findings could be implemented in practice, causing widespread damage to bridges including the collapse of a portion of the Cypress Street Viaduct in Oakland. This collapse was responsible for 42 of 63 fatalities that resulted from this earthquake. While Nigel’s concerns regarding bridge column deficiencies were proven, the earthquake also unearthed other structural deficiencies. One major concern related to the seismic performance of cap beam to column joints, which was partly responsible for the collapse of the Cypress Viaduct. Nigel began to focus on bridge joint work by first evaluating the as-built condition, then developing retrofit strategies and finally producing new effective designs using what is known as the force transfer method. Jason Ingham, and Sri Sritharan, were the PhD students on these projects that also included several large-scale tests. In parallel, several other research projects that related to the seismic design of bridges were undertaken with extensive funding from Caltrans and in collaboration with his colleague Frieder Seible. The late James Roberts was the State Bridge Engineer for Caltrans at that time and Nigel appreciated his full support for advancing seismic bridge design and implementing the findings in design practice. The additional research included topics such as the use of lightweight concrete, relocation of plastic hinges, architectural flares in bridge columns, hanging bridge decks, hollow columns, shear capacity models for reinforced concrete columns, and improving foundation design practice. In 1993 Nigel published a paper entitled “Myths and Fallacies in Earthquake Engineering” (Priestley, 1993), in which he argued that a number of fundamental principles, on which the seismic design of structures is based, are, in his own words, “deeply flawed”. This paper, which received significant attention among the earthquake engineering community and was updated and published by different organizations, prompted Nigel to work on the Displacement-Based Seismic Design procedure. Graduate student Mervyn Kowalsky, and Greg MacRae, worked closely with Nigel on this topic and developed the procedure first for RC bridge columns see Chapter 4. Nigel continued his passion for
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Displacement-Based Design for many years, even after he departed UCSD in 2000, and produced a textbook on this topic with Calvi, Professor at Istituto Universitario di Studi Superiori (IUSS) di Pavia, Italy, and Kowalsky in 2007 (Priestley et al., 2007). As evident from this introduction, it is clear that a handful of people had the privilege and honor to work closely with Nigel on seismic bridge related work, received guidance and learned from him directly in multiple ways. The reminder of this paper presents more detailed accounts of Nigel’s research advancements and impact in seismic design and retrofit of bridges from several of these individuals.
2. Research in New Zealand
In 1978, Nigel published his first paper related to the earthquake response of bridges (Priestley and Stockwell, 1978). The existing South Brighton bridge in Christchurch was an old timber structure built in 1926, and it was proposedthat a replacement bridge be constructed with the superstructure supported on elastomeric bearing pads and steel-cantilever dampers. Dynamic analyses were undertaken that showed that the dampers were relatively ineffective, and it was advocated that a conventional ductile design based on then-current New Zealand bridge design philosophy would be more cost effective without increasing seismic risk. Computer modelling was undert aken for bothlongitudinal and transverse response (see Figure 5.1a) using a program named TWODEE that was developed by Richard Sharpe for his doctoral dissertation at the University of Canterbury, with time history analyses undertaken using the 1940 El Centro N-S record and the 1977 Bucharest N-S record. From this modelling, it was found that the proposed dissipaters had little influence on response, and the findings were elegantly presented by illustrating that the benefit (or detriment) associated with the period shift derived from the inclusion of bearing pads was dependent on the specific attributes of the particular earthquake record (see Figure 5.1b). In 1979, Nigel again published finding based upon computational dynamic analyses using the TWODEE software, which examined the influence of foundation compliance on the seismic response of bridge piers as illustrated in Figure 5.2 (Priestley et al., 1979). Both natural and synthetic earthquake records were used, and the influence of foundation flexibility was modelled using an equivalent spring system. A methodology is presented to show the significant change that foundation
(a) Details of the computer model for transverse loading
(b) Relationship between period shift and response spectra.
Figure 5.1 - Computer modeling of the South Brighton bridge in Christchurch (Priestley and Stockwell, 1978).
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flexibility has on the relationship between overall displacement ductility and local curvature ductility at the pier plastic hinges. Again referring to aspects of period shift, Nigel proposed that then-current New Zealand practice for medium to long period structures may be very conservative, but that for short-period bridge piers the existing practice may be slightly non-conservative. In 1980, Nigel led a discussion group of the New Zealand National Society for Earthquake Engineering to conduct a special study on bridges (Priestley et al., 1980). The focus of this exercise was to identify specific problems associated with specific bridge types and make appropriate recommendations about suitable methods for analysis and design. Whilst a number of bridge types were considered, it is noted that several topics addressed here continued to be features of Nigel’s later research, including bridges with tall piers or piers of differing heights, highly skewed bridges (see Figure 5.3a), long bridges with internal movement joints, and bridges sited across or near active faults where it was proposed that bridge piers be provided extra confinement via an inner confined core (see Figure 5.3b). In 1983, Nigel co-authored an article led by his postgraduate student Robert Park (Jr), son of Nigel’s close collaborator Bob Park, investigating the seismic performance of bridge piles encased within a steel jacket (Park et al., 1983). The reported study had both an experimental and a theoretical component. The study was partly funded by New Zealand Railways, who at that time commonly neglected the presence of the steel casing when considering earthquake response of piles. Six piles were tested with a central lateral load and applied axial compression (see Figure 5.4a) and the theoretical response was developed via determination of moment-curvature relationships that accounted for both the hoop and longitudinal stresses developed in the steel encasement, and the associated elevation of concrete confinement (see Figure 5.4b). From this study, it was concluded that the seismic performance of piles could be adequately predicted in terms of strength and that the
Figure 5.2 - Influence of elastic deflection of foundation on overall ductility capacity of a cantiler bridge pier (Priestley et al., 1979).
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(a)Directionofrotationofskewedbridge
(b)Circularbridgecolumnwithinnerconfinedcore
Figure 5.3 - Topics addressed in a 1980 report on bridges requiring special study (Priestley et al., 1980).
(a)Testset-up
(b)Theoreticalmoment-curvatureresponses Figure 5.4 - Investigation of steel encased bridge piers (Park et al., 1983).
ductility capacity exceeded the then-current requirements for bridges and buildings in New Zealand, but that the local buckling of the steel casing at moderate levels of ductility limited the potential for full ductile response of the system.
5.3 Research at UCSD
Introduction Extending the work from New Zealand, one of Nigel’s first research projects at UCSD was centered on seismic retrofit of bridge columns by steel jacketing. The premise was that full or partial encasement of columns by a steel jacket can reduce the seismic hazards associat ed with older bridges in California. The vulnerabilities of these older Californian bridges were evident after several earthquakes, including the
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1971 San Fernando earthquake, the 1987 Whittier earthquake, and the 1989 Loma Prieta earthquake. Major deficiencies in the substructures, as identified by Nigel in his retrofit program, included: •
Inadequate flexural strength: Base shear coefficients were typically less than in the pre-1971 design of bridges, resulting in lateral strength that is significantly lower than the strength required in the current Caltrans Acceleration Response Spectra (ARS). As satisfactory seismic performance of bridges depends on a ductile response, the low lateral strength in these older bridges results in large ductility demands in the supporting columns.
•
Inadequate flexural ductility: Pre-1971 bridge columns in California were typically detailed with an insufficient amount of transverse reinforcement, resulting in limited ductility capacity. The limited ductility capacity wasfurther compromised byinadequate lap-length ofthe transverse reinforcement in the cover concrete instead of welding of the lap-splices or bending of the reinforcement into 90-degree hooks back into the core concrete. Large lateral displacement demands under intense earthquake ground motions would cause the poorly anchored transverse reinforcement to unravel, rendering the confinement reinforcement ineffective and hence precipitate a brittle failure.
•
Undependable flexural capacity: Pre-1971 guidelines permitted the splicing of column longitudinal reinforcement with starter bars from the footing at a lap-length of times the bar diameter. This lap length was short compared to the current requirements, and was inadequate to fully develop the yield strength of the reinforcement, especially when larger diameter bars were involved. The short lap-splices would degrade rapidly under cyclic loading, resulting in undependable flexural strength of the column.
•
Inadequate shear strength: For pre-1971 bridges supported on short columns, the shear force associated with a ductile yielding mechanism may exceed the shear capacity of the column. The inadequate shear strength arose from the widelyspaced transverse reinforcement and poor anchorage of the transverse reinforcement. Consequently, a common failure mode for short columns involved brittle shear failure, accompanied by low ductility capacity and poor energy dissipation.
•
Footing failures: Pile caps or footings in older bridges were often provided with only a bottom layer of reinforcement. Top steel and shear reinforcement were considered unnecessary and routinely omitted. Such practice was attributed to the use of elastic design in the pre-1971 era, which assumed full gravity load acting during the seismic event while concurrently prescribing unrealistically low values of lateral seismic forces.
•
Joint failures: Joint regions between the column and footing, or between the column and bentcap, are subjected to very high shear stresses during a severe seismic attack. These regions were traditionally not designed to high seismic shear stresses. A joint failure would result in a brittle response in the structure.
Steel Jacketing of Columns Nigel was, as previously noted, a pioneer in seismic retrofit of highway bridges in California and his research program was undoubtedly very successful and results-oriented. He extended his earlier work on steel-encased piles in New Zealand to steel jacketing of columns at UCSD to improve the ductility capacity of deficient columns, as well as their shear strength, and to mitigate undependable strength from inadequate lap-splices. Under the combined action of axial compression and bending, the compression region dilates as the flexural strength of the column is approached. The dilation would be restrained by the radial stiffness of the jacket, placing the jacket in circumferential tension and the column concrete in radial compression, as shown in Figure 5.5a. For a in. (1.52
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m) diameter column retrofitted with a typical 0.5 in. (13 mm) thick A36 steel jacket, the level of confining pressure from the steel jacket is about 590 psi (4 MPa), which would be near the upper limit of confinement by hoops or spirals in current design. The presence of steel jacket enhances both the compressive strength and the toughness of the concrete. The enhanced compressive response of confined concrete is shown schematically in Figure 5.5b. A steel jacket bonded to the column is also effective in resisting a portion of the total column shear force. The shear strength contribution by the steel jacket can be readily derived assuming an equivalent truss mechanism similar to the conventional transverse reinforcement of either hoops or spirals. It may also be expected that the lateral confining pressure from the steel jacket would improve the bond transfer, possibly preventing a bond failure at the lap-splices of the column longitudinal reinforcement. Benefits offered by steel jacketing can significantly reduce the seismic hazard of the older bridge structures. The effectiveness of steel jacketing was verified by Nigel through a comprehensive column test program at the UCSD Powell Laboratory (Priestley et al., 1996; Chai et al., 1991). Figure 5.6 compares the performance of a steel jacketed column with the performance of a deficient column having pre-1971 reinforcement details (e.g., inadequate lap-splices).
(a) Lateral confinement ofsteel jackets
(b) Compressive responses ofconcrete
Figure 5.5 - Confining action of steel jacket on column concrete.
During testing, bond failure, initiated fairly early on during loading, occurred in the lap-splices of the longitudinal reinforcement in the deficient column, as evidenced by the vertical splitting cracks in Figure 5.6a. Strain measurements on the column reinforcement indicated that the maximum stresses of the reinforcement barely reached the yield strength, causing the column to not attain its intended flexural strength. Sliding of the reinforcement, as a result of the bond failure, translated into a rapidly degrading system with poor strain energy dissipation, as can be seen by pinched hysteresis loops in Figure 5.6b. The onset of lateral strength degradation, which occurred at a displacement ductility factor of µ ∆ =1.5, was subsequently used by Caltrans for performance assessment of their old bridges. In comparison, the steel jacketed column shown in Figure 5.6d exhibited a very large displacement ductility capacity. Hysteresis loops, which are shown in Figure 5.6e, were stable up
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(a) Bond failure in lap-splices
(b) Undependable strength from poor lap-splices
(c) Fatigue bar fracture
(d) Large deformation capacity of steel jacketed columns
(e) Ductile hysteresis loops of steel jacketed columns
Figure 5.6 - Lateral response of columns enhanced by steel jacketing.
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to a displacement ductility factor of µ ∆ ≥ 8. The maximum lateral force of the column exceeded the theoretical flexural strength due to strain-hardening of the reinforcement. However, the presence of a steel jacket reduced the column plastic hinge length, causing higher strains in the reinforcement. Under large displacement amplitude cycles, the steel jacketed column failed by low-cycle fatigue fracture of its longitudinal reinforcement, as indicated in Figure 5.6c. Despite the fracture of the longitudinal reinforcement, the enhanced ductility capacity in jacketed columns was deemed more than adequate for the level of seismic hazard that can be expected in older bridges. As a tribute, photographs in Figure 5.7 highlight the success of Nigel’s retrofit program, showing importantly the translation of research from the laboratory to actual implementation in practice.
(a) Steel jacketed column tests at UCSD
(b) Steel jacketed columns, Coyote Wells, California
Figure 5.7 - Nigel’s contribution to seismic retrofit of bridges - translation of research from laboratory to engineering practice.
Bridge Joints and Force Transfer Mechanisms Nigel’s attention to design and retrofit procedures to address the seismic response of concrete freeway bridge bent joints stemmed from his reconnaissance following the Loma Prieta earthquake in Northern California in October 1989 (Priestley and Seible, 1992). In particular, construction of the I-980
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Oakland Southbound Connector had been completed in 1985, just four years before the earthquake, and yet the west column-cap joint of bent 38 sustained considerable damage to the outrigger knee joints in the earthquake (see Figure 5.8). Following the earthquake, Nigel noted the lack of consistent assessment and evaluation models for the treatment of bridge bents (Priestley et al., 2007) and set about developing appropriate assessment and design models in conjunction with Caltrans. A feature of this assessment procedure for bridge joints was a departure from the conventional approach of solely relying on quantifying the shear demand within the joint panel to treating the shear force as part of the force transfer mechanism in the joint zone, which extends outside of the joint panel dimensions. This approach also opened up different concepts for designing bridge joints to minimize reinforcement congestion. The magnitudes of principal compression and tension stresses estimated for the joint region were used to control damage and identify the extent of reinforcement needed in the joint panel region. Figure 5.9 shows examples that Nigel used to demonstrate the possibility of using different reinforcement configurations in the joint region, including the effects of turning column bar hooks into rather than away from the joint as was common practice at the time, the contribution of cap beam reinforcement adjacent to the joint to aid the transfer of forces through the joint, and the effects of applying cap beam prestressing in improving joint performance. With the objective of identifying the need for assessment and design procedures for bridge joints, it was recognized that in comparison to the reasonably comprehensive extent of laboratory test data for the joints of building frames, there was a paucity of laboratory test data pertaining to the earthquake response of concrete bridge joints. The significance of this distinction is that in buildings it is typical
(a)Viewofdamage
(b)Jointreinforcementdetailing
Figure 5.8 - I-980 Bent 38 joint failure following the 1989 Loma Prieta earthquake (Priestley et al., 1996).
