Responding to the extreme pressure released by an explosive mass, each element of the window assembly works together to withstand the blast load and dissipate its energy. Laminated glass takes the brunt of the blast force, the framing connections and glass bites work together to keep the lites in their frames, the mullions and sashes adequately deflect to handle the assembly's changing shape, and ultimately, less energy is imparted to the building structure itself.
As the blast load is transferred from exterior to interior, the window or curtainwall anchorage is the last stop to ensure the most comprehensive protection. Flexibility is key: Too many anchors make the assembly overly stiff, and add unnecessary time and cost to the installation schedule. The type and the number of anchors depend primarily on the substrate and its robustness.
The U.S. Department of Defense's (DoD's) Unified Facilities Criteria UFC 4-010-01, "DoD Minimum Antiterrorism Standards for Buildings," (revised January 2007) is the essential document on blast mitigation in the planning, design, construction, and modernization of U.S. military facilities.
UFC 4-010-01 indicates a clear preference for blast hazard-mitigating window and skylight assemblies that incorporate laminated glass. In contrast, window retrofit products that depend on fragment retention film, or fragment retention as part of a retrofit "catch" system, tend to have higher lifecycle costs because of operation and maintenance problems and a shorter design life.
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In the current standard, ASTM F 2248 is applied to the level of protection, explosive charge weight, and standoff distance to arrive at an equivalent three-second-duration design loading. In turn, this equivalent load is used to select the glass make-up. Framing members are then designed with L/160 deflection limits, and connections and anchors are sized to absorb and transfer blast loads using allowable stress methods similar to conventional design for wind load. As an alternative, or for higher levels of protection, UFC allows testing of representative mockups or dynamic analyses in which computer finite element modeling predicts how the total assembly responds to peak pressure and impulse for a given explosive and standoff distance.
Details of this design process are presented in Wausau's brochure, "Blast Hazard Mitigation," available for download at http://www.wausauwindow.com/resources.
Even when a test report is provided for a specific product configuration or for a particular project, the assembly tested may not serve other applications equally well. Extending blast design criteria to myriad, specific conditions and details on any given project can be a daunting task. It is vital the manufacturer or window subcontractor have licensed professional engineers available to provide the necessary technical input and coordination with blast consultants and in-house detailers. They also can ensure structural adequacy and aesthetic intentions.
DoD's UFC document was used to develop the Advantage by Wausau line of blast hazard mitigating (BHM) INventTM windows. Available in 2 �-, 3 �-, or 4 �-inch frame depths, the BHM product line was pre-engineered in cooperation with a leading blast consultant to meet the requirements of most military facilities - armories, readiness centers, and child development centers - and it is available on accelerated delivery schedules. The BHM product line comes standard with recycled aluminum framing and in a full range of eco-friendly finishes.
Requirements for other government agencies' buildings - courthouses, offices, and essential facilities - are often based on the U.S. General Services Administration Inter-Agency Security Committee (GSA-ISC) "Security Design Criteria for New Federal Office Buildings or Major Modernizations," to be covered in a future Wausau e-newsletter.
Find us online at www.WausauWindow.com.
Email Wausau's governmental and military market manager at [email protected].
