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RE: diaphragm discussions

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Go to, and order "Diaphragm Design Manual Second Edition - No. DDMO2". It was written by Dr. Larry Luttrell for the Steel Deck Institute. There is an example of an open ended structure.

The US Army Corps of Engineers had a slightly different approach as contained in the old TM 5-809-10 (intended for seismic design). It had you calculate a Flexibility Category with diaphram span/depth limits based on concrete or masonry walls or "other" walls.

Basically you need to check the maximum allowable wind induced drift based on servicabilty limits of the masonry walls (contained in AISC Design Guide #3) and then calculate the lateral drift of the diaphram. Some practitioners will go to a 10 year wind for servicability.

The deck will not "rip apart". In failure, the deck flutes will flatten out locally like a match box resulting in large global lateral drift which will then result in badly cracked masonry walls. Left to continue drifting, the P-delta will push the walls over to collapse. Obviously this should be checked against a 50 year wind.

Harold Sprague

From: "Andrew Kester" <akester(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: diaphragm discussions
Date: Tue, 13 Jul 2004 09:30:38 -0400

I have studied the SDI deck book, and asked around to other engineers, and got a myriad of responses on flexible vs rigid diaphragms. Now I am reading this discussion and getting more confused.

I have never had the need to use a wood diaphragm as a rigid diaphragm in rotation, and do not think I will need to do so, and I would avoid it anyway.

BUT, metal deck is separate issue. Let's take the easiest example I can think of, an open ended retail box store. Steel joists and metal deck roof, CMU walls, plenty of solid shear walls on all three sides with an open front.

A) Can you treat the roof deck with NO concrete fill, no bracing, plain deck, as a rigid diaphragm? B) If it is possible, do I have to first calculate the drift at the top of the CMU walls and compare that with the deck diaphragm deflection? C) So if the deck is deflecting greater then the CMU walls, which is what I see happening in a long, rectangular shaped structure, then the deck will "rip apart" prior to being able to translate shear in rotation?

I am asking you guys because the people here in the office already have their minds made up and I want to hear your views on this subject. This is in pure wind country dirty south, no seismic factors or code restrictions...

Andrew Kester, PE
Longwood, FL

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