A Study On Several Issues Of Inelastic Displacement Characteristics Under Seismic Action

Up to now, the main method for seismic design of most national codes is still based on R-μrelationship, which is, to ensure low probability of collapse, the designer makes effectively controls on both the plastic mechanism (ductility capacity) and displacement under rare earthquakes.There are plenty of existed researches on R-μ-T relationship of single-degree-of-freedom system (SDOF), and the conclusions are consistent. However, the"fromμto R"study methods, the researchers took, are not applicable enough for evaluating the existed buildings and the newly built, the hysteretic models in these studies are inconsistence with the real structures. Some researchers had discussed about the effects of hysteretic models on R-μ-T relationship, however, all the hysteretic models are just theoretical, not considering much about the true mechanical characteristics of the structures.In many national design methods, control on displacement under rare earthquakes is the most effective method to ensure"non-collapse uder rare earthquake"of structure. As a result, strict controls on displacement of structure under rare earthquakes are appeared in the national codes and guidance documents. There are many researches on the displacement of the structure, most of them focus on the structural response, but few concern about the influence factors. There are even less researches on how to conveniently identify displacement response rules for multi-degree-of-freedom (MDOF) under multi earthquakes.Based on this background, this dissertation does analysis such as follow:①Study the existed research results of R-μ-T relationship of SDOF, calculate and testimony the existed R-μ-T relationship with a totally different research technique;②Choose the hysteretic models which are closer to the reinforced concrete structures, and also study the effects of hysteretic model on R-μ-T relationship for SDOF system;③With considering the characteristics of SDOF system, take different earthquakes to analyze the main factors influence the displacement ratio of linear SDOF system to non-linear;④To make up the deficiency of lack considering the near-fault ground movement in the present national code, a series of near-fault pulse ground movements are collected to analyze its influence to the response of structure;⑤Design three moment frames with different periods, and make equivalent SDOF systems for them, analyze the displacement response rules of the SDOF systems and MDOF systems under multi earthquake waves, initially prove the applicability of displacement rule for SDOF system in MDOF system.This dissertation gets the initial conclusions from the analysis above, which includes:①The ductility demands of short period structures are generally big, when the earthquake reduction coefficient R is a large number (R>2), almost all the demands of short period concrete structures exceed their abilities. So, the author advices the designers pay attention to it, and earthquake reduction coefficient R may be as small as possible when designing;②Generally, the ductility demands of tri-linear models are smaller than bilinear models, and the range is getting larger at the period gets long or R gets large. For the relatively long period structure, the ductility demands of tri-linear model are close to 1.0, or even smaller than 1.0. Hence, this dissertation supposes the result of Borzi and Elnashai which indicates the hysteretic models have no influence on the R-μ-T relationship is improper. In the inelastic analysis of real structures, to get the more appropriate results, it should take hysteretic models which could reflect the real hysteretic characteristics of the structures;③The shapes and values of elastic displacement response spectrum are influenced by the magnitude and the fault distance, a bigger peak value of elastic response spectrum would be achieved when the magnitude is larger; The period correspond to the peak point is longer when the magnitude is larger; The values of elastic displacement response spectrum are smaller and the displacement amplification coefficient is larger with remoter fault distance;④The effects of magnitude on the inelastic displacement are significant, special at a larger R, it is most significant for the middle period structures. On the contrary, the fault distance influence little to the displacement;⑤The near fault pulse earthquakes significantly influence both linear and inelastic response of structures. The pulse global movements would magnify the strength, displacement and the ductility demand of middle or long period structures;⑥The displacement rule for SDOF system is basically applicable in MDOF system. The designer could approximate evaluate the displacement of MDOF system by checking the displacement response of the equivalent SDOF system.