Blast-Resistant Benefits of Seismic Design

Background
In 1995, terrorists attacked the Alfred P. Murrah Federal Building in Oklahoma City, OK, using a truck bomb composed of an ammonium nitrate fuel oil (ANFO) mixture that was estimated to contain an explosive yield equivalent to approximately 4,000 lb of TNT (Reference 1). The bomb was detonated approximately 15.6 feet from a critical column on the street face of the building, destroying the column and several other adjacent structural elements and triggering a progressive collapse mechanism that involved almost half of the floor area of the building. The term progressive collapse is used to describe a chain reaction of structural failures that occurs following damage to a relatively small portion of a structure; the damage that ultimately results is disproportionately large if compared with the direct change that starts the chain reaction.

The Murrah Building was an older reinforced concrete moment frame structure that was designed and constructed in the 1970s. Because Oklahoma City is located in a region of relatively low seismic activity, the original structural design did not consider the effects of earthquake-induced ground motions in accordance with the applicable building code provisions for that time and location.

An analysis of the damage to the building was reported by the Building Performance Assessment Team (BPAT) in Federal Emergency Management Agency (FEMA) Report 277, The Oklahoma City Bombing: Improving Building Performance Through Multi-Hazard Mitigation (Reference 1). In the report, BPAT members hypothesized that many of the techniques used to increase the earthquake resistance of buildings can also improve blast resistance and progressive collapse resistance.
Since the 1995 Murrah Building attack, engineers in the United States and abroad have discussed the possible parallels between seismic design and blast- and progressive collapse-resistant design. The question that arises is whether a building that has good earthquake resistance will resist blast and progressive collapse damage more effectively than one that does not.

To begin addressing the question using analytical data, FEMA commissioned a two-phase study. The overall purpose of the study was to assess:

• older buildings when new strengthening measures to enhance earthquake response are undertaken; and,
What relative improvements in blast- and progressive-collapse resistance are present

• in new buildings that are constructed with current building code-required seismic detailing as compared with buildings that have not been designed to provide significant seismic resistance? Detailing in this case was the process of designing into structural elements the features needed to ensure that they perform well during any designed loading event.

Phase 1 of this study used the original Murrah Building to focus on assessing reinforced concrete buildings. FEMA Report 439A, Blast-Resistance Benefits of Seismic Design, Phase 1 Study: Performance Analysis of Reinforced Concrete Strengthening Systems Applied to the Murrah Federal Building Design (Reference 2), thoroughly documents that assessment. The report concludes that improvements in blast- and progressive collapse-resistance can accrue both from seismic strengthening of older reinforced concrete buildings and from using seismic detailing that is required for reinforced concrete buildings that are constructed in areas of high seismic activity. However, the report also notes that direct conclusions about construction materials and systems other than reinforced concrete should not be drawn without further study.

FEMA then commissioned Phase 2 of this study, which is described in this report. In Phase 2, an older structural steel moment frame was examined in essentially the same manner as that documented in FEMA 439A.