Study On Failure Mechanism Of Steel-concrete Composite Continuous Rigid-frame Bridge Under Strong Ground Motion

The innovative steel-concrete composite continuous rigid-frame bridge(SCCCRFB)consists of a steel-concrete composite girder and concrete-filled double-skin steel tube(CFDST)columns and shows obvious advantages compared to the conventional concrete rigid-frame bridge with respect to mechanical performance,durability,and construction.In particular,the use of composite bridge piers significantly improves the seismic fortification level and makes this bridge type a promising structural solution in seismic prone areas.This dissertation investigates the behaviors of the innovative composite continuous rigidframe bridge under strong earthquake ground motions using theoretical analysis,model tests,and numerical simulations.The main works include:(1)A new baseline correction method for strong ground motions based on the target final displacement is proposed.(2)Shake table tests of a three-span SCCCRFB model are performed,and control methods are investigated to simulate the across-fault ground motions on shake tables.(3)Based on the test results,the dynamic response characteristics of the SCCCRFB under the near-fault ground motions are analyzed,and numerical simulations are performed to analyze the seismic advantages of the innovative SCCCRFB compared to the conventional one.(4)Based on the test results and numerical simulations,the structural response characteristics and damage modes of the innovative SCCCRFB under the across-fault ground motions are analyzed,and parametric analyses are also performed.(5)Damage mitigation effectiveness of different stiffener arrangements to the CFDST piers of the innovative SCCCRFB is analyzed.The main conclusions are obtained as follows:(1)A new baseline correction method for strong ground motions based on the target final displacement is proposed,which can be used to simulate the across-fault ground motions.The seismic response of a fault-crossing bridge can be classified into dynamic response and quasistatic response,and the target final displacement of the baseline correction dominates the quasistatic response of the structure while shows almost no influence on the dynamic response.(2)A new hybrid input control method to reproduce across-fault ground motions on the multi-shaking tables are proposed,which overcomes the limitations of the traditional acceleration or displacement input and considers accuracy requirements from both lowfrequency permanent ground displacement and high-frequency acceleration.The effectiveness of the new method is validated by the shake table tests.(3)Shake table test results under the near-field uniform-excitation ground motion indicate:the near-fault ground motions with strong pulses can pronouncedly amplify the seismic responses of the bridge structure and result in significantly residual displacements.The bridge damages mainly occur at the upper and lower ends of the CFDST piers with yielding of the steel skins and separation between the steel skins and infilled concrete.The finite element(FE)model developed in Open SEES can accurately predict the structural responses under the uniform earthquake excitations.Compared to the traditional composite rigid-frame bridge with RC piers,the innovative SCCCRFB with CFDST piers can significantly reduce peak and residual displacements.(4)Shake table test tests under the across-fault ground motions indicate: the structural damages of the bridge model mainly occur at the upper and lower ends of the CFDST piers,with yielding and buckling of the steel skins and crushing of the infilled concrete.In particular,obvious residual deformations exist in the bridge piers,and the bridge girder significantly participates in the seismic response of the overall structure.The three-dimensional detailed FE model developed in the explicit FE program LS-DYNA can accurately predict the seismic responses and damage modes of the bridge model under the across-fault ground motions,and the damages of the bridge are mainly governed by the quasi-static deformations.The innovative SCCCRFB shows very good collapse resistance capability under the across-fault ground motions,implying this bridge type has a promising prospect in fault-rupture zones.(5)Numerical results indicate: when the bridge crosses the fault in 90°,the thrust and strike-slip faults can result in very different seismic responses in the bridge structure.Under the TCU ground motion(thrust fault),the CFDST piers mainly show bending deformations,while under the ELC ground motion(strike-slip fault),the CFDST piers show pronounced torsional deformations.Applying proper bi-directional stiffeners to the CFDST piers of the innovative SCCCRFB can effectively mitigate the damages and restrain local buckling of the steel skins of the CFDST piers,which is applicable to cope with the structural damage hazards due to the near-fault ground motions and the across-fault ground motions with different fault mechanisms.