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.

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
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
RE: Conduits through Stud Rails
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
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.
RE: Conduits through Stud Rails
RE: Conduits through Stud Rails
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
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
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.
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RE: Conduits through Stud Rails
thread507-306324: Concerns about stud rail punching shear reinforcement
RE: Conduits through Stud Rails
RE: Conduits through Stud Rails
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.
Quite right. Thanks for the reality check.
Here's the latest from Decon:
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
RE: Conduits through Stud Rails
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
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.
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RE: Conduits through Stud Rails
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
- 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
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
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RE: Conduits through Stud Rails
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
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
In theory, I believe so. You may get some strange looks from the shop floor with such an exact "to the mm" dimension
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
http://www.ancon.com.au/products/punching-shear-re...
Don't know about Decon.
RE: Conduits through Stud Rails
www.SlideRuleEra.net
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RE: Conduits through Stud Rails
RE: Conduits through Stud Rails
RE: Conduits through Stud Rails
Are you missing a few on the bottom row of the East face?
RE: Conduits through Stud Rails
His comments above the photo explain some of that.
RE: Conduits through Stud Rails
RE: Conduits through Stud Rails
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
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
Here is the link to the complete manual: Link
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Conduits through Stud Rails
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
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Conduits through Stud Rails
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
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
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.
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RE: Conduits through Stud Rails
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
www.SlideRuleEra.net
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RE: Conduits through Stud Rails
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