Infilled Frames

SEMINAR REPORT ON

STUDY ON INFILLED FRAMES SUBMITTED TO VIVESWARAIAH TECHNOLOGICAL UNIVERSITY BELGAUM FOR THE PARTIAL FULFILLMENT OF M-TECH (STRUCTURAL ENGINEERING)

BY THIPPESWAMY.N Reg. No: 1st Semester M-Tech Structures

Under The Guidance of: Prof. R. RUDRAPRASAD Department of Civil Engineering

BANGALORE INSTITUTE OF TECHNOLOGY (Affiliated To Visveswaraiah Technological University) Bangalore-560004

BANGALORE INSTITUTE OF TECHNOLOGY BANGALORE -560004

CERTIFICATE This is to certify that Mr. THIPPESWAMY.N bearing university USN

has

submitted the report on “STUDY ON INFILLED FRAMES” in partial fulfillment of the 1st semester M-Tech course in structural engineering as prescribed by the Visveswaraiah Technological University during the academic year 2006-2007, under the guidance of Prof. R. RUDRAPRASAD

Prof. K.JAYRAM H.O.D Dept. of Civil Engg.

Prof. R. RUDRAPRASAD Professor Dept. of Civil Engg.

ACKNOWLEDGEMENT

I express my deep sense of gratitude to Prof. R. RUDRAPRASAD professor, Department of Civil Engineering, BIT, for his guidance and help through out this project work. I will remain thankful to the head of department, PROF. K. JAYRAM and all the faculty members of Department of Civil Engineering, BIT for their support during the course of this work. Finally I express gratitude to my parents, fellow students and friends.

THIPPESWAMY.N M-TECH STRUCTU BANGALORE INSTITUTE OF TECHNOLOGY

LITERATURE REVIEW ON INFILLED FRAMES INTRODUCTION The effect of masonry infilled walls in changing the stiffness, ultimate capacity and failure mode of framed structures has been one of the most interesting research topics in the last five decades. The first report on the contribution of masonry infill in resisting lateral loads came after the completion of the Empire State Building in New York. As reported by Rathbun (1938); during a storm with a wind gust exceeding 90 mph, diagonal cracks appeared in a number of

masonry infill partitions on the twenty-ninth and forty-first floors. Separation cracks between the frame and the masonry walls were also noted. Incidentally, strain guages attached to the steel frame did not register any strains prior to cracking of the masonry despite the presence of strong wind. This was explained by the high rigidity of the masonry infill wall, which

prevented distortion of the steel frame. When the walls were stressed beyond their cracking capacity, there was a marked decrease in the stiffnesses of the infills. Consequently, the strain guages began to register strains indicating that the steel frame had begun to participate in resisting the wind load. Even though it was cracked, the masonry infills confined within the steel frames continued to offer strong lateral load resistance. Ever since this incident and up-to-date, the behavior of infilled frames has been the subject of investigations conducted by researchers throughout the world. Different approaches had been adopted starting from simple strength of materials approach, passing through trials to match experimental results using simple models. Methods based on the theory of elasticity, equilibrium and energy approach, plastic analysis and finally finite element (FE) analysis were also used.

PREVIOUS FRAMES

RESEARCH

ON

MASONRY

INFILLED

The first published research on infilled RC frames subjected to racking load was by Polyakov (1956). This publication reported a test program carried out from 1948 to 1953. In order to determine the racking strength of infilled frames, Polyakov performed a number of large-scale tests including square as well as rectangular frames. Parameters investigated included the effects of the type of masonry units, mortar mixes, admixtures, methods of load application (monotonic or cyclic), and the effect of openings. Polyakov described the history of infilled frame behavior subjected to racking load. First, the masonry infill and the members of the structural frame behave monolithically until separation cracks between the infill and the frame develop around the perimeter of the infill-to-frame interface except for small regions at the two diagonally opposite corners. Secondly, the compression diagonal starts to shorten and the tension diagonal to lengthen until the masonry infill cracks along the compression diagonal in a step-wise manner through mortar head and bed joints. The structural assemblage continues to resist an increasing load in spite of the diagonal cracks. The system is considered to have reached failure after the appearance of large cracks. In a subsequent paper, Polyakov (1960) described experiments performed on a three-bay, three-story model steel frame infilled with masonry. Based on observation of the infill boundary separation, he suggested that the infilled frame system is equivalent to a braced frame with a compression diagonal strut replacing the infill wall. In the same period, experimental work was conducted by Thomas (1953) and Wood (1958) in the United Kingdom and test results provided ample testimony that a relatively weak infill can contribute significantly to the stiffness and strength of frame.

Holmes (1961) proposed a method for predicting the deformations and strength of infilled frames based on the equivalent diagonal strut concept. He assumed that the infill wall acts as a diagonal compression strut, as shown in Fig. 2.1, of the same thickness and elastic modulus as the infill with a width equal to one-third the diagonal length. He also concluded that, at the infill failure, the lateral deflection of the infilled frame is small compared to the deflection of the corresponding bare frame. Also, the frame members remained elastic up to the failure load. By equating the elastic deformation of the frame diagonal to the shortening of the equivalent diagonal strut at failure Stafford Smith (1962) conducted a series of tests on laterally loaded square mild steel frame models infilled with micro-concrete. Monitoring the model deformations during the tests showed that the frame separated from the infill over three quarters of the length of each frame member. These observations led to the conclusion that, the wall could be replaced by an equivalent diagonal strut connecting the loaded corners. The loaddeformation relation recorded showed a high increase in stiffness of the infilled frame compared to the bare frame. Another series of tests were conducted on unframed mortar walls loaded diagonally and measuring the strains along the loaded diagonal. In order to find a theoretical method to predict the experimental results, a stress function was solved for a number of nodes on the wall using the finite element method and the theoretical results were in good agreement with the experimental observations. The theoretical results were translated into what was termed an effective width of the wall, which is the width of an equally stiff uniform strut whose length is equal to the diagonal of the wall and whose thickness is the same as the wall. It was determined that the effective width of the equivalent strut was dependent only on the wall’s aspect ratio. Further tests revealed that the above assumption, which was made based on loading unframed walls, is invalid. The effective width of infill was found to depend on the length of contact between the infill and the

frame, which itself was found to be highly dependent on the relative stiffness between the frame and the infill.

