Stud Rail Stud Height Shorter Than Specified
Stud Rail Stud Height Shorter Than Specified
(OP)
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.
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.






RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
Yup. The rail length overall remains unchanged.
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.
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.
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
BA
RE: Stud Rail Stud Height Shorter Than Specified
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!
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
You most certainly do and if I had an educated opinion I would of course provide input
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
I do miss the sexy sketches. Although your sketch in your OP is probably a 6/10 on the sexiness scale.
RE: Stud Rail Stud Height Shorter Than Specified
https://schulich.ucalgary.ca/profiles/walter-dilge...
https://schulich.ucalgary.ca/profiles/amin-ghali
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
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!
RE: Stud Rail Stud Height Shorter Than Specified
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.
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.
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.
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
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.
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
Just came across your thread. I'm late to the party so I admittedly didn't read everything above, so if I repeat something I apologize
I've used studrails some and have also provided stirrup-reinforced punching shear designs. My gut tells me this sucks. There's a reason the studs are supposed to be that long, just like there's a reason the code says that if that was a stirrup, it needs to be hooked around the flexural reinforcing. The main reason is probably just that no one's ever tested it otherwise, but from an engineering intuition standpoint, it feels inadequate.
As someone wisely alluded to using the strut-tie methodology, you essentially have a truss with webs that don't fully extend to the chords...which isn't much of a truss at all. With respect to your concerns about the strut-tie, I see some of your point regarding the offset of the rails/reinforcing. However:
1. Typically where you have studrails at a column, the spacing of the flexural reinforcing parallel will be pretty tight. Given how the required length detail you have above is drawn and my experience with top reinforcing at a column, I don't think I would have ever had a situation where the 30 degree pullout cone (if you studrails failed that way) ever fell below the bottom of the reinforcing bar by 1". That sounds like the best case scenario they EVER have.
2. If they're going to use that argument that the situation where there's always some gap due to plan-offset... don't they still have the same potential situation? So how would dropping the studheads further down not just make it worse? Unless they're saying they don't have to align with bars by code, but we made them line up. Which at this point, I wouldn't take their word for it anyway...
3. But there also should be at least some redundancy in the load path due to the perpendicular reinforcing that helps the tie force span between the parallel bars. When they drop the stud heads a full inch though, they may have just flown out the window because you probably aren't engaging any of the perpendicular bars either...
Regarding the contractor's statements... ask them to find a registered PE who will stamp their statement that they provided something better than specified. I doubt they'd find anyone who's willing to touch it with a 20' pole. If that was the EIT that wrote that, I'd probably caution him that putting such things in writing without basis are the kinds of ethical violations that can not only lead to the quick dismissal of his ability to ever obtain a license, but also lead to disasters when people who don't know what they're talking espouse opinions with such confidence. I had an old boss that used to joke with me while I was an EIT that as far as the state was concerned, I was still officially incompetent with respect to engineering.
Personally, I'd explore and start discussing retrofit options. And when you get someone that pushes back with the "we did it better" argument, I always reply with "1. Did it match the plans? 2. Better according to who? Clearly by code? 2. Did I approve it for MY design?".
Good luck!
RE: Stud Rail Stud Height Shorter Than Specified
The lower tech support folks stopped tolerating my abuse some time ago. When I call now, they forward me right to "the" guy. I won't name names without having him here to defend himself but, suffice it to say, I'm at or near the top of the food chain with this. "The" guy doesn't have any specific answers per se but he does seem to be well versed on the testing.
"The" guy that I mentioned above seemed quite familiar with the testing and didn't believe that any of it had every resulted in a stud breakout failure which is kind of what we all seem to be concerned with here. However, he did agree that a) there ought to be some concern for stud development / breakout and b) there must be some point at which short becomes too short.
Your area was my area for a good long time. Most of the concrete flat plate work -- especially PT -- seemed to occur in Madison and, to a lesser extent, Green Bay
I'll give it a go. Both those guys are pretty Emeritus-ized, however, so it could take some time.
Yeah, just not for what was actually installed. A second prong to our effort here consists of trying to procure documentation for what it was that was actually installed. The rail supplier won't put their opinion in writing unless they know for certain that it was their product that was in stalled. And that's fair even though I'm sure that it has no theoretical bearing on the issues at hand.
