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Diaphragm to Shear Walls, ...or add intermediate moment frames

Diaphragm to Shear Walls, ...or add intermediate moment frames

Diaphragm to Shear Walls, ...or add intermediate moment frames

(OP)
I am working on design of a 24 foot high, 138 foot by 150 foot "Big box retail" kind of structure - one story, open interior with columns/bays at roughly 50 ft by 46 ft. I am using bar joist roof framing onto joist girders. I am finding that the metal roofing deck (1.5B22, 3 span condition, 5 foot max span) and joists can carry the roof diaphragm shear directly to the exterior (4 sides) cmu shear walls. Thus my interior joist girder and column grid is a gravity system. Even the diaphragm deflection/story drift seem ok. The numbers and examples I have reviewed from SDI and Vulcraft seem to say this is ok, but 150x138 seems like a really large building to me to not have some interior rigid frames (joist girders to columns)to carry some lateral load and help break up the diaphragm. At what point should I be concerned about the size of this building and the roof diaphragm, and start designing in some intermediate/interior rigid frames to share the lateral load with the exterior cmu shear walls? - Seems as soon as I do that, significant load goes to those frames and the moments (joist girders) become fairly significant. Thanks.

RE: Diaphragm to Shear Walls, ...or add intermediate moment frames

I've done numerous big boxes like that with diaphragms up to about 400 ft.

The concern is that the metal deck diaphragm will deform under major wind forces and all your interior columns will lean a bit - small amounts near your side walls and maximum at the diaphragm midspan.
Just be sure to include provision for second order effects from this - we would typically take the deflection across the width and calculate a PDelta reaction which is an additive lateral force. Usually taking the first order deflection times a factor to estimate the iterative amount.

Also make sure that your deck deflection keeps your columns and walls (at diaphragm midspan) within overall building drift tolerances.

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RE: Diaphragm to Shear Walls, ...or add intermediate moment frames

I think that your building is fine. Remember, with diaphragms, it's not so much about size as aspect ratio. The code in your jurisdiction probably has something to say about diaphragm aspect ratio but you're usually fine up to at least 4:1. For comparison, your aspect ratio is only 1.09:1. Once your building gets up to around 300', you start to get into expansion joints and interior lines of lateral resistance become advantageous.

Here's another way to think of it: what kind of moment frame could you possibly install that could compete with your block walls and stiff diaphragm in terms of stiffness?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Diaphragm to Shear Walls, ...or add intermediate moment frames

(OP)
Thank you both. I guess I haven't done enough of these really big buildings to be totally comfortable with the design yet. But your advice is very helpful and reassuring. Thanks!

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