Conduits through Stud Rails 3

[KootK]

Need some quick feedback here. I've got a pour tomorrow and there's a bunch of electrical conduit running through my stud rails. I'm trying to have it moved, of course. However, I called the only rep that I could get a hold of at Decon to confirm my suspicion that this is gawd awful. I was certain they'd back me up. Without even taking a breath, he told me that it's fine.

Anybody have any experience with this? Based on my understanding of how the rails work (sketch below) I have a very hard time accepting the "fineness" of this. At minimum, it would have to affect the Vc portion of things.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

I've never used punching shear stud rails.

However, the presence of the conduit severely decreases the natural shear capacity of the concrete. It also intercepts the central core of the compression strut between the bottom of one stud and the top of the next. It seems like your compression either has to go through the conduit or arch around it. I have no idea how you'd quantify the impact realistically, but it's not good. I probably wouldn't let this stand.

 

[hokie66]

Ditto to TLHS. I haven't used stud rails either, but it is not possible that the conduit has no effect on the shear capacity. If you are given that type advice by Decon, they are untrustworthy.

 

[KootK]

In fairness to Decon, one of my guys spoke with one of their guys. And there may have been language barrier issues on both ends. I've got some voicemails and emails floating around out there trying to confirm. Yankee thanksgiving... pfft. Maybe someone will check their email at half time.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

KootK:
And they did so much work and such a nice neat job of getting it all lined up and in a bunch in one location, only wiping out a 3' wide strip of conc. slab in a most critical location. Just remove the column below, and the punching shear problem and the column strip issues will go away. If anything, the Rep. meant one small conduit wouldn’t hurt anything. And, Happy Thanksgiving to you too.

 

[dcarr82775]

I use studrails every other year or so. I wouldn't allow what is in your picture. As everyone else thinks, it must derate the capacity. You don't have a compression strut anymore.

 

[KootK]

I just spoke with a rep myself and so far they're standing by their conclusion that this is alright. So far, the explanation amounts to "there's enough concrete capacity left over". I'll have to wait until Monday to hear from their technical top dog.

To play Devil's advocate here:

1) If this were a 12" beam with stirrups and some 4" holes located in the middle third of the depth, I believe that many of us would let it pass. Certainly, I've seen many firms' standard details that would indicate this to be the case. What's the difference?

2) Mathematically, we calculate shear assuming a uniform-ish contribution from the cross section (d). In reality, most of the action is probably down at the compression block which is probably below the conduit.

I oscillate between seeing this in strut at tie fashion, as I've shown above, and simply reinforcement crossing a diagonal tension crack. I suppose both models have some validity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

Even if it's just closing a diagonal tension crack, the bottom of the concrete is basically hanging from the top stud. That top stud needs to be restrained somehow, and that somehow is compression through the same zone that the conduit is in. See attached sketch. Without compression in red and restraint from rebar in green the system isn't stable. I've basically gone back to a truss analogy, but I don't see a load path without compression through the zone that the conduits sit in. The only question is how much of an impact the conduit has on that compression zone.

I'm not sure I'd be convinced unless the vendor has some really good math to share, or testing.

 

[SlideRuleEra]

Don't do it. Only a portion of the stud length will have adequate embedment - conduit is in the way.

Need another reason... minimum clearance between conduit embedded in concrete is often considered to be 1.5". Got to let the aggregate though during concrete placement. From the photo, there are plenty of spots where 1.5" is not the case.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hokie66]

And then there was this. Some of the links may be longer active.

thread507-306324

 

[Ron]

Without regard to any other issue, as SRE says, the clearance issue is enough to reject it.

 

[KootK]

Thanks for the great comments so far. Some more devil's advocacy:

Only a portion of the stud length will have adequate embedment

From the photo, there are plenty of spots where 1.5" is not the case.

Stud length, and the need for the 1.5", are both about development, right? My understanding is that stud rail studs are developed 100% at both the head and the rail. So, technically, no need for a discrete development length as with rebar. Of course, the head and rail both need to push against some solid concrete to do their business. Within each stud "lane", I'm planning to have the conduit tied up low against the rail furthest from the column. I'm hoping that will give me the best chance for compression strut development between studs. I know, I'm a shameful renegade cowboy.

Even if it's just closing a diagonal tension crack, the bottom of the concrete is basically hanging from the top stud. That top stud needs to be restrained somehow, and that somehow is compression through the same zone that the conduit is in. See attached sketch.

Quite right. Thanks for the reality check.

Here's the latest from Decon:

Generally speaking, it is acceptable for conduit to run between the studs of the studrail. However, it is not recommended that the conduit run between the studs of the studrails that are within the column's critical section.

Some new information from last night's site vist (cover your ears Hokie):

- new photo below.
- The conduit is 2.125” in diameter about centred in a 250 slab.
- The stud rails were specified as 13mm dia @ 125 o/c. What was provided was 13 mm dia @ 75 o/c . Apparently they just used some extra supply that they had lying around in the yard which I’m not too thrilled about.
- Some of these scrap stud rails are way too long and some are way too short. It makes field review a bloody nightmare and I'll put a stop to it going forward.
- Where rails are short, they're going to lap new, upside down rails to them.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Brad805]

Looks like about 4hrs for the electrician to fix the problem, and it would not surprise me that you have already invested that. Some PVC glue and couplers will fix all of the headaches. Think of it as a teaching event for the electrical trade.

 

[KootK]

and it would not surprise me that you have already invested that.

Accurate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Stud length, and the need for the 1.5", are both about development, right?

Not really. I have not used stud rails, but have done quite a bit with structural steel that included Nelson studs to get a composite floor / bridge deck. Essentially the same thing, but for a different purpose. Go to this page of my website and get the "Nelson Stud Design Manual", located about the middle of the page: Link

The 1.5" (minimum) mentioned for the conduit is horizontal clearance to prevent honeycombing of the concrete. If the aggregate cannot pass between the conduit, then honeycomb is almost guaranteed. An analogy is the minimum spacing of rebar, see ACI 301-05, Paragraph 7.6 "Spacing Limits For Reinforcement". Spacers are manufactured to keep proper spacing between conduit. The following photo shows use in an underground duct bank, but similar products are available for floors.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

Thanks for that SRE. In this case, the minimum clear spacing between conduits will end up being 4". The conduit will be in contact with the studs however. That still bother you?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Continuing from bad to worse:

- spec stud length = 200
- code required = 190
- available this afternoon = 175

General crap QC aside, How much do we really care? I'd cheat the compression side 15 mm. Who really knows what the compression block depth looks like this close to the column anyhow? I await your advice/scorn. I'll concede that this is all adding up to a rather unacceptable story.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

KootK - Yes, it does bother me. For example, note that this ICC-Evaluation Service Report of Nelson studs clearly specifies that: "The anchor must be placed in position before the concrete is cast, to fully embed these anchors." Link

I'll bet that the studs used on Decon's products are furnished by Nelson. To get a definitive answer, suggest that you contact Nelson: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ingenuity]

I'll bet that the studs used on Decon's products are furnished by Nelson

They are not actually. Decon make their own studs from steel coil stock, with a massive hydraulic forge to produce the proprietary head and shank, then the studs get welded to the rail.

 

[KootK]

While we're on the topic, is it possible to get stud rails of any height? If I wanted a 237 mm tall stud rail, could I have it? I'd ask Decon but I've bothered them more than enough for one week.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.