Wind Girder Section Modulous- How to Calculate?

[jcoots]

We have been using the guidlines in the API 650 Code for designing our windgirders. However we want to use a shape and size a little different than what is in the code book. Before we do that we want to be sure we are using the correct method for calculating the section modulous. Currently we keep coming up with numbers real close to the figures listed in the code book, but not exactly. Can someone tell me why this might be. Are we perhaps using the wrong dimensions for angle and plate? (I don't see how) or could we be using the wrong method? Right now I am first finding the Moment of inertia about the y-axis then dividing it by the distance from the netural axis to the edge. S= I/c. Please advise, and thanks in advance.

 

[JoeTank]

Several years ago an inquiry to the API asked about a couple of very small descrepancies in the API tabular values. The inquirer provided a detailed summarry of their results. There were only about two or three values that had to be adjusted in the second or third significant figure. You should consider that values shown as correct. Perhaps the difference is due to the use of rounded corners in the calculations.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

If you check the tabulated dimensions for wide flange beams, you'll find it's hard to exactly match the numbers given, without knowing the exact corner radii used. I think it's a similar issue with the API tables.

Anyway, don't worry about it. If you're calculating section modulus correctly, it doesn't matter if API missed it or not, you still need to be correct. Treating a rounded corner as square or vice versa is a reasonable approximate in this case.

I would take the required section modulus equation as being very approximate- try deriving it, and you can get answers that vary by orders of magnitude- so don't get too hung up on the exact values based on it.

 

[jcoots]

Thanks for the advice.

 

[JoeTank]

JStephen,
The API equation, while based on an idealized wind load pattern, is not approximate. The section modulus requirements for tanks under 200 ft in diameter is intended to limit bending stresses to 15ksi.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

Steve, that equation has come up before, and I think I had mentioned at the time that I have never seen a derivation for it (and still haven't; I'm still not completely convinced that there IS a derivation for it). The actual wind loading on a tank is somewhat variable, and most building codes will give a wind pressure quite a bit different from API's. The assumed wind force distribution around a tank is a great approximation. It would obviously be very variable depending on tank dimensions, site conditions, adjacent tanks, etc., but none of this is taken into acount. An unbuckled tank with uniform loading transmits very little load into a wind girder, so the actual load in the girder, given a particular wind load distribution, is in considerable doubt as well. Anyway, that equation has been around longer than I have, and applies for everything from a 6' diameter by 200' tall standpipe to a 200' diameter by 6' deep clearwell, so I suspect that not too much rocket science went into it.