Analysis of non-uniform torsion in curved incrementally launched bridges

Abstract

Incremental launching is a common and convenient methodology to build continuous girder bridges on several piers. Although it has mainly been applied to straight bridges with box sections, today it is also used for construction of horizontally curved bridges with concrete and composite steel-concrete closed or open sections like I-girders. In these cases the contribution of torsion to the stress state becomes of primary importance when the construction stages of these bridges are analysed. Moreover, the presence of thin-walled cross-sections, makes the analysis of non-uniform torsion fundamental when the angle of twist per unit length is not constant or warping is prevented in those sections where rigid internal diaphragms occur. Consequently the stress state in the launching phases can be strongly influenced by non-uniform torsion, especially for the evaluation of axial stresses in I-girder bridges, where non-uniform torsion presents its maximum influence. In this paper a methodology for the repetitive analysis of launching steps is proposed, based on the Hamiltonian Structural Analysis method, which takes into account the internal characteristics of non-uniform torsion (warping and bimoment) in order to evaluate the influence of prevented warping on the stress state at each stage of launching. The methodology is convenient because it can be considered a sort of generalised beam theory and presents a reduced computational burden with respect to finite element or boundary element procedures, with fast solution of many bridge launching static schemes. A validation of the method is given through a comparison with finite element procedures and literature data. An application is presented on a bridge with different typologies of cross-section in order to compare the different behaviours of thin-walled sections and the different degree of influence of non-uniform torsion on the stress state. The results are given in the form of envelope graphs of internal forces and stresses for the entire launching sequence and for the different cases examined.