Stud Rail Stud Height Shorter Than Specified 10

[KootK]

QUESTION

When stud rail studs are fabricated shorter than specified, how short is too short?

CONTEXT

So... post slab pour my contractor tells me that they couldn't get the stud rails that we specified in time and made an unapproved substitution. The substituted studs are an inch short of spec. This is the same project as was discussed in a previous stud rail thread of mine: Link. The two issues are unrelated, however, other than my ongoing concerns for quality control on site. I've included the before and after data below. I've also clipped the ACI 421 sections that deal with stud length (slab depth less covers less a tolerance of half a flexural bar diameter).

My gut tells me this is not a big deal but, being the aye dotter and tee crosser that I am, I call the stud rail supplier to get their take. As with the other thread, they are very permissive. They tell me the short studs are fine. And that's good. They're problem solvers who do their best to make life easy for their customers. Trouble is, I'm just not sure that I buy it. Why don't I buy it? Here's why:

I told the supplier that, if some deficiency in stud length can be deemed acceptable then, logically, there must also be some degree of deficiency in stud length that must be deemed unacceptable. Surely, 2" tall studs at 1" o/c would not be acceptable, right? Clearly there's a bar somewhere that separates the good from the bad. I feel that, to have confidence in this judgment, we ought to know where that bar is. The supplier agreed with this logic but was unable to comment on the position of the bar of acceptability.

My thoughts:

1) The ACI document clips shown below make it clear that, on some level we do care about the height of the studs. The supplier tells me that the only reason for the limitation is that engineers used to try to sandwich the stud heads between layers of tension steel which was goofy.

2) One story of shear reinforcement is this: the shear reinforcement, be it studs or stirrups, should effectively connect the flexural tension and compression zones. In this respect, having the studs terminate lower than the underside of the flexural steel seems as though it would be a problem.

3) Another story of shear reinforcement is this: so long as the reinforcement, be it studs or stirrups, crosses the potential shear cracks and is developed on either side of the cracks, all is well and it doesn't matter if the shear reinforcement makes it to the tensile and compression zones. This is how the stud rail supplier feels about things, particularly given that the studs are not necessarily placed in the same plan location as the flexural steel.

This would lead one to believe that the minimum height of stud would be that required to "develop" the studs either side of the potential shear cracks without initiating a concrete breakout failure below the studs. Per SlideRuleEra's contribution in the other thread, old Nelson stud catalogs indicated that studs needed to be embedded 8-10 stud diameters to preclude breakout. This would lead one to believe that a 6" stud would be the minimum height stud ever.

ORIGINALLY SPECIFIED

- 8" slab
- 6" tall studs (1" cover T&B)
- 4" stud spacing
- 3/8" dia studs

PROVIDED IN FIELD

- 8" slab
- 5" tall studs
- 3" stud spacing
- 1/2" dia studs
- bottom cover maintained.

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.

 

[PicoStruc]

Very good question ! I am pretty sure that the answer to that question is linked to the ability for shear cracks to develop above shear head.

On other hand, with spacing reducted and diameter increased, the calculated resistance is doubled compared to the original design.

BUT : Does the number of stud was ajuster to meet minimum rail length requirements ?

With the spacing reduced, the clear spacing at head is 1.35 inch, so be sure that the stud rails are installed before rebar placement.

 

[KootK]

Thanks for the response Pico.

BUT : Does the number of stud was ajuster to meet minimum rail length requirements ?

Yup. The rail length overall remains unchanged.

I am pretty sure that the answer to that question is linked to the ability for shear cracks to develop above shear head.

I see what you mean. In an R.A. Gilbert detailing document that I have, they warn designers to generally not to create planes over which cracks could form without crossing reinforcing. This might be an example of that.

On other hand, with spacing reducted and diameter increased, the calculated resistance is doubled compared to the original design.

That was the supplier's argument as well. To me, it seems analogous to reviewing a steel truss and saying: "the tension webs are way stronger than required so don't sweat the compression webs or the connections".

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]

To me, it seems analogous to reviewing a steel truss and saying: "the tension webs are way stronger than required so don't sweat the compression webs or the connections".

Granted, the stud rail supplier repeatedly mentioned that, in his opinion, stud rail design was not a strut and tie thing but, rather, more of a sectional "stitching together". A real strut and tie evaluation of the setup is obviously complicated by the potential lateral distance between the studs and the flexural rebar. So perhaps a "truss" perspective here not appropriate.

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]

Bump. Somebody other than Pico and I must have an opinion on this. Consider:

a) I know damn well that you're all using stud rails on a regular basis and;

b) It's not like I don't regularly go to the mat answering all of your questions.

Step up able minded colleagues! Step up! Guess if you have to.

