Research On Ultimate Load-carrying Capacity Of Long-span Composite Girder Cable-stayed Bridge With Three Pylons

Wuhan Erqi Yangzi River Brige is a composite girder cable-stayed bridge with three pylons, which has the longest span in this kind of bridges in the world, and is a new type of multi-tower cable-stayed bridge. Because the bridge is a very important huge of traffic and transportation and one of landmark architectures of Wuhan city, it posses a very important position in economy and society. Therefore, the safety performance of this bridge is a primary important problem for attention by bridge experts. Because safety performance of structure is determinated by its ultimate load-carrying capacity, it is important to research the ultimate load-carrying capacity of this new type of cable-stayed bridge. Though the ultimate load-carrying load-carrying capacity of a bridge includes the largest loads under static load, earthquake, wind load, explosion load, or ship collision and so on, the failure load under static load and seismic resistance capacity of a bridge are most typical and important, which provide foundation for determining safety performance of bridge under static load and earthquake, so researchs on them are very important. The cable-stayed bridge has very long spans, many pylons and composite girder which is more flexure, so the multiple nonlinearities are very large in ultimate load-carrying capacity state of structure, and calculations of those nonlinear effects are very complex. In order to make clear the ultimate load-carrying capacity of the cable-stayed bridge so as to make sure the safety performance of structure in period of service and understand the mechanic performance of the structure under static or dynamic load, it is quite necessary to research the failure load and seismic resistance capacity of the long-span composite girder cable-stayed bridge with three pylons.This paper takes Wuhan Erqi Yangzi River Brige as the background of research. Firstly methods are studied for calculating geometric nonlinearity, material nonlinearity and interface slip effect in composite girder, so a method considering multiple nonlinearities is formed to analyze the ultimate load-carrying capacity of long-span composite girder cable-stayed bridges. Secondly making full use of advantages of time-history method for calculating seismic responses, a new method of analyzing seismic resistance capacity of a cable-stayed bridge based on plastic hinge is achieved considering concrete cracking and reinforcement yielding with plastic hinges under earchquake and using the bisection method program to make computer automatically analyze and reduce work of calculation. Lastly a method is established to analyze strain-rat effect of structural concrete in earthquake based on damage constitutive model under fatigue loading, in view that bridges may emerge fatigue damage due to fatigue loading during period of service and concrete of structure can appear strain-rat effect under earthquake. The reseach of this paper can improve methods for analyzing failure load under static load and seismic resistance capacity of long-span composite girder cable-stayed bridges, make more clear mechanic characteristics of multi-tower cable-stayed bridges, and provide theoretic foundation for analysis of ultimate load-carrying capacity and for assessment of safety performance of large scale bridges, so the research posses important theoretical and practical significance. The main contents of research are as followings:(1) Studying systemically calculating methods for structural nonlinearity, material nonlinearity and interface slip effect in composite girder, a method is formed for analyzing the failure loads under static load of long-span composite girder cable-stayed bridges considering multiple nonlinearities. By the method the failure load is studied of the long-span composite girder cable-stayed bridge with three pylons and the influence of important nonlinearities is analyzed on structural internal forces and displacements of cable-stayed bridge at bearing capacity limited state in design. The results show that failure load of the composite girder cable-stayed bridge with three pylons is relatively high because the failure load is Pdead+4.1Plive and the safety factor is2.41, and the middle pylon is key structure for ultimate load-carrying capacity of the cable-stayed bridge with three pylons. The study also showes influence of material nonlinearity is relatively large on responses of the cable-stayed bridge, influence of interface slip effect in composite girder and cable sage effect are relatively small.(2) After time-history methods of analyzing seismic responses are studied when considering geometric nonlinearity, multi-dimentional earthquake and travalling wave effect, seismic responses of the long-span cable-stayed bridge with three pylons are analyzed under three dimentional seismic action and the travalling wave effects are also studied. The analyses show that the maximum bending moment of middle pylon is much more than those of side pylons in cable-stayed bridge with three pylons, and the seismic responses of the bridge in transverse are small in whole. The research also shows that traveling wave effect has the largest influence on seismic responses of the side pylon which the seismic wave first arrives to, and the larger influence on those of the side plylon which the seismic wave arrives to last, and the smallest influence of the traveling wave on the middle pylon, and traveling wave effect has very small influence on transverse seismic responses.(3) A time-history analysis method based on plastic hinge is put forward for cable-stayed bridges. Because this method not only fully uses advantages of the time-history analysis method, but also considers the section damages of concrete cracking and reinforcement yielding with plastic hinge and uses the bisection method program for loop calculation to make the computer automatically analyze and reduce work of calculation, it studies the seismic resistance capacity more convenent. Using the method, not only the seismic resistance capacity can be obtained, but also characteristics of seismic responses and structural failure mode are achieved at the state of seismic resistance capacity. Through studing the long-span cable-stayed bridge with three pylons by this method, the result shows that the analysis of seismic resistance capacity of cable-stayed bridge is relatively convenent, and the cale-stayed bridge with three pylons has relatively high seismic resistance capacity because the peak value of acceleration of the largest seismic action is0.302g, and seismic responses of the middle pylon is largest in the cable-stayed bridge with three pylons in state of seismic resistance capacity, so it is the key structure to improve eismic resistance capacity of this kind bridges.(4) A method of analyzing strain-rat effect of structural concrete is established based on damage constitutive model of concrete under fatigue loading. Damage will emerge in concrete in a bridge which is under fatigue loading in service stage and strain-rat effect of concrete in bridge appears under seismic loading, both of wich effect the seismic resistance capacity of structure, so the two factors should be consider in analyzing in order to assess accurately safety performance of structure. Therefore, based on the concept of boundary surface and continuum damage mechanics, a damage constitutive model of concrete under tension-compression fatigue loading is derived which is controlled by strain and can analyze fatigue characteristics of concrete. When the strain-rate effect is considered with Perzyna viscous regularization of strain-energy releasing rate in the above model, a general damage constitutive model is achieved for concrete under static, dynamic or fatigue loading. Comparing with the results of the model and test in a conference showes the proposed model is relatively good for calculating. Therefore, a medthod is obtained for analyzing strain-rate effect of structural concrete material based on damage constitutive model under fatigue loading.The strain-rat effect of concrete is analyzed in the cable-stayed bridge with three pylons under seismic actions in trial application of the above method.