Nonlinear Impact Of Cable Corrosion On Failure Behavior Of Long-span Cable-stayed Bridges Under Extreme Loads

Most long-span cable-stayed bridges are subjected to aggressive environmental exposures,e.g.coastal area,oceanic area or severely polluted area.Under these circumstances,stay cables are susceptible to corrosion damage by those exposures.As one vital component in cable-stayed bridges,that would be a big potential threat for the service life of these bridges.In addition,a substantial proportion of cable-stayed bridges are located in regions with high seismic fortification intensities.The alteration in configuration of girder caused by cable corrosion will potentially affect the seismic performance of cable-stayed bridges,especially for those with corrosion-sensitive floating systems.Thus,it is much of importance to understand the safety concern of those bridges through investigating the failure behavior of long-span cable-stayed bridges subjected to extreme loads considering the cable corrosion.Two key issues,i.e.cable corrosion and failure caused by extreme loads,are discussed throughout the thesis.As a starting point,the influence of in-service cable corrosion of cable-stayed bridges with floating systems on collapse vulnerability and robustness is discussed first.With the aim at the isues in damage and failure mechanism under extreme loads,some methods,i.e.the energy method,the theory of beam on elastic foundation(TBEF)and the plasticity ultimate analysis method etc.are applied to investigate the influence of cable corrosion on the load-carrying capacity,instability and potential failure mode of cable-stayed bridges in order to have a preliminary insight into the influence of cable corrosion on failure behavior of cable-stayed bridges subjected to extreme loads.Some research has been conducted and described as follows,(1)The variant spatial-temporal distribution model of cable correosion,with the consideration of the distribution of chloride ion concentration,cable's service stress level and the distribution of corrosion within cross section of steel wires,has been established for the use as a in-service structural time-variant degradation model of long-span cable-stayed bridges.On the basis of this,the vulnerability of cables and the robustness of some key regions are analyzed and to identify the critical region where the chain-type progressive collapse initiates from vulnerable component in those bridges subjected to combined hazards.The identification indicates that the displacement of girder and tower is very sensitive to cable corrosion.The proposed method can effectively identify the vulnerable cables and the robustness of each key region.Example analysis shows that all vulnerable cables are found to be located around the tower and does not change distinctly with corrosion ratio.The case study exhibits different robustness among five key regions:Region 3(around tower)? Region 2(1/4 main span)?Region 1(mid-span)? Region 5(the end of side span)? Region 4(1/2 side span).The vulnerability and robustness analysis shows that Region 2 and Region 3 are relatively vulnerable and susceptible to local damage.These two regions would become those that induce chain-type progressive collapse with most possibilities under extreme loads.Thus,in the preliminary design of cable-stayed bridges,special attention should be paid to Region 2 and Region 3 to avoid being the most vulnerable regions where structural failure initiates.(2)Proposed is the idea of implementing on cable-stayed bridges the seismic vulnerability analysis considering cable corrosion uncertainties.The uniform design method(UD)is used to consider the uncertainty sources in cable corrosion model,e.g.corrosion rates,the concentration of chloride ions,the corrosion initiation time and the cable replacement time.The effects of service times and uncertainties on the seismic vulnerability of cable-stayed bridges are then investigated.It is shown in the analysis that:the uncertainties in cable corrosion and material strength have significant effects on the vulnerability of components during the service life of the cable-stayed bridges.Girder is not sensitive to earthquake damage at each damage level.The uncertainty in material strength has a relative evident impact on the vulnerability of components,sometimes more evident than cable corrosion.When the uncertainties in cable corrosion and material strength are considered simultaneously,the vulnerability of components may not necessarily increase with increasing service times.In addition,a simplified axial force-moment correlation curve is proposed,more suitable for depicting the force state of the thin-walled steel box girder(bg>>hg)of cable-stayed bridges.In the seismic vulnerability analysis,examples are utilized to analyze the correlation between existing ground motion intensity measures and seismic responses.It is found that velocity-typed measures are more appropriate for predicting the seismic response of large-span cable-stayed bridges,i.e.PGV?PGV/PGA?vrms.SED and SMV.(3)Innovated are an analysis method with the equivalent elastic foundation beam model and the weakest section method to ascertain the influence of cable corrosion on the failure behavior of long-span cable-stayed bridges,and thereby the potential connection between the configuration change caused by cable corrosion and the seismic performance of cable-stayed bridges is unveiled.It is shown in the results of static push-down analysis and incremental dynamic analysis that:the local damages due to cable corrosion could alter the position where the first plastic hinge or the first yielded cable locates.Cable corrosion also could change the sequence of the failed cables and plastic hinges in girder,and thereafter the failure path.It can be also found that significant correlation can be found between cable corrosion and the sequence of plastic hinges and yielded cables.Cable corrosion reduces the seismic capacity of cable-stayed bridges and the buckling safety coefficient of girder but shows insignificant influence on buckling modes.A reasonable predication on such basis can be made that the failure mode of cable-stayed bridges would switch from the strength failure mode preseted to a sudden buckling failure mode with the development of cable corrosion.(4)Demonstrated is a novel elastic buckling analysis with consideration for the discrete distribution the support stifffness and axial force in girder based on the energy principle.On the basis of this,the influence of cable corrosion on the critical buckling load of cable-stayed bridges is investigated.The analysis on a Benchmark model and the actual cable-stayed bridges model proved that:a well agreement with finite element analysis can be achieved by the prosed analysis method with the error below 5%.The case study shows that with the development of cable corrosion,the axial critical load of girder decreases nonlinearly and the buckling failure mode changes from high order to low order.Nevertheless the change is not as obvious as the buckling load.For long-span cable-stayed bridges with a dense cable system,the development of cable corrosion will also increase the force level of some key sections in the girder.And this trend could add to the possibility for the strength failure mode to switch to buckling failure mode.The formula for elastic buckling load with cable corrosion deduced in this paper is a reasonable,simple and conservative method,which can be effectively used in the preliminary design of cable-stayed bridges.(5)Proposed is the idea of using plastic limit analysis to analyze the influence of cable corrosion on plastic failure behavior of cable-stayed bridges.Then the ultimate load and associated failure mode of cable-stayed bridges are identified with the help of linear programming.Through the discussion on the connection between cable corrosion and the failure mechanism of typical single pylon cable-stayed bridges with floating system,it is found that cable corrosion can significantly alter the position of plastic hinges and ultimate load multiplier.When the corrosion is light,only the corrosion in cables near the pylon can change the position of the plastic hinge.However,as the cable corrosion increases,the ultimate load multiplier decrease continuously and the number of cables that can alter the position of the plastic hinges increases gradually,the failure mechanism also changes correspondingly.It is discovered that the ultimate load multiplier of cable-stayed bridges are more sensitive to cable strength.When the strength factor of girder is constant,the increase in cable strength factors could transit the failure mode of girder from a global failure mode similar to simply-supported beam to local failure.When the strength factor of cable is constant,the increase in girder strength factors could transit the failure mode of girder from to local failure to a global failure mode similar to simply-supported beam.