STRUCTURAL PERFORMANCES OF ARCH FRP FOOTBRIDGES

The paper presents a parametric analysis of arch pedestrian bridges entirely made by glass-fiber reinforced-plastic (GFRP) pultruded profiles. In order to understand the performances of this type of structure for different span lengths, 30-, 40- and 50- meter span footbridges are analyzed. The aim of the author is to establish the maximum span length that the proposed type of footbridge can reach without any problem of buckling, vibrations or rupture of joints. The self weight of the proposed footbridges is, respectively, 80, 110 and 150 kN for the 30-, 40- and 50-meter span structure. The ratio between live loads and dead loads lies in the range 3.5-4.5. In the author’s opinion, the lightness of FRP materials can represent a great advantage, especially in presence of a soft soil foundation. Furthermore, the strong corrosion resistance of FRP materials permits to avoid frequent and expensive maintenances. The arches of the three proposed footbridges are achieved by connecting rectilinear profiles so as to realize a parabolic shape. The cross sections of main girders are made by two coupled channel profiles. Tension members are made by two coupled pultruded plates, whereas compression members by square tubes. These are inserted between the channel profiles of the main girders. In the paper some aspects concerning design and finite element analysis are explained. Due to arrangement of fibers, FRP profiles exhibit a pretty low shear stiffness. The value of the ratio between longitudinal Young modulus E and transverse elastic modulus G is about equal to 10 and, consequently, it is necessary to take shear deformation effects into account. The assumed kinematical model is based on Timoshenko’s bending theory and on Reissner’s torsion theory, allowing for the evaluation of shear strain effects due to both nonuniform bending and torsion. Numerical interpolation of the displacement field is based on “modified” Hermitian shape functions. Such polynomials contain parameters depending on both E/G ratio and slenderness, that go to zero when shear strain influence becomes negligible. A few years ago, Reddy demonstrated that these polynomials produce a locking-free element for Timoshenko beams. In a previous paper, the author demonstrated that similar polynomials can be used to interpolate torsional rotation and cross-section warping with excellent results. In order to evaluate the global buckling resistance of designed footbridges, second order terms are introduced in the displacement field, representing flexural-torsional coupling. By using the developed finite element program, the influence of shear deformability on deflection and buckling loads is consequently pointed out. In order to understand the dynamic behaviour of the three compared footbridges under moving loads, several numerical analyses have been performed. The effects of both flexural and lateral pedestrian-induced vibrations on proposed structures have been evaluated. Remembering the highly publicized closure of the London Millennium Footbridge, due to excessive lateral vibration at its opening, the limit-situation of crowd density on bridge deck is taken into account in calculating the vibration frequencies. Due to the low stiffness of pultruded profiles, the design of most of the structural members is governed by the Serviceability Limit State or by interaction phenomena between global and local buckling modes. Hence, the design of the structural members is carried out by taking these possible interaction phenomena into account.