Moment of Inertia of Concrete and steel pipe wall

[ONENGINEER]

I have a steel pipe (6" dia) soldier pile wall. The distance between the pipes are shotcreted (3" thick). What is the moment of inertia of the composite materials around the centroid axis? Is this the sum of the moments of inertia? Should E modulus taken into account? Thank you.

 

[hokie66]

The steel pipes span vertically, and the concrete spans horizontally. So there is no composite action, unless I misunderstand your situation.

 

[ishvaaag]

It may also be worth of consideration a depth of micropile joint connections schedule that ensures that a only a maximum X number of joints coincide at some depth. At the joint the micropile has less strength due to loss of section (and entail). Then you need to average -or consider- the effect of this loss of stiffness through the modular repetition of your pattern, resulting in less stiffness than the fully solid pipes would provide. The averaged loss of stiffness is not much since the lesser value only happens in the joint itself and somewhat through the transfer length through the threads, but there is some.

 

[Ron]

As hokie66 says, there is no composite action. This would be modeled as a series of cantilevered "beams" (the pipe piles) with a concrete "deck" (shotcrete), simply supported, between them. I would probably look at the concrete as one-way slabs.

I think I understand why you might think there would be composite action with the pipe being embedded in the shotcrete, but the pipe does not work like a piece of reinforcing steel. The concrete will not interact with the steel pipe in the same way that concrete interacts with reinforcing steel.

 

[JAE]

Yes, especially at the base of the wall where the pipes continue into the ground and the concrete stops - right there where the moment is close to the maximum value (I think the max. is a bit lower but you'd have to analyze it).