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Conduits through Stud Rails
3

Conduits through Stud Rails

Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

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.

RE: Conduits through Stud Rails

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.

RE: Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

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. smile

RE: Conduits through Stud Rails

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.

RE: Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

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.

RE: Conduits through Stud Rails

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

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

RE: Conduits through Stud Rails

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

Quote (SRE)

Only a portion of the stud length will have adequate embedment

Quote (SRE)

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.

Quote (THLS)

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:

Quote (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.

RE: Conduits through Stud Rails

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.

RE: Conduits through Stud Rails

(OP)

Quote (Brad805)

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.

RE: Conduits through Stud Rails

Quote (KootK)

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

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

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

Quote (SRE)

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.

RE: Conduits through Stud Rails

(OP)
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.

RE: Conduits through Stud Rails

KootK

In theory, I believe so. You may get some strange looks from the shop floor with such an exact "to the mm" dimension smile

I visited the Decon manufacturing facility in Ontario in the early 2000's. Quite an impressive setup, and at the time they were operating on 3x8 hr shifts, 24/7.

RE: Conduits through Stud Rails

Ancon, which is one of the two supplier in Australia, says they can provide studs in any length, but normally in 10 mm increments. Note that these are double headed studs, and the rail is non-structural.

http://www.ancon.com.au/products/punching-shear-re...

Don't know about Decon.

RE: Conduits through Stud Rails

Quote:

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.
Their software does not show any message or warring once height is more than conventional. I would guess that it will do so if it was limited to standard heights.

RE: Conduits through Stud Rails

i'm guessing this is a apt building and the electrical scope wasn't well defined in bid docs... the electrician priced to run the conduit through the slab... and now you are stuck dealing with them because the developer sees a huge price difference to run rigid conduit outside the slab now that it has come up that MEP is half-baked..... don't have anything to offer except that i dabble in structural for things i feel comfortable and this is above my level.... but what is there doesn't look right to me and wasn't done with your permission. the egg should be on the GC and electrician's face, and shouldn't be wiped off. it appears they had more corridor to run this conduit but the direct route was taken..... i had one where they bundled all the conduit right up against the compression side of all the anchors for uniform tendon and intended to do that for every floor.

RE: Conduits through Stud Rails

That's a sh*t tonne of stud rails. Why so many for a 250 slab?

Are you missing a few on the bottom row of the East face?


RE: Conduits through Stud Rails

Trenno,
His comments above the photo explain some of that.

RE: Conduits through Stud Rails

Don't you have a note in your general/concrete notes about minimum distance from column for conduit? For example I just looked at some wsp notes and they have "no pipes and/or conduits should be placed closer than 3 diameter on center nor pass within 24" of the column face".

RE: Conduits through Stud Rails

(OP)
We have 750 mm and 6d (excessive IMO).

Firstly, they didn't read my drawings and just went with what was done on their last job. Secondly, the use of leftover rails that are 600 longer than reauired through me off and caused me to waste a good deal of time on this. We specified 900 and got 1500. And 600 in a few places.

This the first time that I've ever seen 1/2" studs. Usually I go 3/4. They look alarmingly scrawny somehow. Just math I guess, no need to be deterred.

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.

RE: Conduits through Stud Rails

(OP)
Wife was just telling me that a couple of her developers had asked her for 1/2" studs under the assumption that it represented a significant savings over 3/4"? Anyone able to confirm this? Also, I've been working under the assumption that the installed cost of the rails is about $300-500 North American bucks per column. That ring true?

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.

RE: Conduits through Stud Rails

Quote (Decon StudRail Design Manual - Page 15, Paragraph 2.4 Stud Sizes)

Economics should be considered when selecting stud sizes. Generally, designs using either 3/8” or 1/2” diameter studs are the most economical in standard slabs. When the slab thickness exceeds approximately 11”, the larger diameter studs may become a better choice.

Here is the link to the complete manual: Link

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

(OP)
Thanks SRE. I'm not getting out to site enough these days. Thinking back on it, I've actually seen plenty of 1/2" specified. It's just been a while since I've seen it in the wild. Somehow the rust makes it look frailer as well.

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.

