PIPE FILLED WITH CONCRETE

[customgeo]

I am trying to find some information on how a pipe that is filled with concrete will affect it's strength with relation to bending. How will filling a pipe affect it's moment of Inertia?

 

[GregLocock]

You need to develop the equivalent section, which you should have done at university for steel reinforced concrete beams. Hint: the strain is the same in the steel as in the concrete at a given distance from the neutral axis.
Cheers

Greg Locock

 

[ornerynorsk]

I'm a little out of my field, but I've built all of my own retaining walls and pressure walls for various structures on my property ( I live on a hill with heavy frost action). I would venture to say that you are using the concrete's compressive strength coupled with the tensile strength of the steel, much the same as rebar inside of concrete, just inverted. The tube can no longer deform under bending forces without first overcoming the compressive strength of the concrete. I can't help you with the formulae, other than my rule of thumb "It'll be stronger than hell!"

 

[stewbaby]

Strength and Moment of inertia will go sky high! Composite shape analysis can be complicated in that you must calculate each sections Ix about its centroid and transpose to the centroid of the composite section before adding together, but in your case, we would assume the pipe and concrete to be symetrical about the same axis...so just calculate each components Ix and add them together to get the overal composite Ix. Having tried to say that, for the pipe or hollow circle, I=[PI(d2^4 - d1^4)]/64 (where d2 is overall diameter and d1 is inside diameter. For the concrete, I=[PI x d^4]/64. Actually, in this special symetrical case, you could just use the Ix for a circle of the pipes outside diameter.

Section Modulus, S=I/c (in in^3)
Flexural strength (yield stress), Fy, (A-36 is a common steel, it happens to have 36,000 PSI yield stress)

Compare Moment ( M = S x Fy ) of pipe against that of composite section to see impact (watch your units).

 

[GregLocock]

However it is usual to assume that concrete cannot support tension, so it only works in compression. This moves the neutral axis of the composite structure, and makes it non symmetrical.

Cheers

Greg Locock

 

[peglor]

Possibly the best way to use this structure is to cap the ends of the pipe in such a way as to compress the concrete. This will give the concrete an initial compression even when the combined structure is unloaded.

When a bending force is applied to the pipe, the concrete will lose compressive stress instead of going into tension on the outside of the bend. You'll have to make sure you're not overstressing the pipe, particularly if there are any cyclic loads involved, as fatigue in metals needs a tensile force to begin.