Stafford Smith found the theoretical effective width to be consistently less than the experimentally measured values. He attributed this discrepancy to higher strain due to stress concentration and nonlinear load - deformation behavior of the mortar infill at the loaded corner. In view of this finding, he recommended use of experimental curves to estimate the effective width. Mallick and Severn (1967) introduced an iterative technique whereby the points of separation between the frame and the infill, as well as the stress distribution along the length of contact between the frame and the infill, were obtained as an integral part of the solution. Slip between the frame and the infill was also taken into account. Liauw and Kwan (1983) developed a plastic theory of non- integral (without shear connectors) infilled frames in which the stress redistribution towards collapse was taken into account and the friction is neglected for strength reserve. Liauw and Lo (1988) conducted a series of tests on a number of small scale models of micro concrete infilled steel frames. The frame members were hot-rolled mild steel solid rectangular bars. FE analysis was used to model the test specimens, Paulay and Priestley (1992) suggested treating the infill walls as diagonal bracing members connected by pins to the frame members. They also suggested to calculate the stiffness of the structure and hence its natural period based on considering the effective strut width to be one quarter the wall diagonal. Saneinejad and Hobbs (1995) proposed a method of analyzing masonry infilled steel frames subjected to in-plane loading. The method utilized the data generated from previous experiments as well as the results of a series of non-linear FE analyses. The proposed method accounts for both the elastic and the plastic behavior of infilled frames and predicts the strength and stiffness of the infilled frames. The method also accounted for various parameters like different wall aspect ratios

and different beam-to-column stiffness and strength. The method was based on using equilibrium and elasticity equations to generate various parameters governing the behavior of the infilled frame system like the contact stresses and lengths along with the initial stiffness of the infilled Frame. The authors suggested that the resistance to lateral loads was offered by three components. These components are: The force induced due to shear stresses on the beam-wall interface, The force generated by the normal stresses on the column-wall interface and finally the force developed in the steel frame itself as a result of its own stiffness to horizontal loads. Having derived the ultimate load, the area of the diagonal strut was easily derived. Madan et al. (1997) further extended the work of Saneinejad and Hobbs (1995) by including a smooth hysteretic model for the equivalent diagonal strut. The hysteresis model uses degrading control parameters for stiffness and strength degradation and slip Mosalam et al. (1997-a,b,c) reported the results of a series of tests on single-bay single-story and two-bay single-story concrete blocks masonry infilled steel frames tested under quasi-static loading and twobay two-story frames tested under pseudo-dynamic loading. All the specimens were one- forth scale gravity loads designed frames with a semi-rigid connection between the frame members. Along with the variations in the number of bays and/ or stories, the relative strength of the concrete block and mortar and the effect of openings were also considered. Flanagan et al. (1999) reported the results of a number of full scale clay infilled steel frames tested under in-plane loading. A piecewise linear equivalent diagonal strut was used to model the infill. The behavior of the structural clay tile infills was correlated with the absolute story drift rather than the nondimensional story drift. Kappos et al. (2000) conducted an analytical study on the seismic performance of masonry infilled RC framed structures. It was found that

taking the infill into account in the analysis resulted in an increase in stiffness as much as 440%. It is clear that, conditional upon the spectral characteristics of the design earthquake, the dynamic behavior of the two systems (bare vs. infilled frame) can be dramatically different.

Fig.1: - Bare, Infilled and Partially infilled frames

Fig. 2: - A General View of Reinforced hollow concrete block Masonry

A

ECC

Shear connector

AIM AND SCOPE

A

A-A

From literature review it is observed that majority of the work has been carried out on infilled frames made of steel frames with concrete wall panels or reinforced concrete frames with clay brick masonry panels. Hollow concrete block work, which is extensively used in present day construction can be provided with reinforcement in the hollow spaces and tied to the surrounding frame. Grouting with lean mix concrete can provide effective bond and also the stiffness and strength of infill can be enhanced (figure 2). Separation and slip is minimised to a large extent by reinforcing the infill. Such a masonry infill is expected to have better out of plane strength and stiffness and energy absorbing capacity during earthquakes.

SUMMARY AND CONCLUSIONS This Topic summarizes the experimental and theoretical research work conducted in the area of infilled frames. It is now widely recognized that masonry infill walls used for cladding and/or partition in buildings, significantly alter their seismic response, and their effect in changing the stiffness, the ultimate lateral load capacity as well as the ductility supply of the building system should be accounted for in analysis and design. Due to the complexity of the contact problem, the sophisticated composite action of the frame and the infill, and the incomplete understanding of the infill role, as well as the numerous uncertainties involved in modeling the effect of infills; design aids such as manuals and software as well as related code provisions hardly include any detailed guidance to take into account the effect of the infills.