Analytical Method to Estimate the Lateral Restraint for Unbraced Top Chords of Warren-Truss Bridges with Independent Cross-Beam Decks

Abstract

The assessment of the lateral buckling for the unbraced top chord of a truss bridge (a pony truss bridge) requires a finite element analysis of a three-dimensional model of the whole structure or a method of the chord analysis based on the reliable estimation of the lateral restraint for the chord nodes. The latter alternative becomes crucial if rapid assessment is necessary. Majority of the existing analytical methods of assessment of the stability of top chords of pony truss bridges rely on the stiffness of [Formula: see text]-frames that consist of cross-beams and truss bracing members. The available analytical methods of setting the [Formula: see text]-frame stiffness seem to neglect the existence of independent cross-beam decks that, together with Warren-trusses, constitute contemporary truss bridges. Such structures are analyzed in the paper. The stiffness of the lateral restraint provided to the truss top chord by the cross-beams, the diagonals and the bottom chord are computed. The influence of the cross-beams is reflected, for the first time, as an equivalent torsional stiffness of the truss bottom chord. The analytical method is applied against the method included in the DIN design code to the pony Warren-truss bridges of various height-to-span ratios and various arrangements of their member cross-sections. Then the Timoshenko and Gere model of a compressed bar is adapted to find the critical load for the top chords of the aforementioned truss bridges. The results are compared to those obtained from the finite element method (FEM) analyses. It is concluded that the analytical method provides lower estimate for the critical load obtained from the FEM analysis provided the smallest values of cross-sectional properties, within the respective groups, are taken as representative for the computations.