Regional damage estimation of tall pre‐Northridge steel moment‐resisting frame buildings in San Francisco during the Loma Prieta earthquake

For many years, steel moment‐resisting frames (SMRF) with welded beam‐column connections were thought to be the best lateral‐force‐resisting system for buildings in high‐seismic regions. However, the 1994 Northridge earthquake revealed the important vulnerabilities of what are now referred to as pre‐Northridge connections, which experienced unanticipated brittle fractures in many buildings in the Los Angeles Metropolitan Region. There are several tall SMRF buildings with pre‐Northridge connections worldwide, raising serious safety concerns about their performance during future earthquakes. To investigate possible undiscovered damage in these types of connections, an advanced probabilistic regional seismic risk and damage assessment is conducted on 97 tall SMRF buildings in the Financial District of San Francisco, California, with pre‐Northridge beam‐column connections that were subjected to the 1989 Loma Prieta earthquake. This study aims to identify the buildings, floor levels, and orientations more likely to have experienced brittle fractures during the Loma Prieta earthquake. Results indicate that despite this earthquake being only moderate in magnitude with an epicenter approximately 95 km away from the Financial District, peak inter‐story drift ratios in the tall buildings reach 0.65%. Median peak probabilities of yielding and fracture of beam‐column connections do not exceed 37% and 12%, respectively. As a result of ground motion directionality and differences in grid plan in the city, buildings located south of Market Street experienced considerably greater building responses and probabilities of damage than buildings located north of Market Street. Estimates of damage from this study suggest that some pre‐Northridge beam‐column connections likely fractured during the Loma Prieta earthquake, but the fractures were not as widespread as in the 1994 Northridge earthquake. However, earthquakes with either higher magnitudes or closer source‐to‐site distances to the city of San Francisco may cause significant damage to SMRF buildings constructed before the Northridge earthquake.