Did I do something wrong here? Text too long and therefore demotivating? Lack of sexy sketches?

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.

 

[sandman21]

I have not gone a design in awhile, but doesn't the shear stress increase with the smaller depth along with giving a smaller I increase the stress even more. The only have to counter that would to increase the length of the rail not the size and spacing of the studs. Decon has a software, which is not working on my computer, but can run to see if you are ok. I will get my reference material tomorrow and review. And sexy sketches or funny pictures of 'holy' joists get the posts.

 

[rapt]

Kootk,

Sorry for not responding. Maybe no-one feels expert enough on this topic.

I agree with the worry about the gap between the top of the studs and the top reinforcement. There is a failure plane that is relying on concrete tensile strength which is not allowed. This also happens a lot with epoxy anchors which makes the manufacturers details unacceptable in a lot of cases.

Also depends on the design code. EG Eurocode does not allow shear reinforcement including studs in slabs thinner than 200mm. This is effectively making a 200mm slab into a 175mm slab in this regard if we assume the reason (as usual we are not told) that they do not think that shorter shear "legs' can be considered to be effective.

 

[BUGGAR]

As rapt says.

Since I don't know a lot about how these actually work (I've never seen how they really fail), I can't say that some unpredicted failure mode will not happen in the future that teaches us something we didn't know about these things. Thus I wouldn't deviate from Specs. Please take this as my personal opinion.

 

[bookowski]

Tough one - I'll take you up on your guess request.

From a pure gut feel I don't like it. Shear and bending are the same deal, you're progressively turning the corner with that shear to progressively build up your moment. Seems like those mechanical linkages (shear vert reinf and bending horizon reinf/comp block) should be as close as possible. Aren't you relying on a little unreinforced zone to complete the puzzle?

If it was a beam that only needed a little shear reinforcing would you accept floating stirrups mid-beam? Seems even weirder at slab scale.

Why is it not strut and tie but rather 'stitching'?

Did you get shop drawings beforehand? Not that it matters now. You'd be best off to convince yourself it's ok, doesn't seem like there's an easy fix.

 

[BAretired]

I should preface my remarks by saying that I am no expert with respect to stud rails. I believe that stud rails should be anchored above the top steel and below the bottom steel as shown in the photograph in the attached ad. Otherwise, any reinforcement lying outside the head or base of the studs could separate from the rest of the concrete, not so much in a shear crack but rather a tensile crack.

BA

 

[rapt]

Looking at it more closely in terms of truss analogy or strut tie, your studs are tension ties that are supposed to be developing into the top of the member. I would define this as BARetired has as being outside the flexural reinforcement. They are currently stopping at least 25mm short and relying on concrete in tension to provide the remainder of the tension tie. And that tension will be fairly high at the point where the stud terminates and transfer s to the local concrete.

The depth to the underside of the head is in the order of 75mm. In a 200mm slab I would not think that is high enough! Unless it is very very heavily reinforced, I would think it is well below the neutral axis! When it should really be well into the compression zone.

This is one reason why we always tried to avoid slab punching shear reinforcement in the old days before stud rails. It was always too difficult to get the ties to extend outside the flexural reinforcement in both directions, so builders took short cuts and placed them inside the flexural reinforcement. Exactly the case you have here. Too many arguments followed so we usually designed around it, either with drop panels or capitals on the columns. Too late for that for you!

 

[hokie66]

"I know damn well that you are all using stud rails on a regular basis". Nope, never used them.

 

[jayrod12]

a) I know damn well that you're all using stud rails on a regular basis and;

Nope, I'm in the pre-cast capital of Canada (and maybe the world) so I've never once specified them.

b) It's not like I don't regularly go to the mat answering all of your questions.

You most certainly do and if I had an educated opinion I would of course provide input

Step up able minded colleagues! Step up! Guess if you have to.

As a guess, my first inclination was this is NOT ok. I can't give anymore input then that due to the lack of experience

Did I do something wrong here? Text too long and therefore demotivating? Lack of sexy sketches?

I do miss the sexy sketches. Although your sketch in your OP is probably a 6/10 on the sexiness scale.

 

[Brad805]

Have you tried contacting Dr. Dilger or Dr. Ghali thru the university of Calgary numbers? Both were working on the research of these when I was at the University. I recall Dr. Dilger showing his idea a couple of times.

https://schulich.ucalgary.ca/profiles/walter-dilger

https://schulich.ucalgary.ca/profiles/amin-ghali

 

[KootK]

Thank you, gentlemen, for assisting a damsel in distress. It seems that we're all in substantial agreement regarding our concerns here which is a great comfort. That said, I still find it surprising that we're all basically questioning the sentiments that have been expressed to me my the stud rail supplier. To that end, as I respond to your questions and comments, I'm going to somewhat play devil's advocate by attempting to defend some of their positions.

