Average moment of inertia of concrete coated pipe

[firebomber84]

Hi there,

I am considering the concrete coating for the moment of inertia of pipe when analysing s lay using offpipe. Anyone one can provide some help in doing so? currently i use the formula below.

Is = I for steel
Ic = I for concrete
Iavg = Average I
Es = Young's modulus for steel
Ec = Young's modulus for concrete

Iavg = Is + Es/Ec*Ic

IF anyone has got better formula please let me know.

 

[ishvaaag]

The use of such formula in an environment where the concrete can become cracked is not warranted. For a composite beam simply supported where the whole length of the compression head in concrete develops no (hopefully) cracks in tension, it is fine. At the cracks of a cylinder in flexure is less than the formula you use asserts; it is however bigger than Is because the other side it is expected would remain attached without cracks.

Maybe the crack distances along the member can be studied with the formulas proposed for reinforced concrete, but I seriously doubt it, for surely the tests have not comprised composite pipes externally surrounded in concrete. Hence we would be deriving the distances theoretically on assumptions or values of actual slippage at the crack for some length, and driven by the amount of tension the concrete can actually stand and the maximum adherence at the interface. More or less in the same way such distances are derived.

In short one should expect for flexural situations some reduction of the concrete contribution to inertia is required, maybe 60 to 80% of the contribution maybe standing as an average; 50% would be warranted in the compressive side and then who knows around 2/3 average over the length in the other side.

In dynamical situations where vibration may spall the cover I wouldn't consider but the steel.