Retrofit of pre‐Northridge steel moment‐resisting frames using fluid viscous dampers

Abstract

Abstract 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.