Calculation methods for axial load bearing capacity of steel fiber reinforced geopolymer concrete filled square stub steel tubular columns

Axial compression tests were conducted on 20 steel fiber reinforced geopolymer concrete-filled square steel tubular (SFRGCFST) columns, 2 steel fiber reinforced concrete-filled steel tube (SFRCFST) columns, and 2 plain concrete-filled steel tube (CFST) columns to examine the effects of steel tube thickness, concrete strength grade, steel fiber volume fraction and aspect ratio on the failure modes, load-deformation behavior, and ultimate resistance of SFRGCFST columns. The findings indicate that the coefficient index predominantly influences the axial compression performance and failure morphology of SFRGCFST stub columns, particularly when the coefficient index exceeds 0.88, leading to a transition in specimens from shearing to waist-drum failure. The ultimate bearing capacity of SFRGCFST columns shows an average increase of over 30% compared to CFST columns. Furthermore, when the steel fiber volume fraction reaches 0.9%, there is an additional 21.85% increase in the ultimate bearing capacity over that of SFRCFST columns. The inclusion of steel fiber (SF) significantly enhances the deformation and ductility of SFRGCFST and moderates the post-peak slope of the load-displacement curve, effectively decelerating the decline. Utilizing the multi-axial strength criterion and the mechanical equilibrium equation for square CFST, a formula for calculating the ultimate bearing capacity of SFRGCFST stub columns has been developed. This formula enables accurate predictions of the axial compressive load capacity of SFRGCFST stub columns and provides a basis for revising related standards.