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- To: <seaint(--nospam--at)seaint.org>
- Subject: re: multiple subjects
- From: "Andrew Kester, P.E." <akester(--nospam--at)cfl.rr.com>
- Date: Fri, 16 Jan 2009 14:34:05 -0500
Wind on soffits: Agree with Conrad, we always have used the wall pressure of soffits or U/S of overhangs. If you take overhang pressures (which include both surfaces) and compare them to a roof edge pressure + wall pressure, they will be very close. Think about every canopy in front of every retail box, this condition exists. My thinking has always been positive wind pressure hits the wall, goes up, hits the soffit with equal or lesser pressure. Just some common sense, no wind tunnel testing or Bernoulli-type thinking.
Tanks in-ground in Gulf region: Persian Gulf or Gulf of Mexico? All the Gulf Coast states in the US, near the coast, have very high water tables. You must design for buoyancy, the tank can pop out of the ground in the summer wet months. Seen it happen with swimming pools that are emptied when the table is up, still blows my mind! Get a geotech report so you know if you have clay and where your seasonal high water table, as well as your 100yr flood water table is located. In Florida those who design pump stations have to consider this, which just means thicker tank walls which helps you structurally.
Wood box beam- Bill, how much is just a little load? For big fun on a Friday afternoon, I calc’d on Enercalc a 27ft span with low grade S Pine, with a base Fbt and Fbc= 900psi, (2) 2x14 work but not by much complete unbraced (bending stress ratio=0.902). But fully bracing these only reduces it down to 0.827. That assumes self weight and 50plf dead load.
However, regarding sheathing as part of a box beam, will this be built on the ground and then erected as a box, or the plywood will be added after the beams are up. The first really acts as a composite beam with the plywood in compression and tension, and the 2x beams are webs in shear (technically you need to design the fasteners for shear in shear flow, I am sure 12d at 6” oc would be fine).
The second I think would be more like using the sheathing the same as a floor or roof diaphragm, with the beams already loaded there would be little composite action and shear flow through the nails. I think the sheathing in this instance would be more than adequate as a bracing member if it is 12” wide and very lightly loaded. Sounds like if you use reasonable sizes you will be fine either way due to the light load. You could analyze the crap out of this and you will likely find you are being very conservative either way and have a lot more capacity then you think…
Andrew Kester, PE
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