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(a) Lap splice failure in closing knee joint
(b) Column bars bent into tee joint
(c) Incorporating external vertical joint reinforcement
Figure 5.9 - Joint force transfer mechanisms for concrete bridge joints with and without cap beam prestressing (Priestley et al., 1996).
to target plastic hinge formation in beams, whereas in bridge frames it is more common to expect hinging to occur in piers/columns. In addition, the detailing in bridge joints is typically distinctly different from common joint detailing in building frames (see Figure 5.9b), partly due to the differing geometries of the sections framing into the joint. Experimental programs were first undertaken by Ingham to address bridge knee joint response (Ingham et al., 1997; 1998), followed by a companion project undertaken by Sritharan that considered the testing of isolated bridge tee joints and multiple-column bents consisting of knee and tee joints (Sritharan et al., 1999; 2001). The knee joint test program first considered replication (see Figure 5.10a) and then retrofit, repair and new design strategies for the I-980 connector in Oakland, followed by a study of the joint detailing adopted in the then-recently constructed I-105 Airport Viaduct in Los Angeles that contained detailing representative of 1991 design practice in the State of California. The knee joint test program was extended by considering the response of tee joints replicating an interior joint from the Santa Monica Viaduct in Los Angeles (see Figure 10b), followed by three further tests to demonstrate new joint design concepts. The first test minimized the amount of reinforcement in
(a) Bridge knee joint test (see also Figure a)
(b) Bridge tee joint test (test set-up inverted by 180 degrees)
Figure 5.10 - Bridge joint tests undertaken at UCSD (Priestley et al., 1996).
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the joint panel by placing external joint reinforcement in the cap beam while the second and third units achieved the same objective by using partial prestressing and full prestressing in the cap beam. The isolated knee and tee joint testing was then extended by Sritharan to test multiple-column bridge bents consisting of a knee and a tee joint (see example in Figure 5.11). This testing program had the objectives of: a) verifying recently proposed guidelines for designing ductile bridge columns; b) finalizing the external strut force transfer model, a modified version of which is being widely used today for designing bridge joints; c) investigating the feasibility of a precast construction procedure for bridge bents; and d) confirming a suitable pin detail that can be used at the base of the bridge column (Sritharan et al., 2001). The testing and subsequent research formalized the force transfer design methodology (Sritharan and Ingham, 2003) and the modified external strut force transfer design method (Sritharan, 2005), demonstrated the effectiveness of prestressing the cap beam to simply the details and improve the performance of bridge joints, and confirmed the suitability of the straight bar anchorage details for the column longitudinal column reinforcement embedded into the joint and pin details used at the column base. These studies on bridge cap beam joints were further supplemented by testing of footing and pile cap details (Xiao et al., 1996). In conjunction with the above studies Nigel developed mechanical models to describe the anchorage, development and splicing of reinforcement both within the joint and in the members adjacent to
(a) Test set-up Figure 5.11 - Testing of a multi-column bridge bent (Sritharan et al., 2001).
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(b) Response at peak lateral displacement Figure 5.11 - Testing of a multi-column bridge bent (Sritharan et al., 2001).
the joint (see Figure 5.12). Prior to this work, it was common to specify a standard reinforcement development length for column bars projecting into joints, regardless of the force transfer mechanisms that were developed. In some cases, this practice led to inadequate embedment and an incomplete force transfer pathway. Similarly, Nigel developed a model to forecast the onset of lap-splice failure, including the scenario where 90 degree hooked top beam reinforcement may fail when closing moments are applied to a bridge knee joint (see Figure 5.9a).
Precast Spliced Girders Although the vast majority of bridges in California are cast-in-place concrete bridges, the number of precast prestressed concrete bridges constructed in that state significantly declined in the years following the San Fernando earthquake in 1971 through to the 1990’s. In large measure, this decline can be attributed to opinions about the seismic performance of precast girder bridges, as bridge engineers were reluctant to consider the connection between precast girder superstructures and columns as fixed under severe seismic forces. To compensate for this lack of continuity, the
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Figure 5.12 - Lap splice failure of column bars in bridge piers (Priestley et al., 1996).
substructures were made larger, and hence more costly than equivalent cast-in-place bridges. As a result, precast girder bridges were relegated to locations where falsework placement was impractical (Holombo et al., 2000). With funding from Caltrans, Nigel together with his student Holombo and industry partners developed cost-effective integral precast girder superstructure to column connections that can resist severe seismic forces. In their approach, they included the development of a working group of industry professionals from Caltrans and the Precast Concrete Manufacturers of California (PCMAC), now the Precast/Prestressed Concrete Institute (PCI) West. The investigation focused on two new girder shapes for California that could span long distances and were suitable for continuous post-tensioning and splicing (Holombo et al., 2000). These shapes were the California Bulb-Tee and Bathtub girders (Figure 5.13), which have the following characteristics: • Bathtub girders - simulate the appearance of cast-in-place box girder construction and require temporary shoring towers if splicing is used. For the purposes of the study, the girders terminated at the face of the cap beams.
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•
Bulb-tee girders - suitable for cantilever construction if spliced, where continuous segments pass over or through the pier cap (pier segments), and segments within the span (span segments) are temporarily suspended from the pier segments with strong-back beams during construction.
The results of this investigation demonstrated that seismic resistant integral connections between precast concrete superstructures and columns can be constructed cost-effectively. These integral connections relied on many of the methods and details that Nigel and his research staff had recently developed, including external strut mechanisms for column joint design and the use of shear friction for torsion resistance as described in this paper and elsewhere (Priestley et al., 1996).
Figure 5.13 - The bathtub and bulb-tee girder prototypes studied were typical overcrossings, and represented a range of construction methods.
Caltrans and their industry partners were satisfied with this cost-effective approach, with the caveat that the seismic resistance be verified through large-scale tests. This verification required the construction of two large-scale bridge test structures with hydraulic actuators that simulated horizontal seismic forces and displacements, parallel to the direction of traffic. One test incorporated Bulb-tee girders that extended continuously through the cap and were spliced at the dead-load inflection points, and the second test incorporated Bathtub girders that terminated at the face of the cap beam (Figures 5.14 and 5.15). Both model test structures were continuously post-tensioned. Dimensions and forces for both model bridge structures were scaled to 40% of full size, and included a column, a cap beam, precast girders and a deck extending from midspan to midspan of a multispan prototype design example. The midspan was selected as a boundary because it corresponds to the inflection point of the superstructure moment diagram under horizontal earthquake forces acting parallel to the bridge centerline.
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Figure 5.14 - Dimensions for both precast girder model bridge structures were scaled 40% from full size. Large hydraulic actuators simulated realistic seismic forces and displacements.
Significant effort was made to correctly simulate gravity and seismic forces throughout construction and testing. Of particular concern was maintaining correct moments, shears and axial forces within girder end regions near the cap beam without applying forces in areas where resistance was being studied. Force were applied via hydraulic actuators at each end and hold downs were applied near the prototype dead load moment point of inflection (Holombo et al., 2000). The tests validated the effectiveness of the innovative design approach developed during the investigation. Ductile plastic hinges formed at the ends of the columns, while the connections
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between the precast girder superstr ucture and the columns remained essentially elastic during fully reversing cycles applied incrementally to a displacement that exceeded 150% of the design requirement. Further, the data obtained from the tests provided useful insights, which have resulted in improved design and construction practices (Holombo et al., 2000). This investigation under Nigel’s leadership resulted in the development of practical seismic resistant precast concrete girder bridge design practices using rational methods. These details have been further refined and now are in widespread use. As a consequence, precast girder bridges are now routinely being selected for projects in California, even in situations where limitations on falsework are not a concern. This is especially tr ue for larger projects, where the economy of off-site fabrication is enhanced (Holombo et al., 2000).
The UCSD Shear Capacity Model Nigel’s contribution to the development of predictive shear capacity models for reinforced concrete members is often referred to as the “UCSD Shear Model”. However, the srcins of the model date back to one of Nigel’s last students in New Zealand, Ang Ghee. While shear design equations have long been shown to be conservative, their application for predictive purposes often lead to erroneous results. Nigel recognized this early on, realizing that in order to conduct accurate assessments of existing structures, more robust models to predict shear capacity would be needed. Such models, with appropriate reduction factors, could then be safely used for design as well. The development of the UCSD Shear Model can be traced over a series of four publications spanning 11 years, each of which built upon earlier work. In 1989, Ghee et al. published the first of these,
Figure 5.15 - Testing of precast girder bridge details validated the innovative approach and provided useful data leading to improved design and construction practices.
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identifying the importance of ductility on the strength of the concrete shear resisting mechanism (Sritharan et al., 1999). The most important contribution from that first paper (in addition to the extensive series of tests which were reported), was a ductility dependent shear capacity model (see Figure 5.16). While the importance of ductility on shear capacity had been established in the past as early as ATC-6 (Applied Technology Council, 1981), Ghee’s model was the first to quantify this reduction in a non-binary manner (i.e., some codes at this time represented shear capacity at two levels - strength within and outside of the plastic hinge region, but without any consideration of a degradation envelope).
Figure 5.16 - Ghee model for strength of the concrete shear resisting mechanism (Ghee et al., 1989).
In 1994, with Ravindra Verma and Yang Xiao, Nigel proposed a model, which for the first time aimed to decouple the contribution of axial load on shear capacity from that of the concrete. The rationale for the coupling of the two related to the impact that the axial load has on crack widths, allowing an increase in the shear friction along the cracked surface as a function of axial compression force. Verma et al. (Priestley et al., 1994) postulated that the axial load contribution could be separated from the concrete contribution, thus simplifying the model, while also more accurately representing the behavior by suggesting that the horizontal component of the diagonal compression strut formed by the axial load can be directly added to concrete and steel contributions. The Verma et al. model for the concrete contribution is shown in Figure 5.17, while the axial load contribution is shown in Figure 5.18. In 1996, Gianmario Benzoni and Nigel further recognized that the degradation of the concrete contribution did not stop at a displacement ductility of four. Aided by tests on columns with low longitudinal reinforcement, which failed at high levels of ductility, they proposed a change to the concrete contribution to shear strength. The previously constant capacity beyond ductility 4 now reduced further to ductility 8 (Priestley and Benzoni, 1996).
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Figure 5.17 - Concrete contribution to shear strength (Priestley et al., 1994).
Figure 5.18 - Axial load contribution to shear capacity (Priestley et al., 1994).
Over the following four years, the shear capacity model was further studied and the existing database of tests were reexamined. This led to several changes to the UCSD shear capacity model, which would be expressed in its final form by Kowalsky and Priestley (Kowalsky and Priestley, 2000). Amongst the proposed change were: i) constant slope degradation between ductility 4 and 8 (see Figure 5.19); ii) inclusion of the impact of reinforcement ratio and aspect ratio on the strength of the concrete shear resisting mechanism (Figure 5.19); and iii) for the first time, a change to the steel truss contribution, which now recognized the impact of the compression zone on mobilization of the steel
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truss mechanism (Figure 5.20). The axial contribution remained the same in the final version of the model. The final model was shown to greatly reduce the scatter in shear capacity prediction (Figure 5.21). An independent study by a group at the University of Washington (Camarillo and Haili, 2003) confirmed this model to be the most accurate when compared against other models available at the time of that study.
(a) Ductility dependence
(b) Aspect ratio dependence
(c) Steel ratio dependence
Figure 5.19 - The three components of the concrete contribution to shear strength (Kowalsky and Priestley, 2000).
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Figure 5.20 - Impact of compression zone on steel truss mechanism (Kowalsky and Priestley, 2000).
Figure 5.21 - Accuracy of final version of the UCSD shear model (Kowalsky and Priestley, 2000).
5.4 Impact on industry practice
In parallel with leading the seismic research at UCSD, Nigel was instrumental in helping Caltrans retrofit its oldest bridges and design its latest bridges to survive large earthquakes. But more importantly, Nigel made himself available during every step of Caltrans’ journey from a damaged and vulnerable infrastructure following the Loma Prieta earthquake to a seismically resistant highway system with state-of-the-art seismic design procedures.
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When Nigel moved from New Zealand to UCSD, Caltrans was still relying on cable restrainers to hold older bridges together during earthquakes and force-based methods to determine the capacity of new bridges. However, when the “cable restrained” I-605/I-5 Connector almost collapsed due to shear damage of its columns during the 1987 Whittier earthquake, Nigel was ready with a solution, which is to use steel jackets to improve the ductility and shear resistance of bridge columns. He had also already developed procedures to determine the displacement capacity of columns with and without steel jackets. This combination of physical tools to improve a bridge’ s performance along with analytical tools to give designers insight into the bridge’s capacity was what made Nigel so important to Caltrans during the many traumatic events that followed. Nigel joined UCSD at a critical time in Caltrans’ development of seismic analysis and design procedures for bridges. After the 1971 San Fernando earthquake, Caltrans had made progress in developing site-specific ground motions and a ductile bridge design that allowed bridges to take damage without collapse. However, Caltrans was stuck in procedures halfway between the force-based methods of the past and the displacement-based methods of the future. Caltrans was determining the displacement capacity of bridge columns by calculating the displacement at yield and multiplying by a factor. Nigel provided a simple way of determining displacement capacity based on a momentcurvature analysis of the column section. This was the missing piece of the puzzle, which allowed Caltrans engineers to make sure that the displacement capacity of their columns was greater than the displacement demand from the design earthquake. When the Loma Prieta earthquake occurred two years later, Nigel jumped into the many tasks required to come up with solutions to retrofit a variety of Caltrans bridges. Whenever Caltrans had a question, Nigel was able to immediately perform research and come up with solutions that were quickly put into practice. By the time that Nigel left UCSD in early 2000, Caltrans had retrofitted most of its vulnerable bridges and had highlight written modern retrofit (Caltrans, and seismic design criteria(Caltrans, 2013).performance of To Nigel’s ability to focus2008) on research needed to improve the seismic bridges and the corresponding impact on Caltrans seismic retrofit program and improving its design methodologies, selected examples are presented below in a chronological order. •
Steel Column Jacke ting (1988) As previously noted, thisprogram was initiated in1987 and executed in 1988 at UCSD to test large-scale circular and rectangular columns in ‘as-built’ and retrofit conditions for flexural and shear responses. Columns were constructed to a 0.4 geometric scale using materials and design details appropriate for columns designed in ht e 1960’s. The results of these testswere the basis for the thousands of bridges that were subsequently retrofitted after the 1989 Loma Prieta and 1994 Northridge earthquakes.
