NUMERICAL EVALUATION OF THE LOCAL BUCKLING MOMENT FOR PULTRUDED FRP BEAMS

The interest of the scientific community in obtaining the local buckling stresses for pultruded FRP (PFRP) shapes is shown by the huge number of published studies on this subject. The main objective of the present paper is to provide accurate estimates of the local buckling moment for PFRP beams. These estimates are fundamental for defining the nondimensional slenderness parameter introduced by the Italian Design Guide CNR DT 205/2007 [1] for beams in pure bending. One of the most technically sound formulations is that proposed by Kollár [2], who derived explicit expressions for the critical normal stresses of flange and web panels of PFRP beams that depend on the rotational stiffness of the web-flange junction in compression. This formulation is based on a consolidated approach for estimating the flange buckling strength of typical steel profiles [3]. In the present paper, a literature review is initially reported. In particular, it is shown that the approximate expression for the critical stress provided in [3] for steel members tends to underestimate the local buckling strength, especially in the case of wide-flange cross-section profiles. This underestimation becomes particularly evident for PFRP beams. For example, in [4], on the basis of the results of 10 bending tests, an average ratio of 1.2 between experimental and predicted local buckling moments was obtained. Anyway, it has clearly been shown that the web-flange junctions play a crucial role in determining the buckling and ultimate strengths of PFRP beams [5]. Therefore, the experimental and numerical characterization of the junction behaviour is necessary [6]-[7]. The rotational spring stiffness used in Kollár’s model to reproduce the behaviour of the web-flange junction seems sensitive to the shear modulus of the flanges [4] and to the ratio Io,f/Io,w [8], with Io,f and Io,w being the ratios between transverse and longitudinal Young’s moduli for the web and flanges, respectively. Hence, in the present paper estimates of this stiffness based on a series of finite element buckling analysis results are finally reported. The influence of possible differences in the mechanical properties between flange and web panels is suitably taken into account.