Design for buckling and vibration of glass FRP pultruded footbridges

Lightweight footbridges composed by glass fiber-reinforced polymer (GFRP) pultruded profiles are analyzed. For design purposes, a proper two-node beam finite element with seven degrees of freedom per node, already tested by the same authors, is adopted. This element is based on a second-order displacement field holding for a generic thin-walled cross-section and accounting for shear strain influence due to non-uniform bending and torsion. Geometrical effects due to the actual load position with respect to the beam shear centre are taken into account. Interpolating functions of the Hermitian family are adopted, which contain three explicit parameters representing the influence of shear deformations. Hence a locking-free finite element is obtained. The accuracy of the formulation presented is verified by developing a vibration analysis of a well known pultruded structure, i.e.: Pontresina Footbridge, built across the Flaz creek (Switzerland) in 1997. The lateral and vertical vibration frequencies are compared with the results of an ad hoc 3D shell numerical model, with a beam model disregarding cross-section warping effects, and with the experimental results reported in literature. Then, for footbridges with similar truss structure, flexural-torsional buckling of top chords is reported versus the span length. Finally, for a 15-meter span footbridge, lateral and vertical frequencies are reported as a function of the pedestrian load, highlighting the influence of participant masses, geometrical effects and shear deformations