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RE: Open Walled "Shed"

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I have encountered several structures similar to what you are describing and
I too felt uncomfortable with the small loads.  Per ASCE 7-95, the internal
pressure coefficient for an open building is zero.  However, for a partially
enclosed building the internal coefficients are +0.80 and -0.30.  This is
too big of a difference to ignore.
 
So, my solution includes the application of the external pressures on the
roof, columns and beams plus an additional +0.60 internal pressure
coefficient.  One thing I overlooked the first time I designed a canopy was
the lateral load associated with the differential uplift pressures on the
roof.  Depending on the angle of the roof, the difference in the
coefficients can be as high as 0.93.  And since the pressures are acting
perpendicular to the roof surface, a horizontal and vertical component is
present on both sides (whose effects could be additive).  The reason I apply
the internal pressure is because if any object is under the canopy wind will
also flow around it resulting in an upward wind flow that does apply
pressure to the under side of the roof.
 
Another case to consider includes the effect of drift/deflection of the
canopy.  For example, if the wind caused a deflection that exposes more of
the underside of the roof, the whole structure will attract more load due to
the increased exposed area, causing it to deflect more and thus more
exposure ...  If you have ever seen a NASCAR race at a super speedway you
will notice that at 150 mph very little of the underside of the car has to
be exposed to start this process and ends up with the car going airborne.
 
Depending on the importance of the structure (I.e. monetary investment by an
entertainment complex such as Disney/Universal), other effects should be
investigated such as flutter and vortex shedding.  To investigate these
topics involved three days at the local university library reading numerous
theoretical articles and papers.  Neither was a concern due to the period of
the structure, but they still needed to be calculated.
 
Hope this helps ... 

William J. Keil, P.E.