Basic Research On Seismic Design Method Of Eccentric RC Frame Structures Subjected To Pulse-like Ground Motions

The structural eccentricity and near-fault pulse-like ground motion are two extremely unfavorable conditions in the seismic design. Accounting for the coupling effect of these two conditions considered separately in previous work, the present study investigates the seismic demand and design method of eccentric reinforced concrete(RC) frame structures subjected to pulse-like ground motions. In accordance with the current deficiencies, the objects of this thesis are to quantify the seismic demand of these structures using axial load-bending moment interaction hysteretic model developed in this study, suggest the modification method of their seismic ductility reduction factors, and finally recommend a seismic design method based on the strength reduction factor for these structures. The main works of the authors’ study are as follows.(1) The development work of the axial load-bending moment interaction model(N-M model). The N-M model is developed based on the platform of VC++ language and the metal plasticity theory. Its effectiveness and efficiency in structural elasto-plastic analysis are evaluated through the analysis work of a RC frame model with real earthquake damage records and a super high-rise building model. The results show that reliable analytical results can be obtained by using N-M model in structural inelastic analysis, and the efficiency of N-M model is improved greatly comparing to the fiber model or multi-spring model. Besides, N-M model provides a rapid way and validation method for the batch calculations and the analysis for super scale structures, which can be used and need to be improved in the application of scientific research and practical engineering.(2) The investigation on the seismic demand of single-story eccentric structures subjected to pulse-like ground motions. The influence mechanism of different eccentricity types and axial load on the structural seismic demand is revealed. The best relative location of strength center and stiffness center is discussed. The change trend of the elastic and inelastic seismic demand for RC shear wall models and frame models with single-story are studied using N-M model, respectively. The results show that both of the eccentric shear wall and frame structures subjected to pulse-like ground motions suffer much higher elastic and inelastic seismic demand compared to structures subjected to non-pulse-like ground motions. The pulse-like earthquake effect and structural eccentricity have coupling influence on the inelastic seismic response. The axial load has no effect on the elastic demand while it has certain effect on the inelastic one. Among the three eccentricity types, the stiffness eccentricity affects the elastic demand, while the strength eccentricity affects the inelastic demand most. It is suggested that the strength eccentricity be added as a judgment indicator in the elasto-plastic analysis of structures with plan irregularities.(3) The investigation on the inelastic seismic demand and fragility of multi-story and high-rise eccentric RC frame structures subjected to pulse-like ground motions. The effect of different eccentricity types on the inelastic seismic demand of multi-story frames is discussed, and the change trend of the inelastic seismic demand is studied for multi-story frames with general eccentricities. Besides, an analytical method for studying on the seismic fragility of eccentric structures is recommended. The seismic fragility curves are established based on 24,000 dynamic time history analysis for multi-story and high-rise strength eccentric systems. The results show that among the general eccentric systems, the first-story eccentricity has larger effect on the inelastic seismic demand compared to the eccentricity at other stories, and the case with even eccentricity in each story is find to be the most unfavorable one. The multi-story and high-rise strength eccentric frames subjected to pulse-like ground motions suffer much higher exceeding probability of seismic demand compared to structures subjected to non-pulse-like ground motions. The exceeding probability of the maximum element drift, ductility and inter-story rotations increases with the increment of the eccentricities and the exceeding probability of the ductility is most sensitive. The best relative location of strength center-stiffness center to control different response indicatiors is different, and the relatively better location of strength center is located between the mass center and the stiffness center from a comprehensive perspect.(4) The modification method on the ductility reduction factor of the eccentric RC frames and the establishment of the regression expression for the modification factors. A method to determine the ductility reduction factors Rμ of the eccentric structures is suggested by modifying those of symmetric structures, based on the spectrum theory. The four factors of eccentricity ratio, story number, ductility ratio and velocity pulse of ground motions, are investigated to gain insight into this modification factor. A relationship expression between the modification factor and these factors is established by multiple nonlinear regressions, and an example for using it is shown. The results show that the modification factor is mainly affected by the eccentricity ratio, ductility as well as the pulse-like effect of ground motions, and the effect of story number is not apparent. The regression expression of the modification factors takes account of the coupling effect of the structural eccentricities and pulse-like ground motions, which can comprehensively explain the Rμ-μ-T relationship.(5) The establishment of the seismic design method for the eccentric RC frame structures. Based on the evaluation of the current calculation methods for harmful inter-story drifts, the concepts of inter-story average shear angle and inter-story average rotation are recommended, and their relationship with the harmful drift angle are derived and validated. The strength reduction factor R in the current seismic design codes is evaluated, the seismic design method base on this R for the eccentric structures is recommended, and an example is simply shown. This method abides by the R-μ-T relationship, controls the harmful drift, and evaluates the failure probability. Based on some existing conclusions, the coupling effect of structural eccentricities, vertical irregularities and pulse-like ground motions on R is considered, which provides a comprehensive consideration for the strength reduction factors.