This paper presents results of a finite element modeling (FEM) of axially loaded long hollow structural section (HSS) steel columns, strengthened using carbon-fiber reinforced polymer (CFRP) sheet applied in the longitudinal direction. The model provides a lower bound solution as it ignores the contribution of CFRP on the inward buckling face of the column, only at mid-height section, throughout the loading history. The model was verified using experimental results. The non-linear FEM predicted the ultimate loads and failure modes of the columns quite reasonably. The study showed that using 7 layers of 0.54 mm thick CFRP sheets bonded on two opposite sides of an 89 × 89 × 3.2 HSS with 68 slenderness ratio has increased the strength by 22 percent. The model was then used in a parametric study, which showed that the strength gain is significantly affected by the slenderness ratio of the column.