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Seminar on Seismic Assessment and Retrofit of Bridges (199 1) In July of 1991, Nigel with the assistance of Frieder Seible held a seminar, which provided the state-of-the-art in the practice of displacement-based design procedures for existing bridges. The report SSRP-91/03 (Priestley and Seible, 1991) that was published as part of this seminar became the bible used by engineers involved in Caltrans’ big retrofit program of the 1990s.
•
Rocking Response of Bridges (1991) Allowing a bridge to rock during earthquakes is a way to dissipate energy and protect columns that otherwise could be damaged. This concept was used by Nigel for some New Zealand bridges in the late 1970’s. Nigel provided a simple procedure for determining if rocking will occur in the seminar report and it became a popular retrofit strategy by Caltrans.
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Southern Freeway Viaduct (1991) The City of San Francisco was so eager to remove the double-deck viaducts from their city that
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Caltrans was reluctant to tear down damaged or vulnerable parts of the Southern Freeway Viaduct because the city might not allow them to rebuild it. Instead, using Nigel’s research outcomes, Caltrans jacked up the double-deck structure and replaced the columns with stronger, more ductile members and provided additional edge beams to increase the capacity of the superstructure. The resulting retrofit was designed to keep the bridge in service for the largest earthquake likely to occur on the nearby San Andreas or Hayward Faults. •
Santa Monica Viaduct (1992) Caltrans was concerned about how to retrofit the two mile long Santa Monica Viaduct in Los Angeles with its thousands of vulnerable columns. Nigel came up with a retrofit strategy that included the use of struts between vulnerable columns in a bent that would allow considerable damage to the Viaduct during earthquakes, but would prevent a collapse.
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Column-Footing Interaction (1992) Before the 1971 San Fernando earthquake, Caltrans’ bridges were often built without a top mat of footing reinforcement. Testing at UCSD showed that such footings could be so badly damaged during large earthquakes to allow the bridge to fall over. A top mat of footing reinforcement became a part of retrofit whenever the bridge was at risk of collapse if the column-footing connection deteriorated to a pin.
•
Fiber Reinforced Polymer (FRP) Composites Column Casings (1992) Nigel could see the benefits of having FRP Column casings for situations where steel jackets couldn’t be used. He made the UCSD Powell Laboratories available for testing of FRP column casings similar to the testing of steel column casings and found similar performance.
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Shear St rength of Reinforced Concrete Columns (199 4) The shear model that Nigel developed at UCSD included the reduction of shear strength as plastic hinges formed and damaged the column. This model is used as needed in the seismic retrofit of bridges, even though another shear model developed at UC Berkeley is used in new bridge design.
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Damage Analysis of Bridges after the Northridge Earthquake (1994) An excellent contribution that Nigel, Frieder, Chia-Ming Uang and their students made to help Caltrans was a report of their quick analysis of the bridges that collapsed during the 1994 Northridge earthquake (Priestley et al., 1994). This report allowed Caltrans to check if they were addressing all of the vulnerabilities shown by the bridge damage during this earthquake. This report has stood the test of time and was remarkably insightful. Previously unconsidered issues like unbalanced spans, bents, and columns and lack of uniformity such as flared columns, have all been shown to cause bridge collapse and were not considered before the Northridge earthquake.
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Seismic Design an d Retrofit of Bridges (1996) In 1996 Nigel, Frieder, and Michele Calvi published a book that described state-of-the-practice seismic procedures for bridges, based on the last 10 years of working closely with Caltrans. Another seminar was and heldwriting and theseismic new book was distributed to be used by engineers involved Caltrans in retrofitting bridges criteria.
•
Seismic Response Modification D evice ( SRMD) Test System (1997) Retrofitting Caltrans’ big toll bridges usually required using isolation and damping devices to protect the vulnerable substructures from the large inertial force of the long span superstructure. Every giant toll bridge isolator had to have a long proof test before it could be put on a bridge. Nigel and Frieder had a testing facility built on UCSD’s campus, which could test these huge bearings in all directions with the full ground motions from any earthquake.
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5.5 Conclusions
In tribute to Professor Nigel Priestley’s significant contributions to concrete structures, this paper has summarized some of his research and the corresponding impact, especially in California, in the area of seismic design and retrofit of bridges. It is shown that over a period of approximately 15 years, Nigel was able to uncover several basic deficiencies that have existed in seismic bridge design and come up with retrofit strategies and new design solutions. Despite the complexity of the problems, his solutions are generally simple. He always came up with elegant ways to use largescale testing to demonstrate that the proposed solutions would be effective and established suitable recommendations that can be incorporated in design practice. While the 1971 San Fernando earthquake exposed the vulnerabilities of bridges in California, advancements to Californian seismic design practice did not materialize until after Nigel joined UCSD. As reported herein, Nigel’s early focus on studying the seismic response of bridges whilst at the University of Canterbury was the perfect launching pad for further development and then implementation of these innovative seismic assessment and design procedures into practice, particularly at a time when Caltrans was particularly receptive to considering and adopting new approaches. Nigel’s training, his insight, his warmth and humor, his discipline and intelligence, all made him the right person at the right time when Caltrans faced hundreds of structural issues over a decade, during which the Whittier Narrows, Loma Prieta, and Northridge earthquakes continued to demonstrate that poorly detailed bridges would experience significant damage in moderate and large earthquakes. Nigel’s eminent contributions to bridges and other areas can be attributed to his keen mind, which seemed to see deeper into the behavior of structures than most people. When he believed, he stood by ideas even when he was strongly criticized for them. More often than not, he turned out to be right. His hard work, his intelligence, and his willingness to help implement research was critical in getting Caltrans through the big retrofit program in the 1990s. Despite all these successes, Nigel was humble and considerate, and enjoyed the friendships he formed with students, colleagues, practicing engineers and many others.
5.6 References Applied Technol ogy Council (1981) - Seismic Design Guidelines for H ighway Bridges, funded by Federal Highway Administr ation. Buckle I.G., Goodson M., Cassano R., Douglas B., Hegemier G., Imbsen R., Jones D., Liu D., Mayes R., North P., Priestley M.J.N., Rojahn C., Seible F., Selna L., and Viest I. (1990) - Bridge Structures, Earthquake Spectra, 6(S1), pp. 151-187. Caltrans (2008) - Seismic Retrofit Guidelines for Bridges in Califor nia, Memo to Designers, MTD 20-4, Sacramento, CA, 13 p. Caltrans (2013) - Seismic Design Criteria, Ver. 1.7, Sacramento, CA, 180 p. Camarillo Haili R. (2003) - Evaluation of Shear Strength Methodologies for Reinforced Concrete Columns, Master’s Thesis, Depart ment of Civil and Environmental Engineering, University of Washington. Chai Y.H., Priestley M.J.N., Seible F. (1991) - Seismic Retrofit of Circular Bridge Columns for Enhanced Flexural Performance, ACI Structu ral Journal , 88(5), pp. 572- 584. Ghee A., Priestley M.J.N., Paulay T . (1989) - Seismic Shear Strength of Ci rcular Concrete Columns, ACI Structu ral Journa l, 86(1), pp. 45-59. Holombo J., Priestley M.J.N. , Seible F. (2000) - Conti nuity of Precast Spliced-Girder Bridges u nder Seismic Loads, PCI Journa l, 45(2), pp. 40- 63. Ingham J.M., Priestley M.J.N., Seible F . (1997) - Seismic Response of Bridge K nee Joints having Columns with I nterlocking Spirals, Bulletin of the New Zealand National Society for Ear thquake Engineer ing, 30(2), pp. 11 4-132. Ingham J.M., Priestley M.J.N. Seible F . (1998) - Cyclic Response of Bridge Knee Joints with Circular Columns, Journal of Ear thquake Engineering, 2(3), pp. 357-391. Kowalsky M.J., Priestley M.J.N. (2000) - An Improved Analytical Model for Shear Strength of Circular RC Columns in Seismic Regions, ACI Structur al Journal, 97(3), pp. 388- 396. Mander J.B., Priestley M.J.N., Park R. (1998) - Theoretical Stress-Strain Model for Confined Concrete, Journal of Structural Engineering, ASCE, 114(8), pp. 1804-26. Park R.J.T., Priestley M.J.N., Walpole W.R. (1983) - The Seismic Performance of Steel Encased Reinforced Concrete Bridge Piles, Bulletin of the New Zealand National Society for Ear thquake Engineer ing, 16(2), pp. 123- 140.
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Priestley M.J.N. (1 971) - Effects of Transverse Temperature Grad ients on Bridges, Cent ral Laboratories, Report No. 394, Gracefield, New Zeala nd, 30 p. Priestley M.J.N. ( 1972) - Thermal Gradients i n Bridges - Some Design Considerations, New Zealand Engi neering, 27(7), 228 p. Priestley M.J.N., Stockwell M.J. ( 1978) - Seismic design of South Brighton Bridge–A Decision Against Mechanical Energy Dissipaters, Bulletin of the New Zealand National Society for Earthquake Engineering, 11(2), pp. 1 10-120. Priestley M.J.N., Park R.J.T., Heng, N.K. (1979) - Influence of Foundation Compliance on the Seismic Response of Bridge Piers, Bulletin of the New Zealand National Society for Ear thquake Engineer ing, 12(1), pp. 22-34. Priestley M.J.N., Stanford P .R., Carr A. J. (1980) - Seismic design of bridges – Section 11 Bridges Requiri ng special study, Bulletin of the New Zealand National Societ y for Earthquake Engineering, 13(3), pp. 302-3 07. Priestley M.J.N. Park R.J.T. (1987) - Strength and Ductility of Concrete Bridge Columns under Seismic Loading, ACI Structural Journal, 84(1), pp. 61-76. Priestley M.J.N., Seible F. (editors) (1991) - Seismic Assessment and Retrofit of Bridges,” University of California, San Diego, Structu ral Systems Resea rch Project, Report No. SSRP-91/03, 507pp. Priestley M.J.N., Se ible F. (1992) - Performance Assessment of Damaged Bridge Bents Aft er the Loma Pr ieta Eart hquake, Bulletin of the New Zealand National Societ y of Earthquake Engineering, 25(1), pp. 44-5 1. Priestley M.J.N. ( 1993) - Myths and Fallacies in Eart hquake Engineeri ng – Conflicts between Design and Realit y, Bulletin of the New Zealand National Society for Ear thquake Engineer ing, 26(3), pp. 329-34 1. Priestley M.J.N., Seible F., Uang C.M. (editors) (1994) - The Northridge Earthquake of January 17, Damage Analysis of Selected Freeway Bridges, University of California, San Diego, Struct ural Systems Research Project, Report No. SSRP-94/06, 266 p. Priestley M.J.N., V erma R., Xiao Y. (1994) - Seismic Shear Strength of Rei nforced Concrete Columns, ASCE Journal of Str uctural Engineering, 120(8), pp. 2310-2329. Priestley M.J.N., Benzoni G. (1 996) - Seismic Performance of Circula r Columns with Low Longitud inal Reinforcement Rat ios, ACI Structu ral Journal, 93(4), pp. 474-84. Priestley M.J.N., Seible F., Calvi G.M. (1996) - Seismic Design and Retrofit of Bridges, John Wiley & Sons. Priestley M.J.N., Calvi G.M., Kow alsky M.J. (2007) - Displacement-Based Seismic Design of Str uctures, I USS Press, Pavia, Italy. Sritharan S., Ingha m J.M. (2003) - Application of Strut-and-T ie Concepts to Concrete Bridge Joints in Seismic Regions, PCI Journal, 48(4), pp. 2-26. Sritharan S. (2005) - Improved Seismic Design Procedure for Concrete Br idge Joints, Journal of Struct ural Engineer ing, 131(9), pp. 1334-1344. Sritharan S., Priestley M.J.N., Seible F. (1999) - Enhancing Seismic Performance of Bridge Cap Beam-To-Column Joints Using Prestressing, PCI jour nal, 44 (4), pp. 74-91. Sritharan S., Priestley M.J.N. , Seible F. (200 1) - Seismic Design and Experimental Verification of Concrete Multiple Column Bridge ACI Structu ralSeible JournaF.l, (1996) 98(3), pp. 335- 346. Xiao Bents, Y., Priestley M.J.N., - Seismic Assessment and Retrofit of Bridge Column Footings, ACI Structural Journal, 93(1), pp. 79-94. Zahn F.A., Park R.J.T., Priestley, M.J.N. (1986) - Design of Reinforced Bridge Columns for Strength and Ductility, Report 86-7, Depart ment of Civil Engineering, University of Canterbury, Christchurch, New Zealand, 330 p. Zahn F.A., Park R.J.T., Priestley, M.J.N., Chapman H.E. (1986) - Development of Design Procedures for the Flexural Strength and Ductility of Reinforced Concrete Bridge Columns, Bulletin of the New Zealand National Society for Earthquake Engineering, 19(3), pp. 200-212.
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6. Masonry and Earthquakes: Not a Matter for Blockheads By Jason Ingham, Guido Magenes, Katrin Beyer and Rob Chai
6.1 Introduction
In the early 1970s, masonry was almost completely absent from university structural engineering curricula in most earthquake prone countriesworldwide, given its perceived deficiencies when compared to more modern construction materials such as reinforced concrete and structural steel. At that time, the international scientific literature on structural masonry was either dedicated to the study of historical masonry (e.g. Heyman, 1966) orwas developed in non-seismic countries where gravity loading and wind loading were the dominating actions in design (e.g. Sahlin, 1971). However, at that time several factors were decisive in drawing new attention to masonry structures in earthquake-prone countries, namely: - Developments in the construction industry prompted advancements towards more robust masonry structural systems through the use ofconfined and well-detailed reinforced masonry, with equally promising results for earthquake resistance compared to other systems - The growing interest in assessing the safety of existing buildings and their retrofit options when subjected to earthquakes - The spread of strength or ultimate limit state design methods as opposed to working stress methods that were shown to be inadequate to assess the real safety against building damage and collapse, especially when structures are subjected to horizontal loading. Seismic design in masonry was given further impetus in the early 1970s when New Zealand was pioneering the ductile seismic design principles for reinforced concrete structures, mainly through the work of Professors Bob Park and Tom Paulay (Park and Paulay, 1975).It was in this environment that Nigel’s creative work on masonry began, and his subsequent decades of research produced findings and results that remain fundamental to this day for engineers and scholars worldwide interested in understanding the seismic behavior of masonry structures. The authors of this paper had, at different times, the privilege of being introduced to the subject and exposed to Nigel’s thoughts and work. This opportunity has been a constant source of inspiration for their own work on structural masonry and this paper serves as their humble attempt to pay homage, not only to Nigel’s contributions to the subject, but also to his mind-opening, profound and at the same time solid and straight-to-the-point approach to engineering problems. His immense contributions brought forth the fascinating albeit complex subject of seismic design and assessment of masonry structures, much to the attention of structural engineers.
