Can Strengthening for Earthquake Improve Blast and Progressive Collapse Resistance?

Some engineers suggest that current seismic design provisions, both for new buildings and for strengthened existing buildings, can improve resistance to blast loads and progressive collapse. However, there have been few attempts to quantify such improvement. To begin analyzing this possible relationship between seismic detailing and blast and progressive collapse resistance, the Federal Emergency Management Agency of the Department of Homeland Security sponsored a study at the U.S. Army Engineer Research and Development Center. The study was an analysis of the Alfred P. Murrah Federal Building, which was severely damaged in a 1995 terrorist attack. The building was first evaluated for seismic vulnerabilities as if it were located in a seismically active region. Three strengthening schemes were then designed for the vulnerabilities found during the evaluation: a pier-spandrel system and a new special concrete moment frame, both for the street face of the building, and a set of internal shear walls. In addition to these strengthening schemes, the original ordinary concrete moment frame on the street face of the building was redetailed to bring it into compliance with current building code provisions, without including a lateral load analysis. The three strengthening schemes and redetailed frame were then analyzed for their responses to the same explosion that occurred in 1995. Blast and corresponding progressive collapse analyses showed that the pier-spandrel and special moment frame schemes, as well as the redetailed original system, reduced the degree of direct blast-induced damage and subsequent progressive collapse, compared with the behavior of the original building. Internal shear walls, however, were not as effective in reducing the blast and progressive collapse damage. A key finding of the study was that strengthening the perimeter elements using current seismic detailing techniques improved the survivability of the building, while strengthening elements internal to the building envelope was not nearly as effective in reducing damage.