Antiterrorism: Blast-resistant glazing systems and the moving target
The environment of rapidly evolving antiterrorism codes has left estimators and manufacturers in a whirlwind of confusion asking themselves, “Why do blast resistant requirements seem to be a moving target?” It’s important for us to understand what is driving this part of the glazing industry. A great emphasis has been placed on protecting the inhabitants of government buildings from flying shards of glass due to explosion. The U.S. government will be investing great amounts of capital into protective glazing systems during the next 10 to 15 years to make the changes necessary to their existing buildings and for all new construction.
The two major codes driving the changes are GSA/Interagency Security Committee Security Design Criteria and the U.S. Department of Defense Unified Facilities Code UFC 4-010-01, Minimum Antiterrorism Standards for Buildings. The UFC code requires all future DoD programming beginning in 2004 to include blast resistant glazing systems. Over the last couple of years we have begun to see many of these projects entering into construction and it’s essentially the same story for the GSA’s programmed projects.
So, what is required by code for blast resistant glazing systems? The ISC Security Design Criteria defines glazing performance levels based on the predicted location of window glass fragments after the explosion. These performance levels are graphically indicated in Figure 1 and described in Table 1.
For ISC’s higher levels and medium levels of protection (performance conditions 1 to 4) dynamic analysis of the glazing system or blast load testing is required. The GSA has made software titled WINGARD available for restricted distribution for the purpose of dynamic analysis as part of the design. The GSA has also made blast load testing standards available along with many more resources on their website at www.oca.gsa.gov.
The DoD’s criteria UFC 04-010-01 Minimum Antiterrorism Standards for Buildings has different requirements than the ISC requirements. The UFC Code specifies levels of protection for glazing systems similar to the ISC’s criteria. These levels of protection are given in Table 2.
Two different versions of this UFC Code (the 2003 version and the 2007 version) are in most of the DoD specifications currently bidding or under construction. Each version of the UFC is specific in the design requirements for the glazing, frame design and connections design. The connections design includes the glazing systems internal connections and glass stops along with the connections of the glazing system to the structure. Usually the specifications will make an allowance for testing in accordance with ASTM F 1642 in lieu of the design requirements. Both versions of the UFC specify a minimum of ¼-inch laminated glass for single glazing and ¼-inch laminated glass for the inboard pane for insulating glass.
The 2003 version of the UFC is very prescriptive in defining the design blast load required for glazing systems utilizing an ultimate strength design approach—ultimate strength refers to the strength of the component right at failure. Mullions and frames are to be designed for a 1 psi load with L/60 as a limit for deflection. Connections are designed to either 10.8 psi for glazing panel areas less than 10.8 square feet or 4.4 psi for glazing panel areas greater than 10.8 square feet but less than 32 square feet.
The 2007 version of the UFC completely rewrote and substituted the sections for glazing systems and prescribes an allowable strength design approach—allowable strength generally refers to the ultimate strength reduced by a safety factor. The 2007 version also covers only low levels and very low levels of protection. For higher levels, the DoD Security Engineering Facilities Design Manual must be referenced. Design loads are specified with a reference to ASTM F2248 Standard Practice for Specifying and Equivalent 3-Second Duration Design Loading for Blast Resistant Glazing Fabricated with Laminated Glass. The designer must know the stand-off distance for the facility and the explosive weight in order to arrive at a design blast load. Mullions and frames are to be designed to the blast load specified with a deflection limit of L/160 while connections are to be designed to two times the capacity of the glass.
Design and testing requirements for blast resistant glazing systems have been rapidly developing. Differing requirements may appear in the specifications due to the differing codes and versions of those codes. Understanding where these requirements are coming from and the basic knowledge of what is driving these requirements is essential for glazing companies and manufacturers to meet the demands of this rapidly expanding area of the glazing industry.