CR17-2.pdf

CICIND REPORT

Vol. 17, No. 2, September 2001

CICIND REPORT Vol. 17, No. 2

September 2001

EDITORIAL BOARD P.E. Freathy (Editor) - N.R. Bierrum - B.N. Pritchard - D.T. Smith - M. Beaumont CONTENTS

EDITORIAL.................................................................................................................................................................................... 3 FUTURE CICIND MEETINGS .................................................................................................................................................... 3 PRESIDENT’S MESSAGE.......................................................................................................................................................... 4 VIRTUAL WORLD........................................................................................................................................................................ 5 PROFILES OF THE GOVERNING BODY ............................................................................................................................... 6 REPORT ON CICIND's 55th MEETING IN ANTALYA ......................................................................................................... 8 PRESIDENT’S AWARD 2001 .................................................................................................................................................. 10 REPORT FROM THE GOVERNING BODY .......................................................................................................................... 11 COMMITTEE ACTIVITY............................................................................................................................................................ 12 TECHNICAL SECTION: 1.

Seismic load reduction factors for reinforced concrete chimneys: a probabilistic assessment - J.L. Wilson ........................................................................................................................................................................ 13

2.

A challenging chimney retrofit project for a steel producing company in Ontario, Canada - A. Bhowmik ...................................................................................................................................................................... 24

3.

Long-term behaviour of reinforced concrete industrial chimneys in Romania and a retrofit solution for the No 2 chimney of the Isalnita-Craiova power plant - L. Naum, D. Furis ............................................................................................................................................................ 29

4.

ACI 307-98 and CICIND 2001. Comparison of in-line wind loads and reinforcement - N.R. Bierrum .................................................................................................................................................................... 33

5.

Steel stacks for gas turbines - N. Ferlic............................................................................................................................................................................. 36

CICIND PUBLICATIONS .......................................................................................................................................................... 42 CICIND COMMITTEES .............................................................................................................................................................. 43 ORGANISATIONS REPRESENTED IN CICIND .................................................................................................................. 44 INDEX TO TECHNICAL PAPERS IN CICIND REPORT ..................................................................................................... 47 ABOUT CICIND .......................................................................................................................................................................... 52

Articles and papers published in CICIND REPORT represent the views of the authors and are not necessarily endorsed by CICIND. CICIND is not, nor are any of its members, to be held responsible for any failure alleged or proved to be due to adherence to recommendations or acceptance of information published by the Association. CICIND REPORT is published by CICIND, Zurich, Switzerland ISSN 1013-0489 Office of the Secretary CICIND, 14 The Chestnuts, Beechwood Park, Hemel Hempstead, Hertfordshire HP3 0DZ, England Telephone - National 01442 211204, International +44 1442 211204 Fax - National 01442 256155, International +44 1442 256155 email - [email protected] Printed in England 1

CICIND REPORT

Vol. 17, No. 2, September 2001

2

CICIND REPORT

Vol. 17, No. 2, September 2001

Editorial This issue of CICIND Report brings details of your new Governing Body, elected at the NGA in Antalya in April this year. Updated profiles of each member are provided and, before that, a message from our new President, John Sowizal. John has had a distinguished career in industrial chimneys in the US, rising to senior levels within Custodis before staring his own consultancy business and, more recently, taking the role of President of G&H Acoustics in the US. His enthusiasm for CICIND is evident in his message and this will continue to drive CICIND forward, especially within North America but also world-wide. The new Councillors of the Governing Body will bring their fresh thoughts to bear and we look forward to hearing from members about their vision for CICIND.

have already received copies of the revised Model Code for Concrete Chimneys, Part A - The Shell, its associated Commentaries and also the Manual for Thermofluodynamic Design of Chimneys and Chimney Liners. Also in progress are the Metallic Materials Manual, which we expect to start typesetting very soon, and the Seals Manual, which should also be close to completion. Revisions to Parts B & C of the Concrete Code and also to the Customers’ Guide to Specifying Chimneys are also being considered. It is most encouraging to see the level of activity that CICIND members support and maintain, despite the commercial pressures on them. That is what makes CICIND strong and relevant nearly 30 years after its inception.

Technical Papers

CICIND Chimney Design Handbook

This issue of CICIND Report is a little slimmer than some in the recent past, reflecting two things. The first is that the Editorial Board does take its job seriously and we strive to maintain a high standard of published work, upon which CICIND’s reputation depends. This means that we do reject some papers as being unsuitable, while we ask for modifications or clarifications to others. The second reason for the smaller number of papers is that authors are simply not providing the papers for us to review. We have normally been able to rely on those who present papers at the technical meetings following that up with a written paper. Individual cases all have valid reasons for not doing so, I am sure, but collectively it does leave us with a shortfall and I urge all members to both contribute presentations and to provide written papers.

Many of you will already be aware that we are currently preparing an update to the excellent book written by Geoff Pinfold many years ago. The scope has been widened to include both concrete and steel chimney construction, together with the ancillary subjects that contribute to a complete design including wind, environmental concerns, seismic actions, etc. Geoff Pinfold has kindly agreed to act as Editor of the book and has persuaded many of our experienced members to contribute a chapter. It will be published under the CICIND banner and the royalties will flow to CICIND. All of those involved are contributing a great deal to our cause in terms of their time and experience and we should all be grateful to them. First drafts are due to be with Geoff shortly so that the book can be published next year.

I should also mention that having a paper published in CICIND Report is a useful method for advertising your expertise. This is valuable for academic members, because this is a refereed journal, and is also a marketing benefit to commercial members. We do sell a number of reprints of papers each year and even showcase selected papers on the web site. The sales show that professionals outside CICIND do look at our web site and do review our publications list.

Postscript As this issue was being completed we learned of the dreadful attacks in America, only a couple of days before the meeting in Krakow. More eloquent words than mine have been said and printed about these events but perhaps it is worth reminding ourselves about the value of the international cooperation and understanding promoted by organisations such as CICIND.

New Publications By the time you receive this issue of CICIND Report you will

Future CICIND Meetings Dates shown are subject to the agreement of local organisers. Check your circular letters or the web site.

Sydney, Australia March 2002

Cairo, Egypt Sept/Oct 2002 3

CICIND REPORT

Vol. 17, No. 2, September 2001

President’s Message Since this is my first chance to address the full membership of CICIND I would like to thank you all for giving me the opportunity to lead this important organisation in chimney design and construction. It is an honour to serve you as President of CICIND.

participation have enriched both my professional and my personal life. Knowledge I learned in researching, writing and presenting papers has greatly furthered my career as well as providing visibility and a knowledge base of the firms that supported the p aper development.

It has been quite some years since I was first given a copy of CICIND Report shortly after I started work in my career as a chimney engineer. I was impressed at the depth of the technical detail and the apparently complicated design issues. It seemed much more complicated than I first thought in accepting the position. It was truly a humbling experience. To those new chimney engineers in a similar position who happen to read my comments, let me say that it has been very interesting and enjoyable to study the chimney structural design theory as well as the many aspects of wind and seismic design. The opportunity to study past successes as well as failures has benefited my professional growth as well as saved my employer from the cost of repeating past errors and has helped improve their products by taking advantage of good new ideas.

I hope that instead of just reading the CICIND Report, Model Codes and literature you will study them and learn how you can add to the chimney engineering knowledge of your firm. I hope that you meet with your manager to discuss the value of your attendance at our meetings in order to enrich your professional development as well as that of your company. Our codes are far more advanced than when I picked up the first CICIND literature. Our Model Codes provide excellent information to safely design, build and maintain chimneys but they will be much better if you decide to actively participate, share your knowledge and take the time to study the design methods and challenge the committees to make the codes even better and safer. I look forward to seeing new members and especially new guests who are most welcome at the coming meetings. Come, participate and make your opinions count.

Reflecting back over the many CICIND meetings I have attended in these past years, I have enjoyed the many technical presentations as well as the coffee and lunch break small group discussions which made me think and stirred my mind to learn more. The business friendships resulting from my

Thank you for letting me share my viewpoint with you and please let me know your opinions

John Sowizal

Arab Company for Ceramic Products Tel: Fax:

+202 469 8516, 469 8260, 468 2938 +202 469 8304

Producers for:

♦ anti acid and heat resistant masonry bricks used for power

plants and flue liners (including standard and special shapes) ♦ acid resistant mortar ♦ chemical resistant bricks

Our sister company:

MASR AL MOSTAKBAL SPECIALISED CONTRACTS Tel: +202 267 0867 Fax: +202 469 8304 e-mail: [email protected] Executors for:

♦ construction of flue liners for power plants, cement, petroleum, steel & ceramic

factories ♦ paving floor for chemical & petrochemical factories ♦ lining work of sewage tunnels 4

CICIND REPORT

Vol. 17, No. 2, September 2001

Virtual World The graph below shows usage of the CICIND web site, which continues to attract more than 300 visits per month most of the time. A dip in activity from May onwards, seems to have been reversed, perhaps because of the approaching technical meeting in Krakow. Also many improvements that have taken place to the site should increase traffic. Since the last issue of CICIND REPORT we have subscribed to a service that aims to submit sites to search engines and to improve the likelihood of a good listing. Hopefully the benefits will be seen in coming months.

