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Re: Open Building with Gable using ASCE

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> From: Roger Turk <73527.1356(--nospam--at)compuserve.com>

> Paul Ransom wrote:
> 
> . > It would act a lot more like an airfoil if the underside was flat or
> . > nearly flat relative to the exterior profile. Typically, with open gable
> . > sheds, the interior roof is open with the same profile as the exterior
> . > roof.
> 
> . > Also, the windward slope uplift is a result of wind blocked by a wall at
> . > the windward face and forced with a vertical component past the eave.
> . > This condition does not exist with an open structure so the transition
> . > slope is different.
> 
> It is my understanding that an airfoil (uplift) is created by the wind flow
> over the top moving faster than the wind flow under the bottom.  Since there
> is nothing to force the underside wind up against the bottom of a gabled
> roof, the underside wind will travel an essentially level path.

I have no doubt that there will be some airfoil effect for exactly the
reasons that you state. However, an open profile beneath the roof
changes the airfoil effect considerably so that the net pressures may
not be as expected from a wing type airfoil, despite the straight visual
path. Items beneath the roof will most definitely obstruct but also
cause the wind to move faster beneath the roof and possibly evacuate to
the sides rather than toward the leeward eave. The only way to conclude
this would be with specific case tests.

The eave effects are primarily the result of local turbulence due to the
vertical momentum at the blunt windward wall.

The +/- change at 30 deg in, say ASCE 7, comes with a caveat related to
building dimensions including height. When h=0, "zone 2" is neglected
for external surface pressures. Effectively, the 30 deg bifurcation
becomes irrelevent. This reflects the airfoil analogy but still does not
consider the effects beneath the roof.

Much of this is derived from studies done using solid models (e.g. not
open walls). Many simplifications were made and tables provide the
maximum effect conditions for design procedures.

I will have to look for the Akron test information.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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