Study On Simplified Analysis Method Of Nonlinear Seismic Response Of Structures With Vertical Irregularity

As an important tool for the performance based seismic design philosophy, pushover analysis (PA) has been widely caught attention by the experts of earthquake engineering for its merits of simplicity and practicality. The state of the art pushover analysis is reviewed. The methods of calculating the target displacement of PA are summarized and three known methods are introduced in detail, which includes the Equal D isplacement Coefficient Method (EDCM), the Equivalent SDOF History Analysis Method (ESDOF) and the Capacity Spectrum Method (CapS). Several important issues about the selection of the lateral load pattern are discussed.An improved method of calculating target displacement (IMP) is proposed on the base of ESDOF, which is able to take into account the effects of the higher modes on the seismic response of structures. Comparing the results of IMP, ESDOF, EDCM and CapS with the results of nonlinear history analysis (NLHA) in four-story, eight-story and twelve-story regular RC frames demonstrates that the IMP method provides good estimates.Eight irregular frames, which include four stiffness-irregular frames and four strength-irregular frames, are designed by modifying stiffness and strength of the frames in three different locations along the height—first story, midheight story and lower half of structure, for four-storey and eight-storey frames respectively. Comparing the results of PA using five lateral load patterns with the results of NLHA, the optimal lateral load pattern for calculating the target displacement or for story drift of each kind of irregular RC frames are selected and it is indicated that the optimal lateral load patterns for calculating the target displacement and for the story drift are not always identical. Based on the comparison results, a PA procedure is proposed to be carried out in two steps, one for target displacement and the other for story drift, and different lateral load patterns are applied in the two steps.Comparing the target displacements determined by NLHA and pushover analysis with four methods of calculating the target displacement, an optimal method of calculating the target displacement are selected for each kind of vertically irregular RC frames in low buildings and high buildings. By comparing the effect of different methods of calculating the target displacement in vertically irregular RC structures, several conclusions are drawn, which can be used in selecting the calculation method of target displacement for PA of vertically irregular structures.