Time-varying Coherence Of Ground Motion And Its Effect Mechanism On Reticulated Shell Structures

Reticulated shell structure is the most commonly used in long span saptial structure,beacuse of its various forms and good mechanical properties,it is widely used in large public buildings such as stadiums,transportation hubs and exhibition centers.During the service period,the reticulated shell structure is often crowded with people,and after the earthquake disaster,it is usually used as a temporary resettlement place for refugees and a disaster relief command place for relevant personnel.Therefore,once the reticulated shell structure is damaged in the earthquake,it will cause huge losses,and it is particularly important to ensure its safety in the earthquake.The span of reticulated shell structure can reach a few hundred metres,each bearing receives the seismic signals have bigger difference in the earthquake.Previous studies have shown that the traditional uniform excitation cannot have satisfied the engineering requirements,multi-support excitations need to be considered to get accurate results,so the research of the reticulated shell response mechanism under the multi-support excitations has the vital significance.Due to the distance and site conditions in building structure,the spatial variation of ground motion is mainly manifested as the traveling wave effect and the coherence effect.At the same time,the ground motion has time variability,according to the existing seismic observation data,the time-varying characteristics of ground motion spectrum is quite obvious.Therefore,it is commonly believed that the stationary assumption which the spectrum of ground motion does not change with time is not conform to the actual situation,it is necessary to consider the time and spatial variability of ground motion in multi-support excitations analysis,it is great application value to obtain the non-stationary characteristics of ground motion from seismic array data and to establish the method of simulated spatial ground motion which considering the spatial variation with time-varying characteristic.Based on the above key problems about seismic behavior of long-span spatial structures under multi-support excitations.In this paper,the reticulated shell structure and non-stationary ground motion are studied,and the main contents of this research are listed as following:(1)The characteristics of ground motion spectrum are studied,and GMW wavelet is introduced to analyze the time variation of the ground motion spectrum and the coherence of ground motion field by continuous wavelet transform,the wavelet coherence is applied to the analysis of ground motion field,and the relation between the coherence and the distribution of ground motion intensity is studied.Based on the data of LSST and SMART-1 array,the trend of wavelet coherence variation with distance is calculated,and the directional effect of wavelet coherence is analyzed.The results show that the time variation of coherence are objective,and it become more and more obvious with the increase of the distance between the two stations.When the distance reaches the kilometer,the time-varying feature of coherence begins to decrease(2)According to the wavelet spectrum and phase statistical characteristics of the actual seismic records,a method of simulated non-stationary ground motions based on the continuous wavelet transform is proposed,and comparing the spectrum and intensity between original and simulated ground motion record,the difference of ductility spectrum was also studied.On the basis of the proposed method,a method of simulated nonstationary spatial ground motion which matching the target wavelet coherence is presented,and the validity of the proposed method is verified by comparing wavelet coherce of simulated fround motion field with the wavelet coherce.(3)The effect mechanism of reticulated shell structure under multi-support excitations is revealed,and a simplified model of single degree of freedom is set up,the dynamic response of the simplified model under the harmonic load with phase difference is derived,and the relation between quasi-static response and structural dynamic response is analyzed.An energy-based method is proposed to analyze the structural response under multi-support excitations.The time history analysis of three different reticulated shell structures under harmonic load is carried out respectively,considering the influence of nonlinear,seismic wave velocity are used to indicate the phase difference,the influence law of seismic wave velocity(phase difference)on dynamic response of reticulated shell structure was calculated,the relationships among seismic wave velocity,harmonic excitation frequency and structural response are obtained,analyzed the impact of reticulated shell structure response mechanism under multi-support excitations.The results show that the quasi-static response has a great influence on the structure when the frequency of excitation is lower,while the high frequency excitation can cause the coupling between the quasi-static response and the structure vibration.(4)Several groups of spatial ground motions considering time-varying coherence and stationary hypothesis are simulated respectively.The dynamic response of the reticulated shell structure caused by two kinds of spatial ground motions are compared,and confirm the importance of time-varying coherence in structural response analysis.This paper analyzes the response of three kinds of single-layer spherical reticulated shells with different spans under multi-support excitations,the influence of different stiffness substructure is considered,and the ultimate load and failure mode of the reticulated shell structure with different stiffness substructure under multi-support excitations are compared.The mechanism of substructure affecting the seismic spatial effect of reticulated shell structure is studied by analyzing the variation of the force on the members,and the harmonic effect of substructure on spatial variation of ground motion is studied.Finaly,the seismic response of 800 m super long span structure under multi-support excitations is analyzed,and the influence of lower pile foundation on seismic spatial effect is also studied.The analysis results show that the time-varying characteristics of coherence have a great influence on the dynamic response of reticulated shell structure,and the substructure(including pile foundation)can change the difference of excitations and spectrum distribution,and then influence on the seismic spatial effect of the reticulated shell structure.