Stability Analysis for Pitched Portal Frames of Fibre Reinforced Polymer

In this paper the analysis method from Majid (1963) for steel pitched portal frames is progressed so that we can study the influence of shearing action on the overall elastic critical buckling load when members are of Fibre Reinforced Polymer (FRP) material. For solid members of FRP the ratio of the longitudinal Young’s modulus to the shear modulus of elasticity can be significantly higher than for the equivalent sized isotropic members, and so the influence of shear deformability on the buckling resistance is required. Shear-flexibility can also be introduced into skeleton frames by having FRP members that are of braced-batten or of sandwich construction. In the approach developed by Majid the buckling problem is modelled using stability functions that account for the reduced stiffness in beam-column members as deformation increases. To apply the closed-form expressions to frames with shear-flexible members requires the replacement of the shear-stiff stability functions with the shear-flexible equivalents. One set of expressions is from Mottram and Aberle (2002), after they observed an end rotational compatibility issue in the theoretical derivation to the conventional set of shear-flexible stability functions (see Al-Sarraf 1986). To show the influence of shear-flexibility, and the choice of functions, analytical results for the critical load (for symmetrical and anti-symmetrical modes) are reported for two loading cases. A brief discussion is given on how these results impinge on our fundamental understanding of the instability resistance of pitched FRP portals in the presence of shear-flexibility. It is observed that further work is required to find out if the critical load is to be determined by analysis using the 2002 set of shear-flexible stability functions.