Comparative Analysis of Response of Buildings with Setbacks to Seismic Loading

Abstract

During earthquake, failure of the structure starts at the point of weakness. Weakness in any structure arises due to the discontinuity in mass, stiffness and geometry of structure. These type of structures are called irregular structures and can be seen in most of the urban infrastructure. One such type of irregularity is setbacks or vertical geometric irregularity. In the present work, six frames and structures with setbacks at different floors are compared with the regular frame and structure respectively, keeping the dimensions of 2-D and 3-D frames constant throughout the work. The comparison of analysis results of 2-D and 3-D frames was done by two different methods. Static analysis and Response Spectrum Analysis of all the frames was done and various seismic responses like bending moment, shear force and axial force for beams at setbacks were calculated. In Static Analysis it was found that for most of the cases the values of shear force and axial force in beams and columns at setbacks were coming more in irregular frame then in regular frames, whereas Response Spectrum Analysis suggests that the value of axial force in all beams and columns are increasing drastically as we move from regular to irregular structure where all the other forces i.e. shear force and bending moment decreasing as we move from regular to irregular structure. Comparison of both the methods was also done and it was found that the static analysis gave fluctuating results, whereas the response spectrum gave more reliable result. It was interesting to note that, however the mass of the building is reducing but the forces at setbacks increases to a great extent. The analysis suggest that the setbacks introduced in a building can be proven disastrous at the time of earthquake, hence it is not recommended to introduce setbacks in a building.