For now, I thought that you all might enjoy seeing a real, honest to goodness snippet from the offending contractor query.

Yes, better in all respects except, perhaps, the ability to successfully connect the flexural tension and compression fields. Happless contractor EIT appears to be about 15 yrs old in real life and as green as pressure treated lumber. Especially when it comes to stick handling high maintenance EOR's.

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.

 

[BSVBD]

Sorry Koot! I've never designed a stud rail. I wish i could help!

Even when i worked for a fabricator, the only studs i've either seen through a bid or sold to a job are studs for tying masonry to lintels, beams or columns.

In my area, we apparently do more steel than reinforced concrete.

You certainly DO answer more than most and we thank you!

 

[KootK]

Decon has a software, which is not working on my computer, but can run to see if you are ok.

I'm going to look into this a bit more too as it is also what the rail supplier recommended. Unless I can get my hands on some unusually detailed output, however, I'm not sure how convinced I would be by a positive result coming from the software. If the supplier engineers themselves can't tell me whether or not the stud rail height matters, I have limited confidence that the issue is properly addressed in their software. For all I know, the program may just check code specified stud heights and base the reported capacity on Vc and Vs with no specific regard for the stud height. We'll see.

but doesn't the shear stress increase with the smaller depth along with giving a smaller I increase the stress even more.

If I understand correctly, I agree with this. See the sketch below. In this regard, I would much rather have maintained the top cover as opposed to the bottom cover. That way, I might have a reduced effective flexural / shear depth but at least I'd have convincingly engaged the tension steel.

Nope, I'm in the pre-cast capital of Canada (and maybe the world) so I've never once specified them.

"I know damn well that you are all using stud rails on a regular basis". Nope, never used them.

I was using hypberole for dramatic effect so as to prompt some responses. I realize that not everyone/everywhere uses the rails. Regardless, thanks for showing your digi-faces for moral support. You may now consider yourselves to have fulfilled your duty in this matter as banner-men of House KootK.

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.

 

[bookowski]

The decon software is pretty good in terms of interface but it just runs the code calcs so it's not going to address your fundamental question. I also doubt the technical guys that answer the phone know more than you do on the topic.

 

[KootK]

I agree with the worry about the gap between the top of the studs and the top reinforcement. There is a failure plane that is relying on concrete tensile strength which is not allowed. This also happens a lot with epoxy anchors which makes the manufacturers details unacceptable in a lot of cases.

Aren't you relying on a little unreinforced zone to complete the puzzle?

Why is it not strut and tie but rather 'stitching'?

Otherwise, any reinforcement lying outside the head or base of the studs could separate from the rest of the concrete, not so much in a shear crack but rather a tensile crack.

Looking at it more closely in terms of truss analogy or strut tie, your studs are tension ties that are supposed to be developing into the top of the member. I would define this as BARetired has as being outside the flexural reinforcement. They are currently stopping at least 25mm short and relying on concrete in tension to provide the remainder of the tension tie.

The supplier response to all of these items is essentially this:

When we study this in section, we tend to imagine that the rebar is in the same vertical plane as the the studs. And that isn't necessarily true. In general, there will be a plan offset between the tension rebar and the stud heads. At least, to my knowledge, there is no requirement for the studs to align with any of the individual reinforcing bars. They often seem to as a constructability thing but, even at that, it's not as though you are engaging all of the tension rebar right at the stud rails.

Two consequences of this:

1) A complete strut and tie model of the situation would need to incorporate some struts running upwards and laterally, in plan, from the bottom of the rail studs to the rebar at the top of the slab. And where those struts would hit the top of the slab, there would generally be no vertical reinforcing there to pull the force back down to the bottom of the slab.

2) Even with proper height studs, there's still an unreinforced, "concrete in tension" plane existing in the load path.

Another reason cited for this not be a strut & tie thing is simply how the design checks are set up. They mimic conventional beam shear checks. Of course a) this is simply reflects a choice rather than a limitation and b) conventional beam shear checks are based on the truss analogy anyhow which echoes strut & tie in many ways. So maybe this objection is bull.


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.

 

[sandman21]

They have a good design manual DECON® STUDRAIL® DESIGN MANUAL. Reading a little more on this I dont think the location would need to be exactly at the reinforcing. Testing has been conducted with the studs extending to the reinforcing, but I am not sure an offset with the reinforcing would eliminate the effectiveness of the stud rail. The reduced depth does lower the amount of concrete you can use to resist the shear. I would run a number and see where you are at with regards to the stress, if you are comfortable or not with the as-built condition. I would tell the contractor to provide written justification for the substitution from the manufacturer along with supporting calculations or they can start prepping for removal.