LATERAL
LATERAL
(OP)
I am reviewing existing structural dwgs of retail shops where knee brace frames were used in every bay along the front and there are masonry shear walls on other 3 sides. We usually rack the lateral force through the roof diaphragm into the rear walls in such situations. Does anyone know why the lateral force couldn't have been racked in a single story bldg that is 20'high X100'X100' in a non seismic area with 80 mph wind with a metal roof diaphragm? Besides less deflection, what else would be gained by using these frames?






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Some engineers get concerned about the lateral drift that racking the diaphragm produces on the open end. Even with just wind load, the size of the building could be producing lateral drifts in excess of L/400 (or whatever limit is desired). Especially if there is a masonry veneer, a concern for cracking, and a metal deck diaphragm with low rigidity (shear stiffness).
Other than that, I can't think of anything other than the engineer just doesn't like to use the twist of the building to transfer the shear.
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Carl Bauer
www.bauerconsultbotswana.com
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L/230....seems large to me. I checked a table that I use which came from AISC Engineering Journal (First Quarter, 1993) and for an exterior cladding of window wall, they recommend L/400. The footnote indicates that higher deflections could be allowed, but the cladding should be tested with a mock-up for water-tightness. The concern here is for the distress in the sealant and the ability of the connections of the windows to accept that kind of distortion.
1.03" is the deck movement, the top of the window may be proportionately less, but still in the 3/4" range. This would seem to be excessive for a typical 3/8" or 1/2" joint sealant.
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Good joints are expensive...
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It's often less expensive to detail for movement than to increase the stiffness...
Might also want to consider using a trussed header over the window area.
If an architect is involved, then you should advise him of deflections anticipated.