Members Only One key development is the creation of a members’ area to the site. Accessible only with a username and password, this will give access to an up -to-date version of the Members Directory that can be searched by name, company, town or country. Updated more frequently, this will be a useful addition to the annually printed paper Directory. Also available in the new members’ pages are a download page, where copies of CICIND documents may be obtained, and also a discussion area, where questions and comments may be posted for others to respond to. One objective of this area is to provide web space that Committee Chairmen can use to post information about their committee work and also to post draft documents for download. Whilst there is no plan to stop the production of paper copies, these facilities will undoubtedly make it easier for members to communicate quickly across the world.

Number of distinct sites served 1999/2000/2001

600 500 400 300

Of course, some of this information is quite valuable. The contacts list alone has value for people trying to sell their products to you. That is why we need to protect this area with a username and password system. Any paid-up member can obtain one by completing the online form and notifying the Secretary.

200

07-01

05-01

01-01 03-01

09-00 11-00

03-00 05-00 07-00

01-00

09-99 11-99

05-99 07-99

0

03-99

100

5

CICIND REPORT

Vol. 17, No. 2, September 2001

CICIND’s Governing Body At the NGA in Antalya a new Governing Body was elected to serve for a period of 2 years. This year there were many changes, with the retirement of our President and Vice-President as well as 4 Councillors. This new blood, led by the new President and remaining Councillors, will be well placed to guide CICIND through the next period and help it develop further. held the position of Managing Director of OOMS-ITTNERHOF GmbH since 1995. OOMS-ITTNER-HOF is one of the leading companies in the fields of engineering and execution of chimney and refractory lining projects.

John Sowizal President John is a United States citizen. He has been an active member of CICIND since 1991 and was elected to the Governing Body in 1999. John is a member of the steel liner and metallic linings CICIND committees. John is active in a number of United States codes including ACI 307 committee on Concrete Chimneys, ASME STS-1 committee on Steel Stacks, ASTM committee on Brick Liners and ASCE committee on Steel Chimney Linings. John is President of G&H Acoustics, LLC and his firm designs and supplies industrial noise control equipment especially for gas turbine power plant exhaust system inlets and chimneys. After graduating with a Structural Engineering degree from the University of Illinois at Chicago, John joined Custodis Construction Company as a project engineer for concrete and steel chimneys. During the period that John worked with Custodis, he held various positions in engineering and management including Vice President of Engineering, Product Manager and Vice President and General Manager. John is married to Kitty Krambaer and they live in Annapolis, Maryland, USA.

Gangolf is also a board member of the following National Committees for Chimney and Refractory Linings, the Bundesfachabteilung Schornstein - und Feuerungsbau and the Deutsche Gesellschaft Schornstein - und Feuerungsbau, where he is also Chairman of the working group responsible for Research and Development. Gangolf lives with his wife Hannelore and daughter Jenny "links Rheinisch" just outside Cologne.

José del Solar Bermejo Councillor José is a Spanish citizen and joined CICIND in 1980. He graduated in 1971 as Civil Engineer from Escuela Técnica Superior de Ingenieros de Caminos, Canales y Puertos, University of Madrid. In 1971 he joined AGROMAN, where he was responsible for the project management and design of major civil structures in the Civil Engineering Project Department. In 1979 he was appointed Technical Manager of KARMAN Técnicas Especiales S.A. and committed to a one-year stay by KARRENA GmbH in Düsseldorf. Since 1980 he is involved in the field of design, construction and repair of concrete and steel chimneys. In 1985 he was appointed General Manager of KARMAN, continuing to be responsible for design, construction and repair of chimneys, and in 1999 he was simultaneously appointed General Manager of BYGGING ENCOFRADOS DESLIZANTES S.A.

Terry Smith Vice-President Terry is a British citizen but has lived in Italy since 1979. He joined CICIND as a member in 1990, was elected to the Governing body in 1997 and Vice President in 2001. Terry first became involved with chimneys and silos when working as a site engineer for Oscar Faber and Partners in 1966. Joining Tileman & Co. in 1969, he specialised in chimney design and construction. He is a chartered civil engineer, qualified as a Eur. Ing. and obtained the Diploma of Imperial College in Concrete Structures and Technology in 1973. During his time in Italy he has been Managing Director of Tileman Italia, Technical Director for Recchi Energy, and General Manager of HAMON Custodis. He is now General Manager of HAMON Mariani Battista and based in Milan.

José is married to Carmen Serrano Verdes-Montenegro and they have a daughter and son.

Bill Nye Councillor

Terry is married to Harriet and has two grown up daughters.

Bill Nye is a British citizen. He joined CICIND in 1998 and was elected to the Governing Body in 2001. He graduated with a Civil Engineering Degree from the University of Bradford. Bill has spent most of his working life managing construction companies. Besides his experience in the U.K. he was for some years a Director of International Construction Companies based in the Middle and Far East. He was appointed Managing Director of Bierrum in 1998.

Gangolf Stegh Councillor Gangolf is a German citizen. After graduating in Engineering in 1975, he joined the company OOMS-ITTNER where he was responsible for the project management of chimney projects.

Bill is married to Jane and they have a daughter and two sons.

Through the expansion of the company, with the takeover of Franz HOF, Gangolf became a member of the Board and has 6

CICIND REPORT

Vol. 17, No. 2, September 2001 He attended a number of CICIND meetings prior to becoming a member. As a member, he has played an active role on the Steel Chimneys Committee and is also a member of the Metallic Materials Committee. Apart from his work with CICIND he is a member of CEN/TC297 and is involved with the steel chimney and liner working groups, reporting to the BSI on progress. He is further involved with the training of operatives and a UK trade association.

Peder Andersen Councillor Peder is a Danish citizen. He joined CICIND as a member in 1993. Peder graduated with a B.Sc. in mechanical engineering in 1974 and has been employed as an Engineer from 1974-1981 in Denmark and foreign countries. Peder founded Steelcon Chimney A/S in Esbjerg on 1 December 1981 and has been the company's Managing Director since that date. During 1975-1985 Peder obtained a Diploma in Economics as well as in Strategy and Organisation. Over the last few years, Steelcon has become the largest and leading supplier of factory-made steel chimneys in Europe.

Michael lives with his partner Irene in the west of England.

Paul Freathy Secretary Paul is a British citizen. He runs a consultancy business in wind loading, environmental effects and construction product performance, PF Consultants. Previous work experience includes many years at WS Atkins Consulting Engineers, a spell with the Structures Department at BT and being Head of Product Design and Performance at Redland Technology. He is an Executive Committee Member of the UK Wind Engineering Society and Chairman of their Strategy Committee.

Peder is married and has a grown up son and daughter.

Michael Beaumont Councillor Michael is a British citizen. He is the fourth generation of a family that can trace its history back for 125 years in steeplejacking and steel chimneys. Having left college he worked in all departments of the business but had a preference for site related activities and worked all over the UK and the world before taking up management duties. He brings good practical, handson experience to the CICIND Governing Body.

Paul was appointed Secretary in 1998. He is married to Lin and lives in Hertfordshire, England.

G&H ACOUSTICS, LLC Performance & quality guaranteed For gas turbine power plants we provide stacks, silencers, inlet filter systems, weather and diverter dampers. For FGD power plants we provide a full line of dampers built from high alloy and/or carbon steels to suit the environment. Supplied complete with effective controls systems and circuits to control and monitor performance. Worldwide distribution for FGD and gas turbine power plants We provide engineered solutions based on more than 40 years of indus trial and power plant noise control engineering, material development and construction to high quality standards and combined with local fabrication capability to provide cost effective products.

G&H Efficient Damper Designs

A Groupe Vinci Construction company. John Sowizal, President G&H Acoustics, LLC 1419 Forest Dr., Suite 205 Annapolis,MD 21403-1473 USA [email protected] Phone: Fax:

+1(410) 990 4840 +1(410) 990 4841 The leading supplier of high quality resonator FGD silencers

7

CICIND REPORT

Vol. 17, No. 2, September 2001

Report on CICIND’s 55th meeting in Antalya, Turkey CICIND’s 55th meeting was held in Turkey during April 2001 at the Mediterranean resort of Antalya. CICIND had been there before, in 1994, but there should have been no worries about returning. Those who joined the previous meeting were just as impressed the second time around, while the new visitors were able to discover it anew.

a requirement for two separate stacks but no space for separate foundations a creative solution was needed. The one adopted was to re-use the bottom portion of the existing stack and build two D-shaped stacks on the upper portion. This presented some interesting problems of gas flow and stresses. Also in this session, Norbert Ferlic presented case studies of the design of hot stack-bypass systems, highlighting the design issues associated with the thermal shocks arising from rapid start and stop cycles. In between theses papers we received reports from Bill Plant and Bill Evans on the progress of their committee work on metallic materials and seals, respectively.

