Abstract
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The steel tube walls of a biaxially loaded thin-walled rectangular concrete-filled steel tubular (CFST) slender
beam-column may be subjected to compressive stress gradients. Local buckling of the steel tube walls under
stress gradients, which significantly reduces the stiffness and strength of a CFST beam-column, needs to be
considered in the inelastic analysis of the slender beam-column. Existing numerical models that do not
consider local buckling effects may overestimate the ultimate strengths of thin-walled CFST slender beamcolumns
under biaxial loads. This paper presents a new multiscale numerical model for simulating the structural
performance of biaxially loaded high-strength rectangular CFST slender beam-columns accounting for progressive
local buckling, initial geometric imperfections, high strength materials and second order effects. The inelastic
behavior of column cross-sections is modeled at the mesoscale level using the accurate fiber element method.
Macroscale models are developed to simulate the load-deflection responses and strength envelopes of thinwalled
CFST slender beam-columns. New computational algorithms based on the Müller's method are developed
to iteratively adjust the depth and orientation of the neutral axis and the curvature at the column's ends to obtain
nonlinear solutions. Steel and concrete contribution ratios and strength reduction factor are proposed for evaluating
the performance of CFST slender beam-columns. Computational algorithms developed are shown to be an
accurate and efficient computer simulation and design tool for biaxially loaded high-strength thin-walled CFST
slender beam-columns. The verification of the multiscale numerical model and parametric study are presented
in a companion paper.