Research On Seismic Performance Of Existing Underwater Bridge Columns Enhanced With Prefabrication And Assembly Technology

The underwater bridge columns were relatively vulnerable compared to the superstructures.A lot of factors,such as poor service environment,potential natural disasters or human interferes,could affect the performance and durability of the underwater bridge column during its long-term service.Once the damage was caused to those columns,a rapid decline of the seismic performance and an accelerate degradation of the durability could be soon expected.A newly strengthening method with Precast Prestress Concrete Panels(PPCP method)was first proposed in this thesis based on the prefabrication and assembly technology.The PPCP method could achieve a low cost and high efficiency compared to the common strengthing methods which required a cofferdam construction before head,and also significantly improve the bearing capacity,seismic performance,and durability of the underwater bridge columns.This thesis developed a few new underwater strengthening materials such as the precast concrete panel,the FRP grid bars,and the FRP spiral stirrup.The strengthening procedurals with PPCP were also proposed.Based on the theoretical analysis,the characteristics of the momentcurvature(MC)relationship of the strengthened cross-section with PPCP were summarised and described with a new two-stage simplified calculation model based on the "farthest point method" and the energy conservation concept.The positive effect and mechanism of the PPCP method to the concrete columns were proved and studied through the experiments.The semirigid constrain,provided by the precast concrete panels and prestressing steel strands,was found to be restraining the deformation of the concrete columns and enhance the usage of the longitudinal FRP bars.Based on the retrofitting demands of existing bridge underwater columns,this thesis first discussed the designing goals and the realization methods of the PPCP method.The strengthening design method of PPCP was proposed as well as a number of key strengthening technics,such as the development of precast concrete segments,the development of underwater non-dispersive concrete,the selection and preparation of reinforcements,the guarantee of the bonding performance of multiple interfaces,the underwater anchoring method of FRP reinforcement,the damage control method of precast concrete segments,and the design of the circumferential pre-stressing.Two kinds of specific strengthening procedurals were then presented based on the experimental facts.Then,a section-based moment-curvature analysis was conducted through theoretical and numerical analysis.The relationships of the main designing factors,including the equivalent reinforcement ratio,the cross-section enlargement ratio,the elastic modulus of the FRP reinforcement,the axial compressive ratio,and the concrete compressive strength,to the moment-curvature relationship were thoroughly discussed.A new two-stage simplified calculative model was proposed and verified based on the numerical and theoretical analysis respectively,to simplify the actual moment-curvature relationship in design.An experimental study on the axial compressive and seismic performance of the concrete columns strengthened with PPCP were conducted.Nine columns,three of them regarded as the control columns,strengthened either with PPCP or PPCP combined with FRP grid bars were tested under axial compressive loading.The axial compressive performance of the strengthened columns was proved to be significantly enhanced,and the key parameters which significantly affected the axial compressive performance of the strengthened columns were drawn.Six 1/4-scale concrete columns,one of them regarded as the control column,strengthened with PPCP and different types or different reinforcement ratios of FRP reinforcement,were tested with combined constant axial loading and cyclic lateral loading.The strengthening effect and mechanism were comprehensively studied in terms of the stiffness,strength,bearing capacity,ductility,energy consumption,residual displacement,and curvature distribution.A number of design suggestions were proposed based on the experimental results.To compare with the experimental results,a numerical analysis model was built and proved in a good agreement with the experimental results.A multi-parameter numerical analysis was then conducted,the effect of the parameters to the seismic performance,as well as to the bondslip deformations,was thoroughly studied.The simplified calculative model was proposed for predicting the load-displacement relationships of strengthened columns,and the key parameters,such as the drift ratio and the force of yield point and peak point were analyzed and fitted with the numerical analysis results.At last,a restoring force model was established to evaluate the hysteric performance of the strengthened columns based on the theoretical,experimental and numerical studies.To further evaluate the vulnerability of the strengthened columns,a vulnerability assessment method and a corresponding calculative method of DI factor were proposed.In addition,a brief vulnerability assessment was conducted based on a dynamic time-history analysis of a specific strengthened column.