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: Stud Rail Stud Height Shorter Than Specified
Thank you for that. I haven't digested it all but I did not the blurbs below which raise some interesting points/concerns:
1) Apparently the software algorithm doesn't need slab height to calculate shear. That's not encouraging in the context of our current discussion.
2) In sloping slabs, one could easily get the correct hardware installed and still have essentially this same problem (if, in fact, it is a legitimate problem. Granted, I'd expect most slab sloping to be quite gradual.
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: Stud Rail Stud Height Shorter Than Specified
I think that, for small deficiencies, you're probably right. Still, it takes me back to my original, related concerns with this line of thinking.
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: Stud Rail Stud Height Shorter Than Specified
These statements really get to the heart of the matter in my opinion. Do stirrups have to make it all of the way to both the tensile steel and the compression zone? I'd like for that to be the case. And ACI enourages it without being very specific about how close is close enough. They also state the reason as being limited stirrup leg development capability near flexural cracks rather than a specific need to connect the tensile rebar and compression zone.
Consider the common FRP reinforcement scheme below where, in all likelyhood, the reinforcement would not make it into the compression zone.
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: Stud Rail Stud Height Shorter Than Specified
PS - I know NOTHING about stud rails.
RE: Stud Rail Stud Height Shorter Than Specified
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: Stud Rail Stud Height Shorter Than Specified
PS - I know NOTHING about stud rails.
RE: Stud Rail Stud Height Shorter Than Specified
That's precisely the question. Can I look at it as a 7" slab? Seriously. Can I? Most folks seem to think not and, frankly, in the absence of relevant testing, I don't know how to separate the theory from the determination.
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
Did you read all of the preceding comments? I wouldn't blame you if you hadn't but the answer to this question is there in spades. Basically, most feel that the shear reinforcement should make it to the plane of the tension reinforcement.
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
Thanks again sandman. Is it clear that, in my situation, it is the bottom rail cover that has been maintained and the top stud cover that has grown by 1"? I ask because some of you comments lead me to wonder if you might be thinking the reverse. The flexural tension "ties" and shear "ties" in this potential STM model do not, in fact, intersect. Consider Bookowski's salient observation:
I see it the same. While I've no doubt that we have vertical shear capacity on reduced depth as you've described it, I think that the question of horizontal shear capacity remains. And where horizontal shear capacity is compromised, so goes flexural capacity...
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
I do not like the supplier's argument, as it doesn't have much of a basis other than "more steel".
Is there enough room to require bent bars to try to force the failure into the meat of the studs?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
Not easy after the slab has been poured.
BA
RE: Stud Rail Stud Height Shorter Than Specified
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
BA
RE: Stud Rail Stud Height Shorter Than Specified
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
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: Stud Rail Stud Height Shorter Than Specified
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: Stud Rail Stud Height Shorter Than Specified
I am kind of at a loss of words. I like to provide options so the contractor can make the decision themselves. They need to take ownership for their decision to deviate.
1. Provide column capitals. Probably don't need to go as far as drop panels, right?
2. Demo, remove and replace with proper studs
3. Create a mock-up of the current installation and load test
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
I had been thinking of tensioned high strength bolts full depth through the section recessed 25mm into the top and bottom with an anchor plate at each end and tensioning nut at one end. Grouted afterwards. Similar to the approach taken to strengthen bridges for shear except they use Stressed bar to get higher forces. They could be sloped as per the Hilti detail, but it would require a lot of bolts!
The advantage to my bolts is that you are not relying on an epoxy bond for anchorage. Always have doubts about epoxy with how much force you can develop and fire rating! Also, with the bolts as shown, the effective beginning of development would not be at the end of the bolt, it would be significantly down from the end of the bolt, so you still have the same problem as with the short stud rail.
You could drop them down from the top and anchor them at the top to "lap" with the existing short Shear Studs however! That would overcome the short tops of the shear studs combined with the undeveloped end of your bolts.
RE: Stud Rail Stud Height Shorter Than Specified
I agree, but it does depend on how you use the FRP and how much $ you want to spend. I saw a paper a few months back that used FRP rope and threaded it up-across-down-across (repeat) through drilled holes and recesses in the slab.
