Waskow points out that the objective of blast-mitigating design is first to save lives but also to minimize downtime and protect property by: Preventing progressive collapse Controlling level of damage Reducing flying debris Minimizing air-blast pressure that enters the buildingOf these, reducing flying debris—particularly glass fragments—is the primary focus for saving lives and minimizing injury.A high-pressure blast wave can impel glazing fragments into a room at more than 100 ft/second (68 mph) and sometimes twice that. Building components can also become dangerous missiles, along with rocks, dirt and assorted shrapnel. Preventing ultimate destruction through structural collapse is, of course, also an objective.
Given that absolute protection from this onslaught is virtually impossible and that good design is thus a compromise between acceptable damage and product cost, the first question the designer must answer for each project is how extensive is “acceptable” damage. Of course the process begins by keeping the explosives away from the building, which requires ensuring an adequate standoff distance defined by security barriers of various types. Individual components then become the object of scrutiny.
The objective in blast-hazard mitigating fenestration design is to ensure that the glazing, frames and anchorage are all able to survive the blast loading. If any one part of the system fails, then the entire system fails. Also, if the window system has a higher capacity than the supporting wall, when the wall fails the entire window system may be blown into the facility.
The four basic criteria for the design of blast mitigating fenestration systems are therefore:
- The glass should remain intact, emitting minimal fragments; i.e., broken, but not blown out.
- The glass should remain in the frame.
- The frame must stay attached to the wall.
- The wall must remain intact to hold the frame.
In terms of the glass, most recommend the use of laminated glazing. An effective insulating glass unit configuration would be annealed or tempered glass as the exterior lite, with laminated glass for the interior lite.
The frame must have adequate structural integrity and accommodate a deep enough glazing bite so that the glazing does not become dislodged from the frame during the design blast event.
Finally, anchors and trim system must be evaluated for type, size, spacing and embedments to confirm their ability to resist blast loading.
To ensure that components are manufactured and buildings are designed to meet the challenges of blast resistance, standards have been developed. The two most widely used glazing performance standards and hazard-classification schemes are published by the General Services Administration (GSA) and the U.S. Department of Defense (DOD). These variously reference certain ASTM standards for evaluating glass strength and testing for blast loadings.
GSA-TS01-2003,Standard Test Method for Glazing and Window Systems Subject to Dynamic Overpressure Loadings, defines five performance ratings (1 through 5, with 1 being the best) to indicate whether and how far glass shards penetrate into a room when the window and wall segment are subjected to a blast of specified peak pressure and impulse.
DOD’s Unified Facilities Criteria, UFC 4-010-01, Minimum Antiterrorism Standards for Buildings, also defines five Levels of protection based on likelihood and degree of building collapse and personnel injury due to flying glass fragments. These are similar to, but described differently from, the ratings cited in the GSA standard.
Because the application of these various standards can seem conflicting, AAMA developed AAMA 510, Voluntary Guide Specification for Blast Hazard Mitigation for Fenestration Systems to reconcile the factors and help architects develop a meaningful specification.
The best way to confirm blast resistant performance, however, is through mockup testing. This can be done in either of two ways, both of which have advantages and disadvantages:
- Open Arena
- Detonation of an actual explosive device is required
- Specimens are mounted in individual chambers
- Allows for testing of multiple windows at one time
- Shock Tube
- High pressure air pulses are released through a pipe or tube directed at the test specimen
- Specimens are sequentially tested in a reusable test frame
- Allows for testing of one window at a time
Both follow the methodology of ASTM F1642.
Fenestration products can also be certified as blast-resistant, which may take either a “product-specific" path by testing as above at an ISO 17025-accredited laboratory according to UFC, GSA, AAMA 510 or other accepted performance standards or a “project specific" path by testing to confirm compliance with individual project specifications and approved shop drawings. Either path requires continued demonstrated compliance with quality control documentation requirements at the manufacturing plant and verification that each certified product and actual installation details of that product comply with the referenced standards, as confirmed during periodic onsite audits of manufacturing plants and installation sites.