6.2 The University of Canterbury Years
Prior to the early 1970s, little research attention was given to the appropriate seismic design procedures for reinforced clay brick and concrete block masonry (Holmes, 1968), and that lack of attention became the motivation for a number of published studies on the topic in New Zealand over the next two decades. These efforts were partly motivated by the conservative assumptions that had tra ditionally been applied to masonry design prior to this time, and further motivated by the combined development of a new seismic design code for New Zealand (Glogau, 1972; SANZ, 1973) and observed damage in the 1968 Tokachi-oki earthquake and 1971 San Fernando earthquakes. In that period in New Zealand, greater attention was particularly given to the implementation of the limit state design philosophy, and the associated need to quantify behavioral factors for masonry design such as available levels
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of ductility capacity, observed levels of damping, quantification of the extent of strength reduction associated with repeated cyclic loading, and maximum levels of design masonry shear stress. In 1974 Otto Glogau (Chief Structural Engineer at the New Zealand Ministry of Works) reported on the performance of masonry buildings in past earthquakes, including the effects of infilled masonry in framed buildings and the performance of masonry veneer fixed to timber frames (Glogau, 1974). In the same issue of the NZSEE Bulletin, Priestley and Bridgeman (1974) reported on research undertaken on behalf of the New Zealand Pottery and Ceramics Research Association that specifically addressed the cyclic response of reinforced clay brick masonry units using the test setup in Figure 6.1. The study investigated a larger size hollow clay brick unit than had been previously considered, with attentions given to the mechanism and control of shear failure and the extent of load degradation. A total of 18 tests were reported, with 14 test walls constructed from two skins of 230 × 75 × 70 mm clay brick units separated by a 64 mm grout gap, and 4 tests constructed of hollow clay blocks having a nominal width of 140 mm. The main variable in the reported testing program was the quantity of horizontal and vertical reinforcement. Five walls contained horizontal steel confinement plates and two walls contained overburden axial load simulated by a vertical prestressing force. Important conclusions from this research were that distributed horizontal reinforcement was effective in improving the nominal wall shear strength and that cyclic load degradation could be greatly reduced by providing stainless steel confining plates in the bottom few horizontal bed joints at each end of the wall. Nigel left the Ministry of Works in 1976 and joined the Department of Civil Engineering at the University of Canterbury as a Senior Lecturer. His masonry research interests cont inued to be motivated by developments in New Zealand standards, and in particular by a proposed draft for a new masonry code (SANZ, 1980). The specific focusof Nigel’s masonry research efforts at this time were to establish suitable maximum shear stress limits. In 1977 a testing program that investigated six heavily reinforced concrete masonry shear walls was reported (Priestley, 1977). Test parameters included the longitudinal
Figure 6.1 - Masonry wall test set-up used for cyclic testing of clay brick masonry walls (Priestley and Bridgeman, 1974).
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reinforcement ratio, the applied axial load, and the horizontal bed joint confinement plates, similar to his earlier work investigating clay brick masonry walls (Priestley and Bridgeman, 1974). Particular attention was given to ensure that the walls tested were constructed usingrealistic levels of construction quality, and construction detailing included the lapping of vertical starter bar reinforcement projecting from the foundation, the use of open ended masonry units, and the inclusion of clean-out ports in the bottom course of masonry. The concrete masonry units had a width of 143 mm and the associated masonry prism strengths ranged between 18.3-24.5 MPa. The principal finding from this testing was that existing limits for the maximum masonry shear stress were unrealistically low, with all walls exceeding the maximum shear stress value permitted at that time by substantial margins, and with two walls having shear strengths of four times the code level. It was therefore concluded that as long as the provided horizontal reinforcement was adequately anchored, higher shear stresses should be allowed in future masonry design codes. In particular, it was recommended that a maximum shear stress of 1.25 MPa be allowed for walls expected to sustain displacement ductility factors of up to 4 and that a higher shear stress limit of 2.5 MPa should be allowed when the displacement ductility factor does not exceed 2. In addition, it was concluded that the existing recommendation for a strength reduction factor for masonry of φ=0.65 was unnecessarily low, and that the factor should be increased toφ=0.85. In 1979 Nigel again collaborated with researchers from the NewZealand Pottery and Ceramics Research Association to investigate the dynamic performance of brick masonry veneer panels (Priestley et al., 1979). This research was motivated by the poor reputation of unreinforced masonry veneers when subjected to earthquakes, with much of this reputation being attributed to the failure of brick masonry facades and walls during the 1931 Napier and the 1968 Inangahua earthquakes. Seven unreinforced and two reinforced clay brick masonry veneer walls tied to conventional timber-frame backings were subjected to out-of-plane sinusoidal accelerations in the appropriate frequency range imitating earthquake loading, see Figure 6.2, where the stud spacing, veneer-tie type and the initial distribution
Figure 6.2 - Test set-up for out-of-plane dynamic loading of clay brick masonry veneer walls (Priestley et al., 1979).
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of pre-formed cracking were the main variables. Out-of-plane face loading was specifically considered because the draft Code of Practice for light timber frame construction required the entire in-plane load demands to be carried by the timber frame bracing to which the masonry veneer wall is fixed. From this testing, it was concluded that when unreinforced masonry veneers were built to the specifications prescribed in the draft Code of Practice, acceptable response could be expected for earthquake loading levels in excess of those expected for the highest seismic zone in New Zealand. Furthermore, it was found that pre-formed horizontal or diagonal panel cracking had little or no apparent influence on the ultimate performance of the veneers. In 1980 Nigel embarked on a significant undertaking and wrote the background to the draft New Zealand Masonry Design Code (SANZ, 1980a) and dedicated it to the memory of Otto Glogau, his former colleague from the Ministry of Works (Priestley, 1980). The basis for the draft code was based, to a significant extent, on Nigel’s previously published masonry research findings (Priestley and Bridgeman, 1974; Priestley, 1977) that had confirmed the available but limited ductility capacity of reinforced masonry, and on the recognition that reinforced masonry could be designed based on the same principles of reinforced concrete design adapted for low strength materials. Three grades of material properties were prescribed based on the extent of inspection and associated quality of workmanship, and a procedure was presented on how to implement ductile design of reinforced masonry buildings. Criteria were provided for masonry shear wall buildings, in cases of complex geometry, such that individual walls could be classed as being eith er primary or secondary walls, where secondary walls were assumed to not carry in-plane loads but required detailing to sustain the lateral deformations that they would be expected to be subjected to, see Figure 6.3. In the same year Nigel presented a paper at the 7th World Conference on Earthquake Engineering in Turkey (Priestley, 1980a) that dealt with many similar topics, and also referred to Nigel’s research on base isolation (Priestley et al., 1977) and the seismic response of structures free to rock on their foundations (Priestley et al., 1978). Nigel also published a book chapter in 1980 that provided an effective summary of his thoughts and research on masonry up to that time and brought this work to the attention of a wide international audience (Priestley, 1980b). In 1985 Nigel published a revised provisional standard (Priestley, 1985), in response to the code committee’s expressions of dissatisfaction regarding the format of the 1980 document, and recommended that the Masonry Design Code more closely follow the format of the Concrete Design Code, (SANZ, 1982). In 1986 Nigel contributed to a discussion paper related to ‘Structures of Limited Ductility’ (NZNSEE, 1986), where despite good results from his earlier research efforts, he acknowledged that the use confinement plates will ‘inevitably be unpopular, and most masonry will continued to be unconfined’. In 1981 Nigel went on to assess the ductility capacity of masonry wall systems, where the ductile capacity of cantilevered unconfined reinforced masonry walls was computed using an ultimate compressive strain ε m = 0.0025 (Priestley, 1981). Figure 6.4 plots the available ductility of a wall with a height-to-length aspect ratio of 3 for various longitudinal reinforcement and axial load ratios. In this chart the masonry compressive strength has been normalised with respect to the value of 8 MPa assumed the construction complies with Grade B inspection. Charts were developed for both low and high strength grades of longitudinal reinforcement, and a relationship was presented for how the ductility capacity for unconfined masonry cantilever wall with aspect ratio 3µcould be modified to estimate the ductility capacity of similar walls having a different aspect ratio. Shortly afterwards a companion article was presented for the corresponding procedure to determine the ductility capacity of confined concrete masonry shear walls, where the confinement was provided by steel plates inserted into the horizontal bed joints within the plastic hinge zone (Priestley, 1981a, 1982). These procedures for establishing the ductility capacity of unconfined and confined reinforced masonry walls were underpinned by a comprehensive grouted concrete masonry prism testing program (Priestley, 1983).
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Figure 6.3 - Example of subdivision of walls into primary and secondary systems (Priestley et al., 1979).
Two series of prism tests were reported, with Series 1 testing considering 140 mm prisms and Series 2 testing considering 190 mm prisms. The influence of loading rate was investigated by considering applied axial compressive strain rates of approximately 0.000005/sec and 0.005/sec. It was concluded that the unconfined prisms developed a peak stress at a compressive strain of approximately 0.0015, that stress-strain response was not significantly influenced by block width or the presence of vertical reinforcement in the grouted flues, that increasing the strain rate by 1000 times resulted in an average 17% increase in compressive strength, and that a modified Kent-Park stress-strain relationship suitably described the observed experimental response. The studies on the performance of unconfined and confined masonry walls were combined and summarized along with a design example in an article published in an early volume of the journal of the US Masonry Society (Priestley, 1981a). In the early 1980s, Nigel began to conduct research on slender masonry walls. With Don Elder he tested three slender concrete masonry walls (Priestley and Elder, 1982). The aim of these tests was to specifically address the behaviour of slender walls, since previous testing had focused more towards understanding the behaviour of squat walls. These walls were 6.3 m high, which was the limiting height available at the Structures lab. The test units incorporated portions of the first and second floor slabs as well as a bond beam at the wall top, see Figure 6.5. All walls had similar detailing of vertical reinforcement, though Wall 2 had horizontal bed joint confinement plates in the plastic hinge zone, which were absent in wall 1. Wall 3 differed from wall 1 by having a lower level of applied axial load and had a longer lap-splice for the starter reinforcement extending from the foundation. The results of these tests validated the procedures published previously by Nigel (Priestley, 1980, 1981, 1982) but also highlighted a concern about lap-splicing the wall longitudinal reinforcement within the potential plastic hinge region. This work also showed that the capacity of conventional masonry walls may be suspect due to the lack of confinement at the wall toes, and that confining steel plates substantially improve the response of the walls. This study was the last major experimental study related to reinforced concrete masonry walls that Nigel undertook while at the University of Canterbury. In
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(a) Low strength 275 MPa reinforcement
(b) High strength 380 MPa reinforcement
Figure 6.4 - Available ductility capacity of a vertical cantilever reinforced masonry wall with an aspect ratio of 3 (Priestley, 1981).
1986 Nigel published an overview paper for the international audience highlighting all the research accomplishments in masonry stemming from New Zealand thus far (Priestley, 1986). Nigel’s research on masonry walls in the 1980s was driven to a large extent by preferences of the design community and construction industry towards cantilever wall systems. Despite considerable success and promising results from analytical and experimental studies of cantilever walls, Nigel was intrigued by the possibility of re-configuring reinforced masonry walls into ductile moment frames for low-rise buildings, which he later dubbed the “masonry moment-resisting wall-frames” shown schematically in Figure 6.6(a). The beam length in a moment-resisting frame would be longer than the length of the coupling slabs in comparable systems. The premise of his approach to the new system w as that, through capacity-design principles, the masonry wall-frame system can be designed and detailed to emulate the strong column-weak beam mechanism, which has been advocated for the design of concrete momentresisting frames. Plastic hinges, involving yielding of distributed reinforcement, will be forced to form at masonry beam ends at ultimate limit states, ensuring a dependable lateral strength to the system and a stable source of energy dissipation when the structure is subjected to intense earthquake ground motions. Since non-ductile response of elements in the system is to be avoided, a natural concern for the resilience of the new masonry moment-resisting wall-frame system involves the transfer of shear
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Figure 6.5 - Test set-up for multi-storey reinforced concrete masonry wall tests (Priestley and Elder, 1982).
forces from beam hinging to the connecting wall elements, highlighted for the joint region in Figure 6.6(b). The design of the wall-beam joint must also consider potential strain-hardening of the beam reinforcement due to large ductility demand imposed on the structure. It was also recognized that hysteretic response of the joint may degrade under reversed cyclic loading if beam bar forces cannot be adequately transferred to the joint due to poor bond condition, which is in turn dependent on the size of the joint and bar diameter. To demonstrate the potential ductile response of moment-resisting frames in masonry construction, Nigel conducted the first ever full-size masonry wall-frame joint test at the University of Canterbury in 1983. The lateral force versus lateral displacement response, which µ∆ = 6 was monitored at the top of the test specimen and shown in Figure 6.6(c), showed remarkable ductility capacity for what was essentially an unconfined masonry system. A displacement ductility factor of was obtained in the specimen before 15% degradation of lateral strength was observed. Importantly, the wall-beam joint region remained elastic and protected from any significant cracking, as can be seen in Figure 6.6(d), while inelastic rotations occurred at beam ends as intended by the capacity design principles. The success of a single test led to the masonry wall-frame system being accepted by the New Zealand Masonry Code in 1985. During these years, which were nearing the end of his time at the University of Canterbury, Nigel added a new dimension to his research in masonry by investigating the out-of-plane response of unreinforced masonry (URM) walls (Priestley, 1985a). This research was focused on assessing the earthquake characteristics of existing URM walls, rather than the design of new reinforced masonry buildings, and Nigel commented that “the response of unreinforced masonry walls to out-of-plane
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(face load) seismic excitation is one of the most complex and ill-understood areas of seismic analysis”. It is noted that the elastic analysis technique that was commonly applied at that time was focused on masonry stress levels that were “rather insignificant for unreinforced masonry”, resulting in excessively conservative results, and that the seismic capacity of URM walls responding out-of-plane is instead governed by stability and energy considerations. Load paths within unreinforced masonry buildings are discussed, as is the influence of flexible diaphragms. The conditions at wall failure are presented in terms of displacements necessary to cause instability, see Figure 6.7, and it was recommended that dynamic testing and corresponding analysis be undertaken to further refine the presented methodology for assessment. It is noted that the walls in the upper levels of unreinforced masonry buildings are likely to be most critical, and that adequately securing the walls to diaphragms is an essential step for ensuring satisfactory earthquake performance of face-loaded URM walls. The concepts, first presented by Priestley and Elder (1982), were subsequently incor porated, with minor changes, in the URM section of the chapter “Masonry Structures” of the classic book “Seismic Design of Reinforced Concrete and Masonry Buildings” which Nigel co-authored with his close friend Tom Paulay (Paulay and Priestley, 1992). It is possible to assert that the basic concepts outlined by Nigel on the out-of-plane response of URM walls, namely: the load paths, the filtering effect of the building and floor response in determining the out-of-plane demand on walls, the capacity of walls being dictated by displacement and not by force/strength, and that collapse is determined by loss of equilibrium,
(a) Proposed masonry moment-resisting wall-frame systems
(c) Lateral force-displacement response of joint
(b) Transfer of forces across masonry wall-frame joints
(d) Final damage in masonry wall-frame assembly
Figure 6.6 - Reinforced masonry wall-frame system (Priestley and Chai, 1985).
