Moment of Inertia - Corrugated Metal Pipe (CMP)

[sligmalojet]

I have a 16 gauge 24 in. diameter corrugated metal pipe with 2-2/3’’ x ½’’ corrugation. For my analysis I'm looking for the moment of inertia in units of inches^4. The tables that I have found give me 1.892 x10^-3 (in.^4/in) for the CMP dimensions. This seems like a very low value and I'm not exactly sure what the per inch is for in.^4/in. I've hear it called the moment of inertia per unit length of cross section of the pipe wall but I'm not sure what dimension they're talking about. Everything I have tried gives me very low values for I. Is the I for a CMP this size usually really low (like 0.454 in^4)?

 

[KootK]

I suspect that the values are quoted per inch of CMS and would be measured along the longitudinal axis of your 24" pipe. Are you seeking the Ix for the pipe or the Ix of the sheet from which the pipe is fabricated? The "per inch" values are probably the latter

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BUGGAR]

How do you measure section properties for a "per-buckled" shape?

 

[XR250]

My question is why you would need the section properties?
What application is this?

 

[IDS]

As suggested by KootK, the quoted figure is clearly for bending of the corrugated sheet about the longitudinal axis, and the number looks the right order of magnitude.

If you are interested in the bending stiffness of the whole pipe about a transverse axis you can't just use the geometric properties because in this direction the buckles greatly reduce the stiffness. I don't know how it is done, my guess would be that it is based on testing rather than a theoretical approach.

Where did the 0.454 number come from?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[sligmalojet]

I am using the pile analysis program LPILE to model a pile configuration commonly found in foundations of bridge abutments. I have a 40 ft long 12.75" round steel pile driven in the ground with a 20 ft long 24" CMP sleeve enclosing the top half of the pile. The annular void has been filled in with loose sand. I know the section properties (Ix, etc...) of the round pile but I'm not sure about the CMP. The 0.454 in^4 comes from the 1.892 x10^-3 (in.^4/in) value I received from CMP property tables multiplied by the 20ft length (240 in) as suggested by KootK which resolves my per inch dilemma. It sounds like I should expect a low value for Ix considering the corrugation will reduce the bending stiffness, I just haven't worked with CMPs enough to know typical values.

 

[KootK]

I'd ignore the stiffness contribution of the CMP for two reasons:

1) It will be low as discussed above.
2) With sand in the interstitial space, it won't act compositely with the pile.

If you wanted to estimate the E x Ix of the CMP, I would recommend this procedure:

1) Calculate an Ix based on the sheet thickness and the average diameter of the CMP.
2) Modify your E value to account for the "accordion" effect that the the corrugations would have on axial stiffness.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

If the CMP is filled with loose sand then it will not deflect compositely with the pile, and the pipe will have very little effect on the bending stiffness. Presumably the sleeve is provided to reduce the horizontal constraint on the pile. The loose sand will have a greater effect on the lateral stiffness of the pile than the pipe does. I would suggest investigating standard design procedures for structures of this type. Integral abutment bridges would be a good place to start.

Also consider what it is you are checking. If you want maximum deflections and bending moments in the pile then you should use lower bound pile stiffness, so ignore the pipe and reduce the soil stiffness (or treat it as unsupported over the top 20 feet). If you want maximum shear forces at the top of the pile then you should use an upper bound stiffness, so ignore the effect of the loose sand. If the upper or lower bound analyses produce unacceptable results you will have to look at it closer.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/