Seismic Upgrade of a 15-Story Steel Moment Frame Building—Satisfying Performance Criteria with Application of Experimental and Analytical Procedures

This paper summarizes the seismic analysis and rehabilitation design of a fifteen story steel moment resisting frame building constructed with connection details that were found to be vulnerable to fracture in the 1994 Northridge earthquake. This building is located in Oakland, California, less than five miles from the Hayward Fault. Previous testing of the existing moment connections demonstrated that the beam flange to column flange complete joint penetration groove welds were vulnerable to fracture, and consequently the building presented a risk to life safety in the event of a major earthquake. Seismic rehabilitation to meet the requirements of the State of California, Department of General Services resulted in a retrofit scheme including a combination of moment connection strengthening and addition of viscous dampers. To meet the proposed requirements, two phases of sophisticated analysis techniques were performed in the rehabilitation design. First, to estimate the necessary connection strengthening and damping, multi-mode two-dimensional nonlinear pushover analyses were performed along with single-degree-of-freedom nonlinear dynamic time-history analyses. Then to refine the scheme and perform final checks, nonlinear time history analyses of building frames were performed. These models included a nonlinear fiber element that approximated the fracture behavior observed in the existing connection tests and incorporated results from the test program to model the strengthened connection behavior. Concurrent with the analysis portion of the work, various connection modification schemes were studied, designed, and subjected to full-scale laboratory testing. Available design procedures contained in the AISC/NIST Design Guide 12 were modified to include column and panel zone deformation and adapted to alternate strengthening configurations.