Effect Of Aftershock On Seismic Fragilities Of Rural Masonry Structure

A large number of aftershocks usually occurs continuously after an earthquake.Aftershocks will cause potential damage to the structure as well as threaten the safety of life and property.At present,performance-based seismic design and seismic risk assessment have not yet considered the impact of aftershocks.Although the impact of aftershocks on the seismic demand and seismic fragility of structures has gradually gained attention,such researches mostly focus on concrete structures and steel structures.The masonry structure is more likely to cause serious damage or even collapse in the mainshock-aftershock sequence due to its large self-weight,low tensile,flexural and shear strength and poor seismic capacity.The current researches on masonry structures are mostly pseudo-static tests of members or shaking table test under a single earthquake and numerical simulation analysis.There are few studies on the fragility of masonry structures under the action of the mainshock-aftershock sequence.Moreover,the seismic design and seismic risk assessment methods of masonry structures are not yet complete,especially in some villages and towns where there is no rational design and construction system for masonry structures.Additionally,many houses are self-built by villagers which lacking corresponding seismic structural measures,and the construction quality is difficult to guarantee.These problems will largely affect the seismic performance of masonry structures.To sum up,considering the effects of aftershocks when analyzing the fragility of masonry structures is of great significance for earthquake disaster loss assessment and structural improvement and reinforcement.This paper takes the typical single-story masonry structure in the villages and towns of Northeast China as the research object.Based on the dynamic time history analysis and probabilistic seismic demand analysis methods,the impact of aftershocks on the seismic fragility of masonry structures is studied.The main contents are as follows:(1)Using ABAQUS finite element analysis software,the finite element models of unreinforced masonry and constrained masonry structures with ring beam and structural columns were established respectively.The 80 real ground motions were selected from PEER,and repeated sequence ground motions and random sequence ground motions were constructed by amplitude modulation method and random method to study the influence of different mainshock-aftershock sequence construction methods on the fragility of masonry structures.(2)Based on the nonlinear dynamic time history analysis method,the top displacement and inter-story displacement ratio of the structure in each nonlinear time history analysis are extracted,and seismic response characteristics of masonry structure under the action of the mainshock,the aftershock and two different mainshock-aftershock sequences are analyzed,and the influence of aftershocks and the setting of restraint members such as ring beam and structural columns on the seismic response of masonry structures is studied.The results show that increasing the setting of restraint members such as ring beams and structural columns can effectively reduce the displacement of the top layer of the structure and the displacement angle between floors.Aftershocks act on the damaged structure that has suffered from the mainshock,which will cause the structure to produce a larger displacement response than the aftershocks of the same strength alone.That is,the structure will produce more serious damage than a single earthquake,and then adversely affect the structure.(3)Based on probabilistic seismic demand analysis method and structural nonlinear dynamic time history analysis response data,probabilistic seismic demand model of the structure is established,then the fragility curves under different working conditions are obtained.From the perspective of probability,the influence of restraint components such as ring beams and structural columns on the seismic fragility of masonry structures is quantitatively analyzed by comparing the fragility.It fully proved the necessity of setting ring beams and structural columns in the masonry structure;Then the effect of aftershock strength on the seismic fragility of masonry structures is studied.The result shows that it may cause additional damage to the structure to a greater extent,leading to changes in the damage mechanism of the structure and increasing the probability of damage to the structure,even the aftershock intensity is small.And with the increase of aftershock intensity,the seismic fragility of masonry structure is also increasing.Therefore,ignoring the effects of aftershocks may underestimate the seismic response and seismic vulnerability of the structure,making the structure unsafe;Then,the effects of two different main aftershock sequence construction methods on the seismic vulnerability of masonry structures were compared.The results show that compared with repetitive sequence ground motions,random sequence ground motions have a greater impact on structural fragility.If only the amplitude modulation method is considered to construct the main aftershock sequence,the ground motion may underestimate the damage;Finally,the effect of aftershocks on the seismic vulnerability of the main shock damage structure is studied.The results show that the probability of more serious damage to the structure caused by aftershocks increases significantly,as the damage of the main shock deepens.And aftershocks make the damage structure of the main shock reach the maximum beyond the adjacent failure state.The impact of aftershocks gradually decreases,as the damage progresses,...