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Loads on Wind Girders

Wednesday, January 19, 2005   By: Stephen H
API-650 and AWWA D100 include equations for sizing wind girders, which give section modulus of the wind girder as a function of tank dimensions and wind speed. Does anyone know the basis of these equations? Are there any published papers on the topic? I would assume that it is based on some assumed stress level and some approximate moment calculation, but have never seen anything defining how that came about.

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Saturday, February 12, 2005   By: Stephen H
McGrath's 1963 paper "Stability of API Standard 650 Tank Shells" mentions two criteria for the top wind girder section. One is the buckling, and the author gives an expression for the buckling load in a radially loaded circular ring. The other criteria is the Z=0.0001D^2H formula, which he quotes from API-650 as an established rule, with no mention of its derivation or the reasoning behind it.

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Sunday, January 23, 2005   By:
AWWA can get you the basis of the calculations. From the best of my knowledge they are included in thier standards. You just have to purchase the right standard. They use to be in there anyways. The safety factor is 7 the last time I saw it which was several years ago. The National Fire Protection Association use to have simular data related to the subject matter your seeking.

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Monday, January 24, 2005   By: Steve Braune, braune@tankindustry.com
I do a not believe that the factor of safety for wind girders is seven. The basic derivation includes a wind velocity of 100mph and an allowable bending stress of 15ksi. That does not get one a factor of safety of seven.

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Wednesday, January 19, 2005   By: Lawrence
The wind girder structural shape sectional modulus is based on second and first year college courses. These are beam equations. The application of the winder girder with the tank is based on imperical evidence that gives the best placement results which are now qualifing rules. I suggest a reveiw of your course work to help sort out confussion and maybe take a refresher course. These have been around for at least 50+ years. I hope others share their unique insight. Lawrence

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Wednesday, January 19, 2005   By: Stephen H
If it's simply a "beam equation", what beam length and what load assumptions are used to derive the 0.0001D^2H formula? And what allowable stress is used? (Note that the same equation is used in codes with widely varying stress, and remains unchanged even when basic allowable stresses have been changed.) That's the part I've never seen discussed anywhere. Of course the actual wind girder is a continuous ring. Formulas for bending of continuous rings are available in "Formulas For Stress and Strain", among other places. The shell buckling criteria is based on the collapse of a cylinder under uniform external pressure. But applying uniform loading from the shell doesn't give any bending in the wind girder- it gives a compressive load, and application of buckling criteria leads to a minimum moment of inertia, not a section modulus. By assuming different variations of wind loading around the tank, you can get different section modulus requirements, but they don't necessarily match up to the equation above.

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Wednesday, January 19, 2005   By: Lawrence
Remember z = section modulus = moment of interia/centroid. Lawrnence

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Wednesday, January 19, 2005   By: Steve Braune, braune@tankindustry.com
API and AWWA equation is based on analysis of a ring loaded with a uniform transverse load and resisted by a distributed shear. I believe the allowable bending stress is 15ksi, possiblely with a one-third increas for wind. You have to patch a few Roark cases together for this one

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Wednesday, January 19, 2005   By: Stephen H
This is more or less what I had assumed in the past (it's been a while since I looked at the issue) but at the time, I never could come up with numbers to match the tank codes. The wind load can be taken as: 1) Uniform pressure on half of the outside area of the tank 2) Uniform pressure on one half and vacuum on the back side, each equal to half the design wind pressure 3) Uniform loading in the wind direction only as above (as opposed to the pressure cases which give wind force components at varying directions) 4) Non-uniform wind loading based on actual flow around a cylinder 5) Loading around part of the shell only. The resisting shear load can be taken as: 1) Uniform shear in the wind direction around the tank. 2) Shear stress distribution per some rational analysis. 3) Shear distribution around part of the shell only. Each different load case and each shear distribution will give different bending moments in the wind girder. If I remember correctly, the "rational analysis" of shear gives the distribution shown in Roark's 5th Edition, Table 17, case 18. If you apply shell analysis, you get very little shell load transferred to the wind girder. If you assume part of the shell buckles, then you get more load transferred to the wind girder, but you can't very well use that same buckled shell area to furnish shear to hold the wind girder in place- that's why I mention #5 load and #3 shear above. I had wondered if the analysis wasn't based on assuming, say that a shell section "H" long buckles, and is supported by the rest of the shell.

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