Research On Synthetical Three Factors Check Method For Ultimate Load Carrying Capacity Of Long-Span Steel Arch Bridges

More than ever before in China, the application of steel arch bridges have become popular. Some novel and complex steel arch bridges have appeared. The problem of ultimate load carrying capacity is a key point in the design and construction of long-span steel arch bridges. In spite of many scholars have researched this problem and some regulations have been introduced in codes, theoretical studies in our country are still not comprehensive and cannot guide real design works precisely. Non-linear numerical analysis method for the ultimate load carrying capacity has been used in some design works of long-span steel arch bridges. But this method is inefficient and cannot incarnate the essence of collapse of an entire steel arch bridge. Based on Caiyuanba Bridge in Chongqing and Xinguang Bridge in Guangzhou, the main contents of this dissertation are summarized as follows:Nonlinear beam element theories used to analyze ultimate load carrying capacity of steel arch bridges are inducted. After comparing with the results of theoretical analysis and model tests, it is point out that commercial FEA software can be used in calculating ultimate load carrying capacity of steel arch bridges. Residual stress by welding and composite profile could be simulated simply using beam element in commercial FEA software, such as ANSYS. High precision entire steel arch bridge models are established by dual nonlinear supper-parametric shell elements, brick elements and truss elements. In order to get higher accuracy, BFGS(Broyden-Fletcher-Goldfarb-Shanno) method of quasi-Newton method and line search method are combined. To prove the accuracy and convergence of the results, three FE meshes are tested. On this basis, suitable model dimension is determined and this method can further reveal the essence of ultimate load carrying capacity. After comparing the results of this higher accuracy model with those of nonlinear beam element, schemes of modifying nonlinear beam element model are presented considering the influences of integral gusset plates, stiffener and internal diaphragms. Finally, it is pointed that nonlinear analysis is too complicated to guild design work and simplifycheck method is more practical.On the grounds of nonlinear calculation for a real steel arch bridge and two test models, variations of stresses and internal forces have been obtained. Then main emphasis is put on the essence of ultimate load carrying capacity. The reasons for this instability phenomenon might be explained as follows: Due to the yielding of profiles and the diffusion of plastic zones at arch ribs, the stiffness of profiles is reduced. As the result, the nonlinear displacement will become relatively considerable. The effects of design parameters can be reflected by the variations of the internal forces at key profiles. So the effects of many factors can be reflected by the variations of the internal forces at key profiles. Formula of effect index R/; is firstly put forwarded. So the influences of lateral initial crookedness and lateral loads can be embodied in simple check method for ultimate load carrying capacity of long-span steel arch bridges.The calculation formula of the critical load of the circle ribs with non-uniform distributed bracings has deduced with the method of Ritz. Non-linear programming is adopted to optimize of the bracing location for the first time. Formula of the critical load of the circle ribs is imported into software Lingo to solve the problem of bracing location optimization. In order to decrease numbers of total iterations, three strategies, heuristic method for generating a good starting point, successive linear programming to compute new search directions, and the steepest-edge strategy when selecting variables to iterate on, are used. In order to reflect the effects of total lateral stiffness of ribs, formula of effect index R21 is put forwarded. So the effects of stiffness, numbers and locations of bracings on ultimate load carrying capacity of long-span steel arch bridges can be embodied in simple check method.Two preconditions, lateral stiffness of main girder and loads transferred through hangers, resulting in non-directional loads effects are specifically pointed out and certificated. Very few researches, however, have been reported so far on the second condition. Using Ritz method, a formula of lateral critical buckling load of steel arch bridges under non-directional and directional loads is proposed, in accordance with mechanical characteristics of long-span steel arch bridges. For the first time, it is illustrated theoretically that only the loads transferred throughhangers can result in non-directional loads effect of steel arch bridges. Distribution factor for loads is introduced to present the ratio of non-directional loads to directional loads. According to the results calculated, relationship of ultimate load carrying capacity and this factor is founded. In order to reflect the effects of non-directional loads, formula of effect index R3/ is put forwarded. So this factor can be embodied in simple check method for ultimate load carrying capacity of long-span steel arch bridges.Based on the factor index of lateral initial crookedness and lateral loads /?//, the factor index of total lateral stiffness of ribs R21 ,and the factor index of non-directional loads R31, a synthetical index is finally put forwarded to establish Rf=RnR2iR.3i, for checking ultimate load carrying capacity of steel arch bridges. And a safety index K= RtfR is also proposed. Through comparison with the results of model tests, specifications of different countries and four steel arch bridges, accuracy and efficiency of the synthetical three factors method is finally verified. If./? corresponding to the linear internal forces at key profiles under dead loads and full span loads is less than Ri, the entire steel arch bridge is still not reach its ultimate load carrying capacity state.According to a systematical summary of achievements obtained by dozens of domestic and foreign scholars, effects of sixteen factors, such as lateral stiffness of ribs and non-directional loads effects, on ultimate load carrying capacity have been reflected. Differences of Eurocode3, DIN 18800, JHSB, JSSC, AASHTO, JTJ 025-86 and TB 10002.2-99 have been compared through a steel arch bridge model. The synthetical three factors check method proposed in this dissertation has reflected main factors which affect the ultimate load carrying capacity of long-span steel arch bridges.