Bit of a 'Goldberg' solution.
Here is a FRP solution from some Swiss researchers:
Carbon Fiber-Reinforced Polymer Punching Reinforcement and Strengthening of Concrete Flat Slabs: Link
Check out these two FRP-based solutions:
RE: Stud Rail Stud Height Shorter Than Specified
I was thinking something similar to that as well. Though, in that situation, I would want that thru-anchor top plate to grab at least 2 tension bars, and an inch nominal cover seems like too little to pull it off. It would be great if there was a topping slab or terrazzo that you could bury the anchor end in.
Ingenuity, THAT is cool.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
Punching Shear Retrofit Method Using Shear Bolts for Reinforced Concrete Slabs under Seismic Loading: Link
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
1) While I truly hate to see a very bad deed go unpunished, I won't specify a onerous fix for any reason other than technical necessity. And my gut feel conclusion here is that a repair is not a necessity from a technical perspective.
2) I've researched stud rail code provisions, Hilti punching shear repair provisions, and simple ACI/CSA beam shear code provisions going back to 1971. They basically all say the same thing.
- Do extend shear reinforcing as close to the tension and compression faces as possible.
- Do whatever is in your power to convincingly anchor shear reinforcing at the ends.
That said, there are relatively few situations in modern shear reinforcement design where shear reinforcing is truly developed or anchored a) entirely within the compression zone or b) beyond the tension rebar. In general, we seem to accept this as the way of things and, to my knowledge, there have been no consequences. So, to some extent, the stud rail supplier is correct in that our efforts simply constitute a "stitching together" of things.
3) Initially, I had great concern for the potential of an appendix D style anchorage failure at the stud heads. The Nelson stud design manual (clip below) indicates that 8-10 stud diameters worth of embedment is required to prevent that. That would have meant for a minimum 6" stud in my case.
I see now that I was viewing this incorrectly. The upper heads of the studs are able to push back against a concrete strut which significantly alters the character of the problem such that an "anchorage via concrete in tension" interpretation is not strictly valid. I've tried to illustrate this in my sketch below.
4) In the sketch below, I've indicated a zone of perpetual pain and anguish (ZPPA). The ZPPA seems to be, for the most part, where our concerns lie. I still share those concerns but, based on my intuition and free body analysis of the situation, I'm confident that the issues within the ZPPA are sufficiently minor in nature that they can be set aside.
Interestingly, the FBD below provides a qualitative means of assessing the "when is short too short" question that has haunted me. The low stud heads result in something akin to a transverse flexure demand in the concrete struts when those struts are imagined as little beams. Obviously, the shorter the studs, the closer their tension loads are delivered to the "beam" mids-spans. And that means more bending moments and a higher propensity for failure.
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: Stud Rail Stud Height Shorter Than Specified
BA
RE: Stud Rail Stud Height Shorter Than Specified
I ascribe to a structural remediation version of the Hippocratic oath. Above all else, I try to do no harm. I've seen a lot of engineers get lost in the technical spiffiness of their fix only to produce a chopped up result that's probably worse off than the original. Other than the through bolt with top/bottom bearing plates, none of the solutions would seem to connect the compression and tension zones much better than what exists. And most of them would result in a bunch of new holes and micro-cracking the concrete that, hopefully, will resist much of the shear on it's own. My gut tells me to leave it 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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
Latitude:51.062501°
Longitude:-114.09061°
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: Stud Rail Stud Height Shorter Than Specified
Good. Not planning on being near there in the next 30 years anyway!
Just looking at the Canadian code. It would seem to require the same as ACI, maximum top cover = top bar layer cover + db/2.
RE: Stud Rail Stud Height Shorter Than Specified
The following photo is taken from your earlier thread which you referenced in the OP, namely thread507-399858: Conduits through Stud Rails
It appears to me that the top of studs at this particular column is close to the top of the top steel. If the height of stud is one inch shorter than specified, does this mean that the top bars are lower than specified? Or does this mean my eyes are playing tricks on me?
Were any of the electrical conduits moved as a result of your earlier concern?
BA
RE: Stud Rail Stud Height Shorter Than Specified
- the two issues were identified on different floors.
