Pipes/Conduits in concrete slabs 1

[MathMan]

I am unable to find a rule of thumb for maximum size of pipes running horizontally under the top mat of a flat suspended slab. My first guess would be that anything above 1/5th to 1/4 of the slab depth would cause a shrinkage stress concentration, and hence a weak spot in the slab.<br><br>Anyone have any resource that could help me?

 

[HochwaltPE]

Chapter 6.3 of ACI addresses embedded conduit in slab, and limits the maximum size to 1/3 of the overall thickness of the member. It also states that &quot;they shall not impair significantly the strength of the construction.&quot;<br><br>For a non pre-stressed flat slab, the 1/3 rule is probably sufficient to maintain the strength of construction in flexurally controlled regions of the slab, since the conduit will very likely be outside of the compression block. More caution is needed in shear controlled regions since the full thickness of the slab is used to resist shear forces.<br><br>In pre-stressed slabs, the full concrete section is utilized in both shear and flexural regions of the slab. However, since the conduit is well away of from the extreme fiber (and relatively close to the neutral axis), I suspect that the 1/3 rule is sufficiently conservative in the majority of cases.<br><br>The issue of shrinkage is an interesting one. I suspect that MathMan is correct in suspecting that the shrinkage crack will form over the conduit. However, since we assume that the concrete will crack in the ultimate load condition anyway, I don't really see this as creating a &quot;weak spot.&quot; While the floor my be relatively softer under service loads, the shrinkage crack doesn't reduce our ultimate strength.