Conduit spacing in slabs

[Settingsun]

Can anyone point me to guidance on when size/spacing of conduits in slabs has negligible effect on slab strength? It's a two-way slab so the conduits will destroy the strength in one direction if too close together.

 

[SlideRuleEra]

steveh49 - The July 2004 issue of Structure Magazine has recommendations (see the second page).

Our electric generating stations have much embedded conduit. Use steel conduit for elevated and heavily loaded structures, PVC at other locations.

http://www.SlideRuleEra.net

 

[KootK]

SRE's article is awesome and it's going straight into the value. That said, one of their recommendations is inconsistent with my practice. I don't often find that an 18" spacing between conduits is feasible in my market place. We try for a c/c spacing of at least 3 x dia but often get pushed as tight as 2 x diameter by the pressure exerted by our competition. Sadly, I've not ever seen a convincing calculation to determine what should be acceptable. Some folks will look at it as a) shear depth remaining and/or b) vierendeel truss thing. I feel that neither is wholly valid and my experience is that both usually result in problematic spacing requirements.

The clip below is from ACI 314 and is somewhat in-line with what I've been doing in practice.

 

[slickdeals]

And all these rules go out of the window when they come into the electrical room (often from a small riser).
We have some typical details that provides supplementary reinforcement for bunched up conduits and then ask them to fan it out as quickly as feasible.

 

[KootK]

slick's bunching can be used to an almost magical effect in some situations. Before and after below. Deep slab obviously.

 

[Settingsun]

Surprisingly little written on the topic - I thought it may be settled science which was why I didn't give much detail in the first post. Thanks, SlideRuleEra, for the link. That article discusses a different scale of slab though. Mine is 400 mm thick (16") with six 125OD (5") conduits. The cover is 75mm (3") as it is outdoor near salt water with 100 year life - so my cover on one side alone is equal to the total slab thickness from the article.

The ACI requirement for 3*OD centres minimum is a good start. That will push the conduits quite wide. What do people think of a Warren truss analysis? It will rely on the concrete taking tension for every second diagonal (usually a no-no) but I think slabs without stirrups rely on this implicitly anyway.

 

[KootK]

What do people think of a Warren truss analysis?

Not quite sure how you're envisioning this. Can you elaborate or post a sketch.

Below, I've taken my best shot at rational analysis. It boils down to the two checks that are clouded:

1) For generous spacing, you'd be governed by shear taken across the compression block.

2) For tight spacing, you'd be governed by plain concrete flexure across the horizontal plane at the throat.

 

[Settingsun]

I will try to describe the Warren truss in words first. A sketch would be a few hours away.

The analysis would be based on the truss analogy sometimes used for beams. The truss 'web' is diagonal members only; no verticals. They alternate between leaning right and leaning left. The dimension (thickness) of the diagonals is whatever fits between the conduits on an angle of 30-45 degrees, say 36 degrees per my code's simplified shear assumption. With this arrangement, the diagonals alternate between compression and tension. The shear capacity is the vertical component of the unreinforced tension diagonals' capacity. For conduit centres = 3*diameter, I assume bending strength is not significantly reduced.

Thinking about it, it could just be standard truss analogy with concrete in tension vertically instead of a vertical stirrup. The Warren truss just seems 'better suited' to me though.

KootK, your analysis reminds me of castellated beams (iirc, haven't designed one for over ten years). I guess that's essentially what I'm dealing with.

 

[Settingsun]

KootK/everyone, in ACI314 clause 6.8.2.4 posted above, do you take the 2"/25% depth limit to be the larger of those two or the smaller? Eg in my specific case (16" slab), is the conduit size limited to 2" or 4"? And, since this is from Simplified Design rules, OK without additional analysis?

I also spent a bit of time trying to find this clause in ACI318 by mistake. The 2014 version is restricted to motherhood statements, but 2011 had the following, with my questions marked up on the image. I would consider my structure to be 'common conditions'. (I would prefer to stick to 25% rather than 33% though).

 

[SlideRuleEra]

1)...do you take the 2"/25% depth limit to be the larger of those two or the smaller? Eg in my specific case (16" slab), is the conduit size limited to 2" or 4"?

2) And, since this is from Simplified Design rules, OK without additional analysis?

1) IMHO, 25% (4" for your project) has to be acceptable. We use much large embedded conduit and piping, there is no way that that 2" would do the job. The answer, like your project, is to have thick floors for large conduit.

2) My opinion, OK without additional analysis. Again, just keep it simple, use thick floors and keep going.

Edit: There is nothing out-of-the-ordinary or unusual with reinforcing steel used in our (thick) elevated slabs containing large conduits when compared to similar slabs without conduits.

http://www.SlideRuleEra.net

 

[KootK]

X2 for SRE's latest answers. Hard to see how a 5" conduit in a 15" slab is any worse than a 2" conduit in a 6" slab. I'm guessing that the, admittedly ambiguous, statement was written with conventional elevated slab proportions in mind. 7" - 9" condo slab etc.

 

[Settingsun]

Thank you both. The story is that the slab was designed without embedded conduits. They were to be slung underneath. Now that the formwork is down and reinforcement being tied, the services contractor has asked if they can be cast in. No opportunity to thicken further to suit.

The starting position is the same six 125mm OD (5") as were to be slung beneath. That's above the 1/4 limit from ACI314 but just within the 1/3 from ACI318-11 (superseded). I initially thought probably ok but it seems maybe marginal. After the weekend, I'll see if dropping down a conduit size to be 1/4 slab depth is acceptable.