RETROFIT OF PRE-NORTHRIDGE STEEL MOMENT-RESISTING FRAMES USING FLUID VISCOUS DAMPERS P. URIZ 1 * AND A. S. WHITTAKER 1 Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, U.S.A. 2 Department of Civil, Structural, and Environmental Engineering, State University of New York, Buffalo, NY 14260, U.S.A. SUMMARY This paper describes a study of the use of linear fluid viscous damping devices for the seismic retrofit of a three- storey, pre-Northridge steel-framed building. Four part-perimeter moment frames resisted lateral forces in the building. One of the four perimeter frames was analysed using static pushover, incremental dynamic and nonlinear dynamic analysis to calculate likely beam plastic rotations in design and maximum earthquakes. Ten 10–50 (design) and two 2–50 (maximum) earthquake histories were used for the nonlinear dynamic analysis. The maximum beam plastic rotations exceeded 0020 radian for some of the design-earthquake histories and 0045 radian for one of the maximum-earthquake histories. Fluid viscous dampers were added to the frame using the procedures of FEMA 273 with the goal of eliminating plastic rotations in the beams of the frame in the design earthquake. Average reductions in the displacement of the frame by a factor exceeding 2 were achieved in the design earthquake with the addition of approximately 40% equivalent viscous damping. Although plastic rotations in the beams were substantially reduced, they were not eliminated for 7 of the 10 characterizations of the design earthquake; the calculated maximum beam plastic rotations exceeded the measured plastic rotation capacity of pre-Northridge steel moment-frame connections and so some fractures of the beam connections in this frame would not be unexpected. The addition of the dampers to the building frame led to substantial increases in the maximum base shear and column axial forces, which in practice would likely lead to strengthening of the columns and the foundations. The average maximum axial forces in the dampers calculated by response-history analysis exceeded the values calculated using the first-mode procedure of FEMA 273. Higher-mode effects must be considered using either response-history analysis or the procedures of FEMA 274 to adequately size and detail fluid viscous dampers and their connections to the structural framing. Copyright  2001 John Wiley & Sons, Ltd. 1. INTRODUCTION The M67 1994 Northridge earthquake in southern California caused more than US$40 billion of direct and indirect losses (Mahin, 1998). Although the magnitude of the loss was not unexpected for an earthquake of this magnitude and location, the damage to steel moment-resisting frames took most researchers and design professionals by surprise. Damage was identified in more than 150 buildings and was confined mainly to the moment-resisting beam–column connections. Damage took the form of failed complete joint penetration (CJP) groove welds, beam and column flange fractures, and column web and panel zone fractures. The damage from this earthquake forced the structural engineering community and the Federal Emergency Management Agency (FEMA) to search for conventional solutions to retrofit existing moment-frame buildings. The results of these studies that formed the SAC Phase II project have recently been published in FEMA 350 (FEMA, 2000). The use THE STRUCTURAL DESIGN OF TALL BUILDINGS Struct. Design Tall Build. 10, 371–390 (2001) Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/tal.199 Copyright  2001 John Wiley & Sons, Ltd. Received September 2001 Accepted September 2001 * Correspondence to: P. Uriz, Graduate Research Assistant, Department of Civil and Environmental Engineering, Davis Hall, University of California, Berkeley, CA 94720, U.S.A. E-mail: patxi@ocf.berkeley.edu