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GSA

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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 Condition

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.

Table 1 – ISC Security Criteria Glazing Performance Condition

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.