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Re: Steel Stack Design[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Steel Stack Design
- From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
- Date: Mon, 18 Dec 2006 12:18:43 -0700
You should be able to design your stack without any software and without running into any difficulty. Stacks are a bit of a specialty of mine; let me give you a description of how I would go about designing your stack.
There are three considerations regarding loading as applied to stacks. These are downwind loading (or normal wind loading or "drag"), cross wind response or vortex shedding (which is a dynamic loading) and ovalling. All of the stacks I have worked on have been in very low earthquake locations, therefore, I do not have any advice to offer regarding seismic design for stacks.
Ovalling results from positive pressure on the up wind side of the stack and negative pressure on the sides and back. Roark "Formulas for Stress and Strain" can give you anything you need for analyzing uneven loading on a circular ring. If, as I suspect, your stack is simply a piece of 36" diameter standard pipe you shouldn't even have to bother checking this out. If it is very thin wall pipe or rolled plate; and if this is your first stack, you may want to check it out.
Downwind loading is actually a dynamic response for stacks. The Structural Commentaries to the National Building Code of Canada (our standard wind loading procedure) provide an excellent procedure for accommodating this consideration. This same procedure, developed by a man named Davenport, is also contained in the "Handbook of Shock and Vibrations" (or maybe it's the "Handbook of Vibrations and Shock). In any event I would expect that the resulting wind load, considering dynamic response, would be about 30% higher than that obtained by a direct application of older procedures utilizing "fastest mile" or "three second gust" as normally applied to buildings.
Cross wind loading. Wind loading perpendicular to the direction of the wind results from vortex shedding. Vortex shedding causes alternating forces, in the direction perpendicular to the wind, applied at the frequency of the vortex shedding. These forces themselves are quite small, however, if they are synchronized with the natural frequency of the stack the resulting amplitudes, stresses, reactions, etc., can be very large. Vortex shedding is critical and must be checked out otherwise you may have an embarrassing and possibly catastrophic problem to deal with. The procedures in the"Structural Commentaries for the National Building Code of Canada and those in the ASME-SST-1, although somewhat different from one another, give similar results.
I hope you find these comments helpful.
H. Daryl Richardson
- RE: Steel Stack Design
- From: Brian S Bossley
- RE: Steel Stack Design
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