Abstract: Fiber-reinforced polymer (FRP) pultruded profiles exhibit weak mechanical properties in the transverse fiber direction. For square tubes subjected to three-point bending, the directly loaded region becomes a structural weak point due to complex mechanical behavior. In this study, modular glass fiber-reinforced polymer (GFRP) bridge decks for emergency bridges were used as the structural background. Three-point bending tests were conducted on the decks’ basic components: pultruded GFRP square tube and its adhesively bonded double tube specimens. Results indicate that the ultimate load-bearing capacity of both specimens is governed by localized bearing failure of the web in the directly loaded region, with final failure manifested as web local buckling. The adhesive bonding between webs partially restricts cross-sectional distortion and damage progression in the double tube specimens, yielding a 13% higher average single-tube load-bearing capacity compared to single tube specimens. Solid finite element model incorporating damage evolution and a simplified frame model of the directly loaded region were developed. Analyses reveal that damage progression in the tubular profiles is dominated by transverse internal forces, with the transverse interlaminar shear strength and transverse tensile strength of the tube wall being critical factors. Fiber fabric wrinkling caused by the pultrusion process is identified as the primary reason for the reduced strength levels.