This study investigates the effect of underground floor(s) on seismic analysis of buildings. Because the underground floor(s) is more rigid relative to the super structure, some codes have stated that in the seismic analysis of such buildings the base is defined as the top of underground floor(s). A parametric study was established using the response spectrum analysis method. Finite element simulations are conducted using ANSYS program to investigate the effect of underground floor(s). Results are presented for different buildings by varying the number of floors for the super structure (2, 5, 10, 15 and 20), the number of underground floor (1 and 3) and spring support stiffness.
The numerical results of the considered cases show that the seismic loads applied to a building considering underground floor(s) affect the lateral force distribution and member forces of the building. Thus, it is obligatory to include the underground floor(s) in the seismic analysis of the considered buildings.