The first impression of the area would, for many, have been the short plane trip from Istanbul descending Antalya over the mountains for a bumpy final approach. Our hotel was close to the coast and was a splendid location for our meeting. Built around an impressive, full-height atrium with glass, ’scenic’ lifts it gave a good impression even as we walked through the door. Our hosts, Tamer Tunca and his colleagues had worked hard to make this a successful meeting and they managed to do so with a personal touch that was most welcoming. Successfully organising good weather also helped!

After lunch on the terrace our President, Roger Bierrum, presented a comparison of ACI 307-98 with CICIND 2001. Differences in the calculation of inline loads mean that ACI gives up to 25% greater bending moment than CICIND even though the load factors are smaller. This could be particularly significant for foundation design, which is based on unfactored loads. Comparisons of vertical reinforcement requirements show the two codes to be similar but ACI requires rather less circumferential reinforcement, which gives durability problems to smaller RC chimneys. Seismic design Most of the remainder of the day was taken up with a subject of great interest in Turkey - earthquake design. Two presentations on the use of capacity design principles were presented by Michael Angelides and Valeriu Rosetnic. Angelides shows that capacity design methods can lead to high foundation design loads for stiff structures. Rosetnic showed, with examples, the importance of selecting the position of the plastic hinge in capacity design. We then witnessed an excellent presentation by Sami Kilic of Bogazici University, who made full use of modern computer presentation techniques to illustrate his report on a study into the collapse of a 115m stack during the Kocaeli earthquake. He concluded that one of the principal reasons for the failure was the inadequacy of the splices in the rebars around a duct opening.

The meeting opened with a cocktail reception and canapés on the mezzanine level of the hotel overlooking the atrium on one side and outdoors on the other. The meeting was well attended, which made for a lively gathering catching up with old acquaintances and making new ones. The opportunity to relax was particularly welcomed by those who had spent the day in committee meetings and the group dissipated only slowly, with many then going off in groups for dinner. Technical meeting The technical meeting commenced next morning with a good level of attendance. The programme had developed over the previous weeks into a very full and varied one, with a range of both well-known CICIND contributors and new faces, including some local speakers who were most welcome. This interaction with the engineering and scientific community must be one of the most important aspects of CICIND.

The first day was rounded off by an impromptu presentation of a camera inspection system for use with the stack online. A gap had arisen due to the unfortunate absence through ill health of Professor Erdik. Serguei Souchtchev stepped into the breech very well and gave an interesting case study of the camera’s use.

The meeting opened with two papers concerning the wind loading of chimneys and comparisons with other codes and with measured results. The first was the inaugural CICIND presentation by the Secretary, Paul Freathy, who showed the results of comparison calculations using both CICIND and the proposed Eurocode. These indicate that Eurocode gives somewhat higher loads for downwind response and, although there are differences in assumed turbulence for different terrain types this is not sufficient to explain the greater loads. For crosswind response the situation is reversed, with CICIND giving conservative results that provide a safe design whereas the Eurocode results often underpredict measured results, making them unsafe. This is an important issue for the future direction of CICIND Model Codes and the topic was taken further by the second paper of Henk van Koten who presented results in support of the higher crosswind loads predicted by CICIND in comparison with measured data. The DIN code was shown to be unsafe in some cases. This is a matter to be discussed and resolved in the wind committee, with technical papers for CICIND Report to follow. After the break, Arun Bhomik presented a case history of the replacement of a corroded steel stack serving two boilers. With

An attentive audience for Bill Plant and Josef Lettner

8

CICIND REPORT

Vol. 17, No. 2, September 2001 The President then thanked his ‘forgiving audience’ and took the opportunity to congratulate Mr. Tunca and his colleagues for their hard work in making the meeting such a success. The traditional CICIND thank you of an engraved silver salver was presented in recognition of a job well done

Day 2 The following day commenced with a comparison of the performance of a chimney in an extreme earthquake event. John Wilson used a probabilistic approach to establish a probability density function of seismic loading which was overlaid with that for the resistance of the chimney. A comparison of the overlapping probability of failure using existing and proposed code provisions showed how the use of limited ductility in the design significantly reduced the probability of failure.

Bonus event As a bonus, we were offered the chance to sample an evening of traditional Turkish food, music and dancing on Friday evening and, to nobody’s surprise, there were plenty of takers. We sat out on the terrace and enjoyed plenty of good food and wine whilst appreciating the talents of the belly dancer. Perhaps overcome by the occasion (or was it the wine), several of our party felt the urge to have a go themselves. Mrs. Stegh was a particularly fine dancer while perhaps the best that could be said of the male members of our party is that they were enthusiastic!

Continuing the theme, Professor Aydogan of Istanbul Technical University presented a summary of building failures during the recent Turkish earthquakes, with suggested remedies. Bill Plant, Doug Heath and Josef Lettner then spoke on the benefits of using clad-plate linings to reduce the potential for failure in earthquakes. The wallpaper technique was shown to be cost-effective and also to have survived an earthquake in Taiwan with only minor damage. New member Laurentiu Naum authored a paper with Professor Furis, which was presented by a colleague, summarising the performance of reinforced concrete chimneys in Romania. The repair of a particular 200m chimney was described. This led nicely to our final presentation by Paul Arckless and Gary Eastman concerning repairs to five concrete chimneys in Turkey. This practical discussion of repairing chimneys while they remain online brought the meeting to an interesting conclusion. Social programme

Amphitheatre at Demre

Our hosts organised a couple of splendid tours for the ladies on Thursday and Friday, plus one for everyone on Saturday. The principal feature of the Ladies Thursday tour was a visit to the At the amphitheatre in Demre Manavgat Falls. This was followed by a shopping expedition at which various art and craft skills were displayed in a fashion show. Thursday evening also saw the formal dinner at a restaurant close to the hotel.

Stack Design Software Self Supported (STACKDES) ü Graphical Interface ü ASME STS-1-2000 ü Wind per ASCE 7 ü Complete Analysis Output ü Vortex Induced Response ü Metric and English Units

Guy Wire Supported (GUYDES) ü All features of STACKDES…plus ü Determine worst direction for wind ü Check stack/guys for Hot & Cold

Farewell remarks by the outgoing President After the meal, outgoing President Roger Bierrum said a few words of farewell. He noted that he was one of the few remaining members who could claim to have been at the first International Chimney Symposium held in Edinburgh in 1973. At that time, rising fuel costs and new clean air regulations had led to a reduction in operating temperatures and a flood of resulting problems. Professor ‘Hank’ Hancox of Glasgow arranged a meeting of chimney users, builders and designers, expecting that perhaps 20-30 would attend. He was staggered to find that there were, in fact, about 150 delegates, causing an embarrassment of riches to the organisers which had to be spent on entertainment! A malt whiskey tasting was arranged to take care of that. CICIND, which grew from this first meeting, could hardly have had a better start!

Supporting Spreadsheets Ø ASCE 7-98/95/93 Wind Ø Anchor bolt chair/Base Plate Ø Guy Wire Hardware Ø Conical Transitions Ø Roark’s – Plate and Ring Ø Structural Plate Flange

Email: [email protected] Website: www.mecaconsulting.com 9

CICIND REPORT

Vol. 17, No. 2, September 2001

All will be pleased to know that my photographs of the occasion did not come out well enough to publish here. And so, finally, to the Saturday tour which took us to some interesting ancient sites. These included an amphitheatre and some unusual tombs excavated directly into the rock face at Demre, about 25km West of Finike. We also visited an ancient church, with floor mosaics and wall paintings. All this after the Secretary had triumphed with another ‘first’. Having arranged the first CICIND visit to a tractor fest in Austria, the entertainment on this occasion was the arrest of our coach driver for speeding! After many minutes arguing, the driver lost his battle with the policemen and continued the journey as a very unhappy man. All part of the service!

Unscheduled entertainment courtesy of the Turkish traffic police

President’s Award, 2001

The President, Roger Bierrum, giving the 2001 President’s Award to Ray Warren of Warren Environment, Inc., Atlanta , GA, USA. Ray has been an active member of CICIND for many years, carrying the torch for CICIND in the USA and being an exemplary ambassador on our behalf. In Ray’s case the word ‘active’ means exactly that. He was President of CICIND from 1993 to 1994 and has played an important part in the deliberations of the Governing Body both before and since. He has been equally influential in the many CICIND committees that he has contributed to. Ray is a man of strong opinions, never afraid to offer them but always with good humour and often to good effect. Ray and his wife Doris have attended and supported many CICIND meetings, contributing to the fun and enjoyment of the events. Antalya was no exception. The President warmly thanked Ray for his contribution to CICIND and expressed the hope that he will continue to guide us in years to come.

