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.

 

[hokie66]

Ingenuity,
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/public...uds_arranged_in_orthogonal_and_radial_layouts

 

[KootK]

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.

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.

 

[cliff234]

Koot, We've used studrails many times - and what you describe does not sit well with me. The method for designing studrails is clear and the details for their proper use are also clear - and those things have been developed based on calculations and testing. If a contractor deviates from the established procedure and details then we are wandering into uncharted waters. When training the young engineers in our firm on punching shear I always show them a sentence in the PCA Notes on 318 publication..."Two way slab systems are fairly forgiving in the event of an error in the amount and or distribution of flexural reinforcement; however little or no forgiveness is to be expected if shear strength provisions are not fully satisfied." In all of my years as a structural engineer I have never encountered any other statement in any other article, paper or textbook with such an ominous tone to it.

 

[KootK]

Thanks for the input cliff. I share your concerns if not your final conclusion. In response to a couple of your comments:

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.

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.

 

[Trenno]

KootK - might be a stupid question, but would the presence of stud rails influence the integrity reinforcement? Is it common to detail both stud rails and integrity bars?

 

[MacGruber22]

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.

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."

 

[hokie66]

My understanding of integrity bars, maybe different than in the Canadian code, is that they provide flexural capacity for seismic loading. They may have some benefit for gravity caused punching shear, but more top steel is more important.

 

[MacGruber22]

hokie, that is what I thought. In ACI 318, they are merely a prescriptive quantity of continuity bars. And yes, I don't follow the bottom bars doing anything here.

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

 

[KootK]

My understanding of integrity bars, maybe different than in the Canadian code, is that they provide flexural capacity for seismic loading. They may have some benefit for gravity caused punching shear, but more top steel is more important.

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.

KootK - might be a stupid question, but would the presence of stud rails influence the integrity reinforcement? Is it common to detail both stud rails and integrity bars?

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.

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?

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.

 

[cliff234]

I just looked at my ACI 318-11 and noticed that section 11.11.5 is pretty specific with regards to the overall height of studrails. Paraphrasing,... the overall height shall be no less than the thickness of the member minus the sum of the concrete cover to the top bars, the concrete cover below the base rail, and one-half the diameter of the flexural tension bars. (Poorly worded in my opinion.) This says to me that studrails that are "too short" are not acceptable. Of course if an additional thickness of concrete is poured (to accommodate a sloping top of slab elevation) then that's ok as long at that additional thickness of concrete was not considered when the punching shear calculations were performed. The grey area is whether it's ok if the top bars were raised up higher than they would have been if the additional thickness of concrete was not poured. But that seems to be an unlikely situation because the reinforcing steel is sitting on support bars which are chaired off of the formwork, and from my experience the rebar detailer is not going to consider the placement of additional concrete when figuring out support bar heights.

 

[KootK]

I just looked at my ACI 318-11 and noticed that section 11.11.5 is pretty specific with regards to the overall height of studrails

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...

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.

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.

The grey area is whether it's ok if the top bars were raised up higher than they would have been if the additional thickness of concrete was not poured.

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.

 

[rapt]

Re the integrity reinforcement, the Canadian code version can only be for Structural Robustness (i.e. it still stands up if a column is removed). Otherwise it would still be required for PT (d) in the Canadian Code clause) as the tendons will work in the slab in catenary action. So the picture above is not really correct for this case.

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.

 

[hokie66]

All very confusing to me. I agree with rapt about the integrity steel providing robustness against progressive collapse, rather than assisting with punching shear. I wasn't aware than any stud rail products recommended carrying the rails through the columns. I thought the rails were strictly spacers. Sometimes they are installed with the rails at the top.

 

[KootK]

Re the integrity reinforcement, the Canadian code version can only be for Structural Robustness (i.e. it still stands up if a column is removed).

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.

So the picture above is not really correct for this case.

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.

So the bottom bars do not increase the punching shear capacity in any defined way but they make the failure ductile and save lives.

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.

 

[hokie66]

If you remove an internal column and there is continuous reinforcement, the slab can hang as a catenary between remaining columns. Thus rapt's robustness comments.

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.

 

[KootK]

If you remove an internal column and there is continuous reinforcement, the slab can hang as a catenary between remaining columns. Thus rapt's robustness comments.

Oh, I get it. It's just not a correct interpretation of the povidion beeing discussed.

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.

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.

 

[rapt]

Kootk,

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.

 

[KootK]

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!

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.

In ACI 318, they are merely a prescriptive quantity of continuity bars. And yes, I don't follow the bottom bars doing anything here.

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.