The structural configurations of hillside buildings are significantly different than those observed on flat terrain. To study the effect of often observed structural configurations of hillside buildings, collapse fragility of regular "flat land (FL)" and irregular "split-foundation (SF)" and "step-back (SB)" buildings is studied using incremental dynamic analysis. The effects of building height, seismic zone, and near-and far-field sites on collapse fragility are investigated. It is observed that SF and SB hillside buildings exhibit significant torsional effects at the storey just above the uppermost foundation level. In case of FL buildings, collapse occurs due to the flexural failure of beams and columns. On the other hand, in SF and SB buildings, the collapse occurs due to the combined effects of shear failure of short columns and flexural failure of beams and columns in the storey just above the uppermost foundation level. In general, SF buildings are observed to be the most vulnerable, whereas FL buildings are the least vulnerable. It is observed that high-rise SF and SB buildings show unacceptably high probability of collapse at maximum considered earthquake, in Seismic Zone V and for the near-field site in Seismic Zone IV. KEYWORDS collapse probability, fragility analysis, hillside buildings, incremental dynamic analysis, splitfoundation building, step-back building 1 | INTRODUCTION During the past decades, the Indian subcontinent has witnessed rapid urbanization and a dramatic increase in population. Due to these factors, building development not only has been extensive in and around the metropolitan cities but also has been spreading out to the hilly regions of India resulting in the construction of a large number of multi-storey reinforced concrete (RC) frame buildings on steep slopes (hereafter referred to as "hillside buildings"). In some cases, building development in hilly regions is also taking place because of the scenic view and the pleasant climatic conditions during the summer season. In hilly regions characterized by very mild slopes, buildings similar to those existing in flat areas can be observed [1] and are referred to as "flat land (FL)" buildings, in this study. In case of mild to steep slopes, the structural configurations of hillside buildings are significantly different than for buildings constructed on FL, [1][2][3][4][5] in order to adapt to the slope's geometry. Foundation levels of hillside buildings generally follow the natural slope of the ground, known as "step-back (SB)" buildings. [1] In case of very steep slopes, hillside buildings with "split-foundation (SF)" can be frequently observed. [1] These hillside buildings have significant structural irregularities in plan and in elevation due to the presence of shorter columns on the uphill side and variations in mass, strength, and stiffness of successive storeys along the height, respectively. Both these features result in complex dynamic behaviour under earthquake excitation [1][2][3][4][5] increasing the structure's susceptibi...
