Designing Blast Protection
A test explosion shockwave, expanding, then engulfing and destroying two structures, was shown on a video clip at a recent meeting of the Greater New York Construction User Council, a group of building owners and users of construction services focused on improving the management and cost effectiveness of construction. The clip underscored the event’s topic: the vulnerability of buildings to blast.
The main speaker, John Abruzzo of LZA Technology, a construction material evaluation company that’s part of Thornton-Tomasetti, said the General Service Administration (GSA) has performance criteria for window systems in blast situations and is studying how to lessen progressive building collapse. Paul Chistolini, deputy commissioner of the public building services division of the GSA, commented later that these blast mitigation standards were, “developed after the 1995 Oklahoma City bombing and are used in all new GSA designs and on existing buildings as well.”
Owners and designers must estimate a structure’s level of risk—a process that can be difficult. Older structures may become vulnerable if a new high-profile building is built nearby. For example, “buildings next to a high-risk government office will be affected if the government building is a target,” said Abruzzo.
The balance between security and cost is a delicate one. There are aesthetic issues as well; no one wants to live in a building that looks like a bunker.
Solutions vary. Special coatings applied to glass lessen damage from glass shards. It is vital to treat the window and its frame as a single unit. Where land is less expensive, buildings can be set back further from the road—350 feet is ideal, according to Abruzzo. In cities, where land is costly, designers can follow the lead of Israel and Britain and specify blast curtains. Made of various tightly woven materials, these often include high-performance polyester, or fabric reinforced with Kevlar or carbon fiber. Blast curtains, designed for both residential and commercial use, catch exploding glass debris and protect those inside.
The alternative, as Abruzzo put it, “We’ll design better rather than just brick everything up.”