- QC didn't pick up any issues with rebar height on either floor.
- I doubt that QC gave stud height much scrutiny. As an item fabbed in a plant, we rarely have problems with them (other than install) once shop drawings have been reviewed.
- the conduit was not removed. Firstly, the rail suppllier wasn't worried. More importantly, the rails supplied were double the length of the rails specified. The conduit didn't actually impact any of the "specified" studs.
One of the most frustrating aspects of these substitutions is that they wreak havoc on our quality assurance review process.
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: Stud Rail Stud Height Shorter Than Specified
The other photo, shown below, seems to indicate that the elevation of the top of stud is about right relative to the top reinforcement, but it is difficult to tell from a photo. Also, the top steel may not be in its final position at the time the photograph was taken.
BA
RE: Stud Rail Stud Height Shorter Than Specified
I wasn't referring specifically to your link, but to a number of researchers who have concluded that the radial pattern is better. The following is a recent paper from the University of Michigan. Another significant statement in that study is that the ACI Code is not conservative in predicting punching shear capacity of stud reinforced joints which are lightly reinforced flexurally.
https://www.researchgate.net/publication/288701401...
RE: Stud Rail Stud Height Shorter Than Specified
Perhaps this could be the rare occasion where I'm pleased to see the top steel come in a bit low...
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
1) Above, I mentioned the case of a non-prismatic slab (quoted below). It seemed to go under the radar with the group here but I find the example quite salient. In plaza and parkade slabs, having a slab top surface slope that differs from the soffit slope is a common occurrence. Depending on whether or not the designer chooses to slope the top rebar to match the soffit or the top of the slab, a situation may be created where even proper height studs will not make it to the tension steel. A mere 2% difference in slope would be enough to create a 25 mm shortfall at the end of a common length rail. I take this as anecdotal evidence that it is in fact a relatively common occurrence to have less than full height studs. And, to my knowledge, there have been no consequences to that.
2) In Canada, our integrity reinforcing provisions for concrete require us to install integrity bottom steel at all columns to provide a secondary path for punching shear. In that respect, we do have some forgiveness at the ULS condition.
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
So the integrity bars for shear can handle what proportion of Vu in shear-friction? If it is, say 50%, I still don't see how it is rational to distribute 50% of Vu to shear-friction and 50% to the studs - won't they mobilize at different times and not be additive?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
"It is imperative Cunth doesn't get his hands on those codes."
RE: Stud Rail Stud Height Shorter Than Specified
In the Canuck code, most of the integrity steel provisions are just as you've described. Not so for the Slab-Column provision however. The mechanism of resistance is literally a tensile "hanging" of the slab from the bottom steel passing over the columns. It's meant to represent a ULS mechanism that, should it be activated, would surely render the slab unserviceable from that point forward. Technically, no top steel is required.
It is common to detail both stud rails and integrity bars. The stud rails are considered a part of your primary punching shear resistance and not a part of your secondary punching shear resistance. Of course. that's mostly just a choice. I suspect that the presence of stud rails would actually be quite beneficial for the secondary system.
I'd guess that the weak link in the secondary, integrity steel system would be the ability to successfully lap the integrity bars with the rest of the slab bottom reinforcing (depends on the detailing method employed). Having the integrity bars grab the rails which in turn could help grab a healthy chunk of the surrounding slab sounds great to me.
The integrity bars are designed to handle 100% of the shear load in the ULS state and the mechanism of shear resistance is a tensile "hanging" rather than shear friction. As I mentioned above, however, your slab would be pretty much totaled by the time that this mechanism was fully engaged. As such, it's not appropriate to combine the integrity steel contribution to conventional Vc/Vs capacity.
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: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
Yeah, I quoted the same provisions in my original post. But, then, just what is the intent of that requirement? I mentioned the stud rail manufacturer's response to that above...
I found that a tough pill to swallow of course. That said, I was speaking to a legitimate expert on stud rails and he seemed know a ton about other aspects of the design provisions. Take that for whatever it's worth.
Yes, this is the crux of the whole thing. Is it important that the shear reinforcement be developed within/beyond the tension rebar? That's been debated at length above. One path to take is the strict code interpretation path: a short stud is a worthless stud. That can be a difficult line to hold, however when:
1) The experts on the supplier side tell me this isn't necessary.
