Basic Study On Ductility-based Seismic Design Method Of Structures With Vertical Irregularities

Vertical irregularities layout of building structures are one of the major causes which lead tothe damage and failure of structures. The earthquake damage characteristics, seismic response andearthquake resistant design mothod for structures with vertical irregularities have been extensivelystudied by researchers at home and abroad. However, It is a very important and complex issue tostudy the seismic performance of structures with vertical irregularities. Although the studies haveachieved a great deal of progress, there are some new issues with the development of currentarchitectural form and social economy condition. In this paper synthetically considering the factors,such as the vertical irregularity layout of buildings, near-fault velocity pulse earthquake effect anddifferent lateral mode of structure, several issues are exploringly investigated which are a pressingobjective in seismic design philosophy. The main works are as follow.(1) The seismic ductility reduction factor (DRF) for the shear-type structures with verticalregularity layout has been studied. The DRFs for multi-degree-of-freedom (MDOF) systems arestudied by modifying the DRFs for equivalent single-degree-of-freedom (SDOF) systems. Based onthe MDOF lumped-mass shear-type models, nonlinear dynamic time history analysis are performedto investigate the influence of ductility demand increase owing to high mode effect on the DRFs.The results demonstrate that the DRFs for MDOF systems are clearly smaller than those for SDOFsystems due to the effects of high mode effects. The modification factor of DRFs for MDOFsystems is mainly affected by the fundamental vibration period and displacement ductility ofsystem.(2) The seismic demand of structures with vertical irregularities subjected to velocity pulse-likeground motions have been investigated. Specifically, the irregularities are in strength, stiffness, andcombined strength-and-stiffness in the first storey of structures. Nonlinear dynamic time historyanalysis are performed using eight near-fault velocity pulse-like ground motions. Both the structuralseismic demand and distribution mode in terms of displacement ductility, drift and energy, havebeen investigated to account for two disadvantageous conditions of the vertical irregularities and thevelocity pulse effects. The results show that the ductility demands are higher when consideringvertical irregularities and velocity pulse effects. Furthermore, strength irregularities at the first storeyhave more significant effects on the ductility demand than those of combined strength-and-stiffnessirregularities, while the effects of stiffness irregularities are different from those of strengthirregularities. In addition, the displacement demands and dissipation energy at the first storeyincreases for corresponding reference irregular structures, while those at other stories decreases.These increments of displacement and energy could cause the concentration of damages which could lead to poor seismic behavior for structures.(3) The DRFs for shear-type structures with vertical irregularities have been studies. The DRFsfor vertically irregular structures, subjected to pulse-like ground motions, are obtained by modifyingthose for corresponding reference vertically regular structures through a modification factor. Basedon MDOF lumped mass shear type models, the modification method that the DRFs for verticallyregular structures are applied to the vertically irregular structures, is introduced, and the influencefactors of this modification factor are investigated. The results show that the DRFs for irregularMDOF systems are clearly smaller than those for regular MDOF systems. The modification factordecreases with decreasing irregularity ratio and increasing ductility ratio. In addition, themodification factors for velocity pulse-like ground motion are less than those for non-pulse-likeground motion.(4) The difference of DRFs between flexure-type and shear-type systems is compared, and theDRFs for flexural-type structures with vertical irregularities have been studies. Multi-mass columncantilever systems are employed to simulate flexure-type shear-wall structure, while multi-massseries spring connection systems are used to simulate shear-type frame structure. The effects ofstorey displacement ductility and vibration period on the seismic DRFs for flexure-type structure arestudied, and the DRFs between flexure-type and shear-type structures are compared. The DRFs forflexure-type structures with vertical strength irregularities are obtained by modifying those forcorresponding reference vertically regular structures, and the effects of the storey number, ductilityratio, irregularity ratio and velocity pulse effects of ground motion on the DRFs are discussed. Theresults show that the DRFs for flexure-type structure are about40%larger than those for shear-typestructure. Meanwhile, storey number and storey displacement ductility are important factors onseismic DRFs. The DRFs for irregular structures is clearly smaller than those for correspondingreference regular structures. The modification factors decrease with decreasing irregularity, and themodification factors for velocity pulse-like ground motion are less than those for non-pulse-likeground motion, and there are remarkable interactions between velocity pulse-like earthquake effectsand irregularity ratio.(5) Seismic control effect of vertical irregularity factors for reinforced concrete (RC) framestructures are studied and evaluated in detail employing the Monte Carlo simulation method. A5-story and10-story typical frame structures with vertical irregularity of strength or stiffness on thefirst story and on the median story are designed respectively. Only the peak ground acceleration ofearthquake action is considered as a random variable, nonlinear dynamic time history analysis areconducted to investigate the maximum interstory drift of structures. The variation rule of themaximum interstory drift of structures with different vertical irregularities is explored, and the probability of failure for structures exceeding the limit state is counted. The results show that thevertical strength irregularity and stiffness irregularity have a much large effect on the maximuminterstory drift of RC frame structure. With the decrease of strength irregularity and stiffnessirregularity factor, the maximum interstory drifts increase. The vertical irregularities on the firststory have greater effect upon the structure’s interstory drift than the vertical irregularities on themedian story, and strength irregularities have relatively greater effect upon the structure’s interstorydrift than the stiffness irregularities have. The vertical irregularities also have significant effect onthe probability of failure for structures exceeding the limit state criteria. The exceeding probabilityincreases as the vertical irregularity factor decreases.(6) The influences of the ground motion duration on damping reduction factor have beeninvestigated. The effects of the effective ground motion duration on damping reduction factor areinvestigated employing the displacement response spectra of single-degree-of freedom systems withdifferent levels of damping ratios. The ground motion durations are associated with25Chi-chiearthquake records and harmonic sine wave. The results show that damping reduction factordecreases with the increasing of the effective duration of the ground motion and the number ofcycles of harmonic excitation. By employing a nonlinear multiple regression analysis based on thestatistical mean value, a modified damping reduction factor considering the effects of groundmotion duration is suggested.