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RE: Open Walled "Shed"[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Open Walled "Shed"
- From: William Keil <WJK(--nospam--at)brph.com>
- Date: Fri, 24 Sep 1999 09:08:03 -0400
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.
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