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System performance

Equally important to the design of the glass, is the design of the frames and attachments to the structure. To realize the greatest protection, the laminate must be retained within the framing members to enable the PVB interlayer to behave as a blast shield and prevent debris from entering the occupied space. This can be accomplished by enhancing the framing system design, such as: increasing the glazing bite, adding structural silicone adhesive, introducing enhanced anchorage and/or hardware or a combination of all enhanced glazing techniques. It is strongly recommended to solicit the involvement of a qualified blast consultant to verify the overall performance of the glass and framing system combination to resist the specified blast criteria.

Testing

There are basically two test methodologies utilized in blast testing; shock tube and arena. Each methodology can be used to evaluate glass lites or a complete glazed system. The frame in which the glass is glazed, as well as the anchoring of the frame to the building, both play significant roles in the ultimate success of a glazing system. Without proper glazing, the glass could come out of the frame. Without sufficient anchoring, the frame could come out of the wall.

The tests are different, however, in what you can do with them. Typically, the shock tube tests are done on a single fenestration system per blast and can be less expensive than an arena test. The shock tube sends a wave of pressure through a tube towards the fenestration. The pressure and speed of this wave can be manipulated to simulate many different blast levels. An arena test is performed with an actual blast using detonated TNT (or equivalent weight TNT) explosive material. Several systems can be placed around the perimeter of the blast, and they will experience a different blast load depending upon the distance away from the detonated bomb. An arena test serves a good practice for evaluating multiple variables with the same blast load.

The two main documents used as a guidance in testing are General Service Administration (GSA) TS-01 and American Society of Testing and Materials (ASTM) F 1642. Both test methods recognize shock tube and arena testing. The protocols are very similar, with slight differences in the manner in which glass spall from the glazing is measured at the end of the test. Both documents measure the overpressure, duration and the amount of time the load is applied to the glass – as a means of quantifying the test. The characteristics of the test that are recorded are:

• Overpressure (psi)
• Duration (msec)
• Impulse (psi*msec - calculated from overpressure and duration
• Glass condition
• Glazing retention in frame
• Glass fragment number, size and location
• Frame condition