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Re: Internal Wind Pressure for Partially Enclosed Bldg.

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> I am designing a steel framed shed that is 40'x60'.  One side is open on
> the 60' length.  The other 60' wall is sheathed as well as the two 40'
> side walls.  It is obviously a partially enclosed building.  ASCE 7-95
> has an internal pressure coefficient of +0.8 and -0.3.  A windward wall
> coefficient Cp. is also 0.8.  My question has to do whether these are
> additive.  If I have wind blowing on the open face it passes through
> until it hits the back wall.  There it is windward and has a 0.8
> coefficient.  This means the windward pressure coefficient is 0.8.  If I
> add an internal pressure coefficient then this must be equal and opposite
> in nature and not only presses against the back wall and uplifts the roof
> but then blows back out the front of the buildingwhere there is no wall,
> therefore negating the windward pressure.  So it seems to me the correct
> way to do this is to say the inside pressure and the windward pressure
> are one in the same.  It does put uplift pressure on the underside of the
> roof which is additive to the top side uplift, but doesn't add extra
> pressure in the horizontal direction.

You're right if you also included the leeward wall effect.  The way you're
visualizing it, the 0.8 windward pressure and the 0.8 internal pressure are
the same, and are NOT cumulative.  However, you then have to add the leeward
wall pressure coefficient of 0.5 to account for the wind that bypasses the
structure and pulls on the back of the building.  The combined (GCpf+GCpi) =
1.3.

Here's another way to visualize it:  In your scenario with the wind blowing
into the opening, the windward "wall" is the opening itself.  Since there is
nothing there, you will not use the windward coefficient in this load case.
The internal pressure is pushing on the back wall and the non-existent front
wall.  The back wall is the leeward wall, with a coefficient of -0.5
(outward).  Therefore, the +0.8 internal coefficient (acting toward the
wall, or "pushing") and the -0.5 leeward wind coefficient (acting away from
the wall, or "pulling", on the other side) are cumulative.  Since there is
zero effect from the non-existent front wall, the total value is 1.3.

----
Jason W. Kilgore
Project Engineer
Leigh & O'Kane, L.L.C.
Kansas City, Missouri



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