Lateral Confinement of RC Columns by Internal Structural Steel Strip Bands

Abstract

The most common type of lateral reinforcement in reinforced concrete (RC) columns is rectilinear steel ties. Ties provide confinement to the concrete core increasing its compressive strength and ductility. Past studies have shown that tied columns fail while the stress in the ties is below the yield strength, which represents inefficient use of the material. Furthermore, ties are labor intensive making them costly and time-consuming to fabricate and install. Additionally, the presence of end hooks to anchor them into the concrete adds up to the material cost. A new method of lateral reinforcement is explored in this study where the concrete column core is confined by Internal Structural Steel Strip Bands (ISSSBs) in lieu of ties. These ISSSBs are cut from structural steel square or rectangular tubes. This method has the potential of improving the structural behavior of RC columns without increasing the amount of lateral steel. There are two main objectives for this study. The first one is to verify the feasibility of the use of ISSSBs as lateral reinforcement in RC columns, while the second is to develop design recommendations for columns transversely reinforced with ISSSBs. To achieve the stated objectives, five variables are considered: (1) aspect ratio of the cross section, (2) spacing of ISSSBs, (3) thickness to width ratio of the ISSSBs, (4) compressive strength of concrete, and (5) area of the column's cross-section. The experimental work involved the construction and testing of 25 short specimens with different configurations under concentric axial compression. On average, the results of the study showed that the use of ISSSBs can cause 5% increase in the strength, 65% increase in initial stiffness, and 20% increase in ductility of the concrete core compared to the use of ties. Moreover, it was found that the lateral confining steel is more effective in moderate and high strength concrete than in low strength concrete. The lateral steel volumetric ratio has little effect on strength but great impact on stiffness and ductility. Findings of the study showed that the strength of ISSSB-confined columns can be predicted by the tied columns equation of the ACI 318M-14.