• Cover-all Performance
Usually, the specs will have a cover-all performance statement like: “provide design of glazing system to meet the minimum blast requirements of UFC 4-010-01.” But what’s usually missing are the specific performance design requirements - level of protect, explosive weight category and stand-off distance. Getting these items identified at the beginning of a project is essential for the project to flow smoothly.
• Glass Thickness Design
UFC 4-010-01 Tables B-2 and B-3 have minimum thickness listed for single pane and insulating glass. Usually, the minimums work in every case, but it must be shown by calculations according to ASTM E1300.
• Framing Components Design
Calculations are typically required showing that mullions deflections and stress do not exceed allowed limits. The limit for deflection is typically L/160 for static blast loads determined from the UFC criteria.
• Connections/Joinery Design
Calculations are required to demonstrate that all of the internal joinery, including glazing stops and the anchors to the structure, are able to resist the minimum of two times the static blast load from UFC criteria or the glazing resistance determined from ASTM E1300.
• Glazing Frame Bite
The UFC requirements point to ASTM F2248. The glazing pane must be adhered on the inside face of insulating units to the framing either with structural silicone or glazing tape. The only way to have a bite without tape or silicone is under the alternative of blast testing.
• Alternative of Blast Testing
All of the above requirements can usually be neglected with submittal of appropriate blast testing in accordance with ASTM F 1642. Many times this has already been performed by the manufacturer. However, if the proposed size of the glass and span of the mullions exceed that which was tested, you may be required to go back and fulfill all of the other requirements. Usually anchors from the framing to the structure are still required to be designed and submitted even with the blast testing results.
I hope this helps with your decision to go after some of these types of projects. The government recovery money is finally making it down the glazing industry. Just make sure that you have the right help to get the job done. Really these projects aren’t that bad, they just sound much worse than their “bite."
--By Stewart Jeske, P.E., president, JEI Structural Engineering