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RE: Wind Loads to Round Steel Stack

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Daryl,

 

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.

 

Joe

 

Joseph R. Grill, PE

Verde Valley Engineering, PLLC

2220 Sky Drive

Clarkdale, AZ 86324

Ph. 928-600-5459

Fax 928-649-3659

email: VVEng(--nospam--at)cableone.net

 

From: h.d.richardson [mailto:h.d.richardson(--nospam--at)shaw.ca]
Sent: Thursday, October 07, 2010 3:09 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wind Loads to Round Steel Stack

 

Joe,

 

    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.

 

Regards,

 

Daryl

 

----- Original Message -----

Sent: Wednesday, October 06, 2010 6:06 PM

Subject: Wind Loads to Round Steel Stack

 

ASCE wind loads to a round stack give a total psf for wind.  As some may remember I am doing some small steel stack designs and now have one which the schedule will allow time for me to try to model it using RISA3d plates.  I think I have the basic model put together correctly.  I am not an FEM pro  but the model appears to be good.  I would rather put some of the wind load to the windward and some to the leeward faces of the stack and due to the breech configuration I will at a point look at quartering wind loads.  Would 50% to windward and leeward be reasonable?  I think so as it appears the stresses will be pretty small in any case.

 

Since this is one of my first projects using the plate elements in RISA3D is there one of you “Pro’s” out there that would be willing to spend an hour or so looking at my model for problems and comments.  I will reimburse for the time.  Please respond off list if you may be willing.

 

Thank you,

Joe

 

Joseph R. Grill, PE

Verde Valley Engineering, PLLC

email: VVEng(--nospam--at)cableone.net