Effect of Irregular Placement of Infills on Seismic Performance and Fragility of URM Infilled RC Frame Buildings in India

Abstract

Reinforced Concrete (RC) framed buildings with Un-Reinforced Masonry (URM) infills are the most popular structural systems for multi-storey buildings in India and many other parts of the world. Infills are used as cladding at the exterior periphery walls and as partition in the interior of the building. Presence of regular solid infill between the frames contribute significantly in terms of lateral strength, stiffness, and energy dissipation capacity of the composite frame system, but reduces the fundamental time period, inelastic deformation capacity and thereby altering the failure modes as compared to its bare frame counterpart. These buildings have shown poor performance during past earthquakes and suffered severe damage or collapse, even under moderate earthquakes. Despite significant research effort dedicated to such buildings in the past decades, the understanding of seismic behavior of infilled frames is still not adequate and the uncertainty in infill-frame interaction results in complex modes of failure, rendering the simulation of seismic behavior of infilled frames a challenging task. The complex infill-frame interaction is intensified when functional openings in infills due to presence of doors and windows are introduced. Despite significant research efforts, there is still lack of consensus on role of size, shape, and combined effect of opening on seismic performance and consequent fragility of infilled RC frame buildings. Owing to continuing urbanization coupled with ever decreasing available space for construction, a sizeable number of the residential buildings in India are being used for mixed occupancy, where the upper storey(s) are used for residential purpose and the ground storey is used for combination of purposes including parking, small to medium commercial use etc. The complex behaviour of infilled frames with functional openings under lateral loading gets further complicated when infills are placed irregularly in plan and/or elevation to maximize the usage of available space. The consequences of poor performance of RC buildings with irregular URM infills observed in past earthquakes have stirred up the concern regarding in depth study of the inelastic behavior of such buildings for short term and long term mitigation policies. Accordingly, a statistical exercise is carried out in this Thesis to classify the existing URM infilled RC building stock and to develop Model Building Types (MBTs) with prevalent irregular configurations of URM infills and select a representative building plan, based on the analysis of 55 buildings selected from field surveys conducted in Indian cities. Based on the pilot surveys, URM infilled buildings are classified into 7 categories (WD, OGS, EPGS, EPGSIP1, EPGSIP2, EPGSIP3, POGS) depending on type of prevalent infill irregularity at ground storey which are further sub-divided based on the key parameters influencing seismic behaviour of such buildings i.e., framing system, design seismic force levels, detailing of reinforcement and height of buildings; and a total of 14 different MBTs are identified. This Thesis is an attempt to develop modeling guidelines for URM infills, with and without functional openings, and develop a reliable, cost-effective methodology for seismic performance assessment of practical RC buildings compliant to Indian standards and construction practices. Non-linear Static Pushover analyses are performed to study the explicit effect of realistic combination of different sizes of doors and windows on uniformly infilled RC buildings. It is observed that lateral strength, stiffness, and ductility of uniformly infilled RC building reduce significantly with increasing opening ratio. This Thesis work further highlights the combined effect of infill irregularity arising from functional and occupational requirement on the overall behaviour of infilled frame buildings under seismic excitation. Effect of functional openings in upper storey(s), due to presence of doors and windows, on seismic performance of one of the most common vertical irregular configurations of URM infills i.e., Open Ground Storey (OGS) building is studied. It is observed that the lateral stiffness and strength of mid-rise OGS buildings designed as per BIS (2002) provision is reduced to 55% and 65% respectively, when functional opening reached to 30%. Taking a note of the widespread failure of buildings having irregularly placed infills in plan, the seismic behavior of the three most common configurations of RC frames with irregular placement of infills viz. Open Ground Storey (OGS), and Partially Open Ground Storey (POGS), and External Periphery of the Ground Storey without any interior partition walls (EPGS) are studied. To study the seismic performance of buildings with asymmetric placement of infills in the ground storey, Incremental Dynamic Analyses (IDA) are carried out using bi-directional ground motions with a wide range of source and site parameters. It is observed that all the plan irregular infill configurations considered in the study causes premature failure due to damage concentration in the irregular floor. It is further observed that EPGS MBT designed with older Indian seismic standards (BIS 1993, 2002) are comparatively higher vulnerable than the POGS MBT. OGSW building which is not designed for OGS design provision of BIS (2002) shows the lowest capacity and experience the maximum inter storey drift ratio concentration at the first storey level. Fragility curves of the studied buildings are developed considering all the associated variabilities. Capacity of plan irregular infilled structure against “Collapse” damage state and variability in seismic demand are determined from the results of Incremental Dynamic Analysis. The comparative study of the fragility curves and Damage Probability Matrices (DPMs) suggest that functional openings in URM infills result in significant increase in seismic vulnerability of uniformly infilled RC frames. The vulnerability of infilled frames with functional openings is further increased due to irregular infill configurations in plan. The fragility parameters for Indian RC frame building with irregular configurations of URM infills derived in this Thesis are incorporated in the spreadsheet-based open-source seismic risk evaluation software tool ‘SeisVARA’ (Haldar et al. 2013).