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.

 

[BAretired]

I can't see how FRP could be used to resolve this problem.

BA

 

[MacGruber22]

Even this state-of-art computational research only puts FRP punching shear reinforcement increase up to 40%. Testing would likely show that to go down. That is a long long stretch. Again, it seems like an all or nothing choice here.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

Hilti Europe has a solution. I won't employ it though since, somewhat ironically, it probably wouldn't extend any higher than my errant stud rails. Perhaps that, in itself, lends support to sandman's line of thinking. Presumably Hilti would have done some testing.

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]

Spoke too soon... Hilts has an identical requirement for their system.

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.

 

[MacGruber22]

Yeah, I have seen those before. Is there an amount of them that would carry all of the shear?

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."

 

[rapt]

Agreed FRP is no use for a flat slab for shear.

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.

 

[Ingenuity]

Agreed FRP is no use for a flat slab for shear.

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:

 

[MacGruber22]

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.

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."

 

[Ingenuity]

Following from rapt and MacGruber22 comment on using through-bolts, a PhD thesis from U of Waterloo researched the use of Decon manufactured studs (with the forged head) and on the free-end of the stud they cut threads and used plate washers.

Punching Shear Retrofit Method Using Shear Bolts for Reinforced Concrete Slabs under Seismic Loading: Link

 

[hokie66]

Looks like the radial vs orthogonal arrangement discussion still has a ways to go.

 

[Ingenuity]

hokiee66, I am not sure if you are referring to the thesis I referenced, but if you are the research (dated in 2008) did include both orthogonal and radial shear bolts.

 

[KootK]

So... having synthesized a great deal of information on this subject, including that which you all have graciously provided, I've decided that my solution here will be "do nothing". For those who remain interested, here's where I'm at with it.

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.

 

[BAretired]

Well, let us hope you are right. If not, you will experience the zone of perpetual pain and anguish firsthand.

BA

 

[KootK]

Yes. I. Will. I forgot reason #5 in my list above:

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.

 

[rapt]

Can you at least insist on plaques at each entrance so that any of us who doubt the wisdom of your decision know not to enter!

 

[KootK]

A plaque probably isn't in the cards but, for the exclusive use of Eng-Tips members, I can provide coordinates to help the timid steer clear of my zone of perpetual pain and anguish.

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.

 

[rapt]

KootK.

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.

 

[BAretired]

KootK,

The following photo is taken from your earlier thread which you referenced in the OP, namely thread507-399858

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

 

[KootK]

Here's what I can say BA:

- 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.

 

[BAretired]

Yes, that would be frustrating, I agree. It is difficult to know whether or not there is a problem based on the photo above.

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