Rehabilitation of a 1985 Steel Moment-Frame Building

A 1985 steel moment frame is seismically upgraded using passive energy dissipation, without adding stiffness to the system. The design and analysis techniques for sizing the Velocity Braces™ and their impact on the demand capacity ratios are reviewed. The structure was built in the San Francisco Bay Area in compliance with the 1985 Uniform Building Code (UBC). The moment frame contains the classic pre-Northridge nonductile moment connection, complete with weld backup bars left attached. Nonlinear time-history analysis procedures were implemented to verify the demand capacity ratios at the critical beam-column connections. Flexural demand capacity ratios of .6 achieve elastic behavior in the design basis earthquake with R=1.0. The response spectra of the time history chosen for design exceed the requirements of the 1997 UBC Zone 4. Torsional response to earthquake excitation is minimized by strategic placement of nonlinear viscous dampers. Nonlinear dampers that reduce the flexural demand on joints and control interstory drift without inelastic excursions of the beam flanges are achieved. Floor spectral accelerations and maximum drift limits are reduced to be consistent with immediate occupancy performance. The damper driver mechanism, being velocity driven, reduces moment frame demands and allows flexibility in configuration.