The applicability of semi-active control for seismic protection of elevated highway bridges is investigated through comparison with active and passive systems. A bridge pier-bearing-deck structure is modeled as a linear two-degree-of-freedom system and three design goals are studied: reduction of pier response, reduction of bearing response and reduction of both responses. The passive system is assumed to have a high-damping rubber bearing and linear quadratic regulator (LQR) control is used for the active system. Normalized peak displacements are used to optimize bearing damping and LQR parameters. LQR-based clipped optimal control is used to command a magneto-rheological (MR) damper in semi-active control, where the MR damper is designed according to deterministic analysis of the active system. Numerical simulations show that semi-active control can reach active control performance if the design goal is to reduce bearing response, while it shows similar behavior to the passive system if the design goal is to reduce pier response. All strategies showed similar performance for the reduction of both responses. Q (C) 2002 Elsevier Science Ltd. All rights reserved.