SEISMIC VULNERABILITY OF IRREGULAR RC BUILDINGS

Abstract

Irregular reinforced concrete (RC) buildings constitute a significant portion of the existing housing stock. Modern seismic design codes converge on the basic design principles for regular buildings; however, the presence of irregularity in the building structural system is handled differently in different national codes. Most seismic codes restrict irregular buildings and provide design guidelines that are essentially applicable to only regular buildings, while some codes specify an increased demand and more stringent design requirements for irregular buildings, as compared to regular buildings. With this in view, the present study evaluates the provisions for some of the commonly found irregularities in RC buildings, given in the chosen modern seismic codes, viz., Indian Code (IS 1893 Part 1 2016), Eurocode 8 (EN 1998-1 2004), the US American Codes (ASCE 7-16 2017 and ASCE 7-22 2022), and New Zealand Loading Standard (NZS 1170.5 2004). During seismic shaking, columns are invariably subjected to bending in two orthogonal vertical planes, which leads to a complex interaction of axial force with the biaxial bending moments. However, the existing practice to estimate the seismic performance of a building is to carry out nonlinear time history analysis using two-dimensional models subjected to unidirectional excitations, even though the multiple components of ground motion can affect the seismic response significantly. In the present study, a fiber-hinge lumped plasticity model is duly calibrated with the biaxial cyclic experimental results to simulate the inelastic behavior of columns under bidirectional bending. The calibrated biaxial model of columns is then utilized to evaluate the effect of bidirectional excitation on the seismic performance of regular and symmetric RC moment frame buildings. The buildings are designed and detailed according to the Indian codes, which are at par with the other modern seismic codes. A comparison of the estimated seismic collapse capacity is presented, illustrating the importance of considering the bidirectional effects. The results from fragility analysis indicate that the failure probabilities of regular buildings at the maximum considered earthquake level (return period 2475 years) under the bidirectional excitation are significantly higher than those obtained under the unidirectional excitation. Indian seismic code (IS 1893 Part 1 2016) defines a building to have “lateral story irregularity in a principal plan direction” if the fundamental translational periods in the orthogonal directions are closer to each other by 10% of the larger of the two values. In the present study, the effect of different proportioning of lateral load-resisting system in the orthogonal directions on the seismic collapse capacity and fragility of RC buildings is investigated. Buildings with different structural systems and configurations are designed using the current Indian seismic design code. To simulate the structural behavior up to collapse, the hysteretic models of beams, columns, and shear walls are duly calibrated using experimental cyclic loading test results. Results show that if the buildings of a given height are designed for the same hazard level, these show equal levels of safety margin against collapse, irrespective of the proportioning of lateral load-resisting systems and hence the translational periods in the orthogonal directions. The asymmetric distribution of mass, stiffness, and/or strength in the plan of the building results in lateral-torsional coupling, where the building experiences both lateral displacement and floor rotation. In the present study, the seismic collapse performance of three-dimensional multistory buildings with varying degree of torsional irregularity and designed following the recommendations of each of the considered seismic codes are assessed using bidirectional incremental dynamic analysis (IDA). The effect of stiffness and strength eccentricities and torsional flexibility is studied on the seismic performance of the buildings. The collapse resistance of the buildings is evaluated in terms of collapse margin ratio and collapse fragility curves. The results are compared and discussed, emphasizing the adequacy and limitations of the design provisions and recommendations in different codes. It is seen that the design provisions for torsionally irregular and torsionally flexible buildings in most of the codes are inadequate in one respect or the other. There is a need to provide a complete set of guidelines considering the good practices of all the codes. A common type of vertical irregularity is in the form of discontinuity in the vertical framing elements, which can exacerbate their seismic vulnerability. The present study evaluates the vulnerability of RC frame buildings with discontinuity in columns (commonly known as ‘floating columns’) in terms of seismic collapse capacity, collapse resistance against maximum earthquake demand level, and failure mechanism. Analysis results show that the sequential analysis of buildings considering the construction stage effects considerably affects the design and hence the collapse failure mechanism of even low- and mid-rise buildings. The results also underline the importance of the ‘strong column-weak beam’ ratio in the seismic performance of the floating column buildings. The effect of the vertical component of ground motion is also observed to be relatively more crucial in floating column buildings. The floor acceleration demands for irregular RC buildings have also been studied for the seismic design of non-structural components (NSCs). It is observed that the horizontal peak floor acceleration (PFA) demand reduces with an increase in the inelasticity (or ductility demand) of the building. The floor response spectrum (FRS) is observed to be better correlated to the ground response spectrum (GRS) rather than the peak ground acceleration (PGA), as used in modern seismic design codes. In the case of torsionally irregular structural configurations, the lateral-torsional coupling of modes affects the floor acceleration evaluated at the stiff edge and the flexible edge at a given floor level. The RC buildings with discontinuity in columns are found to have a parasitic vertical component of floor response due to the horizontal vibrations of the building. It results in increased vertical floor acceleration demand on NSCs in floating column buildings. The parasitic contribution of horizontal components of ground motion to the vertical response of the building floors is seen to be dependent on the location of column discontinuity in the building.