Counter terrorism: Tougher fenestration standards strive to reduce casualties
Explosive terrorist attacks during the last 13 years have made counter-terrorism a national priority. Protection goes beyond the high-profile news of detection, interdiction and military action. The mission of protecting civilians from building hazards has officially expanded from fire safety and mitigation of natural disasters to include intentional attack, particularly with explosive devices.
The force of blasts and their threat to structural integrity often represent the major source of injury in an explosive attack. In mitigating this hazard, first ensure that the building has a balanced window system, meaning that the glazing, frames, anchors and structural components must all survive blast loading. Otherwise, the weakest link will cause the whole system to fail.
The government response
This mission has led federal government officials to require blast resistant and hazard-mitigating design for new federal facilities within the United States and overseas. One of the initial steps was the January 1997 publication of the U.S. General Services Administration’s “Security Criteria” for government buildings, addressing risk from window glass fragments.
The Interagency Security Committee, composed of 26 member agencies, set out to adapt GSA’s criteria for all federal agencies. The result was the May 2001 release of the ISC’s “Security Design Criteria for New Federal Office Buildings and Major Modernization Projects.” It applied to all federal buildings except those under Department of Defense jurisdiction, as DOD had its own requirements.
The GSA also developed its “Standard Test Method for Glazing and Glazing Systems Subject to Dynamic Overpressure Loadings”; the current version bears the numbers GSA-TS01-2003. It employs both open-arena testing using actual explosive charges, or contained, simulated blasts utilizing a “shock tube” to assess blast resistance of fenestration systems. The GSA standard defines five performance conditions–numerically defined from one, “safe,” through five, “low,” to indicate whether and how far glass shards penetrate into a room when the window and wall segment are subjected to a blast of calculated impact pressure.
Meanwhile, DOD published its Unified Facilities Criteria 4-010-01—the most recent version dated October 2003—titled “Minimum Antiterrorism Standards for Buildings.” The UFC 4-010-01, more stringent than the GSA-ISC Security Criteria, includes 23 prescriptive standards in four major categories: site planning, architectural, structural and electrical-mechanical.
To minimize hazards from flying glass fragments, UFC 4-010-01 sets forth prescriptive provisions for new and replacement glazing and window frames. These include laminated glass of minimum 1¼4-inch thickness and metal window frames capable of resisting a static load of 1 pound per square inch, with the resulting deformation not exceeding 1¼160 of the unsupported member lengths. The UFC standard also defines its own five levels of protection ranging from “below AT [antiterrorism] standards” to “high,” that must be met under different conditions of occupancy. These are similar to, but described differently from, the performance conditions and protection levels cited in the GSA-ISC standard. Fenestration industry leaders also noted that the DOD standard proposes to minimize hazards through reduction in window numbers and sizes.
Migration to the private sector
Members of a relatively small nonprofit dedicated to the study of blast-resistant and other forms of security glass, the Protective Glazing Council of Topeka, Kan., sit in a good position to detect trends in this market. As recently expressed by PGC leaders in a white paper “An Information Paper on the Fenestration Industry Focus on Protective Glazing Systems,” published by PGC, dated June 20, 2005: “Private building owners and developers are increasingly adopting and implementing the [federal] standards.” However, there has been some confusion in applying the various government standards because of the differences among them.
A major step in the private sector was the development and subsequent revisions to ASTM International’s “Standard Test Method for Glazing and Glazing Systems Subject to Air-blast Loadings,” ASTM F 1642-04. This standard describes six hazard ratings for glass breakage ranging from “no break” to “high hazard.” Again, these levels are similar, but do not directly correspond, to those specified in GSA-TS01-2003 and UFC 4-010-01.
“As the requirement for [blast-resistant] products was growing, it became obvious that those responsible to specify or buy did not understand the intricacies of the products and were writing very vague if not incorrect [requests for proposals] or specifications,” says Dennis Kelly, senior vice president of Graham Architectural Products in York, Pa., the chairman of an American Architectural Manufacturers Association task group assigned in 2001 to look into the situation.
“The National Institute of Building Sciences in Washington, D.C., handed the challenge to reconcile blast-mitigation specs off to AAMA in much the same way as we handed off the InstallationMasters program concept, because of AAMA’s 40-year track record in window certification,” says Earle Kennett, NIBS vice president of technical programs. Members of the AAMA task group set out to rectify the problem by developing a performance-based, rather than prescriptive, guide specification that allows manufacturers, specifiers and end-users to design and specify fenestration systems that may be subjected to defined blast-load criteria.
The result, published in March: AAMA 510-06, “Voluntary Guide Specification for Blast Hazard Mitigation for Fenestration Systems.” Referencing ASTM F-1642, UFC 4-010-01 and GSA-TS01-2003 where appropriate, the document reconciles differing criteria, performance definitions and testing protocols by defining a rating system and establishing test-report requirements.
“The intent was to develop a ‘road map’ to follow when buying, specifying or developing blast-resistant products,” Kelly says. “The plan was to use all existing testing protocols when at all possible. This way, anyone who had already tested to acceptable methods did not have to re-test.”
The suggested guide specification requires that fenestration systems meet the load conditions described in terms of peak blast pressure in kiloPascals (metric) or pounds per square inch, equivalent triangular load duration in milliseconds or impulse in kiloPascals per msec or psi per msce, and the performance conditions. The latter are specified as numerical values (1 through 5) defined by ISC/GSA-TS01-010. These are related in a separate table within AAMA 510-06 to the fenestration glazing response levels described in ASTM F 1642-04 and the “level of protection” nomenclature of UFC 4-010-01.