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