Seismic Response And Seismic Isolation Of Skew Continuous Beam Bridges Subject To Strong Earthquakes

In order to satisfy the needs of the terrain and the route,the skew bridges have been widely used in expressway,urban road and transportation hub projects.Due to the particularity of skew bridges,their seismic response is more complicated than straight bridges.In previous destructive earthquakes,the earthquake damage of the skew bridges are prominent,such as beam falling,rotation of main beams,pier damage and collision between adjacent structures or components.The seismic isolation technology,as an important mean to improve the seismic performance of bridges,has been widely used in bridge structures and building structures.However,seismic isolation technology is rarely used in skew bridges,and seismic damage control methods and seismic isolation of skew bridges are still in the exploratory stage.Taking a four-span skew continuous beam bridge as the research object.The seismic response and damage of non-isolated skew bridge with traditional laminated rubber bearings and fixed piers different types of earthquake excitation was analyzed.The seismic isolation effect of lead rubber bearing(LRB)on skew bridge was studied.On this basis,the effects of ambient temperature and lead core heating on the seismic response of LRB isolated skew bridge under near-fault ground motion were studied.Finally,based on the seismic isolation design concept of functional separation,a seismic isolation and damage controlling system comprised of bearings and braces was proposed,the seismic isolation effect of the system was studied and the self-centering performance of the isolated skew bridge was evaluated.The main work and results are summarized as follows:(1)The earthquake damage and its mechanism of skew bridges in strong earthquakes at home and abroad were summarized.The earthquake damage of all skew bridges during 2008Wenchuan earthquake was investigated,and the problems that should be paid attention to in the design and seismic isolation of skew bridges were summarized.In addition,the research status of skew bridges was reviewed from the aspects of dynamic calculation models,seismic response,numerical and experimental researchs,seismic damage control,and the influence of temperature on lead rubber bearings and the design method of isolation bridge were reported in detail.(2)In order to study the influence of different types of ground motions on the seismic response of skew bridges,the near-fault ground motions records with forward rupture directivity,fling-step effect,no-pulse and the far-field ground motion records were selected as seismic excitation.The seismic response and damage of skew bridge added with laminated rubber bearings or lead rubber bearings were investigated by time history response analysis.The results showed that the fixed piers and shear keys of the skew bridge with laminated rubber bearings were seriously damaged under the action of near-fault ground motions and far-field ground motions.Stronger seismic response of the skew bridge might occur under the pulse near-fault ground motions,and the fixed pier and shear keys were more likely to be damaged.The use of LRB in the skew bridge could achieve better mitigation seismic effects,and the decreasing ratio of seismic response could exceed 50%.(3)Under the cyclic reciprocating motion,the stiffness and strength of the bearing are degraded due to the lead core heating.At the same time,LRB is exposed to the external environment for a long time,and its mechanical properties are also changed under the influence of the ambient temperature.The mechanical properties of the LRB is mainly determined by the characteristic strength Q_d and the post-yield stiffness K_d.The material properties of rubber and lead were modified to consider the effect of low temperature on the characteristic strength Q_d and the post-yield stiffness K_d of LRB,and the influence of the lead core heating on the mechanical properties of the LRB was considered.The relationship among the seismic response of the isolated skew bridge under the action of near-fault ground motion and the ambient temperature,the lead core heating,and the skew were studied.The results showed that the influence of low temperature environment on the seismic response and bearing shear force of the isolated skew bridge was notable,and the influence of lead core heating on the seismic response and bearing shear force of the isolated skew bridge could be neglected to some extent.When the ambient temperature and the lead core heating worked together,the shear force and torsional moment of piers were significantly amplified,while the rotation of the main beam was reduced.The magnification of the longitudinal shear force and torque of the bridge pier increases with the increase of the skew.At a low temperature of-30℃,the longitudinal shear force and torsional moment of a 60°skew bridge pier were amplified by 20%and 39%,respectively.(4)Based on the seismic isolation design concept of functional separation,a seismic isolation and damage controlling system comprised of sliding bearings and braces was proposed.The sliding bearings afford the vertical loads of the beam and the braces(SCEDB,BRB)control the horizontal displacement of the beam.A displacement-based seismic design method was developed to determine brace parameters.The seismic performance of the isolated skew bridge were analyzed,and the seismic isolation effect of the system equipped with SCEDB or BRB were evaluated.Finally,from the perspective of recoverable functions,an optimal seismic isolation and damage controlling system with a combination of SCEDB and BRB was proposed for skew bridge.The results showed that the seismic isolation and damage controlling system with sliding bearing and energy dissipation brace can control the relative displacement between the pier and beam effectively,and the SCEDB brace can reduce or even eliminate the residual displacement between pier and beam.For the skew bridge,when the self-centering force ratioζwas 0.033,using the combination of SCEDB and BRB was able to achieve the best seismic isolation effect,and the energy consumption of BRB and the self-centering characteristics of SCEDB were exhausted fully.