Buckling equation of steel pipe as cantilever beam 5

[robbce]

Good Day all,

I had a question regarding a relatively simple problem my boss left on my desk for me yesterday. It is a seemingly simple problem of structural design, however I am missing a few tools to determine the solution. The design problem is to size (find diameter of) an 8' segment of exposed API 5L Grade B pipe based on this situation:

The 8' span of pipe in question is to be exposed piping fixed at one end and free at the other. I need to size the pipe appropriately so it won't buckle under a uniform loading of 322#/ft. I solved for the maximum moment, which occurs at the fixed support, but I don't know how to proceed from here.

I am a Civil Engineer that would otherwise have no issues designing this problem for anything other than round steel pipe (i.e. W, S, HP, C shapes, etc.). But, in my experience, I have never dealt with having to design for pipes acting as beams (only acting as columns with axial loading).

It is my understanding that the info I need to determine the failure modes and stresses of round steel pipe can be found in ASME B31.1 Power Piping (equations for buckling of steel pipe is not found in my bible- AISC Manual of Steel Construction).

If anyone would be able to help me out with the equations for finding maximum/allowable shearing stress, maximum/allowable bearing stress, and/or ways to test for buckling in round steel pipe I would greatly appreciate it.

Thanks very much for any and all of your input,

Rob B., C.E.

 

[JohnBreen]

Hi Rob,

Well, as you suspected it is a simple beam equation but with one important difference. What Code are you designing it to?

You stress (M/Z) must be less than the allowable stress for the Code required. The B31.x Codes will give you the allowable stress at temperature but this might be too conservative for your application. AISC will allow 60 percent of yield and that may be what you put into your beam equation (temperature would be an issue to consider). Given the allowable stress just back calculate to see what section modulus you need for the moment you have and round that up to the next size pipe/wall thickness.

Regards, John.

 

[jte]

(equations for buckling of steel pipe is not found in my bible- AISC Manual of Steel Construction).

Rob-

Don't be such a pessimist. Your bible does deal with it - if you have the green version (9th ed) take a quick look at F3 on page 5-48. "Circular Tubes" applies to pipes... Most pipes I've dealt with qualify as compact sections per Table B5.1 page 5-36 (circular hollow sections) as long as you keep your D/t below 94 for 35 ksi yield pipe. But as John pointed out, you need to check the piping code to make sure you're ok from that perspective also.

jt

 

[BigInch]

Sounds like its not a pressure pipe design, so the structural code covers it, not the BXs.

If I remember correctly beams have a lateral buckling mode based on L/ry. Don't you just use L/r in the lateral buckling sense, but instead of rx and ry, you just use r for the circular section. Wow, that's been a long time.

http://virtualpipeline.spaces.msn.com

 

[ApC2Kp]

robbce,

The buckling failure mode might imply that elastic buckling would occur with deflections that are beyond linear analysis limits. You might want to set some deflection limits to check your selection of pipe size. The bending deflection of pipe beam with uniform load is straight forward calculation with section modulus of pipe in tables.

Some might use a 1" as reasonable allowable deflection, or even one pipe diameter as limit of allowable deflection, but check it along with the deflections of smaller diameter pipes for comparison. This small deflection probably would be at lower stress than any code allowable stress for the pipe.

By the way, what is the pipe wall thickness, standard weight?

 

[robbce]

Thanks all for your help, it appears I may have overlooked page 5-48 in my haste. The pipe in question would be standard weight, yes. Tomorrow I will revisit the code and see if I can't get to the bottom of this seemingly easy design problem.

 

[robbce]

Funny, but the outdated version of AISC that I am currently borrowing, is the 7th edition (blue) and doesn't have anything about circular tubes on page 5-48 in section F3.

 

[BigInch]

I think for that edition, you would calculate maximum Fb allowed based on the r for the circular pipe section.

http://virtualpipeline.spaces.msn.com

 

[jte]

...the outdated version of AISC that I am currently borrowing, is the 7th edition (blue)...

Man, you need to get into the 1980's! The ninth ed is copyright 1989...

jt B-)

 

[ApC2Kp]

robbce,

The ANVIL International "Pipe Hanger Design Handbook" has pages 76-82 with pipe data including cross section moment of inertia and section modulus, along with weight per foot.

http://www.anvilintl.com/db/catalogs/Pipe_Hanger_Design-07.pdf

Much of the pipe data that was in the old Grinnell Piping Design and Engineering handbook is now split between that book, the Pipefitter's book, and the Pipe Hanger catalog.