Strengthening of reinforced concrete columns by direct fastening steel jackets

Abstract

Reinforced concrete (RC) columns are the primary type of vertical support used in RC moment-resisting frame building that sustains vertical and lateral loads. However, their strength and flexural stiffness may be insufficient due to fire, earthquake or the scenario that actual seismic load is larger than the designed seismic load. Insufficient flexural stiffness may incur great seismic displacement demand during a strong earthquake, which can lead to the failure of columns. However, previous strengthening methods mainly focused on enhancing the strength but not the flexural stiffness of the RC columns. In response, a novel steel encasement that can increase both the strength and the flexural stiffness is developed. This novel strengthening method features easy installation and quick strengthening as direct fastening is used to connect the four steel plates surrounding the column. This new connection method is used to connect two steel components quickly and stably by driving high strength fastener into them. In this developed steel encasement, the steel plates are composited with the concrete column and hence can directly sustain axial and lateral loads. The connections behave like transverse reinforcement, which can provide shear strength and also passive confinement to the concrete. As a result, the axial load capacity, lateral load capacity, flexural stiffness, and deformability of the column can be enhanced. The experimental study on the shear connections jointed by direct fastening is first conducted. Based on the experimental results, the failure modes, ductility, shear strength, effectiveness stiffness, and ultimate displacement of the direct fastening shear connection are examined. A series of axial load strengthening tests are then carried out. Special attention is given on the robustness of the shear connection jointed by direct fastening. By inspecting the failure mode, the axial load-deformation curves, the performance of the ductility, and the strain variation in steel plates, the critical parameters which affect the load bearing performance and deformation behavior are identified. A third experimental investigation of strengthened columns under reversed cyclic lateral load with a constant vertical load is conducted. The robustness of the shear connection jointed by direct fastening is checked against the test results, and the critical variables that affect the performance of the strengthened RC columns are subsequently identified. Furthermore, a theoretical model is developed for the strengthening method to predict the lateral load capacity and the generalized force-deformation relation is also examined. To quantify the confinement effect of the direct fastening steel jacket and stirrups, a theoretical model that applies to rectangular columns subject to axial load is developed. On the basis of parametric study, the optimum stress ratios of the stirrups and the optimum shear force ratios of connectors for engineering designs are obtained. A detailed design procedure for the application of the direct fastening steel jacket in strengthening RC columns is reported. To illustrate the application of the proposed design procedure, a working example is presented.