Lessons from damage to steel buildings during the Northridge earthquake

One of the important overall surprises of the Northridge earthquake of 17 January 1994, was the widespread and unanticipated brittle fractures in welded steel beam-column connections. The economy, versatility and presupposed high plastic deformation capacity of welded steel moment-resisting frame (WSMF) buildings led to their common usage in Los Angeles and elsewhere in the US. No casualties or complete collapses occurred during the Northridge earthquake as a result of these connection failures, and WSMF buildings in areas of moderate shaking were not damaged at all. However, a wide spectrum of brittle connection damage did occur, ranging from minor cracking observable only by nondestructive testing to completely severed columns. This paper reviews the performance of steel buildings during the Northridge earthquake, and examines some of the results of studies undertaken as part of a project initiated by US Federal Emergency Management Agency (FEMA) to reduce the earthquake hazards posed by steel moment-resisting frame buildings. The objective of this project is to develop and verify reliable and cost-effective methods for the inspection, evaluation, repair, and rehabilitation of existing steel frame buildings and the construction of new ones. The project is conducted by the SAC Joint Venture, consisting of the Structural Engineers Association of California, the Applied Technology Council and the California Universities for Research in Earthquake Engineering.