Study On The Influence Of Seismic Velocity Pulse On Structural Response And Isolation Properties

The near-fault strong ground motion is very complex, mainly affected by the length of fault rupture, the development process of fault dislocation, fault rupture velocity, sliding direction and other factors. The basic characteristics of near-fault ground motion include the concentration of strong ground motions, surface rupture, rupture directivity effect, large velocity pulse and hanging wall effect. For all of these basic characteristics, the large velocity pulse with very high energy has the greatest influence on the engineering structures, and the seismic responses (such as stress, displacement, ductility and etc.) of structures subjected to velocity pulse are much greater than those ground motions without velocity pulse.The purpose of this study is to reveal the engineering characteristics of velocity pulse, and to provide valuable advice for the seismic design of structure near fault. In order to eliminate the influence to results caused by different response spectrums, this paper proposes a synthetic method of seismic motion. Several strong motion records with velocity pulse and corresponding synthetic time histories with same response spectra are used as ground motion inputs, and the seismic responses of structure are compared and analyzed to highlight the effect of velocity pulse on engineering structures. Reinforced concrete (RC) frame structures, base-isolated structures and large span cable-stayed bridge structure are modeled with Finite Element Method for the seismic responses analysis. For the selection of ground motion with velocity pulse, the forming mechanism of velocity pulse is specially considered. Typical ground motion with forward directivity pulses and fling-step pulses are used as ground motion inputs. In the paper, the influence of velocity pulse to the seismic responses and the damping effect are discussed. At the same time, the differences of two kinds of velocity pulse to the seismic response of RC frame structures and base-isolated structures are compared.The main contents and research results include the following5aspects:(1) A synthetic method of seismic time-history with the absolute acceleration response spectrum and the peak velocity as target is proposed. Under the premise of fitting the target acceleration response spectrum, the peak ground velocity is obtained by a separate adjustment of long period component of the ground motion Fourier Spectrum. The results show that this fitting method has a high precision for the target response spectrum and target peak velocity, and it has theory and practical significance on the study on velocity pulse.(2) In order to study the different influences of two types of velocity pulses to the seismic response of RC frame structures, the seismic responses of3RC frame structures (4stories,9stories and14stories) under the velocity pulse are analyzed with three dimensional finite element models. Comparative analyses were carried out for the seismic responses of the RC frame structures under6ground motion records with velocity pulse and corresponding6synthetic time histories with same response spectra but without velocity pulse. The results show that the seismic response of RC frame structures under the ground motions with velocity pulse is significantly greater than the ground motions with no velocity pulse. The influence of forward-directivity pulses and fling-step pulses to median-low RC frame structures have no obvious difference in elastic stage, but for the seismic displacement response of the longer period of high-rise RC frame structures, the influence of forward directivity pulse is greater than fling-step pulse.C3) To study the validity of base isolating for a building under near-fault velocity pulse,3base-isolated buildings(4stories,9stories and14stories) are selected as typical base-isolated structures. Comparative analyses were done for the seismic responses of the RC frame structures and the base-isolated structures under6near-fault ground motions with velocity pulse. The results show that the lead rubber bearing has the good ability of energy dissipation under near-fault ground motions with velocity pulse. The seismic responses of base fixed structures under near-fault ground motions with velocity pulse are apparently bigger than the seismic responses of base-isolated structures. The lead rubber bearing can significantly reduce the seismic responses of base-isolated structures under near-fault ground motions with velocity pulse.(4) In order to study the different influences of two types of velocity pulses to the seismic response of base-isolated buildings, the seismic responses of3base-isolated buildings (4stories,9stories and14stories) under the velocity pulse are analyzed with three dimensional finite element models. Comparative analyses were carried out for the seismic responses of the RC frame structures under6ground motion records with velocity pulse and corresponding6synthetic time histories with same response spectra but without velocity pulse. The results show that the seismic displacement response of base-isolated buildings under the ground motions with velocity pulse is significantly greater than the ground motions without velocity pulse. For the4-story and9-story base-isolated buildings, the impact of fling-step pulse is a little bigger than forward directivity pulse. For the14-story base-isolated buildings, the impact of forward directivity pulse is bigger than fling-step pulse. The fling-step pulses lead to large displacement response in the lower layer of base-isolated building, and the forward directivity velocity pulse has a certain influence on each layer of base-isolated building.(5) In order to study the influence of velocity pulse on the seismic response and seismic control for a long-span floating cable-stayed bridge, three dimensional finite element model are set up, inputting with4strong motion records with velocity pulse and corresponding4synthetic time histories with the same response spectra but without velocity pulse. In general, most seismic responses of a cable-stayed bridge subject to ground motions with velocity pulse are greater than those without velocity pulse. Ground motions with velocity pulse can make bad influence on seismic responses of non-control, semi-active control and passive control for the cable-stayed bridge, and the influence on semi-active control is almost same as the influence on passive control.