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Figure 6.7 - Consideration of seismic loading and out-of-plane wall stability for unreinforced masonry buildings with flexible diaphragms (Priestley, 1985).
pioneered the topics that many subsequent researchers have further pursued and, with some corrections (mostly regarding the simplified methods that can be used to estimate the displacement demand on walls) coming from more recent research, are still retained as valid in present methodologies for the seismic assessment or URM walls as reflected in several modern assessment codes, such as the Italian codes developed since 2005 (OPCM 3531, 2005 and NTC08-IST, 2009) and the seismic assessment methodology published recently by the New Zealand Society for Earthquake Engineering (NZSEE, 2016).
6.3 The TCCMAR years at the University of California at San Diego
When Nigel arrived at the University of California at San Diego (UCSD) in early 1987, he immediately became involved in activities associated with the U.S. Coordinated Program for Masonry Building Research, under the direction of the Technical Coordinating Committee for Masonry Research (TCCMAR/US). The aim of this project was to support the development of new design criteria for reinforced masonry buildings in seismic regions. At UCSD, Nigel continued his research on the behavior of reinforced masonry structural components focusing, as regards experimental investigations, on flanged cantilever walls, and teamed up with Frieder Seible and Gil Hegemier and a numerous and talented group of research engineers and graduate students to carry out, at the UCSD Charles Lee Powell laboratory, an ambitious 5-storey full-scale building test to validate the design and analysis procedures developed in the TCCMAR project and in previous research. These were also the years in which Nigel was perfecting and finalizing his book with Tom Paulay, which was published in 1992 (Paulay and Priestley, 1992). We believe that the masonry chapter of this book represents the refined summa of Nigel’s knowledge and experience on masonry at that time, and
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is still nowadays a treasure of mind-opening and stimulating ideas as well as rational and practical design principles for anyone with an interest in the response of masonry structures subjected to seismic loading. The chapter on masonry in this book is in great part dedicated to reinforced masonry solutions for seismic resistance, but a final section dedicated to the assessment of URM buildings is also present, drawing mostly from his previous work at the University of Canterbury described above. The chapter starts with a clear discussion on the limitations of elastic working stress design for masonry structures, also referring to two simple and effective examples, the first being the loadbearing capacity of an eccentrically loaded slender URM wall, and the second being the influence of the distribution of flexural reinforcement in reinforced masonry walls (see Priestley, 1986a). The chapter is then developed by presenting Nigel’s understanding of the key features of differing masonry structural systems. First a discussion of the different typologies of systems for seismic resistance is presented (cantilever systems, coupled walls with pier hinging, coupled walls with spandrel hinging) together with the expected displacement ductility associated with each system. Then the design for flexure (out-of-plane and in-plane) of reinforced masonry walls is thoroughly discussed, first with reference to strength, then to ductility, including simple rectangular and then complex flanged sections. Then shear design principles are discussed, followed by thorough considerations of geometric requirements and structural detailing. A section dedicated to masonry moment-resisting frames is then presented, where the topics discussed in the previous work with Rob Chai (Priestley and Chai, 1985) have been supplemented with further experimental and analytical work carried out at UCSD and at the University of California at Los Angeles (Hart et al., 1992). A brief but effective section on the design of masonryinfilled reinforced concrete frames follows, after which a section dedicated to minor (low-rise) masonry buildings is presented. A precious complement to all the principles discussed in the chapter are two detailed worked examples of the design of a seven-storey cantilever wall and a three-storey masonry wall with openings, including the foundation system, with complete and detailed drawings of the reinforcement and of the reinforcement details (splices, hooks). Does anybody recall “The art of detailing” chapter of another famous book? (Park and Paulay, 1975). Research on the behavior of flanged reinforced masonry walls carried by Nigel in the early 1990s was an essential complement towards a better understanding of the response of three-dimensional wall assemblages and systems. This study was carried out with quasi-static cyclic and dynamic (shake table) tests on T-section cantilever walls (Priestley and He, 1990). In these tests the effect of confining steel plates in bed joints was further investigated. The main findings of this work were relevant to the non-symmetric nature of the force-displacement response, see Figure 6.8, highlighting the difference in strength, stiffness and deformation capacity, as well as hysteretic behavior, that the wall displayed depending on the sign of the shear force (inducing either compression or tension in the flange), as well on the effective flange width that could be assumed in design/assessment models. The non-symmetric response of the flanged wall also had implications for the capacity design principles to be applied to prevent shear failure, recognizing that the required shear reinforcement was dictated by the flexural strength in the strong (flange in tension) direction. The design principles inferred from this study of T-section walls were added to the TCCMAR seismic design philosophy, which was the basis for the development of the 5-storey test carried out at the Charles Lee Powell Structures Laboratory at UCSD (Seible et al., 1994), and which form the basis of the 1991 NEHRP provisions for the seismic design of masonry buildings (NEHRP, 1991). This ambitious test, see Figure 6.9, had three main objectives: (i) to provide a test bed for the TCCMAR design principles; (ii) to provide benchmark data for the calibration and verification of TCCMAR analysis models; (iii) to advance the state of art in full-scale laboratory testing of stiff multi-degree-offreedom structures subjected to simulated seismic loading. Nigel was mainly involved in the activities related to design principles and analysis methods. Among the design principles stemming from Nigel’s
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(a)Testset-up
(b) Non-symmetricalforce-displacementresponse
Figure 6.8 - Experimental behaviour of flanged reinforced masonry walls (Priestley and He, 1990).
previous research that found their new implementation in the TCCMAR building were: the use of uniformly distributed vertical reinforcement which simplifies construction and enhances performance; reduced vertical and increased horizontal reinforcement to better control shear cracking and enhance shear capacity; no lap splices in the first storey vertical wall reinforcement to eliminate possible brittle bond failure; and proper detailing for ductile behavior of coupling elements between walls. The testing procedure, which included pseudo-dynamic testing sequences, was also able to reproduce higher mode contributions that constituted a meaningful reference for the evaluation of shear dynamic amplification factors to be used in capacity design. The test was successful in showing how the available design and analysis (simplified and refined) models were able to closely predict the various design and behavior limit states. It was then shown how the design principles for reinforced masonry that had been developed over the previous years, where Nigel had played a fundamental role, had reached a level of maturity that allowed a reliable design of reinforced masonry wall systems for buildings.
6.4 The Direct Displacement-Based Seismic Design Years
It can be said that Nigel’s strong involvement in masonry research came to an end as more compelling as the damage to infrastructure during the 1989 Loma Prieta, 1994 Northridge and 1995 Kobe earthquakes drew his attention. In the early 1990s Nigel also developed what we know today as the direct displacement-based seismic design of structures, see Chapter 4. The growing collaboration and relationship with Michele Calvi led him, as known, to be a co-founder, together with Michele, of the
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Figure 6.9 - The full-scale five-storey TCCMAR reinforced masonry building seismic test carried out at the Charles Lee Powell Structures Laboratory at UCSD.
ROSE graduate school in Pavia, Italy, which was established at the turn of the millennium. By then, Nigel had started to spend extended periods of time in Pavia. As said, at this time the design of masonry structures played only a subordinate role in Nigel’s research activities. However, one chapter of his latest book deals with the displacement-based seismic design of masonry structures (Priestley et al., 2007). One cannot quite help but notice that this chapter seems to be one of the least developed in the book, and few aspects seem to have been validated with numerical simulations or experimental results. However, and maybe for this very reason, this observation allows us to value their engineering intuition and their unique understanding for the seismic behavior of structures. Naturally, there are also some aspects we might today - with ten years of additional research - not fully agree with. The following paragraphs highlight some aspect of this chapter, which we consider remarkable and trend-setting for future research on masonry structures. By the time of publication of the book, it was generally accepted that displacement-based methods led to more realistic results than force-based results. Because strengthening of structures can be very costly, displacement-based methods found their way in particular into assessment codes and had by that time also been applied to URM structures. However, one large challenge was the estimate of the displacement capacity of URM piers, which is typically expressed as the interstory drift ratio. Up to
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then, only empirical formulae were available, which were derived from the results of quasi-static cyclic tests and the estimates that were implemented in codes led to large coefficients of variations when compared with experimental results. Nigel and Michele proposed to our knowledge the first simple mechanical model for the drift capacity of URM walls, which was applicable for walls failing in flexure: θd,fl
=
cm
c
lw–c 2
(1)
where εcm is the strain capacity of masonry (the suggested value isεcm =0.004), lw is the wall length and c the compression zone depth. The model seems almost too simple to be true - but we could meanwhile show that it outperforms classical empirical drift capacity models and performs similarly well than newer, more complicated mechanical models. Furthermore,while this drift capacity model was intended for walls failing in flexure, it seems that models of similar form can also provide reasonable estimates for walls failing in shear, provided the walls fail due to crushing of the bricks. Nigel and Michele further advocated the use of bed-joint reinforcement to increase the drift capacity of URM walls. Such reinforcement is easy to place and does not increase the construction costs significantly. Today, the first results of URM walls with bed joint reinforcement areavailable and this idea seems to be very promising. It is expected that future research will continue to develop this idea and will develop mechanical models for predicting the increase in drift capacity that can be gained from such reinforcement. One aspect that Nigel and Michele gave significant attention to was the role of spandrels and their effect on global building response. Up to the time of publication, isolated spandrel elements had not yet been tested experimentally although some numerical studies in the literature had pointed out the importance of such elements in determining the global response mechanisms, and somevery simplified, if not simplistic, code proposals could be found regarding spandrel capacity (e.g. in the 2005 Italian seismic code OPCM 3531). Nigel and Michele had the merit of bringing toa broader audience the attention to this issue, despite the inability of the proposed models forspandrel strength to be validated via existing experimental data, and they might seem from today’s point of view somewhat rough. However, their attention to the spandrel behavior contributed in anticipating the research focus on spandrels that followed in the decade after the publication of the book and continues to remain a research topic with many open questions. The final aspect of the chapter on masonry structures treats the out-of-plane response of URM walls, which Nigel had already studied in 1985 (Priestley, 1985) and where he had set the path for works by other researchers by determining the overturning capacity and the displacement capacity from kinematic analysis. In 2007, Nigel and Michele noticed that “it is now fully recognized and confirmed by recent experimental and theoretical research that only by considering displacement parameters is there a reasonable prospect of predicting the actual response”. In agreement with the direct displacement-based design methodology, the out-of-plane response of URM walls is addressed by an equivalent secant stiffness model up to maximum displacement. The displacement demand is determined for an equivalent viscous damping of 10%. To our knowledge, the methodology proposed by Nigel and Michele has not yet been validated by numerical simulations by the time of publication or today, but the appealing simplicity of the proposed approach has been the reference and starting point for further simplified models by other researchers, which once fully validated would be highly suitable for code implementation. Meanwhile, displacement-based assessment procedures of the out-of-plane response have already found their way into codes, such as the Italian structural design and assessment code (NTC08, 2008; NTC08-IST, 2009), which today is one of the most advanced codes for the seismic design and assessment of masonry structures.
