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Blast Load Engineering

For Windows, Storefronts, Curtainwalls and Glazing Systems

2 MAIN BLAST LOAD CODES

What type of project do you have? Is it a Government Services Administration (GSA) Project or a Department of Defense (DoD) project? If it is neither of these, and it is not a Department of State project then it is likely a private party project, with criteria based on GSA or DoD criteria. the two main governing codes for the majority of projects in the US are:

US Department of Defense (DoD) Unified Facilities Criteria UFC 4-010-01, Minimum Antiterrorism Standards for Buildings
GSA/Interagency Security Committee (ISC) Security Design Criteria

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.

DoD BLAST LOAD CRITERIA FOR GLAZING – UFC 04-010-01

The DoD’s criteria UFC 04-010-01 Minimum Antiterrorism Standards for Buildings has different requirements than the ISC requirements. The UFC Code allows a prescriptive approach to design based on an equivalent static blast load and charted glazing systems based on level of protection. Levels of protection for glazing systems are specified in a manner similar to the GSA/ISC’s criteria with small differences. These levels of protection are given below in Table 1.

Table 1 – UFC 4-010-01 Levels of Blast Protection for Glazing Systems

Protection Level	Hazard Level	Description of Glazing Blast Response
High	None	Glazing does not break. Doors will be reusable.
Medium	Minimal	Glazing will fracture, remain in the frame and results in a minimal hazard consisting of glass dust and slivers. Doors will stay in frames, but will not be reusable.
Low	Very Low	Glazing will fracture, potentially come out of the frame, but at a reduced velocity, does not present a significant injury hazard. Doors may fail, but they will rebound out of their frames, presenting minimal hazards.
Very Low	Low	Glazing will fracture, potentially come out of the frame, and is likely to be propelled into the building, with the potential to cause serious injuries. Doors may be propelled into rooms, presenting serious hazards.
Below Anti-Terrorism Standards	High	Doors and windows will fail catastrophically and result in lethal hazards.

- What UFC blast resistant design criteria is usually in the specifications? Two very different versions of this UFC 4-010-01 (the 2003 version and the updated change 1 2007 version) are in most of the DoD specifications currently bidding or under construction. Each version of the UFC 4-010-01 is specific in the blast resistant design requirements for the glazing, frame design and connections design. The blast resistant design of connections includes the glazing system's internal connections and glass stops along with the connections of the glazing system to the structure.

Specifications adhering to the 2003 version of the UFC 4-010-01 usually require mullions and frames to be designed for a 1 psi load with L/60 as a limit for deflection. connections design load is usually required 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 big difference is that these loads are intended for design to the ultimate strength (i.e. no safety factors). This all changed with the 2007 version of the UFC 4-010-01.

Specifications adhering to the 2007 version of the UFC 4-010-01 will usually specify a level of protection, sometimes a standoff distance and sometimes a specific equivalent static design blast load. The 2007 version of the UFC 4-010-01 completely rewrote the sections associated with windows and skylights for a prescribed equivalent blast load associated with level of protection and standoff distance. The designer is required to utilize this information with ASTM F2248 to find the equivalent design blast load. The equivalent design blast load is intended for use with allowable strength methods (i.e. safety factors are involved).

- What do window blast load calculations need to include? According to each version of the UFC 4-010-01, Appendix B calculation are required for the frames and the connections. The requirements for the connection design calculations are broad and ranges from all internal glazing system connections, including glazing stops, to anchor connections of the frame to the structure.

- Are window blast testing reports acceptable to submit by DoD blast resistant criteria? Usually project specifications will make an allowance for testing in accordance with ASTM F 1642 Standard Test method for Glazing and Glazing Systems Subject to Airblast Loadings in lieu of the design requirements. If not, the UFC 4-010-01 has a section where it allows blast testing in accordance with ASTM F 1642 in lieu of calculations.

- What type of glass is required by DoD blast resistant criteria? Both versions of the UFC 4-010-01 specify as a minimum ¼” laminated glass for single glazing and ¼” laminated glass for the inboard pane for insulated glass. Specific glass requirements associated with levels of protection and standoff distance are listed in a Tables B-2 and B-3 located in Appendix B.

- What glass bite is required by DoD blast resistant criteria? The 2003 version of UFC 4-010-01 gives the minimum frame bite of 3/8" for structurally glazed systems and 1" for mechanically glazed systems. The 2007 version of UFC 4-010-01 points to ASTM F 2248 for the glazing frame bite.

GSA/ISC BLAST LOAD CRITERIA FOR GLAZING

The ISC Security Design Criteria defines glazing performance levels based on the predicted location of window glass fragments after the blast event. These performance levels are graphically indicated in Figure 1 and described in Table 1.

Table 1 – ISC Security Criteria Glazing Performance Conditions

Performance Condition	Protection Level	Hazard Level	Description of Glazing Response
1	Safe	None	Glazing does not break. No visible damage to glazing or frame.
2	Very High	None	Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.
3a	High	Very Low	Glazing cracks. Fragments enter space and land on floor no further than 3.3 ft. from the window.
3b	High	Low	Glazing cracks. Fragments enter space and land on floor no further than 10 ft. from the window.
4	Medium	Medium	Glazing cracks. Fragments enter space, land on floor and impact a vertical witness panel at a distance of no more than 10 ft. from the window at a height no greater than 2 ft. above the floor.
5	Low	High	Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 10 ft. from the window at a height greater than 2 ft above the floor.

Figure 1 – ISC Glazing Performance Criteria

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.

- What ISC/GSA blast resistant design criteria is usually in the specifications? Many times specifications will include specific blast design pressures (psi) along with an impulse (psi-msec). The impulse is a pressure vs. time area which is associated with blast energy during the time period of the blast event. These two pieces of information are intended for dynamic plastic analysis which will indicate an acceptable level of damage to the system. The acceptable level of damage to the glazing is given in the above figure 1. The acceptable level of damage for the framing members is sometimes specified as end rotation or pocket rotation and ductility.

- What do window blast load calculations need to include? Usually ISC/GSA specifications require calculations proving the window glazing meets the hazard level specified. This can be done with WINGARD. Calculations are also required for the framing and connections. The framing system must be shown to meet the rotation, deflection and ductility criteria specified. The internal system connections and framing anchors should be designed for the peak dynamic reactions at the capacity of the window system as glazing is released from the frame. This provides a balanced design and does not allow the connections and anchors to fail before the glazing.