An analytical method to estimate the natural bending frequency of the spans of railway through truss bridges with steel-and-concrete composite decks

The Eurocode 1 standard requires railway bridges to be checked against the hazard of resonance. To perform this task, the first natural bending frequency of a bridge span has to be computed. The analytical method based on the single-beam analogy is sufficiently accurate for simply supported beam-girder bridges. This paper proves the applicability of the method for through truss bridge spans with steel-and-concrete composite decks; this is despite the fact that the spans do not strictly comply with the assumptions of this method. The flexural stiffness of the bridge span is computed taking into account the joint action of the truss girders and the composite deck; the limited extent of span shear stiffness due to diagonal bracing is also considered. A uniform distribution of the span mass over the span length is assumed. The mass is computed based on geometrical characteristics of truss and deck members, taking into account gusset plates, stiffeners, connectors and fittings. The analytical method and finite element method are applied to find the natural frequencies of two through truss bridge spans with composite decks. Computed results are compared with frequencies recorded during bridge testing. Taking into consideration the joint action of the truss girders and the composite deck, as well as the limited shear stiffness provided by diagonal bracing, significantly improves the accuracy of the assessment provided by the analytical method. The natural bending frequencies computed for the two bridge spans fall within the range of 0.95–1.05 of the recorded values.