Seismic strengthening of RC columns by steel encasement with direct fastening connections

Abstract

Reinforced concrete (RC) columns are crucial structural members that resist both gravity and seismic loads. The seismic resistance of existing RC columns designed based on old concrete design codes may not meet the latest design standard. Hence a new strengthening method that uses a steel encasement to increase the axial and seismic shear capacity and deformability of RC columns with a square section is proposed. This method is quick and convenient because direct fastening is used to assemble the system. In this method, unique hardened fasteners are driven into steel material by a powder-actuated gun or battery-actuated gun. Four columns including two control columns and two strengthened columns have been tested under reversed cyclic loads to examine the reliability and effectiveness of the proposed method. Special attention has been paid to the robustness of the connections joined by direct fastening. Although direct fastening plays a crucial role in the structural performance of the proposed method, current design guidelines have not incorporated this new connection technique. Hence, extensive connection tests have been conducted to evaluate the key factors that affect the bearing strength of the connections. New design equations for the connections that use direct fastening are presented.