Elastic buckling analysis of pultruded FRP portal frames having semi-rigid connections

Presented in this paper are results for the elastic instability of Pultruded Fibre Reinforced Polymer (PFRP) portal frames with semi-rigid connections. The kinematical model used is based on a second-order displacement field, accounting for the shear strain influence on both non-uniform bending and torsion. A two-node locking-free finite element with seven degrees of freedom per node is adopted. Joint flexibility at member ends is included by means of a simple manipulation of the stiffness matrix to the finite element, such that the influence of joint behaviour on membrane, shear, bending and torsion deformations, as well as to cross-section warping, can easily be taken into account. Numerical examples are reported and discussed to illustrate the influence on elastic buckling loads of in-plane and out-of-plane moment-rotation behaviour of joints, as well as the role played by the base warping restraint and deformable lateral bracings.