NONLINEAR STATIC PUSHOVER PROCEDURE OF ATC-40-A PARAMETRIC STUDY FOR AN ELASTO-PLASTIC SYSTEM

Abstract

The goals of earthquake resistant design are: i) No damage for frequent minor earthquakes ii) Little damage for infrequent moderate earthquakes and iii) Life safety for rare major earthquakes However, building codes usually specify only one level of seismic loads. These loads are a very small fraction of seismic loads that are expected to occur for elastic behaviour in case of a major earthquake. Codes imply use of linear elastic analysis for these loads. Adequate performance in major earthquake is expected for a structure that behaves in a ductile manner. However, there are no checks whether these goals are indeed achieved. These goals are partly achieved by using the Capacity Design Method in which inelastic ductile behaviour is allowed at specific locations in the structural members while other parts of the structure still remain elastic. Performance Based Engineering (PBE) in association with existing concepts of earthquake resistant design requires nonlinear analysis to obtain estimates of deformations for damage assessment for different levels of earthquakes. In the performance based procedure, the desired levels of seismic performance for a building for specified levels of earthquake ground motions are specified. The performance is checked in terms of post elastic deformations. ATC-40 gives the Capacity Spectrum Method for implementing PBE for buildings. It uses Nonlinear Static Pushover (NSP) analysis to develop the capacity curve (a plot of base shear versus roof displacement). 111 In this dissertation, a three-storey building located in seismic zone IV is taken. The capacity curve is taken as elasto-perfectly plastic instead of obtaining the capacity curve by nonlinear analysis. Effective equivalent viscous damping due to the hysteretic behaviour of the building has been taken into account, to reduce the elastic demands. The conditions where, the demand becomes equal to the capacity have been found by plotting capacity spectrum and demand spectra. The points of intersection of capacity spectrum and demand spectra are called Performance Points, which are measures of displacement demands. If the displacement demands for all the EQ levels are less than the permissible values for the desired performance objectives for the corresponding EQ levels, then the design is acceptable, otherwise it needs revision. These performance points have been computed for three levels of Earthquakes for different spectral yield levels for a building having initial time period of 0.3 seconds. Two global measures of earthquake damage — displacement ductility and drift index — for the building have been obtained at the performance points. These values are compared with permissible values. From the results, it is seen that for this building the design is governed by displacement ductility values and not by drift. This is as expected for such a short period (0.3 sec) 3-storey building. Drifts are expected to control the design of long period multi-storey buildings. For this building, the performance points are determined for different levels of earthquakes. iv