10

CICIND REPORT

Vol. 17, No. 2, September 2001

Report of the Normal General Assembly 27 April, 2001 - Antalya, Turkey There were 30 members present at the NGA and the Secretary had 7 notified proxy votes, giving a quorum for the meeting. The following report covers the main discussions and decisions taken during the meeting - Minutes have been sent to all members for their records.

Elections Messrs. Clark Brownscombe were reappointed as Auditors. In the elections for officers of CICIND the following were elected:

President’s Report Outgoing President, Roger Bierrum, noted that CICIND’s most important activities are publications, research and meetings. It was encouraging to see these activities continuing so strongly. During the year the Concrete Code Part A and its Commentaries, as well as the TFD Code had been completed and progress was made with the Metallic Materials Manual. He remarked that it had been an honour to serve as CICIND President and thanked members for their support before wishing his successor well for the future.

John Sowizal Terry Smith

- President - Vice President

Peder Andersen Michael Beaumont Bill Nye Jose del Solar

- Councillor - Councillor - Councillor - Councillor

The Editorial Board for CICIND Report was confirmed as Messrs. Freathy, Bierrum, Beaumont, Pritchard and Smith. Presentation As the incoming President was unable to be present at the meeting, the new Vice President took the chair following the elections. He thanked Roger Bierrum for his contribution to CICIND during his 4-year term and presented him with a token of our thanks, a pair of silver champagne flutes.

Secretary's Reports The Secretary noted that member numbers had fallen slightly during the year, reflecting the economic conditions. However, moves were needed to maintain a continued influx of new members. CICIND’s financial reserves remained at an acceptable level but the annual expenditure was now outstripping income so that action was required. Indeed, the 2001 budget, approved by the meeting, showed a deficit. Costs were being targeted, especially printing, but the Governing Body was forced to recommend the first increase in subscriptions for more than 10 years. This motion was approved. Changes to Statutes The meeting approved 5 minor changes to the Statutes, reflecting the changes to subscriptions and some administrative changes concerning the notification of proxy votes and the use of electronic communication. These changes were subsequently printed in a revised booklet and were circulated to all members.

Report from the Governing Body The Governing Body has met once since the last edition of CICIND Report was published; in Antalya, Turkey (April 2001) at the technical meeting described elsewhere in this issue. The following notes provide an update on their discussions. As always, the Governing Body would welcome contributions to their debates from members. This can be done by contacting the Secretary or any GB member.

proposes computational analysis. Both are actively under consideration by the Governing Body and it is hoped that at least one project will be approved fro support for this year. Of course, there are many other topics that we could consider supporting and the Governing Body welcomes any suitable proposals. If you don’t ask, you don’t get! CICIND subscriptions

Research proposals

You will be aware from the report of the NGA and from Circular Letters that subscription rates will be increased from 1 January 2002. This is the first rise for more than 10 years, which represents a very good track record for the growth and management of CICIND. The Governing Body continues to

In the last issue, the lack of suitable proposals was discussed and I am pleased to say that we now have two proposals concerning further work on the seismic behaviour of reinforced concrete chimneys. One takes an experimental approach, while the other 11

CICIND REPORT

Vol. 17, No. 2, September 2001

take this very seriously and I am pleased to say that the initial indications are that the measures we have taken to reduce the cost of our core activities are working. As always, the other side to this equation is to increase membership or the sale of other products. New Codes and Manuals are in production to help in this and we are actively searching for new areas in which to promote CICIND.

Code and the Customers Guide to Specifying Chimneys should be reviewed and updated where necessary. Volunteers to help in this process will be sought by the Secretary but please, if you have an interest, don’t wait to be asked! In addition, the preparation of a new book is progressing well under the guidance of Editor Geoff Pinfold.

New Publications The GB has recommended that Parts B & C of the Concrete

COMMITTEE ACTIVITY Concrete Chimneys - Revisions to Part A of the Code and the Commentaries regarding crack width and seismic design are currently being printed and should have been circulated to members prior to distribution of this edition of CICIND Report.

Metallic Materials - Chairman Bill Plant reports that progress continues with the preparation of the Metallic Materials Manual. Sections of the Manual are being developed in loose-leaf form for ease of updating and to permit publication in part rather than await total completion. This is particularly pertinent in that as the potential benefits of the manual as a reference work become increasingly apparent, additional proposals have been made. As an example, work has now commenced on a section entitled “Low Temperature Applications.”

Thermofluodynamics - The TFD Code is currently being printed with circulation to members planned for late September. Wind loading - Following the comparative calculations carried out by the Secretary and Mr Pritchard there remains a difference of opinion within the committee about the suitability of the Eurocode wind model. Further calculations and discussions are needed to resolve this.

The intention of the meeting of the Metallic Materials Committee in Krakow will be to finalise as many sections as possible to permit formal Governing Body approval to be gained and enable publication to commence.

Maintenance - Chairman John Turner has reported that he intends to circulate a first draft of the maintenance booklet to the committee, hopefully before the Krakow meeting, to enable responses to be made. He has also signalled his intention to take advantage of the CICIND website to extend the boundaries of his committee in order to elicit responses from the wider membership.

Seals - There remains only one section of this manual to be completed and it is hoped that it will be possible to start preparing the document for publication in the new year.

12

CICIND REPORT

Vol. 17, No. 2, September 2001

Seismic load reduction factors for reinforced concrete chimneys: a probabilistic assessment J.L. Wilson, University of Melbourne, Australia Presented at CICIND’s 55th meeting, Antalya, 2001

1.

INTRODUCTION

(c) Life Safety performance level: 10/50 or 475 year return period

Current code recommendations for the aseismic design of tall reinforced concrete chimneys in regions of high seismicity are generally non conservative and result in brittle and expensive chimneys. The design requirements needed for a chimney to be designated moderately ductile have been investigated both experimentally and analytically [1,2]. It has been demonstrated that chimneys designed for moderate ductility result in cheaper windshields and foundations and improved performance under extreme earthquake events. A moderately ductile chimney has the important characteristic that the acceleration to cause failure is in excess of four times the elastic design acceleration (i.e. af ≥ 4ae ). This observation has been translated into practical aseismic design recommendations suitable for codes of practice. The selection of a suitable structural response factor or global ductility factor which satisfies both the serviceability and structural stability limit states has previously been reported using deterministic techniques [1,2].

(d) Collapse prevention performance level: 2/50 or 2475 year return period. These mean return periods are typically rounded to 75, 225, 500 and 2500 years respectively. The performance levels recommended in FEMA 273 are consistent with the three limit states outlined by Paulay [5] consisting of: (a) Serviceability Limit State - SLS (b) Damage control or damageability limit state - DLS (c) Survival or structural stability limit state - SSLS The SLS is generally associated with no damage requiring repair and no interference to the operation of the facility. Adequate stiffness and strength should be available so that displacements are controlled and the response is essentially elastic. Hairline cracking of concrete may result but significant reinforcement yielding or concrete crushing should be prevented. The DSL is generally associated with damage that is repairable. Such damage may involve epoxy grouting cracks and locally replacing damaged concrete. The SSLS is associated with extreme ground shaking with the emphasis placed on the prevention of collapse and loss of life. It is expected that the structure would be severely damaged following large inelastic deformations however the residual strength would be sufficient to support the gravity loads.

This paper presents a probabilistic approach to compare the performance of a chimney designed using existing and proposed code provisions to an extreme earthquake event. The seismic hazard will be represented by a Weibull distribution relating a peak effective ground acceleration to a return period, from which a probability density function for seismic loading can be developed. The structural resistance will be expressed by a Normal or Gaussian distribution. Numerical integration techniques will be used to compare the probability of failure values calculated for the different designs.

The acceptable risk associated with each limit state depends on the relative importance and design life of the structure. For building structures the return periods associated with the SLS, DLS and SSLS are typically 75, 500 and 2500 years respectively. These return periods are equivalent to probabilities of exceedance of 50%, 10% and 2% for a 50 year design life (ie. 50/50, 10/50 and 2/50).

2.

DESIGN PHILOSOPHY AND CODE RECOMMENDATIONS 2.1 Design philosophy and performance levels The seismic design philosophy traditionally adopted around the world has been to ensure life safety at the ultimate limit state, defined with a return period of 475 years which is equivalent to a 10% probability of exceedance in 50 years (10/50). In addition a damageability limit state criteria is specified so that earthquakes that could be expected during the life of the structure do not result in excessive damage. These limit states have been to date the philosophy of the Uniform Building Code [3] of the USA. Following damaging earthquakes in 1989 (Loma Prieta), 1994 (Northridge) and 1995 (Kobe) which included significant damage to code compliant buildings, greater emphasis has been placed on the development of performance based codes.

