Finite element investigation on the capacity of slender HSS steel columns strengthened with multilayer CFRP strips

Abstract

This thesis presents a numerical finite element (FE) investigation on the behavior of steel square hollow structural section (HSS) columns (mild steel) strengthened with CFRP materials. Three dimensional finite element (FE) models of square HSS sections were developed using shell elements considering both material and geometric nonlinearities whereas CFRP strengthening was incorporated with additional layers of both conventional and continuum shell elements. Damage properties of CFRP and GFRP materials were incorporated in the FE model. The developed FE models were used to simulate experimental studies done by past researchers. Good agreement was found in between present analysis and past experimental results, which validated the acceptability of the FE model to carry out further investigation. Study was then focused on some selected non-compact AISC square HSS columns and the effects of number of CFRP layers, slenderness ratio and cross-sectional geometry on the increase in axial load capacity of those columns were studied. From the parametric study it was found that the axial strength of column specimens can be possible to increase by up to two-thirds to that of unstrengthened specimens. For columns of short and intermediate lengths, variation in strength is less significant. When column length increases, strength decreases significantly as expected. The capacity generally increases with the increase in the number of CFRP layers. In some cases, the strength gains, however, do not correlate directly to the number of CFRP layers. Medium sized HSS sections benefit most from CFRP strengthening compared to larger or smaller sized sections. This is because addition of CFRP layers decreases the b/t ratio more for medium sized sections and hence brings the section closer to compactness and delays local buckling which contributes to significant increase in capacity.