Investigation On Duration Characteristics Of Ground Motion And Its Effect On Nonlinear Seismic Response Of RC Frame Structures

Damage investigation during the previous earthquakes indicates that the ground motion duration has obvious influence on the accumulated damage of structures.Up to now,the frequency content characteristics of of ground motion is not decoupled reasonably for the investigation on the influence of ground motion duration on seismic response of structures.The prescriptive provisions for ground motion duration based on the probabilistic specific site design spectrum in current seismic design code are not clear.So the duration characteristics of ground motion and its influence on nonlinear seismic response of structures are researched thoroughly in this paper.The main research work and conclusion are as follows:(1)To study the duration characteristics of ground motion,the five ground motion duration indices used in current study have been summarized,namely,uniform duration,bracketed duration,5-95% significant duration,5-75% significant duration and effective duration.The definition,characteristics and key influence factors of these five indices are analyzed.The ground motion records during Wenchuan earthquake have been selected to validate the accuracy of duration evaluation using prediction equation based on the least squares algorithm considering rupture distance.The results indicate that the definition of theses duration indices have large difference,which are influenced by earthquake magnitude,rupture distance,site conditions and threshlod in different degree.The duration prediction equation based on the least squares algorithm considering rupture distance can predict the duration of Wenchuan earthquake ground motion records effectively.(2)To investigate the selection of reseanable ground motion intensity measure considering the duration effects,a series of finite element models of single degree of freedom(SDOF)systems with different periods,strength and stiffness degradation characteristics are built.And the duration indices Ds5-95%,and intensity measures such as Arias intensity(AI)and commulative absolute velocity(CAV)of 60 ground motion records are also obtained.Then,the correlation analyses between these three indices and peak seismic responses of SDOF systems,the duration index and intensity measures based on maximum seismic response of structures are conducted.The influence of ground motion duration on total hysteresis energy dissipation of SDOF systems with different periods are studied.The results indicate that the intensity measure CAV are preferred in the dynamic analyses considering duration characteristics of ground motions.The long-duration ground motions can make obvious influence on the hysteresis dissipation of long-period structures.(3)To investigate the influence of ground motion duration on the collapse resistance capacity evaluation of structures,the numerical model of a multi-storey infilled RC frame structure is built.The 30 suits fitted artificial ground motion records based on seismic design code and 60 suits natural ground motion records matched spectrally are selected as earthquake excitation to evaluate the collapse resistance capacity of multi-storey infilled RC frame structures.The results indicate that with the same intensity measure,ground motion records with longer duration may induce larger inter-story drift ratio and higher collapse probability for the structure.The collapse resistance capacity of the structure tends to decrease with the increase of the significant duration of ground motion.For the cases of artificial ground motion and natural ground motion records,the influence of ground motion duration on residual deformation of structures is not same completely.(4)To reflect the influence of deformation mode at beam-column connection regions on the degree of structural nonlinear deformation,a six-storey infilled RC frame structure is selected as an example.Three kinds of numerical model are built,which are the beam-column joints without considering inelastic deformation,with considering the bar bond-slip,and with considering the bar bond-slip and the shear deformation together.The ground motion records with long and short duration are selected as inputs to study the influence of ground motion duration on the seismic fragility,repairable capability and collapse resistance capacity evaluation of RC frames with different failure modes at beam-column connection regions.The results indicate that the exceeding probability for each damage limit state and collapse probability of structures are obviously higher under long-duration ground motion records than short-duration ground motion records.And with the increase of nonlinear deformation of beam-column connections,this kind of increase becomes more obvious.(5)To explore the influence of all kinds of duration indices on seismic response of RC frame structures,the truncated ground motion records tbased on bracketed duration and the 5-95% significant duration are taken as input for the numerical model of a six-storey RC frame structure.The nonlinear dynamic response and hysteresis energy dissipation of the structure under different ground motion intensity are analyzed.The results indicate that the residual deformation obtained from truncated ground motion based on bracketed duration may be underestimated comparing with the results from truncated ground motion based on 5-95% significant duration.Under certain ground motion intensity,the ground motion that aretruncated based on 5-95% significant duration can induce more obvious nonlinear deformation on structures.