Running Conduit Horizontally Thru Concrete Slab 4

[steellion]

My project features a cast-in-place, two-way flat plate concrete slab ranging from 10"-14" thick. The contractor wishes to run conduit horizontally through the slab. The structure was not specifically designed for this allowance. Does anyone have any experience with this? If we allow it, to what parameters should we limit the conduit? Any code guidance on this subject? I'm located in the United States.

 

[VAStrEngr]

I am guessing that it is cast against the ground? If that is the case, we have allowed it in several instances where we have equipment which must be powered on the slab without any issues or problems. Our typical conduit size is less than 2" OD which might also be different in your case.

Will pull out the ACI code later and check...I am fairly certain I remember seeing it in there.

 

[slickdeals]

I am not sure I can answer that specifically, but take a look at ACI-318 Section 6.3 : Conduits and pipes embedded in concrete.

 

[steellion]

VAStrEngr, we do have slab-on-grade, but I was referring to elevated slabs, not slabs cast against the ground.

 

[Gumpmaster]

I've allowed it in thick slabs (10" is probably too thin), but only in the middle 1/3 of the slab.

Too low and the conduit takes a tension load, too high and it displaces compression concrete.

It's not ideal, but if you only have a couple of pieces of conduit in a large slab, it shouldn't be a problem.

 

[JAE]

I'd agree with Gumpmaster and also second the idea of slickdeals with 6.3.

Also keep conduit AWAY from columns and high shear areas - I think 6.3 might talk about that.

 

[steellion]

Thanks, everyone. I had read 6.3 previously (even had it highlighted in my ACI book) but forgot it was there. According to ACI, conduit through the slabs/beams/columns is permissible, but only if it's accounted for in the design. You can't just allow conduit in the slab and call it a day.

 

[slickdeals]

JAE:
You bring up a good point. ME's always like to run up a conduit adjacent to the column which means they have a 90 degree elbow close to the face of the column and kills you in punching shear.

Sometimes I wish they had a little bit more well rounded knowledge of how they affect other disciplines.

 

[steellion]

JAE, you're right, that's certainly a consideration. ACI 6.3 does list rules on the size and amount of conduit that can pass thru a slab without structural engineer's approval. A 2" dia. conduit would not have a major negative effect on punching shear capacity IMO. We'll take a close look at any large openings, especially near columns. We already have several drop caps to account for punching shear.

 

[ron9876]

Have seen this many times in PT slabs. Always seems to show up in the field after the bids are set and the conduit is in place. A lot of time it just won't work. However you have to allow some conduit in a residential project because there isn't anywhere else to place it. Just need to review it before constrcution. I have started requiring a conduit drawing submittal before I approve the reinforcing steel shop drawings. In a 10" slab that isn't PT you can place a decent sized conduit without damage to the capacity but not at columns.

 

[mijowe]

It is difficult to get an electrical sub to put the conduit in the middle third of the slab because they like to tie it to the bottom rebar mat. Putting it in the middle third is a battle every time because they then have to supply their own chairs/supports. (I do have a PTI publication that recommends conduits over 1” be put in the middle third, 1" and less anywhere in the slab)

Anytime the conduits enter the slab from an electrical room the number of conduits is large, this always tends to be a problem area as well. For this I have them run the conduits under the slab until they are spread out enough.

Other items that i put in my notes:

No conduit In the shear cone
Not conduits larger than 2” or ¼” the slab thickness, whichever is less.
No crossing of conduit (again always a problem to get the sub to comply)
No displacement of tendons or rebar
No conduits in parking slabs
No aluminum conduits.
Some limit on center to center spacing, ie 3xDia
Some limit on the number of adjacent conduits in a slab, ie only 3 in a 6’ width

I have also provided additional top and bottom steel in instances where the above rules could not be followed.

 

[hokie66]

With normal size conduit, I have never had an issue with it being placed in slabs, except at concentrations. I don't think runs of 30 mm or 40 mm conduit on the bottom reinforcement are a problem as long as they are separated. Aluminum is a no-no, of course.

Then there is the sparky who wants to put his conduit on top of the top mat....

 

[csd72]

Beware of junction boxes, it is not always just round conduits.

You have not stated what type of building it is. The worst ones are hospitals and hotels where the conduits can be very numerous and clumped together like crazy.