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Too much conduits in slab

Too much conduits in slab

Too much conduits in slab

Hi, Please see attached photo showing spreading electrical conduits over 140mm thick slab. Cover to steel is no issue but I doubt about amount of concrete actually in the slab as conduits consume considerable volume. I'm searching for limitations, if any, for amount of voids allocation withing a slab.

Please help.


RE: Too much conduits in slab

It looks like your slab is not thick enough to accommodate all those conduits. The ones running parallel to the main bars are not terrible, but then the ones running perpendicular look like they are on the top of the reinforcement. No conduits are allowed with the cover concrete.

RE: Too much conduits in slab

Thanks @hokei66. My concern is not that. Is there a sufficient concrete amount to bear the design load?

RE: Too much conduits in slab

Not where the conduits are laid on top of the reinforcement. Not sure of your concern, but I told you my concern.

RE: Too much conduits in slab

I’m not sure of any strict rule in this regard, but I would not be overly happy with that. Where closely spaced conduits run through the compression zone you have a considerable loss of cross sectional area.

RE: Too much conduits in slab

Also, the conduit appear to be too close to each other. The concrete will likely have honeycomb because of this. Additionally, if conduit are too close to rebar, honeycomb will make that rebar "worthless".

In the photo, the conduit look like PVC, not steel. IMHO, PVC conduit are structurally "less than worthless", for the reasons stated by others, especially if this an elevate slab.

www.SlideRuleEra.net idea

RE: Too much conduits in slab

ACI 314R has the recommendations shown below. I've seen the general notes of many firms stretch the allowable opening size to whatever can fit withing the middle third of the slab depth. Similar to what others have already mentioned, the concerns are usually:

1) Not compromising the flexural compression block.

2) Not compromising cover or bond on the reinforcing steel.

3) Retaining a shear mechanism that has an arching feel to it rather than Vierendeel truss action. That said, it's not unheard of to pound in rebar above and below a wide opening and attempt to tell that Vierendeel truss story.

RE: Too much conduits in slab

Thanks a lot @KootK. Really helpful.

RE: Too much conduits in slab

Based on the thickness of your slab i think the whole section will be in tension side and the compression section will be located not greater than the concrete cover. In this case the embedded pipe will not be a concern any more. Just maintain a good concrete at the top of your slab.

RE: Too much conduits in slab

And what about shear resistance, with a hight 14cm it is kind of symbolic? And with all that conduits goes drasticly down. In a 25 cm slab 25 mm isn't really an issue but in 14 cm it's a thing to consider.

RE: Too much conduits in slab

Has this already been poured?

RE: Too much conduits in slab

@Juan: 15mm layer out of 70mm compression zone is affected and it is unavoidable. Large number of conduits is the issue as it considerably reduce the amount of concrete in compression zone. According to ACI minimum spacing is 3 times the conduit diameter.

RE: Too much conduits in slab

@Tommy385: I have the same concern

@MIStuctE: Yes. Done.

RE: Too much conduits in slab

I run into this all the time. Your slab does not look that bad. If, as others have said, the ACI guidance can be followed, that's ideal. Maybe on the top of the picture, they can spread apart the conduits to get the spacing required.
The sparkies have limited choices. They can put the conduits under the slab. But then they need to mobilize before the rod busters. Plus then you have a mess of conduits under the slab with poor compaction around them. Or, under the dead of night, they sneak in between the layers of reinforcing. That way, they have a good reference of where everything is. There's no good way, just a couple of bad ways.
As structural engineers, our first priority is the load carrying capacity of the structure. But the building is put there for a reason. Whether it's a doghouse, outhouse or cathouse, it will have supporting utilities, like power, plumbing and HVAC that we need to accommodate.
I've attached a bad layout. And it's not a slab on grade, it's an elevated slab.

RE: Too much conduits in slab

I don't understand your comments. If the conduits are under the slab, they have to go in after the slab is in place and the formwork is removed. In the OP's case, that is the only way this situation could have been avoided properly. The ACI guidance should be taken as the bible, not a suggestion. In no case should conduit be allowed below the bottom bars or above the top bars.

RE: Too much conduits in slab

I'm assuming that this is a slab on grade. At second glance, the picture isn't clear, but there seems to be grade adjacent.

RE: Too much conduits in slab

I never considered that. If it is a slab on grade, it is a heavily reinforced one.

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