Based on a quick review, here are the major changes/impact that I see:
1) The biggest change – Conventional Construction Stand-Off Distances. This is a huge change!
They have re-identified the conventional construction stand-off distances in Table B-2 based on the wall type construction. I anticipate the net effect will be a moving to wall construction which minimizes the construction stand-off due to spacial constraints associated with military bases. So for reinforce masonry and concrete and wood studs w/brick that means about 30 feet and 16 feet and 36 ft respectively! Glazing systems with these requirements will not likely be able to have static equivalent calculations performed – They will be forced to dynamic analysis. I anticipate that these types of installations will start showing up late summer to fall of this year.
2) Another major impact – Testing Requirements. This is also huge! The requirements state that the testing must meet the appropriate pressure and impulse from the applicable standoff and explosive weight. So, given the changes with the Conventional Construction Stand-Off Distances most all of the manufacture’s blast testing (at 82′ Ex weight II and 144′ Ex weight I) will be obsolete! I feel badly for all of the manufacture’s who have invested a great deal of effort and resource to testing in the past 5 years.
3) Dynamic analysis is pushed. Under paragraph B-3.1.3 ASTM F 2248 Design Approach (This is the 3-second static equivalent method) they state, “In order to reduce the conservatism associated with using the ASTM methodology, the window systems may be designed using dynamic analysis or tested …”
4) Framing calculations under the static equivalent method is changed. Framing must now be shown adequate with a design load of 2x the glazing resistance instead of the 3-second equivalent blast load. In some cases (with small glass lites governing – Its usually the case that the Architect is unaware) this will be the difference between 50psf (typical 3-sec load) and 400 – 1200 psf! L/60 is used as a deflection limit now instead of L/160, but I don’t think that will help much – sections will need to be much stronger using this method.
5) Connections and anchors under static equivalent method is changed. The connections and anchors must be designed for either 1x or 2x the glazing resistance instead of the typical loop holes that we have seen 2x the 3-second load (usually 100psf). Anchors and connections at these loads will be difficult if the controlling glazing lite in an elevation is small. I think anchors into masonry and wood and light-gage steel will not be workable with this method.
6) We are now able to Use ultimate strength design instead of allowable stress, but again this helps little in light of the major changes above.
Stewart Jeske, PE
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