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6.5 References Glogau O. (1972) - The Objective of the New Zealand Seismic Design Code, Bulletin of the New Zealand National Society for Earthquake Engineering, 5(4), pp. 113-127. Glogau O. (1974) - Masonry Performance in Earthquakes, Bulletin of the New Zealand National Society for Earthquake Engineering, 7(4), pp. 149-166. Hart G.C., Priestley M.J.N., Seible F. (1992) - Masonry Wall Frame Design and Performance, The Structural Design of Tall Buildings, 1(2), pp. 133-158. Heyman J. (1966) - The Stone Skeleton, International Journal of Solids and Structures, 2, 249 p. Holmes I.L. (1968) - Masonry Construction for Earthquakes, Bulletin of the New Zealand National Society for Earthquake Engineering, 1(1), pp. 113-127. NEHRP (1991) - Recommended Provisions for the Development of Seismic Regulations for New Buildings: Parts 1 & 2. In Earthquake Hazard Reductions Series (Vol. 65). US Federal Emergency Management Agency (FEMA). Washington, D.C. NTC08 (2008) - Nor me Tecniche per le Costruzioni, D.M. 14 Gennaio 2008, Ministero delle Infrastrutture, S.O. No. 30 alla G.U. del 4/2/2008, No. 29, Rome, Italy (in Italian). NTC08-IST. (2009) - Istruzioni per l’applicazione delle Nuove Norme Tecniche per le Costruzioni di cui al Decreto Ministeriale 14 Gennaio 2008, Circ. C.S.Ll.Pp. No. 617, 2/2/2009, Consiglio superiore dei lavori pubblici. S.O. n.27 alla G.U. del 26.02.2009, No. 47, 2009 (in Italian). NZC 4203P (1985) - Code of Practice for the Design of Masonry Structures, Standards Association of New Zealand, 1985. NZNSEE (1986) - Structures of Limited Ductility, Bulletin of the New Zealand National Society for Earthquake Engineering, 19(4), pp. 285-336. NZSEE (2016) - Section C8 - Seismic Assessment of Unreinforced Masonry Buildings, The Seismic Assessment of Existing Buildings: Technical Guidelines for Engineering Assessment, Accessed at: http://www.eq-assess.org.nz/new-home/part-c/c8/ 14 May 2017. OPCM 3431 (2005) - Ulteriori Modifiche ed Integrazioni all ’Ordinanza n.3274 del 20/3/2003, recante ‘Primi Elementi in Mteria di Criteri Generali per la Classificazione Sismica del Territorio Nazionale e di Normative Tecniche per le Costruzioni in Zona Sismica’. Suppl. ord. No. 85 alla G.U. del 10/5/2005 No. 107, Rome, Italy (in Italian). Park R., Paulay T. (1975) - Reinforced Concrete Structures. John Wiley and Sons. 769 p. Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concrete and Masonry Buildings. Wiley, 768 p. Priestley M.J.N., Bridgeman D.O. (1974) - Seismic Resistance of Brick Masonry Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, 7(4), pp. 167-187. Priestley M.J.N., Crosbie R.L., Carr A.J. (1977) - Seismic Forces in Base-isolated Masonry Structures, Bulletin of the New Zealand National Society for Earthquake Engineering, 10(2), pp. 55-68. Priestley M.J.N. (1977) - Seismic Resistance of Reinforced Concrete Masonry Shear Walls with High Steel Percentages, Bulletin of the New Zealand National Society for Earthquake Engineering, 10(1), pp. 1-16. Priestley M.J.N., Evison R.J., Carr A.J. (1978) - Seismic Response of Structures Free to Rock on Their Foundations, Bulletin of the New Zealand National Society for Earthquake Engineering, 11(3), pp. 141-150. Priestley M.J.N. Thorby P.N., McLarin M.W., Bridgeman D.O. (1979) - Dynamic Performance of Brick Masonry Veneer Panels, Bulletin of the New Zealand National Society for Earthquake Engineering, 12(4), pp. 314-323. Priestley M.J.N. (1980) - Seismic Design of Masonry Buildings - Background to the Draft Masonry Design Code DZ4210, Bulletin of the New Zealand National Society for Earthquake Engineering, 13(4), pp. 329-346. Priestley M.J.N. (1980a) - Masonry Structural Systems for Region of High Seismicity, Proceedings of the 7th World Conference on Earthquake Engineering, Istanbul, Turkey, pp. 441-448. Priestley M.J.N. (1980b) - Masonry, in Design of Earthquake Resistant Structures, Editor E. Rosenblueth, John Wiley & Sons, New York, 1980, pp. 195-222. Priestley M.J.N. (1981) - Ductility of Unconfined Masonry Shear Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, 14(1), pp. 12-20. Priestley, M.J.N. (1981a) - Ductility of Unconfined and Confined Concrete Masonry Shear Walls, Journal of the Masonry Society, 1(2), pp. 28-39. Priestley M.J.N. (1982) - Ductility of Confined Concrete Masonry Shear Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, 15(1), pp. 22-26. Priestley M.J.N., Elder D.M. (1982) - Cyclic Loading Tests of Slender Concrete Masonry Shear Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, 15(1), pp. 3-21. Priestley M.J.N., Elder D.M. (1983) - Stress-strain Curves for Unconfined and Confined Concrete Masonry, Journal of the American Concrete Institute, 80(3), pp. 192-201. Priestley M.J.N. (1985) - Seismic Design of Masonry Structures to the New Provisional New Zealand Standard NZS 4230P, Bulletin of the New Zealand National Society for Earthquake Engineering, 18(1), pp. 1-20. Priestley M.J.N. (1985a) - Seismic Behaviour of Unreinforced Masonry Walls, Bulletin of the New Zealand National Society for Earthquake Engineering, 18(2), pp. 191-205. Priestley M.J.N., Chai Y.H. (1985) - Seismic Design of Reinforced Concrete Masonry Moment-Resisting Frame, The Masonry Society Journal, 4(1), pp. T1-T17. Priestley M.J.N. (1986a) - Flexural strength of rectangular unconfined masonry shear walls with distributed reinforcement, The Masonry Society Journal, 5(2), pp. 1-15. Priestley M.J.N. (1986) - Seismic Design of Concrete Masonry Shearwalls, Journal of the American Concrete Institute, 83(1), pp. 58-68. Priestley M.J.N., He L. (1990) - Seismic Response of T-Section Masonry Walls, The Masonry Society Journal, 9 (1), pp. 10-19. Priestley M.J.N., Calvi G.M., Kowalsky M.J.N. (2007) - Displacement-based Seismic Design of Structures, IUSS Press, Pavia, Italy, 720 p.
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Sahlin S. (1971) - Structural Masonry, Prentice Hall, 290 p. SANZ (1973) - DZ 4203 General Structural Design and Design Loadings, Draft New Zealand Code of Practice, Standards Association of New Zealand, Wellington, New Zealand. SANZ (1980) - DZ 4210 Code of Practice for Masonry Buildings, Part B, Draft New Zealand Code of Practice, Standards Association of New Zealand, Wellington, New Zealand. SANZ (1982) - Code of Practice for the Design of Concrete Structures, NZS 3101, Part 1: 1982. Standard Association of New Zealand, Wellington, New Zealand. Seible F., Priestley M.J.N., Kingsley G.R., Kürkchübasche A.G. (1994) - Seismic Response of Full-scale Five-story Reinforced-masonry Building, Journal of Structural Engineering, ASCE, 120(3), pp. 925-946.
Cracks in columns… 1994.
Even bigger cracks in even bigger columns, July 1996.
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7. From the Pioneering Work on Presss-Technology to the New Paradigm of Low-Damage Design By Stefano Pampanin, James Conley, Suzanne Dow Nakaki, Sri Sritharan, Christopher Latham and John F. Stanton
7.1 Introduction Sometimes, a person stands out for their keen intellect, their encyclopaedic knowledge of a subject, or
their commitment to a cause. Very occasionally, an individual possesses all three characteristics. But of them, only a rare few also impact directly the lives of others, positively affecting both the individuals and the larger community. Nigel Priestley was one of this rare breed, and we were all members of a small group of people, drawn from all over the world, who were fortunate enough to have benefitted from Nigel’s mentorship under the auspices of the PRESSS (Precast-Seismic Structural Systems) research program of the 1990s. Tom Paulay, one of Nigel’s mentors, had a tongue-in-cheek description for such individuals: “research victims”. As a small tribute to Nigel, some members of the PRESSS Phase III team, see Figure 7.1, has reconvened after approximately 20 years to provide an overview paper on his pioneering work on jointed precast systems. Those systems are often referred to as PRESSS-Technology, from the name of the
Figure 7.1 - Nigel and the PRESSS Building Test Team during a working break in 1999. Clockwise from bottom left, Nigel Priestley, John Stanton, Jim Conley, Stefano Pampanin, Christopher Latham and Susie Nakaki (note: Sri Sritharan who took this photograph is missing in this picture).
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Research Program PRESSS that in the 1990s embodied the innovative “jointed ductile connections” approach for precast concrete structural systems. The introduction of jointed systems is a breakthrough in earthquake engineering. Previously, ductility and the corresponding inevitable and often irreparable damage had reigned supreme, but these new systems maintained the concepts of Capacity Design yet at the same time opened the door to the use of Damage Control, instead of just Life Safety, as a design criterion. In the past two decades, significant, further developments have followed this ground-breaking work; the concept and technology have been extended to different construction materials (cast-in-place concrete, masonry, steel and timber) and structural systemsand (frames, and coupled walls, dual systems, floor-diaphragms and bridges) for both new design retrofitsingle applications. In parallel with these new design concepts, Nigel was leading the development of the Direct Displacement Based Design (DDBD), which is discussed in Chapter 4. DDBD has been proven to be powerful, particularly for structures in which deformations and displacements dominate the response. It also provides an approach to design that is more transparent and effective than the force-based methods. It has naturally become the primary design methodology for innovative structures such as those used in the PRESSS program described in Section 7.3. This Chapter provides an overview of Nigel’s pioneering work over a period of almost twenty years, starting from the srcinal concepts in the late 1980s/1990s, through the various phases of development, to preparation of code provisions and implementation in practice. His vision, intellect and personal qualities mark every step of the way.
7.2 The New Zealand Legacy of Ductile Design
Current seismic design philosophies promote the design of ductile structural systems that can undergo inelastic cyclic deformations while maintaining their structural integrity. Ductile design was successfully developed in New Zealand, and in other countries, to ensure adequate nonlinear response in the critical regions of a structural system. In the late 1960s and early 1980s, Professors Bob Park and Tom Paulay went further and developed capacity design, to ensure that a suitable mechanism on inelastic deformation could develop and be maintained during a strong earthquake. The basic principle in capacity design is to ensure that the “weakest link” within the structural system is located where the designer wants it, and that it will behave as a ductile “fuse”, protecting the rest of the structure from potentially brittle failure mechanisms by limiting the forces that can act on it, see Figure 7.2 for a demonstration. This approach would allow the building to sway laterally without experiencing collapse through, for example, a “soft-storey” mechanism or a “pancake” collapse. Regardless of the structural material adopted, i.e. concrete, steel, or timber, traditional ductile systems rely on the inelastic behaviour of the material. The inelastic action is intentionally concentrated within selected discrete “sacrificial” regions of the structure, typically referred to as plastic hinges. Until recently, the development of inelastic action in traditional monolithic as well as in emulative concrete connections has led inevitably to structural damage, thus implying that “ductility = damage”, with associated repair (or replacement) costs and business downtime. This is largely a consequence of bonding the steel reinforcement to the surrounding concrete. To provide the desired ductility, reinforcing bars must yield, but the associated strains in the reinforcement and concrete inevitably result in structural damage. Achieving structural ductility without accepting damage was considered impossible. Nigel understood the limitations of the ductility based designs and propelled the technology towards damage-avoidance (or damage-controlled) design philosophy in the PRESSS program, while ensuring that the capacity design is followed through in the process.
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Figure 7.2 - Basic concept of capacity design: the “weakest link of the chain” concept (left) and its implementation in a frame system to prevent a soft-storey mechanism (centre) in favour of a beam side-sway mechanism (right) (Paulay and Priestley, 1992).
7.3 Next Generation of Earthquake Damage-Resisting Systems
Communities affected by the severity of the damage caused by an earthquake demand both costeffective solutions and better seismic performance of the building stock. For example, the significant socio-economic impact of the Canterbury earthquake sequence in 2010-2011 called into question the appropriateness of structural systems designed focusing on life-safety alone. Even today, codes embody that philosophy on the basis that they should be the guardians of public safety by enforcing collapse prevention, while design to a higher standard to avoid the costs of damage was an economic choice for the owner. The economic impact of the 2010-2011 Christchurch earthquake sequence was in fact shared by the whole community, and not just the building owners. Consequently, awareness is now growing among the public, building owners, territorial authorities and insurers, that the costs of deaths, dollars and downtime need to be accounted for when determining the suitable design solutions. This, in turn, is facilitating wider acceptance and implementation of cost-efficient high-performance building and bridge technologies in countries like New Zealand, Chile, Japan and Ecuador, which have recently experienced significant disturbance and loss of life because of strong earthquakes. Nigel coordinated the PRESSS research program that resulted in the development of advanced structural precast concrete systems capable of achieving high-performance (low-damage) at costs comparable with traditional systems (Priestley, 1991, 1996; Priestley et al., 1999). This research program culminated with the pseudo-dynamic testing of a large scale five-storey fully precast concrete building in the third phase, see Figures 7.3 and 7.4. The new structural systems, based on dry jointed ductile connections, developed within PRESSS were conceived and developed for precast concrete buildings (frames and walls) in seismic regions with the intent of creating a sound alternative to the traditional “wet” and/or “strong” connections. The intend of the traditional connections is to emulate the behavior of equivalent cast-in-place systems. In PRESSS frame or wall systems, precast elements were jointed together through unbonded posttensioning tendons/strands or bars, creating moment-resisting connections. The choice of debonded, prestressed reinforcement was critical in reducing structural damage and promoting the re-centring behavior when the lateral load is removed. However, it was not an obvious choice, because it violated the professional wisdom of the time. Then, the ACI 318 building code did not allow seismic reinforcement to have a yield strength greater than 550 MPa, thereby effectively banning the use of prestressing
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Figure 7.3 - Memento of the completed construction of the Five-storey PRESSS Building, time to test. (from left to right: Stefano Pampanin, Jim Conley, Nigel Priestley, Randy Clark, Sri Sritharan).
Figure 7.4 - The Five-storey PRESSS Building tested at University of California, San Diego (Priestley et al., 1999) and the expla1. nation by Nigel Priestley in the UCSD-TV Video Five Stories for the Future
1
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in seismic design. This approach is consistent with the legacy of the response of the Four Seasons apartment building in Anchorage, which was being constructed using unbonded post-tensioned floors when it collapsed during the 1964 Alaska earthquake, promoted the idea that debonding the posttensioning was particularly unwise. Of the various systems, an efficient solution to framing precast members is given by the “hybrid” concept (Stone et al., 1995; Priestley, 1996; Stanton et al., 1997), see Figure 7.5a, which combines unbonded post-tensioned tendons passing through corrugated metallic ducts with bonded mild steel used primarily for energy dissipation. The mild steel reinforcement also adds strength and stiffness to the connection. To date, the hybrid concept has been extended to the column to foundation and wall to foundation connections. An alternative to using mild steel reinforcement at the foundation interface, a wall system can accommodate an energy dissipating element to create a jointed coupled wall system, see Fig. 7.5b. In the PRESSS building, such a system was included with U-shape Flexural Plates (UFP) proposed by Kelly et al. (1972) as the energy dissipation element, see Figure 7.6. Because in the all PRESSS jointed structural systems, tendons were left partially or completely unbonded, the member lengthening caused by rocking and associated gap opening at the joint interfaces induced only a small incremental strain that was accounted for in the design to ensure that the tendons would remain elastic when the systems experience the target design drift. As a result, the tendons enable the precast systems to re-centre, minimising the residual displacements that are commonly associated with traditional structural systems. As with other jointed concepts, the hybrid system accommodates the earthquake-induced displacements within the connections through opening and closing of an existing interface, rather than through inelastic deformation occurring within the members, as is the case in a conventional ductile structural system. This is demonstrated for a wall to foundation connection in Figure 7.7. Since the members are not subjected inelastic deformations, they sustain negligible or no structural damage. The combined dissipative and re-centring mechanisms of hybrid systems lead to a flag-shaped hysteretic response, whose properties and shape can be modified by the designer by varying the amounts of prestressing steel and mild steel reinforcement. This in turn will provide different moment contributions at the connection interface with one contributing re-centring and the other providing energy dissipation, see Figure 7.8. A 50-50 combination between these components would thus generate the maximum level of energy dissipation (typically in the order of 15-20% equivalent viscous damping ratio) while maintaining the re-centring capability. In the view of those intending to
Figure 7.5 - Jointed precast a) “hybrid” frame, and b) jointed wall systems (fib, 2003; SNZ, 2006).
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Figure 7.6 - U-shape Flexural Plate Dissipaters (rendering courtesy of Nakaki and Stanton) and hysteretic response.
