EFFECT OF URM INFILLS ON SEISMIC PERFORMANCE OF RC FRAME AND FRAME-SHEAR WALL BUILDINGS

Abstract

Unreinforced masonry (URM) infilled reinforced concrete (RC) frames are among the most commonly used structural systems, especially in developing countries such as India. Infills are known to affect the behavior and performance of the RC frames significantly. However, they are ignored during structural design and are considered as dead weight. This work studies the effect of infills on the performance and floor acceleration demands of RC frame and frame-shear wall buildings. Indian seismic codes (IS 1893 : Part 1, 2016; IS 13920, 2016) were updated in 2016. The major changes included the introduction of cracked stiffness modification factors and minimum column-to-beam flexural strength ratio. The first study in this thesis is a parametric study on the effect of infills on the collapse probabilities of RC frames designed for different design code levels. The parameters being varied include the quality, thickness, and aspect ratio of the infills, design code, and number of stories. A total of 33 frames divided into 8 sets are included. Each set includes a bare frame, a bare frame designed using the period of the corresponding infilled frame, a frame with an open ground story, a frame with an open ground story designed for 2.5 times the story seismic forces, and a fully infilled frame. The collapse probabilities are obtained through multiple stripe analysis (MSA) with eight stripes or return periods. Results indicate that the adopted design code has the most significant effect on the collapse performance, followed by the number of stories. The effect of adding infills on the collapse performance of bare frame is negligible to severely negative depending on multiple factors. Most tall buildings in India use a dual system consisting of RC frames and shear walls as their lateral load-resisting system. These RC frame-shear wall buildings are often infilled with URM infills as partition walls. A study is undertaken to estimate the effect of adding infills on the collapse performance of a 25-story RC frame-shear wall building. The bare frame, fully infilled, and open ground story versions of the building are considered. The performance is quantified through the probability of exceedance of collapse prevention (CP) limits for seven Engineering Demand Parameters (EDPs). The ratio of story shear carried by the moment frames to the total story shear is studied at five return periods (approximating five levels of nonlinearity) to comprehend the frame-shear wall interaction in different buildings at different levels of damage. The bare frame-shear wall building has higher probabilities of exceedance for most EDPs indicating that infills have a beneficial effect on the performance. The probabilities of exceedance of different EDPs are similar for the two infilled frame-shear wall buildings (open ground story and fully infilled) indicating that presence of an open ground story doesn’t affect the performance significantly. The effect of URM infills is also estimated on the floor acceleration demands in frame and frame-shear wall buildings. These demands are calculated at five return period to examine the effect of nonlinearity. The observed demands are also compared with the floor acceleration demands specified by major international codes. The shape of observed peak floor spectral acceleration/peak ground acceleration PFSA/PGA profiles is significantly different from that of the corresponding code-specified profiles. For all buildings, the peak floor acceleration/peak ground acceleration (PFA/PGA) profiles specified by IS 16700 (2016) and Eurocode 8 (1998) are significantly conservative. In contrast, the PFA/PGA profile specified by ASCE 7-22 (2022) is non-conservative except in the top few stories. The observed floor response spectra/ground response spectra (FRS/GRS) values decreased with an increase in return period or non-linearity. The FRS/GRS values were higher in the flexible direction of the building. The torsion due to asymmetric infills placement is often unnoticed as infills are usually ignored during the design. The effect of irregular placement of infills is studied in terms of induced torsion and its effect on seismic performance of the RC moment frame buildings. 8-story RC frame buildings are considered with different infills patterns, and designed for modern codes, old codes and only for gravity loads. The performance is compared in terms of probabilities of collapse at MCE, obtained from MSA conducted at five stripes. The adverse effects of presence of infills generally overshadow the torsion introduced due to asymmetric infill layout leading to consistently poor performance of buildings with more infilled frames. At higher return period, most of the building eccentricity is lost upon yielding of infills. The effect of asymmetric placement of infills is also studied on the floor acceleration demands in RC frame buildings. In addition to the infilled buildings having stiffness and mass eccentricities in one and both directions, a bare frame building, and a fully infilled building are also considered for comparison purposes. The floor acceleration demands are estimated at five return periods (representing different levels of nonlinearity). The eccentricity along one direction also led to a difference in acceleration demands at flexible corner (FC) and geometric center (GC) along the other principal direction. Interestingly, the difference in floor acceleration demands at FC and GC in the case of bi-eccentric building was relatively less than the difference in the case of uni-eccentric building.