This paper numerically investigates the behavior of hollow-core fiber-reinforced polymer-concrete-steel (HC-FCS) building columns under combined axial compression and flexural loadings. The HC-FCS column for buildings consists of an outer circular fiber-reinforced polymer (FRP) tube, an inner square steel tube, and a concrete wall between them. A three-dimensional numerical model has been developed using LS_DYNA software for modeling of large scale HC-FCS columns. The nonlinear FE models were designed and validated against experimental results gathered from HC-FCS columns tested under cyclic lateral loading. The FE results were in decent agreement with the experimental backbone curves. These models subsequently were used to conduct a parametric study investigating the effects of the concrete wall thickness, steel tube width-to-thickness (B/t) ratio, and local buckling instability on the behavior of the HC-FCS columns. The obtained local buckling stresses results from the FE models were compared with the values calculated from the empirical equations of the available design codes. Finally, an approximated expression based on the available empirical formulas and the FE model results has been proposed in this paper to calculate the local buckling stresses of HC-FCS columns
