Seismic analysis of low rise and high rise building and its foundation design

Abstract

The design of foundations for tall structures is an interesting and challenging task. In structural design usually, the effect of foundation soil is generally overlooked. In this study, 3D analysis of the superstructure is carried out considering the self-weight, imposed load and earthquake load. The provisions of (BIS Code of practice for criteria for earthquake resistant design of structures: Part-1, general provisions and buildings, IS 1893, Bureau of Indian Standards, New Delhi, 2016a) are used to carry out the dynamic analyses. Three-dimensional analysis of the building system is used to design and analyse the superstructure in which the structural performance parameters viz., base shear, lateral load distribution, natural time period, storey drift ratio, beam-column capacity (BCC) and percentage reinforcement were evaluated for low-rise building (G + 4) and high-rise building (G + 15) located in seismic zone- II & III. The support reactions of the superstructure were exported and used to design the foundation. SAFE software was used to design the raft foundation for low-rise buildings (G + 4) whereas the piled raft was designed for high-rise buildings (G + 15). The parametric study was carried out considering different values of soil bearing capacity as 165, 200 and 250 kN/m² and the thickness of the raft was varied from 0.3 to 1 m in the case of low-rise building. For high-rise buildings, the diameter of the pile was kept constant and the length of the pile was varied to arrive at a suitable dimension. Parameters like settlement, soil pressure and punching shear failure were compared and discussed. It was found that the column size of G + 15 building was increased when compared to G + 4 building. The time period and base shear of the building evaluated by the equivalent static method and response spectrum method were found to be in good agreement. The storey drift ratio and beam-column capacity were found to be within the prescribed limits in relevant IS codes. For G + 4 building, located in SZ II and III a minimum raft thickness of 1000 mm is recommended for soil with a bearing capacity of 165 kN/m² while for soil with a bearing capacity of 200 and 250 kN/m² the minimum thickness of the raft is 500 mm. For G + 15 building, located in SZ II and III even a raft with a thickness of 2.0 m was not found sufficient and hence piled raft was used with a raft thickness of 1.75 m and a pile length of 10 m for all bearing capacity of soil considered.