Figure 7.7 - Comparative response of a traditional monolithic and a precast concrete hybrid wall system (fib, 2003).
maximise the hysteretic energy dissipated per cycle (a prevailing view up until the 1990s and a view well-entrenched in the profession currently), the flag-shaped hysteretic loops are often deemed to be inferior, because of the low hysteretic energy dissipation capacity, which were associated with larger lateral displacement demands. Nigel proved this to be just a myth and noted that only short period structures may experience large displacements (Priestley and Tao, 1993). During the three Phases of the PRESSS Program, Nigel involved a large and exceptional team comprising of many university and industry representatives. The breadth of the team can be judged from the Acknowledgments, reproduced below, of the PCI paper entitled “Preliminary Results and
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Self-centering
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Figure 7.8 - Flag-shaped hysteresis loop idealized for a hybrid system (after fib, 2003). Effects of varying the ratio between re-centring (post-tensioning and axial load) vs. dissipative (mild steel and dissipaters) contribution to the flag-shape hysteresis loop.
Conclusions from the PRESSS Five-Story Precast Concrete Test Building” summarising this work (Priestley et al., 1999):
The project described in this paper has involved a large number of individuals and organizations, all of whom deserve individual thanks and acknowledgment. A full list would be impossibly long. Of particular importance are Dr. Chris Latham (UCSD) and Professor Akira Igarashi (Kyoto University) whose efforts in solving the extremely difficult problems of controlling the pseudodynamic tests were essential to the test success. The efforts of the building designers, Professor John Stanton (plus University of Washington graduate students), and Ms. Suzanne Nakaki, who not only did a superb job of the building design, as evidenced by its excellent performance, but also were present during most of the long-night testing sessions, are particularly acknowledged. Primary financial support for the PRESSS research program was provided by the PCI (Precast/Prestressed Concrete Institute), the NSF (National Science Foundation), and the PCMAC (Precast/Prestressed Concrete Manufacturers Association of California). The extent of industry support, in terms of financial assistance, material donation, technical advice, and provision of precast products is unparalleled in major United States
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structural research projects. Special thanks are due to Mario J. Bertolini, chairman of ATLSS and PRESSS Ad Hoc Committee, and Thomas J. D’ Arcy, chairman of the PRESSS Phase III Advisory Group. In addition, contributors to the testing program include A. T. Curd Structures, Inc.; BauTech, Co.; California Field Iron Workers Administrative Trust; Charles Pankow Builders, Ltd.; Clark Pacific; Coreslab Structures, L.A.; Dayton Superior; Dywidag Systems International; ERICO; Florida Wire & Cable, Inc.; Fontana Steel; Gillies Trucking; Headed Reinforcement Corporation; Horizon High Reach; JVI, Inc.; LG Design; Master Builders, Inc.; NMB Splice Sleeve; Pomeroy Corporation; Precision Imagery; Spancrete of California; Sumiden Wire; and White Cap. While it was indeed a great team effort, Nigel’s leadership stood out and made the PRESSS program a great success. The team members genuinely appreciated his inclusive and inquisitive nature and elegantly rigorous way to tackle complex issues, by breaking them down to small problems that he could then solve “on the back of an envelope”. He did all of this with a disarmingly humble attitude and inspiring approach towards his postgrads, treating them as young research colleagues rather than “students”.
7.4 Implementation of the PRESSS Technology in Building Standards
Immediately after the PRESSS building test, codification of the PRESSS building systems began. The focus was on the hybrid frame and unbonded post-tensioned walls system. With support from the Precast/Prestressed Concrete Institute (PCI), these two systems have been integrated within the ACI building code by developing Innovation Task group reports. The first version of the hybrid frame documents was published in 2001 and the jointed wall system was codified in 2007. These documents facilitate the design, detailing, and construction of hybrid frames and walls systems with unbonded post-tensioning in conjunction with the ACI 318 building standard. More recently, the ACI 550 committee of the American Concrete Institute has published standards ACI 550.2R (2013) and ACI 550.3 (2013). These two standards provide information on design, detailing, and construction of connections between precast members in jointed systems, including moment frame and structural wall systems that allows such designs in combination with the ACI 318 building code. New Zealand was another early adopter of the PRESSS technology. In 2005, the New Zealand Concrete Society held a series of seminars aimed at implementing the PRESSS structural systems, which were going to be covered in the upcoming New Zealand Concrete Standard NZS3101 (SNZ, 2006), and followed by the issue of a comprehensive PRESSS Design Handbook (NZCS, 2010).
7.5 Applications of the PRESSS Technology
The acceptance and standardization of the structural systems stemming from the PRESSS programme has resulted in a wide range use of these systems and in the development of PRESSS-like systems for structural steel and cross-laminated timber (Buchanan et al., 2009). The first and most glamorous PRESSS technology application was the Paramount Building built in San Francisco, see Figure 7.9. This 39-storey apartment building is the world’s tallest precast concrete structure in a high seismic region at the time of construction (Englerkirk, 2002). Perimeter precast hybrid frames, combining unbonded tendons and grouted mild-steel bars for hysteretic energy dissipation, were used in both primary directions of the building to provide lateral resistance. Given the evident structural efficiency and cost-effectiveness of these systems, e.g. high speed of erection, off-site quality control, as well as the flexibility in the architectural features (typical of precast
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Figure 7.9 - Thirty-nine storey Paramount Building in San Francisco (Englerkirk, 2002). Left, overall building view, right, tendon stressing operation. Photos courtesy of J. Sanders of Pankow Builders.
concrete), several precast jointed framing systems have emerged in Italy, through the implementation of the “Brooklyn System”, see Figure 7.10, developed by BS Italia of Bergamo, with draped tendons to increase the bay length and a hidden steel corbel (Pampanin et al., 2004). Several buildings, up to six storeys, have been designed and constructed with this concept in regions of low seismicity in Italy (gravity-load dominated frames).
Figure 7.10 - Application in Italy of the Brooklyn System with draped tendons in Italy (Pampanin et al., 2004).
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The Alan MacDiarmid building at Victoria University of Wellington, designed by Dunning Thornton Consulting Ltd. (Cattanach and Pampanin, 2008), is the first multi-storey PRESSS-building in New Zealand. The building has post-tensioned frames in one direction and coupled post-tensioned walls in the other direction, with straight unbonded post-tensioned tendons. The second PRESSS-Building in New Zealand is the Endoscopy Consultants’ Building in Christchurch, designed for SouthernCross Hospitals Ltd by Structex Metro Ltd. (Pampanin et al.,2011), see Figure 7.11. Like the Alan MacDiarmid buillding, lateral resistance in the Endoscopy Consultant’s Building is provided by hybrid frames and coupled walls acting in the two orthogonal directions. The post-tensioned frame system relies upon a non-symmetric section reinforcement with internal mild steel located on the top ofthe beam only and cast on site along with the floor topping. The unbonded post- tensioned walls are coupled with UFPplates.
Figure 7.11 - Souther n Cross Hospital Endoscopy Consultant’s Building (Pampanin et al., 2011).
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Figure 7.12 - The world first Pres-Lam building utilising unbonded post-tensioned rocking/dissipative timber walls. Nelson Marlborough Institute of Technology, (NMIT), New Zealand (Devereux et al., 2011).
The timber industry in New Zealand took up the findings of the PRESSS program and comprehen sively developed comparable alternatives using post-tensioned wood. NMIT building, constructed in 2011 in Nelson, is the world’s first commercial building utilizing this technology (Devereux et al., 2011). This building has vertically post-tensioned timber walls resisting all lateral loads as shown in Figure 7.12. Coupled walls in both directions are post-tensioned to the foundation through high strength bars with a cavity allocated for coupling of these bars along the building height. Steel UFP devices link the pairs of structural walls together and provide hysteretic energy dissipation capability to the system.
7.6 Strong Earthquake Testing of a PRESSS Technology Building: When Reality Meets Expectations
The Southern Cross Hospital endoscopy building was severely tested during the Canterbury Earthquake Sequence, and survived with only negligible damage, see Figure 7.13. Unlike many of the surrounding structures, which experienced severe damage and were repaired extensively or demolished, this building resumed operation the day after the 22 February 2011 earthquake and remained operational during the other major events of that earthquake sequence.
Figure 7.13 - Negligible damage, to both structural and non-structural components, to the Souther n Cross Hospital designed with the PRESSS systems following the Christchurch earthquake of 22 February 2011.
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One of the main features in the design of a rocking-dissipative solution for this building is the possibility of tuning the level of floor accelerations (not only drift) to protect both structural and nonstructural elements including content and acceleration-sensitive equipment. More information on the design concept and performance criteria, modelling and analysis, construction and observed behaviour of the building can be found in Pampanin et al. (2011).
7.7 The PRESSS Program and Nigel Priestley’ s Influence on the Team
Susie Nakaki and John Stanton were in charge of the design of the PRESSS Building. As they like to recall:
Nigel Priestley’s engineering education had been greatly influenced by the two senior “Ps” in New Zealand (Paulay and Park), and his pre-PRESSS thinking was deeply imbued with the ideas of achieving ductile deformations in conventional cast-in-place systems. Those ideas were the seismic gold-standard of the era. He had a phenomenal grasp of the underlying principles, and had values in his head for the appropriate limits for all the important variables involved. These inevitably became known, behind his back, as “Nigel Numbers”. Given that background, it is a measure of his vision that he was able to recognize a promising idea, and embrace it, even though it violated most of the principles that had guided his professional life up to that time. Most people are unable to let go the past and embark on a new and largely untested concept, because of the enormous professional and intellectual risks to which they would expose themselves. But Nigel did just that with the unbonded post-tensioned systems that provided the backbone of the PRESSS program. He not only embraced the ideas, but he also did what only true leaders do, which is to place trust in their team members by delegating authority to them, and not micro-managing their work, but rather being there in the wings when help was needed. Knowing that you were being trusted to produce, and that the team leader was relying on you, was at once exhilarating and terrifying, but that trust was a critical element in the success of the PRESSS project. Nigel’s trust is all the more remarkable in view of the bizarre srcins of the unbonded posttensioned technology. In the early 1980s, an engineer had designed a conventional seismic moment frame for an office building in Seattle, but had added unbonded post-tensioning to the beams purely to control gravity deflections. Through a tortuous logic, entirely associated with money, the owner had his own building condemned as unsafe for occupation, on the basis that the beams violated the ACI building code of the time because they used post-tensioning “to resist earthquake forces”. Litigation followed, a frame test was conducted (Ishizuka et al, 1984), and the engineer lost his license and left the profession. Despite many inappropriate details in the frame, the test performed remarkably well, and that outcome provided the impetus for further investigation of the concept (Stone et al., 1995) in which, for the first time, unbonded post-tensioning was introduced as an intentional component of the seismic resisting system. The “Hybrid Frame” was thus born, and unbonded post-tensioning technology was adopted into the PRESSS program largely because the early investigators had suggested it to Nigel Priestley and were subsequently invited to play a role in the PRESSS program. The role of Dean Stephan, then the head of Charles Pankow Builders, also proved critical. He, too, was prepared to put faith in the system, and, in addition to sponsoring the first investigations, was subsequently active in helping Nigel Priestley to embrace it as well. His company used the system in many buildings, including the Paramount Building in San Francisco, at the time the tallest concrete building in a high seismic zone. The jointed rocking wall was developed during the PRESSS program, and followed principles similar to those of the moment frame that had been developed earlier.
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This process of placing faith in students and younger colleagues was instrumental in creating very strong bonds within the PRESSS team and producing outstanding results. But it had other, more personal and long-lasting effects as well. Many of the members of the PRESSS team have forged careers that were almost certainly more successful than they would have been without Nigel’s example, and they have been able to pass on to their own junior colleagues some of the same lessons of professional courage and personal encouragement that he embodied. During a PRESSS design meeting in San Diego, Rebecca Hix, then a student on the design team, at one moment pointed out “Professor Priestley, the code does not allow you to do that.” Priestley snorted in disgust and responded, “We are not here to satisfy the code, we are here to re-write it!” The whole team felt enormously liberated by those words, both at the time and for years afterwards. Nigel set high expectations, especially professionally, but he also had the capacity for letting his hair down. He learned to love good food, and to cook, early in life and he exercised those skills to the benefit of all those who worked with him. A room-mate from his undergraduate days, (who also went on to an illustrious career in engineering) is happy to admit now to accepting the subservient role of dish-washer then, given that Nigel was going to do the cooking. Later in life, Nigel’s idea of a celebration was still to cook a meal for everyone and to entertain them lavishly at his house. For example, at a conference in Lake Tahoe, put on in honour of his own impending retirement, Nigel hosted all the attendees for dinner at his cabin in the woods, and was in total charge of the cuisine. He should have retired more often! Nigel Priestley was a man of many parts: visionary professional engineer, articulate and persuasive author, lover of life, and mentor, both professionally and personally, to many fortunate individuals who crossed paths with him. We celebrate him, and at the same time sorely miss him, for a life filled to overflowing. Sri Sritharan served as the Project Manager for the building test conducted at UCSD as part of Phase III of the PRESSS program. His reflection of his involvement in the PRESSS III project and interactions with Nigel during this project period follows:
Following my PhD, which was supervised by Nigel, I was looking for an opportunity to move on. To my surprise, I managed to find a position on the East Coast in a short time. Nigel and his colleague Frieder Seible convinced me to stay in San Diego; their pitch was “enjoy the weather and take charge of the PRESSS test program”. This turned out to be a life changing opportunity for me because a large part of my research since leaving UCSD has focused on precast/prestressed structures. Above all, I met all the authors of this paper, except Chris Latham, during the PRESSS program. PRESSS III challenges were very different as we were not only in uncharted waters, but we were also trying to accomplish a plethora of additional milestones. These included testing four different frames, one wall system and two floor diaphragms in one large-scale building model; constructing the largest test building in the U.S. (at that time) inside a laboratory; verifying the DDBD methodology; and conducting the test using a pseudo-dynamic (or hybrid) procedure. If I recall correctly, the building was brought to the laboratory in 98 precast pieces that were fabricated at four different precast plants in California. On top of that, from the day we moved into the laboratory until the day we completed the tests, we faced lpenty of challenges with regards to project cost, construction, erection and testing. We didn’t hesitate to overcome these challenges individually or in groups, but when everything failed we knew we could rely on Nigel to help us. The assurance that Nigel gave us to overcome the challenges, which he might not have even realized that he was giving, was extraordinary and uplifting for the PRESSS group consisting of academic and industry-based researchers-all striving to accomplish an innovative concept for
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the precast industry. In addition, I believe, his passionfor research and larger-than-life quality to influence researchers made programs like PRESSS so successful. Prior to the construction of the building, the PRESSS III team met every four to six weeks at UCSD under Nigel’s leadership. These meetings, which were crucial for the success of the project, were very educational and transformative for me as this was when design details were worked out. In those meetings, it was clear who was in charge. Nigel didn’t hesitate to question the young and old when he wasn’t convinced of a suggestion or an argument, but would offer a rationale for his own opinions. He was so skilful at using small sketches and back of an envelope calculations to demonstrate his viewpoint, which was hardly challenged. Once the construction started, Nigel was comfortable and left me in charge. I relied on Susie Nakaki and John Stanton along with Randy Clark who from Clark Pacific took charge of the erection of the building. As the building went up, Susie, John and I could work around the clock, due to differences in our schedules. Though I never confirmed it, I surmised from the fax and email messages (no email attachment was possible then), that Susie went to bed when I was preparing to head to the lab and John was in high gear when I was ready to quit by 3 pm. Nigel was busy with other commitments and travel. Every time he returned to the lab, he made sure to check the construction progress and asked the right questions to ensure things were going smoothly. His ability to do this in such a short amount of time was incredible. Nigel was so involved during the tests. They lasted weeks, and most of them had to be done during night to minimize the thermal movements of the laboratory building, which was used as reference points for some of the instruments. We typically started the tests after 9 pm and ended around 2 am. Nigel was there with us, providing explanations for the test observations and suggestions for the next steps. But just like a graduate student, he was also busy taking his own notes and marking cracks. In addition, he made sure we had food and celebrated with the team as we completed each critical test. As always, he was friendly and approachable during testing and never hesitated to educate anyone about jointed construction, the PRESSS program, seismic design or DDBD. During the PRESSS III project, I met Rich Wargo, an employee of UCSD-TV. Rich and I decided to work together and we produced a documentary entitled PRESSS: Five Stories for the Future2. Nigel was very supportive of this effort and agreed to do his fair share of participation and interviews. Figure 7.14 shows different shots taken from the documentary, demonstrating how active Nigel was during the test. Rerunning the video has brought happy memories of a great time, reminding me of our synergistic team effort on PRESSS III under Nigel’s leadership, and left me wondering if any one of us thought, at that time, that unbonded post-tensioning would become a norm for creating resilient seismic structures regardless of the construction material (i.e., concrete, steel, masonry or timber). Jim Conley, at that time Graduate Student at UCSD, likes to remember:
By the time of the PRESSS Project, structural analysis software had advanced to a level that the use of multiple iterations on very complex software models was possible. However, Nigel’s passion for simplicity showed through again in the analysis model used to predict the performance of the walls to compare to the actual live pseudo-dynamic testing. As one of the most junior members of the team, I remember struggling with the Ruaumoko
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Figure 7.14 - Participation of Nigel during the PRESSS building test: a) Explaining to Rich Wargo about the industry-academic partnership; b) monitoring the test; c) marking crack; and d) observing the building performance. (Clockwise from bottom: Nigel Priestley, Christopher Latham, John Stanton, Sri Sritharan).
