An element-by-element approach is adopted to integrate the stresses over the cross sectional areas of steel and FRP. The cross section was divided into four areas (A1 to A4), as shown in Fig. 4(a). The flat part of the flanges oriented normal to the plane of buckling (A1) was divided into 12 strips through the thickness, where the strain is constant across the width of each strip. The flat part of the flanges parallel to the plane of buckling (A2) was divided into 12 x 80 elements to capture the strain gradient along the depth of the section and also the residual stress distribution within the thickness. The conjunction between the flange and the web (A3) was idealized as a square and divided into 12 x 12 elements. Area A4 represents the FRP layers attached to area A1, and was divided into 1 x 2 elements. To model specimen 3L-4S with FRP sheets on four sides, an additional area of FRP, A5, attached to area A2, was also modeled. The distance between the center of each element and the centroid of the cross section is yi, as shown in Fig. 4(b). It is assumed that perfect bond exists between the FRP sheets and steel
surface and strains are linearly distributed along the depth of the section. The stresses at the centroid of each element are assumed constant within the element area.