Case study of Expected seismic performance of an instrumented steel moment frame building

A relevant number of modern welded steel structures suffered unexpected damage during the January 1994 Northridge earthquake. The damage of welded special moment resisting frames (W-SMRF) typically involved the brittle fracture in the welded girder to column connection areas. In the aftermath of the Northridge earthquake the research and structural engineering communities addressed the issue of the seismic safety of damaged and undamaged W-SMRF’s. However, even though seismic design codes in the United States were significantly revised after the Northridge quake, still there are hundreds of existing W-SMRF’s in the US and in other countries, designed and detailed according to pre-Northridge criteria. This work is a case study on seismic response of a W-SMRF instrumented by the California Strong Motion Instrumentation Program (CSMIP) which was shaken during the 1987 Whittier, 1991 Sierra Madre and 1994 Northridge earthquakes. A detailed finite element model of the structure is developed to analyze the building dynamic response during recorded strong motions. Model accuracy was assessed by comparing prediction of the analytical model with actual CSMIP response records. In order to assess its vulnerability in response to future and possibly stronger earthquakes, nonlinear analyses were conducted subjecting the model to the Newhall and Sylmar records of the 1994 Northridge and to the 1995 Kobe earthquakes. Analysis results show that the structure behaved linearly during these earthquakes. The predicted response to larger and more severe ground motions show that plastic deformation occur at many column ends and inelastic deformation demands exceed the expected capacity of the welded pre-Northridge beam-tocolumn connections.