The Multi-scale Finite Element Analyses And Experimental Studies On The Principles Of Seismic Isolated Bridges

In the past two decades,there have been numerous newly built beam bridges in the traffic system of China,in order to protect these bridges from being destroyed during an earthquake,many researchers have been focused on the seismic isolation technology and its engineering application.At present,a preliminary system for the seismic isolation technology of bridges has been developed in our country,and the corresponding standards and codes have been drafted and improved.However,there are still some projects need to be further investigated,for example,the accurate numerical simulation of seismic isolation devices,the local mechanical behaviors of key components of bridges during an earthquake,the damage and failure mode of seismic isolated bridges subjected to major earthquake,the more accurate and practical inelastic-response-spectra-based seismic design method for seismic isolated bridges,etc..On the basis of massive previous studies,the main focuses and main research results of this dissertation are presented in the following:(1)The theoretical,experimental and fine numerical studies on three types of seismic isolation devicesThree types of seismic isolated bearings with the diameter 100mm are first produced,the mechanic performances of these bearings are then theoretically,experimentally and numerically analyzed.Since the diameter of the bearing is typically small,a specialized loading device was crafted for the compression shear tests of bearings.Besides,due to the uncontrolled computation cost of implicit algorithm for simulating bearings,as well as the difficult simulation of contact relations,the explicit finite element(FE)software ANSYS/LS-DYNA is utilized and several simplifications in the FE model are verified to have negligible influence on the behaviors of bearings.(2)The multi-scale FE modeling method for reinforced concrete(RC)members in ANSYS/LS-DYNAThe theory of MAT159 and MAT174 for concrete in ANSYS/LS-DYNA are first expounded,and the approximate equations for each parameter in the models are then presented.After that,the multi-scale FE modeling method in ANSYS/LS-DYNA is discussed,and taking a common pier as an example,four different models of the pier are developed,including solid element model,multi-scale model I,multi-scale model II and simplified beam element model.Based on these models,the pseudo static pushover tests are conducted and the dynamic characteristics of them are investigated.It is found that results from the four models coincide well with each other,and the computation cost of the two multi-scale models is greatly reduced by 3/4 compared to that of the solid element model.(3)The shake table tests and the multi-scale finite element analysis for the scaled seismic isolated beam bridgeTaking a practical RC beam bridge as background,the scaled model with geometry similarity ratio 1:4 is fabricated.Then the dynamic characteristics of the bridge and the seismic responses of the bridge including acceleration,displacement of the girder and the relative displacement between girder and pier,are measured and analyzed.Results show that as the peak acceleration of the input record increases,the bridge with different seismic isolated bearings get different variations of the acceleration response of the girder,while the relative displacement between the girder and pier are all nearly linear even with different isolated devices.Besides,it is found that the relative displacement is negatively correlated with the ratio of peak ground acceleration to peak ground velocity(PGA/PGV).After that,the multi-scale model for the bridge is developed,the seismic responses of the bridge subjected to 9 records are analyzed based on the model,and the numerical results of the acceleration and displacement of the girder are compared with the measured ones.It is found that they are in consistent with each other,which further verifies the accuracy of the model.In the end,the shear force of all piers and the mechanical behaviors in the local part of the bearings,including sliding and pounding,are studied based on the model.(4)Studies on the influence of earthquake characteristics on the responses of seismic isolated bridges.The multi-scale model for a 3-spans RC seismic.isolated bridge is first developed,and the dynamic characteristics of the bridge are then obtained.After that,the WA wave with different intensities is selected as input to investigate the influence of the peak acceleration on the acceleration and displacement response of the girder,the damage state at the bottom of piers,and the positive vertical stress within the rubber layer.Similarly,the WB1-WB15 waves with different PGA/PGV ratios are selected to investigate the influence of the ratio on the seismic responses of girder and piers.(5)Developing of the strength reduction factors based on the massive far fault records for seismic isolated bridgesBased on the record pool with 1410 far-fault ground motions and nearly 2 million nonlinear analyses,the mean strength reduction factor R for seismic isolated bridges is statically computed,and the influences of the ductility ??the natural period T?the PGA/PGV ratio?the post yield stiffness ratio a and the site conditions on R are studied.An equation is then proposed to fit the obtained mean R which takes all the above factors into account,and 62 additional records are selected to verify the accuracy of the equation.(6)Developing of the inelastic displacement spectra based on the near fault pulse-like records for seismic isolated bridgesBased on 67 near-fault pulse-like ground motions,the mean inelastic displacement ratio for seismic isolated bridges is statistically computed as a function of T/Tp,here Tp is the pulse period of record,and two indicators?????are introduced to assess the maximum difference due to different ? and ?.Similarly,the elastic acceleration response spectra is also developed based on the records.In the end,the inelastic displacement spectra for seismic isolated bridges are developed.(7)The inelastic-displacement-spectra based design for seismic isolated bridges subjected to near fault pulse-like ground motionsThe premise to simplify the seismic isolated bridge as a single degree of freedom(SDOF)bilinear system is first analyzed,the theoretical equations to combine all bearings as one "total bearing" for seismic design are then presented,after that,the detailed iterated processes for design are proposed.In the end,taking a 3-spans RC continuous bridge as an example,the inelastic-displacement-spectra based design for seismic isolated bridges subjected to near fault pulse-like ground motions are realized.