Behavior of short and deep beams made of concrete-filled fiber-reinforced polymer tubes

Behavior of short and deep beams made of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) was compared experimentally to that of their slender counterparts. Ten specimens made from four types of glass FRP tubes with different fiber architecture and lamina lay-up were tested with shear span to depth ratio between 0.9 and 6.25, diameter to thickness ratio between 16 and 63, and reinforcement index between 0.11 and 2.2. The study extended the test database of CFFTs to the lowest practical limit of shear span to depth ratio. None of the CFFT beams tested, even with the lowest shear span to depth ratio of 0.9, failed in shear. Tensile bending strains at the bottom of the midspan section of the beams always remained higher than the respective diagonal tensile strain at the midpoint of the shear span. Web shear cracks were observed only in the concrete core of deep CFFT beams with high reinforcement index. Following the first flexural crack, the concrete core began to slip relative to the FRP tube. This lack of composite action made shear less critical than flexure. Finally, short and deep CFFT beams exhibited higher bending capacity than their slender counterparts, primarily due to the direct diagonal compression strut that develops in the concrete core through arching action.