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.

 

[Ingenuity]

KootK

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.

 

[hokie66]

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-reinforcement/sizesconfigurations

Don't know about Decon.

 

[SlideRuleEra]

Thanks, Ingenuity, I stand corrected.

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[strucEng36]

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.

 

[darthsoilsguy2]

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.

 

[Trenno]

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?

 

[hokie66]

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

 

[bookowski]

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".

 

[KootK]

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.

 

[KootK]

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.

 

[SlideRuleEra]

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

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[KootK]

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.

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

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.

 

[KootK]

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.

 

[SlideRuleEra]

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.

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[KootK]

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.

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

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.

 

[cliff234]

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.