RE: Conduits through Stud Rails

Yes, the studs need 8 to 10 times their diameter embedment for the pull-out concrete conical failure mode to exceed the tensile strength of the steel in the stud. When the slab / bridge deck is thick enough, the bigger the better. Larger diameter studs are more resistant to physical damage when they get hit during the placement of heavy rebar.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

(OP)

Quote (darth)

i'm guessing this is a apt building and the electrical scope wasn't well defined in bid docs... the electrician priced to run the conduit through the slab... and now you are stuck dealing with them because the developer sees a huge price difference to run rigid conduit outside the slab now that it has come up that MEP is half-baked

Exactly that. I was reading in another thread where a fellow refused to return rebar shop drawings until after he'd received conduit submittals. To the extent that's enforceable, it sounds great.

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.

RE: Conduits through Stud Rails

(OP)

Quote (SRE)

Yes, the studs need 8 to 10 times their diameter embedment for the pull-out concrete conical failure mode to exceed the tensile strength of the steel in the stud.

Now that's an interesting statement. My assumption, based on my understanding of how stud rails work, has been that:

1) On the compression face, the studs are restrained by a diagonal compression strut and the flexural compression block acting as a compression strut.

2) On the tension face, the studs are restrained by a diagonal compression strut and the flexural tension rebar acting as a tension tie.

In both cases, there would be no concrete conical failure mode as the studs would be restrained by reinforced concrete elements (struts and ties). There would be a development requirement, of course, but I thought that studs were developed right at the head/rail, similar to the case with a bearing plate. Can you supply a reference for the 8-10d conical failure mode business? I'd like to learn more about it. I apologize if that's in the Decon manual. I haven't read that cover to cover yet.

Additional thoughts:

1) This ties in with my question above regarding how concerned we are with the height of studs. Code requirements very much make it sound like the strut and tie model that I've described in which case full height studs would be rather important. In a breakout cone scenario, one ought to be able to tolerate short studs, so long as their anchored either side of the diagonal tension crack.

2) There's an analogous situation in common beam stirrups that has always bothered me. To be code effective, a stirrup only needs to be developed either side of the assumed diagonal tension crack. Somehow, it's not necessary for them to be developed across the compression block / tension tie as seems to be the case with studs and as one would expect with a truss model in play. Often, with smaller bars, they are developed across the truss chords but, per code, they don't have to be.

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.

RE: Conduits through Stud Rails

KootK - I thought that statement might get your attention...



This comes from page 8 of the 1977 Nelson Stud Design Manual that I linked to in my "27 Nov 15 19:10" post. Read the Introduction and pages 8 and 9, for sure... and don't pour that concrete until the "mess" in the field is straightened out. wink

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

(OP)
The thing with the Neslon catalog information is that it is speaking to studs that are anchored by some form of concrete tension. It's the pre-App D version of AppD.

As I described above, I believe that stud rail studs are anchored by a different mechanism: the struts and ties that make a reinforced concrete slab reinforced concrete.

Consider the not so trivial example of a 1/2" stud in my 10" slab (d~8"). At 10d = 5" either side of the assumed diagonal tension crack, I wouldn't be able to anchor even one stud per diagonal tension plane.

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.

RE: Conduits through Stud Rails

Ok... I won't dispute that studrails may be different, but what I take away from the Nelson manual is that full-embedment of the studs (no matter what their length relative to diameter) is important. For studs that are intentionally embedded less than 8 - 10 diameters full embedment is even more critical. If conduit is immediately adjacent to studs... there goes meaningful full embedment. I see that as a problem.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Conduits through Stud Rails

(OP)

Quote (SRE)

If conduit is immediately adjacent to studs... there goes meaningful full embedment. I see that as a problem.

I agree. And I'm very surprised that Decon seems so liberal on this issue. Thanks for taking the time to help me work through this. Quality control, and enforcement of it it, is on of the things that I find more appealing about bridge work relative to buildings.

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.

RE: Conduits through Stud Rails

We have used studrails on many projects and we prohibit anything going through areas where there are studrails. When you need studrails, you already have high shear stresses. Putting things like conduit through those high shear stress zones is not a good idea. Too many things can go wrong. That is just my gut feeling. I suspect that you feel the same way - otherwise you would not have posted your message.

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