Experimental study on torsional behavior of thin- and thick-walled RC box-girders using BFRP bars and steel stirrups under combined torsion-shear-moment

Abstract

Experiments on hollow FRP-RC members subjected to combined torsion are uncommon. Six specimens of identical size and length, five rectangular and one trapezoidal, were constructed and tested in this investigation. About 2.5 percent reinforcement is included in all of the specimens. The study variables were the effect of shape (rectangular vs trapezoidal), concrete wall thickness (thin vs thick), cover thickness, and reinforcement layout (single-layer vs double-layer). Strength and stiffness, deflection and angle of twist, cracking pattern, and modes of failure were all evaluated. It is discovered that a trapezoidal thin-walled box-girder can attain around 85% of the ultimate strength of its rectangular counterpart, with 9% less deflection and 11% more twist. Thin-walled box-girders have a 17 percent higher ultimate torsional capacity than thick-walled members with thick covers. Thin-walled box-girders have the least deflection and twist, but their ultimate torsional capacity is limited compared to thick-walled counterparts. In thick-walled members, utilizing a double layer of reinforcement instead of a single-layer, can result in just a 4% increase in ultimate strengths, while deflection and twist both drop by 23%; in addition, a double-layer reinforced box-girder with thicker webs than flanges can reach the largest ultimate strength with the least angle of twist at failure.