The inclusion of the 2500 year return period event addresses the possibility of infrequent large magnitude events that may not be accounted for at the 10/50 hazard level. The design philosophy and code recommendations proposed by the author for the aseismic design of reinforced concrete chimney structures are based on satisfying both the SLS and SSLS limit states, which by default will also satisfy the DLS. The probabilities of exceedance selected for ordinary and special chimneys for a 50 year life correspond to 50% and 25% for the SLS and 2% and 1% for the SSLS.

The 1997 NEHRP guidelines for the seismic rehabilitation of buildings - FEMA 273 [4] recommends up to four performance levels consisting of:

2.2 Proposed provisions for aseismic design (for 2001 Revision to the CICIND code )

(a) Operational performance level: 50% exceedance in 50 years (50/50) or return period of 72 years.

(a)

(b) Immediate occupancy performance level: 20/50 or 225 year return period.

Overview The seismic design approach described in this section is based on performance based criteria:

13

CICIND REPORT

Vol. 17, No. 2, September 2001

(a1) designing the chimney elastically to resist earthquake induced loads considered reasonable for a serviceability limit state earthquake event (SLS).

ground accelerations and velocities (PGA and PGV) with a probability of exceedance between 5% and 20% (i.e. return period ranges from 225 years to 975 years). These mean or 50 percentile values were used to scale normalised response spectra defined at the 84 percentile level (mean plus one standard deviation). In addition, the ATC published generic recurrence charts of earthquake hazard ranging from very low seismicity to high seismicity regions. The hazard charts were presented as a plot of effective peak ground acceleration vs return period as shown in Figure 1.

(a2) designing the chimney with sufficient ductility so that the chimney will survive an extreme earthquake event without premature failure and collapse at the structural stability limit state (SSLS). (b)

Seismic Actions

The distribution of annual peak accelerations can be approximated by a Weibull distribution of the form:

(b1) Return Period: The design basis earthquake is a representative earthquake associated with a return period of 475 years (i.e. 10% chance of exceedance in 50 years).

(

)

T = exp f1 ⋅ af 2 or a = f1−(1/ f2 ) ⋅ [ln(T) ](1/ f 2 )

(1)

where

(b2) Elastic Response: The elastic response of the chimney shall be calculated using the response spectrum method and the design basis earthquake.

• Assume uncracked properties.

T

=

Return period (years)

a

=

effective peak ground acceleration

f1 f2

=

seismicity dependent constants

Croft [7] fitted a Weibull distribution to the hazard curves by optimising the constants f1 and f2 and obtained a very good match as shown in Figure 1. The constants f1 and f2 and the resulting peak ground accelerations corresponding to the 475 year and 2475 year events are listed in Table 1 for levels of seismicity ranging from very low to high.

• Use a response spectrum with 5% critical damping and 50% shape bound probability appropriate for the site conditions.

• Sufficient number of modes shall be included so that at least 90% of the gravity load of the chimney is accounted for in the modal analysis.

These generic curves and resulting acceleration ratios between the 2475 year and 475 year events appear consistent with studies undertaken in low seismic regions such as Australia and higher seismic regions such as New Zealand as reported

(b3) Seismic Design Actions: The seismic design actions shall be obtained from the elastic response by multiplying the actions by an importance factor (IF) and dividing by a structural response factor (R) to account for ductility. (a) Importance factor The importance factor is dependent on the importance class of the chimney: Class 1: IF = 1.2 (R = 1), IF = 1.0 (R = 2) Class 2: IF = 1.4

Seismicity

f1

f2

a475

a2475

a2475/a475

Very low

17.3

0.36

0.057g

0.11g

1.9

Low

14.2

0.37

0.10g

0.19g

1.9

Moderate

12.3

0.42

0.19g

0.34g

1.8

High

12.3

0.72

0.38g

0.53g

1.4

Table 1 Weibull constants and earthquake accelerations for different regions of seismicity and return periods

(b) Structural response factor The structural response factor is dependent on the level of seismic detailing: R = 1.0 No specific seismic detailing R = 2.0 Specific design requirements (c)

General capacity requirements

design

and

seismic

detailing

The design of the chimney should be consistent with the principles of capacity design. The foundation system and the shell in the vicinity of openings should be designed for overstrength (flexure and shear) so that inelastic flexural behaviour will develop in the ductile regions of the shell away from significant openings. In addition a number of specific requirements relating to reinforcement ratios and splice details need to be satisfied. The background to these recommendations is provided in references [1,2].

3 SEISMIC HAZARD 3.1 ATC-3-06 Recommendations Figure 1 Earthquake acceleration versus return period [6,7]

The Applied Technology Council [6] in 1978 published seismic hazard maps for the USA in the form of effective peak 14

CICIND REPORT

Vol. 17, No. 2, September 2001

by Dowrick [8]. For example, special studies for low seismicity sites near Sydney and Adelaide indicated that extrapolation of results for a 2500 year return period would lead to a doubling of the 475 year ground acceleration. In contrast the New Zealand studies representative of moderate to high seismic regions suggested an acceleration ratio in the order of 1.7 between the 475 and 2500 year events. Interestingly, the draft joint Australia/New Zealand Earthquake Loading Standard [9] recommends an acceleration ratio of 1.8 between the two return periods.

fault. Based on these studies estimations of the magnitude, rupture length, rupture mechanism, slip rates of the faults, and dates of rupture are made, and added to the earthquake recurrence database. The 475 year return period hazard maps prepared by the USGS are similar to those recommended in the ATC-3-06 document for the central and eastern United States regions. In the very low seismicity regions the acceleration ratio of the 2475 year to 475 year events was in the range 2-3 which was slightly larger than the value of 2 implied from the ATC-3-06 recommendations. However in the moderate seismicity regions near existing faults the acceleration ratios between the 2475 and 475 year events increased significantly to values in the range 4-6, particularly in the areas surrounding the New Madrid and Charleston faults. This large increase has caused significant conjecture amongst the earthquake engineering profession.

3.2 1997 NEHRP Recommendations The NEHRP recommended provisions for seismic rehabilitation of new buildings and other structures [10] is a document published by the Federal Emergency Management Agency (FEMA) in the USA. The document is intended to serve as a resource document for the development of model building codes. The seismic hazard is described in terms of the 84 percentile (mean plus one standard deviation) response spectral accelerations for periods of 0.3 seconds and 1.0 seconds. Spectral acceleration hazard maps have been developed for the whole of the USA corresponding to return periods of 475 years (10/50 years) and 2475 years (2/50 years). The effective peak ground acceleration can be estimated by dividing the 0.3 second spectral acceleration by a factor of 2.5. These hazard maps were developed and published by the USGS in 1996 and are based on a number of studies [11] and are intended to replace earlier 1978 maps published in ATC-3-06.

Hwang [12] notes that the seismic hazard in the Memphis area is dominated by the characteristic earthquake assumed for the New Madrid fault. The USGS hazard maps are based on a magnitude 8 characteristic earthquake with a recurrence interval of 1000 years derived from paleoliquefaction studies. There is however, considerable uncertainty with respect to the exact location of the fault, the size of the characteristic earthquake (i.e. M= 7 or 8) and the recurrence interval, with some researchers suggesting a much longer recurrence time. In summary, the 1997 NEHRP provisions present the latest findings from seismological studies of the USA carried out by the USGS. A comparison of the acceleration ratios between the 2475 year and 475 years events using the 1997 NEHRP and 1978 ATC-3-06 provisions indicates general agreement except for some moderate seismicity regions that are dominated by large and infrequent earthquakes. In such cases, the ATC-3-06 approach seems to underestimate the ground accelerations associated with the 2475 year event, although it is recognised that significant uncertainty is associated with such infrequent events. Interestingly, the ATC-3-06 generic recurrence recommendations appear consistent with independent seismic studies undertaken for Australia and New Zealand.

During the 1980's and 1990's extensive strong motion data was recorded from a number of moderate to large magnitude earthquakes in California. Based on this data it was concluded that the 0.4g effective peak ground acceleration previously assumed to be representative of ground motions associated with a return period of 475 years (10/50 years), significantly underestimated the motion that could be experienced in the near field of major faults. Consequently at some near-field sites the peak ground acceleration has increased from 0.4g to 0.6g for the 475 year event. A comparison between the effective peak ground accelerations corresponding to the 2475 year and 475 year events in the high seismic regions of California indicated that the acceleration ratio was low with values ranging from 1.0 to 1.5. Interestingly these values were consistent with the ratios interpolated from the ATC-3-06 recurrence charts (Figure 1).

4.

JUSTIFICATION OF SEISMIC LOAD FACTORS - PROBABILIS TIC METHOD

The probability of failure for chimneys designed using a number of different methods in regions of seismicity ranging from very low to high are compared in this section. The different design methods include the 1998 CICIND code [13] and the proposed limited ductile design approach outlined in Section 2, to be included in a 2001 Revision to the Code.

