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RE: Wind Loads to Round Steel Stack[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Wind Loads to Round Steel Stack
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Sun, 10 Oct 2010 03:17:00 +0000
We engineers always focus on the extreme loadings on stacks. Do not overlook the vortex shedding for much lower wind velocities. We are fighting one now that has a vortex shedding problem with winds between 15 and 20 mph. This causes excessive cycling at the EJ at the building. Again, we may have a very stiff stack, but the cycling at other interfaces may be a problem.
Even if you determine that you do not need strakes, provide enough margin for the addition of strakes if you determine later that you have a problem.
Tuned mass dampers can be used, but that is a specialty contractor that will cost some money.
Regards, Harold Sprague
Subject: RE: Wind Loads to Round Steel Stack
Date: Thu, 7 Oct 2010 15:40:34 -0700
Thanks for all the information. Yes, I agree with the FEM, to a degree. The stack for the most part will not be a problem for buckling, vortex shedding, ovaling, etc as they are pretty “beefy”. These were not a problem with the last short stack I did. These stacks are short, 35’ +/- and standard pipe thicknesses. This particular stack has four breech openings very near the base. As I have a little time for some FEM self education I thought I would go through the process for, again the learning experience, and secondly to see what may be happening around the breech openings. Thank you very much for the reply.
Joseph R. Grill, PE
Verde Valley Engineering, PLLC
2220 Sky Drive
Clarkdale, AZ 86324
From: h.d.richardson [mailto:h.d.richardson(--nospam--at)shaw.ca]
I think that a 3D plate analysis is overkill for the stress analysis. The elements of a stack are essentially prismatic beam elements and can be treated as such (with the possible exception of the breach area). I think you need to put more emphasis on the loading and allowable stress.
There are four major considerations you need to look at. These are
1.) Wind loading parallel to the wind direction. Due to variation in wind speed wind is a dynamic loading; the deflections are greater than would result from static wind; and Hook's Law applies, therefore, you need to increase the static wind loading by a factor that you must determine. Usually this factor is between 1.3 and 1.5; but I have seen it at 2.0. There is a procedure in the Shock and Vibration Handbook (or perhaps it's the Vibration and Shock Handbook) which is also part of earlier editions of the National Building Code of Canada. The procedure was developed by a man named Davenport at Canada's National Research Council.
2.) Cross wind vibration, or vortex shedding. The procedures I use are included in the National Building Code of Canada but you can probably find other references more suitable to you.
3.) Ovaling. Due to uneven wind pressures around the stack varying from positive for upwind to negative for down wind there will be bending moments in the horizontal direction. Once you assess the horizontal wind profile Roark will have something you can use.
4.) Local buckling of thin wall buckling. I think the ASME - STS - 1 will cover this (and probably vortex shedding too) adequately.
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