EFFECT OF URM INFILLS ON SEISMIC PERFORMANCE OF RC FRAME BUILDINGS

Abstract

Unreinforced masonry (URM) infilled RC frame buildings are common structures in India. Masonry infill is used in buildings for its good sound and heat insulation property. The analytical modeling of infilled frame is a complex issue because these structures exhibit highly nonlinear inelastic behavior resulting from the interaction of the masonry infill panel and the surrounding frame. Due to this frame —infill interaction, the behavior of masonry infilled RC frame buildings under lateral load gets changed than that of bare RC frame building. The common practice of ignoring infills and designing the buildings as bare frame is not always conservative. The modeling approaches for masonry infills can be grouped into Micro models and Macro models. Micro models capture the behavior of infill and its interaction with the frames in much detail, but these models are computationally expensive. On the other hand, the Macro model try to capture the gross behavior of the infill, are approximate but computationally efficient. In this study, infill panels have been modeled by strut elements with axial hinges. The required force-deformation properties for hinges have been derived from the drift limit states given in FEMA 356. Chord rotation model with some modifications has been used to simulate the frame elements. Two stiff zones have introduced at the ends of the element to represent the joints. All the inelasticity has been concentrated at the faces of the joint and element by the introduction of hinges. The force-deformation properties of hinges have been adopted from FEMA 356. 4 storey, 8 storey and 16 storey buildings, having identical plan have been considered in this study. Linear analysis and design of each bare frame building has been performed as per the provisions of IS: 1893-2002 with and without the correction factor mentioned in the clause 7.8.2 of IS: 1893-2002. 150mm and 230mm thickness of masonry infill has been considered. Non-Linear Staged Construction analysis has been performed to avoid the effect of gravity loads in infills. The effect of infills on dynamic characteristics, yield patterns and seismic performance has been studied with the help of Non-Linear Push-Over Analysis. The behavior of buildings on stilts has also been studied. Adequacy of codal provisions for the design of the stilt buildings has also been examined.