Buckling Calculations 1

[FDS2008]

Does anyone know if there is some type of rough calculations that we can use to verify whether a larger pipe (D/t>100) will buckle if we have a compressive load, bending and external pressure?

Or do we need to carryout FEA on this?

Thanks

FDS2008

 

[TGS4]

If you are talking pipe, then I would look at ASME B31.3 - and then look at 304.1.3. This, of course, bounces you to ASME Section VIII, Division 1, UG-28 through UG-30. To be conservative, you could figure out what the maximum compressive stress is from the axial load, bending, and external pressure, and compare that to the maximum compressive load from UG-28 through UG-30. (BTW, this is indeed done all the time with tall pressure vessels - with weight being the axial load, wind or seismic being the bending moment, and vacuum being the external pressure.)

 

[BigInch]

You can check the Euler buckling load formula for a quick look.

I hate Windowz 8!!!!

 

[fegenbush]

You could refer to the procedure in Chapter H of the AISC Steel Construction Manual (14th Edition). Alternatively, you could look at 3.4.3.1 of AWWA D100 (2005 or 2011 Edition) to determine the allowable compressive stress.

 

[FDS2008]

Thank you all!

TGS4: ASME Section VIII, Division 1, UG-28 through UG-30 only deals with external pressure. How do you convert max. allowable external pressure to compressive force in order to add up all the components (i.e. pure axial force, axial force due to bending moment and axial force due to external pressure)?

Big Inch: Euler buckling load will give me the critical force which is good, but my issue is calculating the forces in the pipe to verify that they are below the critical force.

fegenbush: I will look into the references but I still need to include external pressure in the equation.

 

[TGS4]

FDS2008 - to answer your question to me: my algebraically adding them together. That the axial external load ans axial load due to external pressure add seem to me to be obvious. For the bending moment, the axial force actually varies in a first-order Fourier series (i.e. sinusoidally). However, at the point of maximum axial force from bending moment, you could conservatively add that to the other values.

 

[FDS2008]

TGS4: If I use Pr/2t to convert external pressure to axial force, I get 4 lbs of axial force. I calculated a max. of 92 psig external pressure using Section VIII DIV I and somehow I cannot believe that the external pressure (1 bar) translates to this minimal effect on the axial force. If I had 6 times the external pressure (close to the max. allowable) it would still only translate to 24 lbs. Does this make sense to you?

 

[TGS4]

What's are the values of P, r, and t? While we're at it, what's the length that you are considering?

 

[FDS2008]

The design pressure is 15 psig (vacuum)
The piping is 316L, 42"dia, 3/8" thick and 36" long (connects two vessels)
I have calculated the max external pressure to be 92 psi per ASME Sec VIII DIV I
The forces and moments are yet to be calculated, but I just want to make sure I am on the right track!

 

[Duwe6]

agree that the Bending Moment will be a major concern. The usual pipe design criteria is to allow spans between supports short enough that no more than 1/2" of deflection occurrs. Don't forget the weight of fluid inside the pipe; at 42" it is a *major* concern in the calc's.

 

[BigTank]

Roark's has some formulas for bending with axial compression. Treat the pipe like a beam for the bending and compressive load. Re-arrange the Code formulas to find stress (pressure) if possible [I'm too lazy to crack Section VIII sitting right here beside me]. Put them in a blender...or add them algebraically.

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