It seems to me that if the box is fully enclosed except for the missing
roof, the wind blowing over the top would create an internal vacuum on all
four walls, more or less equal to the lift pressure if the box had a flat
I would use the code inward and outward pressures on the exterior faces of
the windward and leeward walls, respectively, but add a negative pressure on
the windward wall equal to the lift pressure on a "virtual" roof, which is
probably greater than the negative pressure on the leeward wall.
I would ignore the negative internal pressure on the leeward wall for a
conservative design. This load condition would create simultaneous negative
pressure on the sidewalls, which you may want to consider.
From: James Lane, P.E. [mailto:jamesalane(--nospam--at)hotmail.com]
Sent: Friday, March 03, 2000 8:01 AM
Subject: Wind load on open box
Have an existing 20'x20'x16'height box with the top open. The box sets on
the top of a roof.
Question: Does the second (leeward wall) pick up full wind pressure as the
first wall? Any good references for how much the second wall will pick up.
In the past I have always been conservative and used full pressures on both.
What if the bax were smaller the first wall would shield the second wall and
you would think pressures would be less.
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