In addition, hazard studies were undertaken in the lower seismic risk regions of central and eastern United States. The concept of the 2475 year return period event (2/50 years) was introduced so that the effects of infrequent but very large magnitude events could be included in the design process. It was felt that structures designed only for the 475 year event may not have sufficient lateral resistance in regions where large earthquakes have previously occurred such as around the New Madrid and the Charleston faults. (The New Madrid seismic zone generated four M=8 earthquakes during 1811 1812).

4.1 Reliability Theory The load and resistance factors used in the ultimate limit state design approach are usually derived from reliability theory. In doing so, it is implicitly assumed that all loadings and all structural resistances such as material and member strengths can be represented as random variables with known probability distributions [14,15].

Defining the earthquake hazard at a return period of 2475 years is associated with significant uncertainty, particularly since the database of ground shaking only extends for one to two hundred years. Paleoseismic techniques have been used to extend the database by carbon dating the historic movements that have occurred along faults. This is a time consuming and expensive process and usually involves trenching across an existing fault that has outcropped at the surface, and studying the geological layers either side of the

The load and resistance factors are selected so that the probability that the demand or design load, S, exceeds the capacity or design resistance, R, is acceptably small. The associated notional probability of failure p f is expressed as: pf

15

=

Pr (R < S)

(2a)

=

Pr (R/S < 1)

(2b)

CICIND REPORT

Vol. 17, No. 2, September 2001

=

Pr (R-S < 0)

(2c)

=

Pr (ln (R/S) < 0)

(2d)

Another method for assessing the structural reliability is the safety index β. ?ISO 2394 defines the β index in the following generalised form:

The notional probability is defined in Figure 2 as the area beneath the overlapping portions of the R and S probability distribution curves. The term 'notional' rather than 'true' probability of failure emphasises the uncertainties associated with the assumed probabilistic distributions of parameters.

=

10-4 . Ks . nd / nr

(

β= R- S

=

(3)

social criterion factor dependent on type of structure (Table 2)

nd

=

design life in years

nr

=

average number of people in or near the structure during the period of risk Type of Structure

Ks

Public assembly, dams

0.005

Domestic, office, industry

0.05

Bridges

0.5

Towers, Masts, Offshore

5.0

(

)

β = ln R / S

where Ks

(4)

where φ(-β) denotes a cumulative frequency distribution with zero mean and unit variance. The two commonly accepted (and equivalent) expressions for β are as follows:

The selection of an appropriate probability of failure is a complex issue and dependent on social, economical and environmental issues. Bierrum [17] recommends the following formula for calculating the acceptable total life time probability of failure (or risk) p f : pf

φ (-β)

pf =

)

σR 2 + σS 2

5(a), 5(b)

2

CVR + C VS

2

R

= expected strength or resistance

S

= expected load

σR σS

= standard deviation of R and S

CVR CVS

= coefficient of variation of R and S

β

1.28

2.33

3.09

3.72

4.26

4.75

5.61

pf

10-1

10-2

10-3

10-4

10-5

10-6

10-7

Table 3 - Relationships between β and pf for Gaussian distribution The relationship between the safety index and the probability of failure is illustrated in Figure 3 and listed in Table 3 assuming both the load and resistance distributions are Normal or Gaussian. A value of β = 3.7 is approximately equivalent to a lifetime probability of failure, p f = 10-4.

Table 2 - Social Criterion Factors

The Normal or Gaussian distribution is commonly used to describe the distribution of dead loads and structural resistance parameters. In contrast the Gumbel distribution (FT-1) is commonly used to describe the population of extreme wind loads and live loads, whilst the Weibull distribution (FT-3) is often used to represent the probabilistic distribution of earthquakes.

A probability of failure, pf = 10 -4 is commonly accepted as a reasonable figure for tall reinforced concrete chimneys [17], based on Ks = 0.05, nd = 50 years and nr = 2.5. ISO 2394 [18] states that a failure rate of 1.0E-6 per annum appears reasonable for buildings, which is equivalent to a probability of failure of 0.5E-4 over a 50 year life.

Figure 2 Load and Resistance Probability Distributions [16] 16

CICIND REPORT

Vol. 17, No. 2, September 2001

The probability of failure pf can be calculated relatively simply with a closed form solution when both distributions are Gaussian. When one or both distributions are non-Gaussian (which is the case for earthquake loading) numerical integration techniques may be applied.

of n years can be approximated by a Binomial or Poisson distribution [7]. 0 n n n   n  1   1 1  Pn (a ) =   Q o P n =      1 −  = 1 −  (8a) T T  0  0  T  

[

Pn ( a ) = 1 − exp( − f 1 ⋅ a f 2 )

The probability of failure pf can be calculated from the following integral: ∞∞

pf =

∫∫

pR ⋅ p S ⋅ C u ⋅ dRdS

Pn(a) =

where probability distribution for factored resistance

pS =

probability distribution for factored loads

Cu =

1 if S > R and 0 if S < R

n

∑∑ i

1

(7)

f2

n −1)

i

The mean (expected) ground accelerations an and associated variance υn can be directly calculated from the probability density function.

where ∆p R ∆p S =

Probability that a ground acceleration greater than or equal to ‘a’ will occur over an exp osure period of n years (Equation 1), which is equivalent to a cumulative probability function (cpf).

[1 − exp(−f ⋅ a ) ](

n

∆ pR ⋅ ∆ pS ⋅ C u

(8b)

In order to calculate the mean and coefficient of variation of this distribution, the probability density function (pdf or p n(a)) must be derived. pn (a ) = Pn (a ) = n ⋅ f1 ⋅ f2 ⋅ a (f 2 −1) ⋅ exp(−f1 ⋅ a f 2 ) ⋅ (9) da

This integral in practice can be represented by a finite summation:

pf =

n

where

(6)

00

pR =

]

interval probability density for parameters R and S respectively.

∞

an =

In the following sections the probability density function distributions assumed for the seismic loading and structural resistance will be presented and the probability of failures calculated using numerical integration techniques.

∫a ⋅p

n

( a ) da

(10)

0 ∞

νn =

∫ (a − a )

2

⋅ p n ( a ) da

0

4.2 Seismic Loading - pdf

(11)

These integrals can be calculated simply using a spreadsheet and numerical integration techniques such as Simpsons rule. The co-efficient of variation CVn, can be calculated (by a definition) from n and υn as follows:

The probability density function for seismic loading can be developed from the following two assumptions: that the distribution of annual peak ground accelerations follows a Weibull distribution and that the probability that a peak ground acceleration, a, will not be exceeded over a design life

CV n =

υn

an

Figure 3 Relationships between β and pf for Gaussian distribution [19]

17

(12)

CICIND REPORT

Vol. 17, No. 2, September 2001

These integrals have been calculated using numerical integration techniques (Simpsons rule) and are summarised for the four different seismic environments in Table 4. Seismicity

f1

f2

a475

a 50

CV50

Very Low

17.3

0.36

0.06g

0.027g

0.78

EARTHQUAKE DEMAND High seismicity acceleration y475=0.38g n 50 * WEIBULL DISTRIBUTION f1 12.3 * f2 0.72 * MEAN 0.246 * CV 0.38 * S.D 0.093

Low

14.2

0.37

0.10g

0.049g

0.73

i

Moderate

12.3

0.42

0.19g

0.095g

0.54

High

12.3

0.72

0.38g

0.240g

0.38

Table 4 - Statistical parameters for different seismic regions The pdf and cpf values corresponding to the following 10 discrete acceleration values ai were then calculated: ai

=

0

ai

= a50 + (i - 3) σ50

= 2-10

Pj 0 3.87 4.25 1.77 0.63 0.22 0.08 0.03 0.01 0.01

Yj(g) 0.059 0.342 0.702 0.890 0.960 0.985 0.994 0.998 0.999

R = 1.4 a475 / [f . (1 - 1.65 CVR)]

0.135 0.199 0.283 0.375 0.468 0.562 0.656 0.750 0.843

Delta Pj 0.1186 0.4473 0.2726 0.1036 0.0366 0.0132 0.0049 0.0019 0.0008

(13b)

For reinforced concrete chimney structures it can be assumed that φ = 0.85 and CVR = 0.10 resulting in: R = 1.96 a475

and

σR = 0.20 a475

(b1) Proposed CICIND Code Limited Ductile Design (LDD)

2001

(13c)

Revision

-

The proposed limited ductile design approach recommends ordinary chimneys be designed for 0.5 times (R = 2) the 475 year return period design earthquake event (i.e. ae = 0.5 a475). Special chimneys are assigned an importance factor IF = 1.4 resulting in a design load of ae = 0.70 a475. The pdf associated with the structural resistance needs to be investigated for both the shear and flexural failure modes.

