Calculating moment capacity and section modulus for pipes/cylinders 1

[yahoo123]

The AWWA M11 gives
Section modulus = I/c = ? * r^2 * t

However, I found in the McGraw Hill Civil Engineering Formulas textbook,
Section modulus = I / c = ? (OD^4 - ID^4)/(32 * OD)

The results are totally different.

Has anybody done any bending moment checks on large pipes/cylinders?

Thanks.

 

[DWHA]

McGraw Hill is correct.

 

[yahoo123]

So I take it that M11 is wrong? For anyone who are interested, this is in the M11 pg 86.

 

[swearingen]

The first one is a close approximation for thin-walled pipes. For instance, if I use it to find the modulus of a 30" diameter, 0.05" W.T. pipe, I'd get:

15^2 * 0.05 = 11.25 in^3

Using the McGraw formula:

(30^4 - 29.9^4)/(32 * 30) = 11.19 in^3

A change of less than 1/2 of a percent.

It doesn't work so well when the wall gets thick compared to the diameter, though:

10" diameter, 1/2" W.T. pipe:

5^2 x .5 = 12.5 in^3

Now if I use the McGraw formula:

(10^4 - 9^4)/(32 x 10) = 10.75 in^3

which comes in at 16% off...



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