| Precast segmental bridge piers have fast construction speed and provide a practice platform for introduction and promotion of new material and technology.Due to uncertainty of seismic performance and imperfection of seismic design,insufficient research on self-centering,required additional design of energy dissipation capacity,and prestressing loss sensitive to toe crushing,the bridge piers are limited to bridges in moderate and high seismicity areas.Due to its high strength,superior ductility and enhanced durability,ultra-high performance concrete(UHPC)is applied in civil engineering to ensure structures with high reliability in limit state,high resilience under the disaster action,high durability within long service time,and high environmental friendliness over the life cycle.Specially,when UHPC is used to fabricate precast segmental bridge piers,on one hand,the material is used to fabricate precast segmental hollow bridge piers to reduce damage at column toe and improve energy dissipation in earthquake shock;on the other hand,UHPC is used to develop a novel structural type of precast segmental bridge piers,which can rapidly recover their function after earthquake by adopting reasonable details considering a new concept of earthquake resilience.Therefore,precast segmental UHPC hollow bridge piers are selected as research objective,and experimental,numerical and theoretical researches are conducted to study their seismic performance.The relationship between design parameters and seismic performance objective is established based on the results of those researches.The design parameters can be determined in design process according to the expected seismic performance objective,which can realize performance-based seismic design.The detailed research contents and results are listed as follows:(1)Large-scale cyclic loading experimental research on self-centering precast segmental UHPC hollow bridge piersUHPC is used to fabricate precast segments.Separated segments are connected to a whole by using unbonded post-tensioning tendons.Energy dissipation capacity is enhanced by using energy dissipation(ED)bars with unbonded length at the base joint.Trial parameters include post-tensioning force level and amount of ED bars.A new earthquake resilient UHPC bridge pier is proposed and fabricated with precast segmental construction.The experimental research is used to examine the connection between replaceable UHPC plates and base segment and the connection between replaceable ED steel rods and the corresponding threaded ends.Experimental parameters include post-tensioning force level and amount of replaceable ED steel rods.Cyclic loading were conducted on repaired bridge piers to study their seismic performance.(2)Seismic deformation capacity of precast segmental UHPC hollow bridge piers based on the equivalent plastic hinge modelThe equivalent constitutive relationships of the post-tensioning tendons and the ED bars are analytically derived to meet the plane section assumption for sectional analysis.Based on the equivalent plastic hinge model,the lateral force-displacement curve is calculated and compared with experimental results.Using the proposed model,a parametric study is conducted to investigate the effects of nine main design parameters on the seismic deformation capacity of the bridge pier.Based on the results of the parametric study,a simplified formula for the effective stiffness is developed through regressive analysis.(3)Energy dissipation and self-centering capacities of precast segmental UHPC hollow bridge piers based on fiber elementBased on OpenSees calculation platform,the equivalent fiber section,meeting the plane section assumption,is established to simulate the base joint by using the equivalent constitutive relationships of the post-tensioning tendons and the ED bars.The equivalent plastic hinge length is used to consider the bond-slip between rebar and concrete,and the effective stiffness is calculated to take the effect of shear deformation into account.The finite element model is verified by the results of cyclic loading test.The effects of design parameters on energy dissipation and self-centering capacities are studies based on the finite element model.(4)Design criterion for self-centering capacity of precast segmental UHPC bridge piersAt first,the qualitative relationship,between the residual displacement and two influencing factors including lateral displacement maximum value and self-centering coefficient,is determined by mechanism analysis.Then,the quantitative calculation formula of residual drift is obtained and composed of the maximum of lateral displacement and the self-centering coefficient based on the results of pseudo-static loading test The results of nonlinear dynamic time history analysis are used to verify the quantitative calculation formula of residual drift under seismic excitation.Eventually,using the quantitative calculation formula of residual drift,limit values of self-centering coefficient are obtained at different seismic design levels.The limit values can be used to guild the design of self-centering capacity for precast segmental UHPC hollow pier.(5)Hysteretic behavior of precast segmental UHPC bridge piers based on the idealized flag-shaped modelThrough theoretical and parametric analysis,the idealized flag-shaped hysteretic model parameters are deemed to be dependent on the yield point and the peak point of the lateral force-displacement skeleton curve.Key parameters of the idealized flag-shaped hysteretic model are obtained by establishment of the analytic calculation method about the yield point and the peak point.The analytical calculation method of those key parameters is verified by the pseudo-static test results.Dynamic time history analysis is conducted respectively based on the finite element model and the idealized flag-shaped hysteretic model with parameters determined by the proposed method.Based on the two models,the two indicators including lateral displacement maximum value and cumulative energy dissipation maximum value are calculated compared to evaluate the applicability of the idealized flag-shaped hysteretic model with parameters determined by the proposed method in the dynamic time history analysis.(6)Feasible region of post-tensioning force for precast segmental UHPC hollow bridge piers based on performanceFirstly,the basic idea is introduced that the upper and lower limit values of post-tensioning force is determined by three performance requirements including "residual displacement controlled","joint shear demand" and "no collapsing with strong earthquake".Then,the analytical calculation method for the upper and lower limit values of post-tensioning force is established based on the analytical calculation results of the limit state to determine the feasible region of post-tensioning force.The influence of design parameters on the feasible region of post-tensioning force is studied based on the proposed analytical calculation method for post-tensioned prestressed upper and lower limits.Through discussion about the influence of the compressive zone height of the bottom section at the limit state on the feasible region,a non-iteration simplified calculation is proposed for the feasible region of post-tensioning force.(7)Seismic fragility of precast segmental UHPC hollow bridge piers considering deformation and self-centering capacitesA reinforced concrete bridge pier is selected from practical projects and regarded as a reference to design a precast segmental UHPC hollow bridge pier through five equivalent design principles.Due to the proposed method by existing research,a quantitative standard,expressed by displacement ductility factor and residual drift,respectively,is determined for bridge piers at damage states.According to the quantitative standard,the results of increment dynamic analysis are dealt with for the two bridge piers with different structural types subjected to three different types of ground motions including far field,near field with no pulse and pulse.For the two bridge piers with different structural types under different types of ground motions,the probability seismic demand models are established,and the fragility curves are obtained to evaluate the seismic performance of the two bridge piers. |
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