Seismic performance of self-centering structural walls incorporating energy dissipators

This paper presents elements of seismic design for jointed precast cantilever wall units designed to rock about their foundation. Gravity loading and prestressed unbonded tendons provide the restoring force in these walls. Lateral displacements eventually result in a separation gap forming only at the wall-foundation beam connection. The gap reduces the wall stiffness and results in nonlinear response. Design of these walls is made with the explicit objective of ensuring a self-centering response. That is, the wall returns to its preearthquake position upon unloading from a large displacement excursion. The integrity of the walls is maintained because no plastic hinges form and there are no residual postearthquake displacements. Energy dissipators, in the way of longitudinal mild steel reinforcement crossing the joint between the walls and the foundation, are incorporated into these walls to add significant energy dissipation capacity while preserving the self-centering response. This paper also describes the results of an experimental program performed on three half-scale precast concrete jointed walls. The wall units were tested under quasi-static reversed cyclic loading conditions. These units were designed for various performance objectives. The test units underwent a prescribed quasi-static reversed cyclic testing regime and attained drift ratios in excess of 3% while preserving their lateral-force capacity and ability to self-center. Two units incorporated energy dissipators. These displayed a characteristic "flag-shape" hysteretic response and had equivalent viscous damping ratios of 14%. The performance objectives selected in the design of the walls were closely met in the experimental program.