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RE: Glazing Blast Loads[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Glazing Blast Loads
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Mon, 17 Nov 2008 18:12:58 +0000
I think that is Jon Schmidt's recommendation. Send him an e-mail and ask him for the rationale.
There are many ways to design glazing and glazing connections. And there are variances between the State Department methodologies and DoD. There are differences between dynamic and static methodologies. You must be consistent and not mix and match.
The UFC selectively applies elements of the ASTM's. Since you are referencing DoD, stay with the UFC and the methodology contained in the UFC which is contained in section B-220.127.116.11.
Connection Design. ...The connection design load will be determined in accordance with ASTM F 2248 based on the applicable explosive weight at the actual standoff distance at which the window is sited, but not greater than the conventional construction standoff distance. Additionally, the allowable fastener loads will be as recommended by the fastener manufacturer for the materials to which the window or skylight systems are being connected.
...Note:The actual connection design load is dictated by the glass type and thickness determined by ASTM E 1300. Therefore, in order to keep the connections loads reasonable, use a glass type and thickness that just exceeds the required glazing resistance....
DoD "guides" you to design for the calculated dynamic pressures to get the least expensive glazing system with a good degree of conservatism. In addition, most window manufactures have tested their assemblies and can give you what was used in the test. The test is the real deal for performance including the fasteners.
What the UFC is trying to prevent is using a 1" thick laminate glass in a 3' x 3' window in a LLP and designing the connections for the glazing resistance. THAT would be pricey. The UFC does not want to preclude the use of the big bad blast glass, but you only have to design for the minimum glazing and minimum glazing connection requirement. The risk is having the blast glazing and the connections might drive the design of the rest of the building.
Regards, Harold Sprague
Date: Fri, 14 Nov 2008 19:53:33 -0500
Subject: Glazing Blast Loads
For blast loads on glazing per UFC 4-010-01, what is the load that the connections and glass frames have to take?
In this Structure Mag article it says:
"Framing members and connections are designed for a service load equal to two times the resistance of the glazing"
1. There is a huge difference in designing for 2 times the resistance of the glazing as compared to what ASTM F2248 says:
"This practice assumes the framing system supporting the blast resistant glazing shall attach mechanically to the structural framing system with fasteners that will resist forces generated by a uniform load acting on the blast resistant glazing that has a magnitude AT LEAST 2 TIMES THE MAGNITUDE OF THE 3-SECOND EQUIVALENT DESIGN LOAD AS DETERMINED HEREIN." ("HEREIN" lies loads that are based on standoff distances and amount of explosive, not on the resistance of the glazing system)
2. Also, the UFC says on page B-15:
"The actual design load is dictated by the glass type and thickness determined by ASTM E1300" ( I take this to mean the connections should equal at least 1.0 times the glazing resistance?)
"The connection design load will be determined in accordance with ASTM F 2248 based on the applicable explosive weight at the actual standoff distance at
which the window is sited, but not greater than the conventional construction standoff distance."
3. ASTM E1300 has nothing about connection design that I can find.
So where is 2 times the resistance coming from?
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