Analysis On Saddle Pushing Of Tripple-Tower Suspension Bridges During Erection Stages

Before the erection of the stiffening girders of suspension bridges, the main saddle at the top of the tower is usually set pre-offset toward side span while supporting the naked main cable. Along with the erection of stiffening girders and the action of deck system, the main saddle on the top of tower should be pushed toward the center span step by step in order to change the relative position between tower top and saddle and therefore to ensure the safety of tower. So, the length of pushing step should be governed by the structural strength of the tower.Differing from the double-tower suspension bridge, the triple-tower suspension bridge has an extra middle tower and therefore possesses more complex structural behavior and saddle pushing process. A triple-tower and double-span suspension bridge, which is nowadays under construction in China, was employed as case study to illustrate the proposed method in this paper.The finite element models for the whole bridge and the separate tower were established respectively by general structural analysis software. The pre-offset and the pushing step length were carried out after simulating the erecting process by backward analysis. During the backward erecting process, the function of live/dead elements was used to determine the free slip displacements and off-centre vertical forces of saddle in each erecting stage.The separate tower model was established as well to calculate the maximal allowable displacements of top end due to each erecting stage on the cases with or without consideration of p-A effect. The criteria to determine the allowable displacement was based on the critical tower sections without tension stress. The solution for the saddle pushing schemes of the example triple-tower suspension bridge was carried out after analyzing and comparing the calculation results. The differences with or without consideration of p-Δeffect are finally discussed in this paper.