Mechanism And Randomn Robustness Of Progressive Collapse Of Isolated Structures Subjected To Multi-direction Coupled Dynamic Excitation

Progressive collapse of the structure often brings serious economic losses and adverse social impacts,and thus has become a focusing topic in the field of civil engineering.However,studies on the progressive collapse of isolated structures have rarely been reported.The horizontal stiffness of the isolation system and its ability of rotational restriction for the super structure are both small,which reduces the redundancy of the isolation layer against progressive collapse.So the collapse mechanical behavior and collapse resistance of the isolated structure are very different from the traditional fixed-base structures.In addition,in the study of structural progressive collapse,the analysis of structural robustness and the exploration of evaluation indicators is also a relatively new and important topic,but for the isolated structure,the study is almost blank.Moreover,the current research on structural robustness does not fully considering the influence of randomness,but in fact,the structural robustness is inherently random.Therefore,in this paper,takes the initial damage caused by the accidental loss of the isolation bearing as the precondition to carry out the following studies on the progressive collapse mechanism of the isolation structure subjected to the multi-directional dynamic coupling excitation and the proposal of random robustness index.(1)Progressive collapse mechanism of isolated structure subjected to multi-directional dynamic coupling excitationThe motion equation of the isolated structure subjected to multi-directional dynamic coupling excitation is established.The nonlinear dynamic response of the isolated structure subjected to the vertical imbalance and multi-directional coupled dynamic excitation is contrastively analyzed.The stress state under dynamic coupling excitation and the evolution process of damage are discussed.The basic law of dynamic progressive collapse of the isolated structure subjected to multi-directional dynamic coupling excitation is obtained.(2)Influence of ground motion characteristics on the anti-collapse ability of isolated structuresAccording to different ground motion characteristics,60 ground motion records were selected and grouped according to their spectral characteristics,amplitude characteristics and duration characteristics.Based on the random IDA method,the anti-collapse ability analysis of the isolated structures was carried out,and the relationship between the ground motion characteristics and the ultimate load coefficient is analyzed.The collapse failure probability of isolated structures under different ground motions is calculated to reveal the influence of vibration characteristics on the collapse resistance of isolated structures.(3)Judgment of collapse failure mode of isolated structure subjected to multi-directional dynamic coupling excitationThe three failure modes of the isolated structure subject to the vertical imbalance andmulti-directional coupled dynamic excitation are listed.The structural damage index is calculated based on the two-parameter damage model considering deformation and hysteretic energy.The damage index is used as the performance index to establish the limit state equations of different failure modes of the isolated structure.The structural failure probability under different failure modes is calculated according to the reliability theory,so as to judge the most likely failure mode of the structure.(4)Proposal and application of random robustness indexConsidering double random of structure and seismic wave,a random robustness index was proposed based on the reliability theory.The influence of randomness on robustness is revealed by comparative analysis on the differences between robustness indexes of the isolated structure,which is based on the consideration of the randomness of the structure only,or the randomness of the ground motion only,as well as the double randomness of the structure-ground motion.Based on the structural limit performance,cumulative damage and risk models,the random robustness indexes with difference performances were established,and combined with different failure modes,the random robustness index of the isolated structure were calculated to provide effective criteria for evaluating the robustness of isolated structures against progressive collapse.