2) Some experts here tell me this isn't necessary.
3) The software that we all use doesn't even take explicit account of stud height.
4) Anecdotally, I can think of situations where similar conditions occur without incident.
5) Given the possible lateral separation between stud and tension steel, the whole STM notion of developing the shear reinforcement beyond the tension steel seems questionable.
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: Stud Rail Stud Height Shorter Than Specified
The interesting fact about Shear Studs is the rails themselves. In the initial testing in Germany as I was told many years ago by a PhD involved in the testing, they tested studs with continuous rails and others with rails that stopped at the edge of the column and did not continue through the column in any direction.
The tests with the rails stopping before the column suffered a normal brittle punching shear collapse.
The tests with continuous rails still failed at a similar loading but did not collapse basically giving a ductile punching shear failure as shown in the pretty picture above.
It was decided that the rails (or bottom reinforcement)continuous through the column head at the bottom acted as shear dowels preventing the collapse.
So the bottom bars do not increase the punching shear capacity in any defined way but they make the failure ductile and save lives.
RE: Stud Rail Stud Height Shorter Than Specified
RE: Stud Rail Stud Height Shorter Than Specified
It is for robustness but, I believe, not in the sense that you've described. Consider: if a column were removed, there would be nothing to hang the slab from and our spiffy integrity bars would be useless. The goal, as I understand it, is to prevent a punching shear failure at one column from instigating punching shear failures at adjacent columns and subsequently pancaking a large section of slab.
It's the picture that CSA provided in the CSA concrete code to explain those provisions. More or less the same material shows up in the European provisions and ACI352 as shown in the clips below.
Based on the snippets below, Canada, Europe, and the USA seem to have found a defined and quantifiable way to to increase the punching shear capacity of the secondary punching shear resisting mechanism.
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: Stud Rail Stud Height Shorter Than Specified
I am unfamiliar with the term "secondary punching shear", but it looks like to me the bottom bars through columns represent strictly integrity reinforcement to prevent seismic collapse.
RE: Stud Rail Stud Height Shorter Than Specified
Oh, I get it. It's just not a correct interpretation of the povidion beeing discussed.
It's not jargon of any kind. The primary punching shear path is Vc/Vs. The secondary punching shear load parth is the integrity bars. And, were there yet another load path available, it would be there tertiary punching shear mechanism.
Seismic loads do not enter into the computation on if the integrity bars in CSA. But, then, we don't explicitly consider vertical seismic load either. To my knowledge, there is little if anything that is seismic specific about the general integrity reinforcing requiremeets in CSA/ACI.
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: Stud Rail Stud Height Shorter Than Specified
I realise that, but CSA23 3_04 clause 13.10.6.3(d) says it is satisfied for prestressed slabs as long as draped tendons are placed over the column capital. So no bottom reinforcement required.
In an RC design this bottom reinforcement is lapped with the normal bottom reinforcement so that you have both Robustness if the column disappears because of the continuous bottom reinforcement and ductile punching behaviour.
In the prestressed slab, the tendons will provide robustness in catenary action, but not ductile punching behaviour.
Which leads me to think that the Canadian Code clause is for Robustness and not ductile punching behaviour as one of the possibilities they define will not provide that!
Further to that, in an earthquake, I would not be trusting unbonded tendons, which rely on the anchorage friction grip holding load, to provide robustness.
Hokie66,
I do not know what the US/Canadian/Australian recommendations are for this. It came out of testing for the German company that was involved in the development of Shear Studs along with the Canadians and their recommendation back then was to use continuous rails because of the ductile behaviour benefits.
RE: Stud Rail Stud Height Shorter Than Specified
I never said anything about the intent of the provision being a ductile punching shear failure although, in many cases, that would be an added benefit. Rather, I was very careful to always refer to the intent of the provision as being a secondary punching shear load path. Viewed as a secondary punching shear load path and nothing more, I remain convinced that:
1) the provision for prestressed concrete is compatible.
2) the diagram that I supplied is entirely applicable for the RC case.
As I mentioned above, the bottom bars are being used identically in ACI352.
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.