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United States Aluminum: Blast Mitigation Design Guide

Blast Mitigation Systems

In the blink of an eye, the contemporary concept of facility security was forever changed on the morning of April 19, 1995. The attack on the Oklahoma City Murrah Federal Building, and the implications of the complex physical interactions caused by an event of that magnitude compelled Architects, Engineers and the Federal Government to reexamine the design process and expectations for government facilities. A blast event of that nature turns conventional glass into a multitude of tiny shards propelled at over 200 feet per second into a structure. Studies estimate that over 60% of the casualties in the Murrah Building were glass related. Reports of shattered glass were received from up to 14 blocks away from the Murrah Building, demonstrating the vulnerability that structures near a possible target site face for collateral damage.

United States Aluminum has designed and tested the Defender Series of Blast Mitigation Glazing Systems, manufactured
to the highest standard, to provide protection for GSA, DoD and private sector facilities. These systems cover the range of glazing requirements, including storefront, entrance, curtain wall and window systems designed to meet a variety of protection levels. While our dedicated blast mitigation project management team and application engineers will provide assistance in selecting the appropriate system for a project, it is strongly recommended that each project design team work with an experienced blast consultant to ensure compliance.

Blast Mitigation Design Guide

An emerging trend of the green building movement is the emphasis placed on daylighting in buildings. Daylighting is the design practice of placing windows, and sometimes reflective surfaces, so that during the day natural light provides effective internal lighting. Design consideration must be given to glazing used in buildings assessed to be at risk of explosions, both accidental and intentional. Buildings considered at risk include GSA and DoD Federal buildings, Office buildings, research facilities and buildings in proximity to possible target buildings.

As shown in this illustration, a blast wave is an extreme energy release manifested in 4 forms; light, sound, heat and a shock wave. The chronology of a blast wave consists of a instantaneous positive pressure wave that quickly reaches the peak overpressure point measured in psi exerting positive pressure on a glazing assembly which decreases exponentially in milliseconds to the zero pressure point followed by a negative pressure phase where air rushes into the void behind the blast wave pulling the glazing assembly and debris outward.

Blast mitigation glazing assemblies have three critical stress points; anchor, frame and laminated glass. Both monolithic and insulated blast mitigation glass units typically include a thin interlayer material, usually polyvinyl butyral (PVB), to bond annealed or tempered layers of monolithic glass together. The glazed assembly must have a secure anchoring system to keep the framing in place, the framing system must keep the laminated glass unit in place, and though the laminated glass unit may fracture, the glass must remain adhered to the PVB layer according to the Performance Condition of the project. The design purpose for these blast resistant glazed units is to absorb energy and debris from a blast event by deforming in its framing, instead of being rigid in an attempt to deflect the energy.

A host of government and non-government agencies have addressed the issue of blast mitigation for facilities. The United States federal Interagency Security Committee (ISC) developed a blast-resistance standard outlining criteria for design. The General Services Administration (GSA) implemented the ISC Security Design Criteria as its design standard. The GSA is responsible for nearly 60 agencies, the U.S. Courts and Congress; encompassing over 340 million square feet of space in office buildings, courthouses, border stations and warehouses. The GSA guideline for glazing system performance, the GSA-TS01-2003, outlines approved methods for shock tube testing, arena testing and test modeling with software.

This illustration shows a cross section of a test structure defining the protection levels correlated in table 1: ISC Security Design Criteria Blast Protection Levels for Windows. Table 1 describes the six defined ISC Protection Levels and the associated glazing response expectations.

The GSA and the Department of Homeland Security use a standard called the Federal Security Risk Management (FSRM) as a threat/vulnerability assessment and risk analysis to determine the protection level required for a facility.

The Department of Defense (DoD) developed a blast mitigation standard for use in designing DoD facilities,the Unified Facilities Criteria (UFC) – DoD Minimum Antiterrorism Standards for Buildings.

Table 2-1 Levels of Protection (New and Existing Buildings) and Table 2-2 Levels of Protection (Expeditionary and Temporary Structures) detail the varying levels of protection, the potential structural damage and potential injuries. Section B-3.1 Standard 10 Windows and Skylights of the DoD’s UFC 4-010-01 establishes design expectations for blast mitigation glazing systems in response to the designated Levels of Protection.

UFC 4-010-01 establishes a minimum standoff distance of 82 ft. from roadways and parking lots with certain
exemptions like post exchanges that may require different blast mitigation performances due to the proximity
of vehicular access.

Section B-3.1 of the UFC specifies that hardware, connectors, glazing stops and anchors be calculated to
a load of 10.8 psi for glazed units of 10.8 square feet or less, and 4.4 psi for glazed units of more than
10.8 square feet but less than 32 square feet.

Section sets minimum glazing requirements as two 1/8” annealed glass panes bonded together
with a minimum of a .030” polyvinyl-butyral interlayer, as shown in Table B-2 and Table B-3. For insulating
glass units a minimum ¼” laminated glass is required for the inboard pane. These standards are based on
application of the ASTM F 2248 Standard Practice for Specifying an Equivalent 3-Second Duration Design
Loading for Blast Resistant Glazing Fabricated with Laminated Glass
and the ASTM E 1300 Standard
Practice for Determining Load Resistance of Glass in Buildings