huangyhg

wind upload calculations
what is the formula to calculate the uplift load on a wood post column? lets say i want to build a 12 ft. sq. open picnic shelter in a 80mph wind zone with 4x4 wood post columns at each corner. how do i determine the uplift load for each column?
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is the roof of ur picnic shelter pitch or flat?
if pitch tell the elevation of ridge?
also roof loading in ur area?
i'd check your local building code (boca, sbc, etc.), and there should be a wealth of information on calculating uplift forces.
i guess for this application you just want a rough idea of what to do to hold your roof down. the code requirements will probably give you a very safe force. if there was a typhoon people would be unlikely to be picnicing so failure is not going to be life threatening. your maximum uplift will be roughly 0.6kpa. that should give you a safe pressure unless you are in a very exposed area.
carl bauer
using the asce 7-95,
use the formula
qz=.00256*kz*kzt*v^2*i
kz=.9 for open terrain (use 1.08 if your on the edge of a large body of water (1 mi accross))
kzt=1.0 (unless you are on top of a large hill, then =2.0)
v=80 mph (this is given by you, check your local building code official because this will very important and may vary if your by the coast)
i=1.0
i get qz=14.7 psf
then use formula
f=qz*g*cf*af
g=.85
cf = 1.3 (for a 30% sloped roof or greater, conservative for slopes < 30%)
af=total area divided by number of posts
i get 16.2 psf *12 ft * 12 ft/4 posts = 583 lb/post
this does not take into account loads from overturning due to wind of the entire structure.
get yourself a simpson abu 44, and use the bolt option.
make sure you provide a large enough footing to hold down 583 lb of uplift (at least 2 ft x 2ft x 1ft)
hth
there is also a provision in the boca code which determines the center of action of the uplift force. in your case, it would be about 0.4 of the distance from the windward edge. therefore the two windward posts would need to resist 60% of the force.

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