Influence of combined imperfections on lateral-torsional buckling behaviour of pultruded FRP beams

By way of computational analyses using Finite Element (FE) software this paper presents, by way of sensitivity studies, lateral-torsional buckling resistances of I-beams made of pultruded fibre reinforced polymer. Parameters changed in the studies are the geometric imperfections, the vertical load position and the load eccentricity. Measured geometrical and material imperfections are incorporated into the geometrical nonlinear FE simulations. Constants in the FE work are three-point bending loading and the imperfection condition from having different elastic constants in the four flange outstands. Numerical results from the sensitivity studies are verified by comparing them with equivalent buckling load results from a series of physical tests conducted previously. It is found that the influence of combined geometrical and material imperfections on LTB failure can be significant, such that an imperfect beam can be put into a near 'perfect imperfection' condition, in which it possesses a higher buckling resistance. The opposite can happen for a 'more severe imperfection' with poorer beam response under loading. The load eccentricity on I-beams confirms a complex structural response. To be able to have a recognised design procedure for pultruded I-beam members the influence of combined imperfections on lateral-torsional buckling resistance needs to be reliably quantified.