4.3 Structural Resistance - pdf Normal or Gaussian probability density functions have been used to represent the seismic resistance provided by structures designed using the following methods: (a) 1998 CICIND code, (b) proposed 2001 CICIND code recommendations for (b1) limited ductile design (R=2) and (b2) elastic design (R=1).

(i) Shear Capacity

In each case the pdf has been divided into 3 intervals either side of the mean with areas of 0.422, 0.076 and 0.0023, corresponding to median acceleration values of µ ± 0.56σ, µ ± 1.75σ and µ ± 3.05σ. Clearly, the mean (µ) and standard deviation (σ) need to be evaluated so that the structural resistance pdf can be defined. In addition, it is desirable to express the structural resistance pdf in terms of an equivalent acceleration so that the loading and resistance pdf's are compatible, which in turn enables the probability of failure to be directly evaluated.

It was recommended in [1] that nominal design shear forces be enhanced by a dynamic shear magnification factor Ω vd = 1.5 over most of the chimney height to account for inelastic effects. The ratio of the failure acceleration to elastic acceleration b = af / ae (assuming shear failure occurs when the inelastic shear forces exceed 1.5 times the nominal design forces) resulting from some 35 inelastic analyses [2] and, not including the effects of overstrength, had a mean ratio of b = 4.2 and minimum and maximum values of 2.4 and 6.0 respectively. This is equivalent to a standard deviation of σR = 0.6 (or CV R = 0.15) assuming a normal distribution with the minimum and maximum spaced ± 3σ either side of the mean. The mean resistance R and standard deviation σR can therefore as follows: φR = 4be .2expressed a e ⋅ φ0

1998 CICIND Code The CICIND code recommends the chimney be designed to resist a nominal load of 1.4 times the 475 year return period design earthquake event (i.e. ae = 1.4a475). The resulting resistance, Ruφ is assumed to be normally distributed and defined as follows: R u φ = φ ⋅ R u = φ ⋅ R ⋅ (1 − 1 . 65 CV R ) = 1 . 4 a 475 (13a)

∴

The term Ru = R (1 - 1.65 CVR) is defined as the ultimate or characteristic resistance with 95% exceedance. The capacity reduction factor φ accounts for the uncertainties associated with the available resistance due to material variations and modelling assumptions. Equation 13a can be re-arranged to enable the mean resistance to be evaluated:

PDFi

0 0.1186 0.5659 0.8385 0.9421 0.9787 0.9918 0.9968 0.9987 0.9994

Figure 4 - Discretised probability distribution for the high seismic region

The resulting cpf values were then averaged for each of the 9 discrete intervals and the associated acceleration values aj back calculated using equation 8b for each interval. These acceleration values aj were then assumed to have a discrete probability of occurrence, Pj , equal to the area under the pdf curve in each interval, j. This methodology was put into a spreadsheet program and the discretised probability distribution values calculated for each of the four seismic regions. An example of the spreadsheet calculation for the high seismic region is presented in Figure 4 (note the acceleration coefficient ai has been represented by the parameter y i).

(a)

CPFi 0 0.153 0.246 0.339 0.433 0.526 0.620 0.713 0.807 0.900

0.9994

for i = 1 for i

Yi(g) 1 2 3 4 5 6 7 8 9 10

R = 4 .2 a e ⋅ φ0 / φ

(14a, 14b)

where φ = 0.75 = Capacity reduction factor for shear forces φ0 = 1.4 = Moment overstrength factor Hence R = 7.8 ae and σR = 1.2 ae 18

(14c)

CICIND REPORT

Vol. 17, No. 2, September 2001

(ii) Flexural Capacity

It should be noted that a fair degree of uncertainty is associated with the results since the analyses focus on the extremes or "tails" of the load and resistance pdf's. However, the analyses provide a useful basis for comparing the relative merits of the different design approaches.

The ratio of the failure acceleration to elastic acceleration b = af /ae (assuming flexural failure occurs when the inelastic curvature demand at a plastic hinge exceeds the curvature capacity) resulting from some 40 inelastic analyses and ignoring the effects of overstrength had a mean ratio of b = 7.7 and a minimum and maximum value of 4 and 14 respectively. This is equivalent to a standard deviation σR = 1.23 (or CVR = 0.15) assuming the minimum value is located 3σ from the mean.

The proposed limited ductile design (LDD) approach appears reasonable with the probability of failure less than the desirable level of pf = 1.0E-4 over a 50 year life for all seismic regions. Further the application of IF = 1.4 reduces the p f by more than an order of magnitude. Clearly the analyses suggest that the chimneys designed for LDD have a negligible probability of failure, and indicate that the selection of R = 2 whilst appropriate for the SLS is conservative at the SLSS.

The mean resistance R and standard deviation σR can therefore be expressed as follows: R = 7.7 ae . φ0

The proposed elastic design approach for special chimneys is equivalent to the 1998 CICIND code and results in much higher failure probabilities with values ranging from 20E-4 to 170E-4 (i.e. 0.2% to 1.7% exceedance over 50 years). These p f values increase further for ordinary chimneys (IF = 1.2, R=1) with values ranging from pf = 60E-4 to 250E-4. These indicative results highlight the inherent dangers of designing structures elastically with no consideration for ductility. The results also suggest that designs based on the 475 year return period event are associated with higher probabilities of failure in the lower seismic regions. In reality, these p f values would generally be lower since wind loads typically dominate the design of chimneys in low seismic regions providing some additional overstrength.

(15a)

where φ0 =

1.4 = Moment overstrength factor

hence R = 10.8 ae

and

(b2) Proposed CICIND Elastic Design (ED)

σR = 1.6 ae

Code

2001

(15b)

Revision

-

Ordinary chimneys designed without consideration to seismic detailing are assigned an importance factor of IF = 1.2 and a structural response factor of R = 1 (elastic design) resulting in a design load of ae = 1.2a475. This design load is increased for special chimneys (IF = 1.4) to ae = 1.4a475.

A separate study was undertaken to investigate the likely probabilities of failure associated with the 1997 NEHRP provisions [10]. This document recommends that new and retrofitted buildings be designed for a level of ground shaking equal to 0.67 times the 2475 year return period event (i.e. 0.67a2475) instead of the 475 year event. Design seismic loads are then obtained by reducing this nominal load by appropriate structural response factors (R factors) to account for the ductility and overstrength of the structural system. The commentary to the NEHRP document notes that buildings are expected to be severely damaged, but not collapse at an event equivalent to 0.67a2475. The commentary also notes that buildings typically have a margin against collapse of at least 1.5, suggesting that structures should be at the point of collapse for a 2475 year earthquake event (a2475).

The pdf associated with the structural resistance for the chimneys designed without seismic detailing in similar to that discussed for chimneys designed to the 1998 CICIND code (assuming φ = 0.85 and CVR = 0.10): R = ae / [φ . (1 – 1.65CVR)] = 1.4ae and

σR = 0.14ae (16a)

hence for: IF = 1.2, R = 1.7a475 and σR = 0.17a475 (16b) and for: IF = 1.4, R = 1.96a475 and σR = 0.20a475 (16c)

The resulting probability of failure associated with an exposure period of 50 years and a coefficient of variation CVR = 0.15 was calculated to be in the order of 190E-4 to 200E-4 (i.e. consistent with 2% exceedance in 50 years). Whilst there is a degree of uncertainty associated with this simplified analysis, the results suggest that the acceptable failure probabilities for ordinary buildings subject to severe seismic activity are significantly larger than the p f = 1.0E-4 typically recommended for other structural loads such as dead, live and wind loads. The study also indicated that the adoption of a 2475 year rather than a 475 year return period event to define the notional seismic loads results in probabilities of failure which are fairly uniform for all seismic regions

4.4 Probability of Failure The probability of failure was calculated using equation 7 and the loading and resistance pdf's described in Sections 4.2 and 4.3 respectively. The process was semi-automated using a spreadsheet program. Sample calculations and plots of the pdf's are presented in Figures 5 and 6 for the case of an ordinary chimney located in a high seismic region and designed in accordance with the proposed LDD provisions (IF = 1.0, R = 2) and the 1998 CICIND recommendations (IF = 1.4, R = 1) respectively. The probability of failure associated with the LDD design was in the order of p f = 0.02E-4 and hence negligible

5

[ae = 0.38g/2 = 0.19g; R = 7.8ae = 1.48g; σR = 0.15 R = 0.23g].

CONCLUSIONS

1. Recommendations have been developed for the elastic design (ED, R=1) and limited ductile design (LDD, R=2) of both ordinary (IF = 1.0, 1.2) and special (IF = 1.4) chimney structures to satisfy the serviceability limit state (SLS) and structural stability limit state (SSLS). The probability of exceedance over a 50 year life for ordinary and special chimneys are in the order of 50% and 25% for the SLS and 2% and 1% for the SSLS.