models of the walls. Nigel in his infinite wisdom sat down with me and sketched up his thoughts on the best approach. He broke the very complex system into just 4 elements: elastic column elements for the walls, spring elements for the unbonded tendons, no-tension springs at the ends of rigid links for the wall boundary zones and springs at the ends of rigid links for the U-shape Flexural Plate Dissipaters. Much to my surprise the prediction aligned very well with pseudo-dynamic testing results, prompting me to jump out of my seat and do the ‘prediction dance’ with the team, Nigel included. What was most profound to me was the notion of simplicity that Nigel carried beyond the university borders. I loved that he drove a VW bus as his main transportation. When he was asked to present the preliminary results of the testing at the 45th Annual PCI Annual conference in Palm Springs, he invited Stefano and me to attend along with him and the other team members. It was exciting and well received by all attending. On the way back from the convention Nigel invited us to his cabin in Idyllwild. We toasted the success of the event with a bottle of wine and some grilled fare. To this day I will always remember Nigel’s grill. A simple round grilled rack seasoned like a skillet leaning against the cabin wall. Nigel grabbed the grill and balanced elegantly between the igneous rock common to the San Jacinto Mountains. A few broken sticks from the surrounding Pines provided the fuel. Here we were being hosted by one of the pioneering leaders in the field at the time and to this day. The campfire meal was just right and one of the best meals of my life. Nigel really had a gift for taking the very complicated and simplifying it to get the right results. He was instrumental in encouraging that approach in all of us, certainly for me. I’ve carried
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on that approach in professional practice to this day, sharing knowledge with others along the way. It’s certainly an approach that I feel is even more valuable in professional practice given the advances we have seen and the ever-increasing reliance on computer analysis in our blossoming millennial Structural Engineers. Christopher Latham, who carried out the control of the test and was responsible for data acquisition, likes to remember:
Nigel Priestley arrived at UCSD when I was most of the way through my PhD work so did not have too much influence on me at that time. My first real interaction with him was when my advisor, Frieder Seible, was on sabbatical and Nigel was serving as my advisor in absentia. At the time I was going through some health issues which affected my ability to work and Nigel was very supportive and understanding. One thing about Nigel is that he was always Nigel to everyone and not Prof. Priestley. After I finished up and started working at the Powell Labs as a staff member I started to have more interaction with Nigel providing technical support for him and his graduate students. While, of course, I had an interest in the results of the testing programs what I was most involved with was the process of structural testing; the practical science of the whole endeavour. What I observe here is that Nigel was the consummate experimentalist. I remember that he was a stickler for good documentation and correct scientific method. One phrase I remember and repeat myself to graduate students in the lab to this day is “Scraps of paper, scraps of paper” in reference to people taking notes on loose sheets of paper instead of lab notebooks. Stefano Pampanin fondly remembers:
I met Nigel for the first time at the University of California San Diego (UCSD) in 1998, where I was as a Fulbright Visiting Scholar, flying across the Atlantic to the US West Coast from Pavia in Italy to work with ‘the number one’ in Earthquake Engineering. Nigel Priestley had joined UCSD as a Professor in 1986 from the University of Canterbury (UoC) in Christchurch, New Zealand, where he was already established as one of the ‘3Ps’; the unique ‘kiwi’ legacy in earthquake engineering based at UoC: Professors Tom Paulay, Bob Park and Nigel Priestley, born 10 years after one another. Nigel’s contribution and impact on earthquake-engineering in the United States (and worldwide) in the 1990s had already been extraordinary, with major impacts on research, developments, code provisions and practical implementations following the Loma Prieta 1989 and Northridge 1994 earthquakes. He had played a primary role in making UCSD one of the top universities in the world for Earthquake Engineering. I clearly recall the very short brief I had received before my departure about the on-going research projects at UCSD. It had vaguely mentioned something about ‘concrete prefabrication in buildings’, which sounded like a topic of no immediate excitement for a young researcher. It did not take me too long to realize that the team I was joining, led by Professor Nigel Priestley, was pioneering the way of designing and constructing the next generation of seismic-resisting buildings, based on rocking-dissipative systems. These are now known as PRESSS (Prestressed Seismic Structural Systems) technology, and more recently as ‘low-damage systems’. They have now been extended and adapted to different materials (starting from precast concrete, moving to steel, and more recently to timber) and structural systems (frames, walls, dual systems, bridge piers and decks). Twenty years later the impact of such earlier developments and concepts is evident and impressive….
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We have been working on this paper on the return from the New Zealand Society of Earthquake Engineering Annual Conference, NZSEE 2017, held in Wellington from 27-29 April 2017 and combined with the 15th World Conference on Seismic Isolation, Energy Dissipation and Active Vibration Control of Structures. The theme of the conference was “Next Generation of LowDamage and Resilient Structures” and it has been impressive and overwhelming to observe the emergence of the ‘new normal’ in earthquake engineering, with base isolation, supplemental damping, dissipative bracing systems and rocking/dissipating (PRESSS-technology in all materials) systems regularly featuring in the discussion between engineers, architects and clients, both in the reconstruction of the city of Christchurch as well as for the retrofit or new design of other cities. The design of such jointed ductile connections, which rely upon a rocking mechanism, and thus on the development of concentrated rotations at the interface between connected elements, can naturally exploit the elegant simplicity of the emerging DDBD approach. This design approach was already included in the book “Seismic Design and Retrofit of Bridges” (Priestley, Seible and Calvi 1995), for the design of isolation devices for bridges, and was later presented in its first comprehensive form as a Keynote Lecture at the European Conference in Earthquake Engineering in Paris (Priestley, 1998). As a result, also DDBD is no longer seen as a strange and scary academic beast, but rather - once you have learned its basic tricks - as an obvious step forward where advanced design methodologies can be combined with advanced seismicresisting technologies”
7.8 Acknowledgments
It is a severe challenge to write a semi-technical tribute to honour Nigel Priestley, while trying to convey the huge impact he has had, not only on the whole world of earthquake engineering but, more particularly, on the lives and careers of a number of fortunate people. He has raised bars, both technical and personal, to previously unknown heights. We are humbled and sincerely appreciative of the great fortune we have had in being able to know and work with Nigel, and we hope that this and the other papers prepared for this special tribute publication will provide some insights of his legacy. If any of us can now see further, it is because we have had the privilege of standing on the shoulders of a giant.
7.9 References ACI 550.2R. (2013) - Design Guide for Connections in Precast Jointed Systems, American Concrete Institute, Farmington Hills, Michigan, United States. ACI 550.3. (2013) - Design Specification for Unbonded Post-Tensioned Precast Concrete Special Moment Frames Satisfying ACI, American Concrete Institute, Farmington Hills, Michigan, United States. Buchanan A.H., Pampanin S., Palermo A., Newcombe M. (2009) - Non-Conventional Multi-Storey Timber Buildings using Post-tensioning, 11th International Conference on Non-Conventional Materials and Technologies (NOCMAT), University of Bath, United Kingdom. Cattanach A., Pampanin S. (2008) - 21st Century Precast: the Detailing and Manufacture of NZ’s First Multi-Storey PRESSS-Building, NZ Concrete Industry Conference, Rotorua, New Zealand. Devereux C.P., Holden T.J., Buchanan A.H., Pampanin S. (2011) - NMIT Arts & Media Building - Damage Mitigation Using Post-tensioned Timber Walls, Proceedings of the Ninth Pacific Conference on Earthquake Engineering, Auckland, New Zealand, paper 90. Englekirk R.E. (2002) - Design-construction of The Paramount: a 39-story Precast Prestressed Concrete Apartment Building. PCI Journal, 47(4), pp.56-71. fib 2003 (2003) - International Federation for Structural Concrete. Seismic Design of Precast Concrete Building Structures. Bulletin No. 27, Lausanne, 254 pp. Ishizuka T., Hawkins N.M., Stanton J.F. (1984) - Experimental Study of the Seismic Resistance of a Concrete Exterior Column Beam Sub-assemblage Containing Unbonded Post-Tensioning Tendons, Dept. of Civil Engineering, University of Washington, Seattle, Washington, United States.
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Kelly J.M., Skinner R.I., Heine A.J. (1972) - Mechanisms of Energy Absorption in Special Devices for use in Earthquake Resistant Structures. Bulletin of the New Zealand Society for Ear thquake Engineering, 5(3), pp.63-88. NZCS (2010) - PRESSS Design Handbook. New Zealand Concrete Society, Pampanin, S., Marriott, D. and Palermo, A, editors. Pampanin S., Pagani C., Zambelli S. (2004) - Cable Stayed and Suspended Solution for Precast Concrete Frames: the Brooklyn System, Proceedings of the New Zealand Concrete Industry Conference, Queenstown, New Zealand, September 16-18. Pampanin S., Kam W., Haverland G., Gardiner S. (2011) - Expectation Meets Reality: Seismic Performance of Post-Tensioned Precast Concrete Southern Cross Endoscopy Building During the 22nd February 2011 Christchurch Earthquake NZ Concrete Industry Conference, Rotorua, New Zealand. Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concrete and Masonry Buildings. New York: John Wiley & Sons, Inc. Priestley M.J.N. (1991) - Overview of the PRESSS Research Programme, PCI Journal, 36(4), pp. 50-57. Priestley M.J.N., Tao J.R. (1993) - Seismic Response of Precast Prestressed Concrete Frames with Partially Debonded Tendons. PCI Journal, 38(1), pp.58-69.
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Nigel Priestley Symposium, Lake Tahoe, August 2008.
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Authors
Editors: José I. Restrepo Professor of Str uctural Engineering University of California, San Diego, United States
[email protected]
Gian Michele Calvi Professor, IUSS Pavia Director of the section “Seismic Input and Design”, Eucentre Foundation, Pavia, Italy Director, Studio Calvi, Pavia, Italy
[email protected]
Contributors:
Katrin Beyer Assistant Professor, Ecole Polytechnique Fédérale de Lausanne, Switzerland katrin.beyer@epfl.ch
Carlos Blandón Professor of Structural Engineering University EIA, Envigado, Colombia
[email protected]
Athol J. Carr Emeritus Professor University of Canterbury, Christchurch, New Zealand
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Rob Chai Professor of Structural Engineering University of California, Davis, United States
[email protected]
Michael P. Collins University Professor The University of Toronto Ontario, Canada
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James Conley Vice President Hope-Amundson, Inc., San Diego, United States
[email protected]
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André Filiatrault Professor State University of New York at Buffalo in Buffalo, NY, United States and School for Advanced Study of Pavia (IUSS), Italy
[email protected]
Jay Holombo Senior Bridge Engineer & Project Manager T.Y. Lin International, San Diego, California, United States
[email protected]
Jason Ingham Professor of Structural Engineering and QuakeCoRE Flagship Leader University of Auckland, New Zealand
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Mervyn Kowalsky Professor of Structural Engineering North Carolina State University, Raleigh, United States
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Christopher Latham Research and Development Engineer 5 University of California, San Diego, United States
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Greg MacRae Associate Professor University of Canterbury, Christchurch, New Zealand
[email protected] Joe Maffei Principal Maffei Structural Engineering, San Francisco, United States
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Guido Magenes Professor University of Pavia, Italy
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Suzanne Nakaki President Nakaki Structural Design, Inc., Tustin, United States
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Stefano Pampanin Professor University of Canterbury, Christchurch, New Zealand and University of Rome “La Sapienza”, Italy
[email protected]
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Sri Sritharan Wilkinson Chair in the College of Engineering, Interim Assistant Dean and Professor of Structural Engineering Iowa State University, Ames, United States
[email protected]
Tim Sullivan Associate Professor University of Canterbury, Christchurch, New Zealand
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John F. Stanton Professor University of Washington, Seattle, United States
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Nihal Vitharana Australasian Dams and Seismic Leader Arup Partners, Sydney, Australia
[email protected]
Mark Yashinsky Senior Bridge Engineer Caltrans, Sacramento, California, United States
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At Anza Borrego in 1990.
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memoria nostra durabit, si vita meruimus
