Conduits In Concrete Slabs

[RIDGEHILLSTRUCTURES]

I am in the process of designing a slab with embedded electrical conduit running horizontally to the flexural steel. ACI (current edition in Massachusetts) is not very helpful. ACI no longer has spacing requirements for embedments. ACI says "Embedments shall not significantly impair the strength of the structure.." Interpretation of this statement is laid onto the designer, as it says nothing more than just that. The most important thing in my opinion is to if need be, provide data that supports the decision making for the design, which will hold up an argument in court.

That being said... I prefer to have the conduits between the mats of steel in the middle third of the slab cross section in effort to avoid conduit in the compression zone. While I understand the compression zone does not always extend to the middle third of a cross section, this middle third requirement helps navigate construction error and tolerances. We can prove that conduit displacing concrete in the compression zone decreases the flexural capacity of the slab, which is evidence to back our decision to keep it in the middle third.

I do have a couple questions Id appreciate getting opinions on, from other design professionals.

1. How conduits running parallel to tension steel in the compression zone effects the flexural capacity.
2. If the ACI embedded conduit spacing requirements of older codes (3Diameters of the embedded item, spacing center to center, min.) is followed by contractors, and the conduit is below the compression zone and above the tension steel can we assume flexural capacity as not being affected.
3. Should we be specifying spacing requirements for conduit in relation to the tension steel to provide full concrete coverage around the tension steel. This question stems from seeing conduit placed directly on top of and parallel to tension steel.
4. Realistically, these conduits that follow the ACI spacing requirements eventually need to converge and sweep up into large junction boxes. This convergence impairs the structural capacity of the slab as it essentially separates the engagement of tension steel to the compression zone in large swaths of conduit. Do we not allow them to converge in the slab or to we integrate some sort of haunch section, or other ideas....?

 

[Lomarandil]

Conduits between the mats makes sense for a lot of reasons, although it requires coordination between the trades. I'd argue that the effect of any conduit with diameter less than 25% of the slab thickness, spaced reasonably (use ACI's old 3D rule as a starting point, although I could be flexible here), and between the mats would have negligible effect on the flexural capacity and insignificant to very little effect on the shear capacity of the slab.

(with lightly reinforced slabs, I don't often see the compression zone extending beyond the "compression side" reinforcing, so the distinction of "compression zone" and between the mats is perhaps not needed)

If your conduit diameter within the 25% rule stated above, I wouldn't be overly worried about the conduit disrupting bond either. We practically don't often adjust slab development lengths for lapped reinforcement or congested reinforcement scenarios, it's not worth the cost savings. And I don't see conduit that differently.

I think the convergence is fine, so long as it occurs in a section of the slab that isn't critical structurally. Don't put it at the punching shear perimeter of your column or a major flexural zone of a raft footing