In contrast, the failure probability increased to pf = 21E-4 for the CICIND98 design [ae =1.4 x 0.38g = 0.53g; R = 1.4 ae = 0.74g; σR = 0.10 R = 0.07g]. A summary of the failure probabilities associated with the three different approaches and the four seismic regions is summarised in Table 5. 19

CICIND REPORT

Vol. 17, No. 2, September 2001

EARTHQUAKE DEMAND High seismicity acceleration y475=0.38g n 50 * WEIBULL DISTRIBUTION f1 12.3 * f2 0.72 * MEAN 0.246 * CV 0.38 * S.D 0.093 i Yi(g) CPFi PDFi Pj Yj(g) Delta Pj 1 0 0 0 0.059 0.135 0.1186 2 0.153 0.1186 3.87 0.342 0.199 0.4473 3 0.246 0.5659 4.25 0.702 0.283 0.2726 4 0.339 0.8385 1.77 0.890 0.375 0.1036 5 0.433 0.9421 0.63 0.960 0.468 0.0366 6 0.526 0.9787 0.22 0.985 0.562 0.0132 7 0.620 0.9918 0.08 0.994 0.656 0.0049 8 0.713 0.9968 0.03 0.998 0.750 0.0019 9 0.807 0.9987 0.01 0.999 0.843 0.0008 10 0.900 0.9994 0.01 0.9994 RESISTANCE Ae 0.19 * NORMAL DISTRIBUTION b 7.8 * b*Ae 1.482 CV 0.15 * S.D. 0.2223 i Ai Delta Pi 1 0.804 0.0023 2 1.093 0.076 3 1.358 0.422 4 1.606 0.422 5 1.871 0.076 6 2.160 0.0023 1.0006 i 1 2 3 Ai 0.804 1.093 1.358 j Yj Delta Pj Delta Pi 0.0023 0.076 0.422 1 0.135 0.1186 0 0 0 2 0.199 0.4473 0 0 0 3 0.283 0.2726 0 0 0 4 0.375 0.1036 0 0 0 5 0.468 0.0366 0 0 0 6 0.562 0.0132 0 0 0 7 0.656 0.0049 0 0 0 8 0.750 0.0019 0 0 0 9 0.843 0.0008 1.76E-06 0 0 1.76E-06 0 0

4 1.606 0.422 0 0 0 0 0 0 0 0 0 0

5 1.871 0.076 0 0 0 0 0 0 0 0 0 0

6 2.160 0.0023 0 0 0 0 0 0 0 0 0 0

0.000002

5

PDF (%)

4

3 Load Resistance

2

1

0 0

0.5

1

1.5

2

2.5

3

Accel (g)

Figure 5 - Failure probability calculation and plot of probability design functions for LDD chimney located in a high seismic region

20

CICIND REPORT

Vol. 17, No. 2, September 2001

EARTHQUAKE DEMAND High seismicity acceleration y475=0.38g n 50 * WEIBULL DISTRIBUTION f1 12.3 * f2 0.72 * MEAN 0.246 * CV 0.38 * S.D 0.093 i Yi(g) CPFi PDFi Pj Yj(g) Delta Pj 1 0 0 0 0.059 0.135 0.1186 2 0.153 0.1186 3.87 0.342 0.199 0.4473 3 0.246 0.5659 4.25 0.702 0.283 0.2726 4 0.339 0.8385 1.77 0.890 0.375 0.1036 5 0.433 0.9421 0.63 0.960 0.468 0.0366 6 0.526 0.9787 0.22 0.985 0.562 0.0132 7 0.620 0.9918 0.08 0.994 0.656 0.0049 8 0.713 0.9968 0.03 0.998 0.750 0.0019 9 0.807 0.9987 0.01 0.999 0.843 0.0008 10 0.900 0.9994 0.01 0.9994 RESISTANCE Ae 0.53 * NORMAL DISTRIBUTION b 1.4 * b*Ae 0.742 CV 0.10 * S.D. 0.0742 i Ai Delta Pi 1 0.516 0.0023 2 0.612 0.076 3 0.700 0.422 4 0.784 0.422 5 0.872 0.076 6 0.968 0.0023 1.0006 i 1 2 3 4 Ai 0.516 0.612 0.700 0.784 j Yj Delta Pj Delta Pi 0.0023 0.076 0.422 0.422 1 0.135 0.1186 0 0 0 0 2 0.199 0.4473 0 0 0 0 3 0.283 0.2726 0 0 0 0 4 0.375 0.1036 0 0 0 0 5 0.468 0.0366 0 0 0 0 6 0.562 0.0132 3.03E-05 0 0 0 7 0.656 0.0049 1.13E-05 0.000374 0 0 8 0.750 0.0019 4.39E-06 0.000145 0.000806 0 9 0.843 0.0008 1.76E-06 5.82E-05 0.000323 0.000323 4.78E-05 0.000577 0.001129 0.000323

5 0.872 0.076 0 0 0 0 0 0 0 0 0 0

6 0.968 0.0023 0 0 0 0 0 0 0 0 0 0

0.002077

6 5 Load

PDF (%)

4

Resistance

3 2

1

0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

Accel (g)

Figure 6 - Failure probability calculation and plot of probability design functions for CICIND designed chimney located in a high seismic region

21

CICIND REPORT

Vol. 17, No. 2, September 2001

2. The LDD approach is strongly recommended for the design of tall chimney structures. This method allows a 50% reduction in earthquake loads (R=2) to account for ductility effects, provided some basic design guidelines are followed. In contrast, the ED approach which assumes R=1 and does not allow a reduction in seismic loads, results in a chimney that may be brittle. The ED approach is consistent with the method recommended in the 1998 CICIND code. The R factors are applied to the elastic seismic forces corresponding to the 1 in 475 year event.

codes of practice generally accept higher values in the order of p f = 100 - 200E-4 (i.e. 1% - 2% exceedance in 50 years). 7. The probability of failure values calculated using numerical integration techniques for the 1998 CICIND code were in the order of pf = 20-170E-4, which were consistent with the results of the ED method for a special chimney (ie. IF=1.4 and R=1). Applying the ED method to an ordinary chimney (IF=1.2, R=1) slightly increased the failure probability to p f = 60-250 E-4.

3. The earthquake recurrence rates implied in the 1978 ATC3-06 and 1997 NEHRP seismic hazard provisions are generally consistent except in some moderate seismicity regions that are dominated by large and infrequent earthquakes. In such cases the ATC approach underestimates the ground accelerations associated with the 2475 year event compared with the NEHRP provisions. However significant uncertainties and conjecture are associated with these NEHRP provisions. In addition, independent seismic studies undertaken in Australia and New Zealand were in good agreement with the recurrence relationship implied in the ATC-3-06 document.

8. In contrast, the probability of failure associated with the LDD method (R=2) for both ordinary (IF=1.0) and special (IF=1.4) chimneys is extremely low with pf ≤ 1.0E-4, indicating the significant advantages of designing structures to possess at least limited ductility.

6.

REFERENCES

1. Wilson, JL, 2000 "Code recommendations for the aseismic design of tall reinforced concrete chimneys", CICIND Report, Vol. 16 No.2, Sept. pp 8-12. 2. Wilson, JL, 1999 "Earthquake design and analysis of tall reinforced concrete chimneys", CICIND Report, Vol. 15 No. 2, Sept. pp 16-26.

4. The recurrence rate of peak ground accelerations implied in the ATC-3-06 provisions can be described by a Weibull distribution with the application of different constants for different seismic regions.

3. International conference of building officials, 1997, "Uniform Building Code, Chapter 23: Earthquake Design", ICBO, California, USA.

5. A probabilistic approach was used to calculate the probability of failure, pf, associated with chimneys designed for a 50 year life using the ED method (IF=1.2/1.4 and R=1) and the LDD method (IF=1.0/1.4 and R=2) in regions of seismicity ranging from very low to high. The seismic loading (ATC-3-06 provisions) and structural resistance were represented by Weibull and Normal distributions respectively and the mean and standard deviations for each pdf were evaluated.

4. FEMA 273/274, 1998, "NEHRP Guidelines for the seismic rehabilitation of buildings", Report No's. FEMA 273 (Guidelines), FEMA 274 (Commentary), Federal Emergency Management Agency, Washington DC, USA. 5. Paulay, T, Priestley, MJN, 1992, "Seismic design of reinforced concrete and masonry buildings", John Wiley and Sons Inc.

6. Acceptable probability of failure values for the design of structures to resist dead, live and wind loads are typically in the order of pf = 1.0E-4, whilst for earthquake actions

6. ATC 3.06, 1978, "Tentative provisions for the development of seismic regulations for buildings", Applied Technology Council, USA.

Seismicity

Parameter Very Low

Low

Moderate

High

a475

0.057g

0.10g

0.19g

0.38g

(a)

Current CICIND code

150E-4

170E-4

80E-4

20E-4

(b)

LDD method (R=2) IF = 1.0

0.2E-4

0.2E-4

0.1E-4

0.02E-4

IF = 1.4