Buckling design of steel tied-arch bridges

• Out-of-plane buckling behaviour of steel-tied arch bridges is studied.
• An extremely detailed FEM is developed for many bridges to study actual buckling.
• Two design methods for tied-arch bridge buckling are proposed, following Eurocode.
• Buckling curve “b” for steel tied-arch bridges seems quite conservative.

The structural behaviour of steel tied-arch bridges is determined by the introduction of a large compressive force. As a consequence, slender steel arches are highly sensitive to in-plane as well as out-of-plane buckling. At present, no specific buckling curves for out-of-plane buckling exist for non-linear or curved elements in the international codes and calculation methods. Hence, the buckling curves for straight columns, as determined by ECCS, are used, which leads to considerable inaccuracies in the assessment of the critical buckling load for arch bridges.This paper presents two practical calculation methods to design for the buckling behaviour of slender steel arch bridges. The first one follows the calculation method of the Eurocode, but proposes some augmented empirical formulas for the buckling length of the arches. This allows for a better representation of the out-of-plane stiffness of the arch cross section and of the wind bracings between both arches.In addition a second method is proposed, based on the use of simplified finite element models to determine the relative slenderness of the structure. Both methods are validated using results from very detailed three dimensional finite element models. Finite element models of several tied-arch bridges have been created. These models include variations of the bridge length, dimensions of the arch cross-section, boundary condition, and load type. The conclusion of these calculations is that for both of the proposed methods a higher buckling curve can be used than proposed by the code, thus resulting in a more slender bridge design.