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.

 

[ARKeng]

KootK,

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!

 

[KootK]

I also doubt the technical guys that answer the phone know more than you do on the topic.

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.

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.

"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.

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

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

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.

I'll give it a go. Both those guys are pretty Emeritus-ized, however, so it could take some time.

Did you get shop drawings beforehand?

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.

 

[KootK]

They have a good design manual DECON® STUDRAIL® DESIGN MANUAL.

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.

 

[KootK]

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.

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.

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...

...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.

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]

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.

If it was a beam that only needed a little shear reinforcing would you accept floating stirrups mid-beam?

just like there's a reason the code says that if that was a stirrup, it needs to be hooked around the flexural reinforcing.

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.

 

[IFRs]

All this theory is nice, but does the actual construction serve the need? Can you look at this as a 7 inch slab with 5 inch stud rails properly installed and an extra 1 inch of cover?

PS - I know NOTHING about stud rails.

 

[KootK]

As a hypthetical that I would never do in practice, consider the beam shown below. I believe it code compliant depending somewhat on your definition of "as close as possible". I also think that it is quite analogous to the stud rail situation.

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.

 

[IFRs]

All this theory is nice, but does the actual construction serve the need? Can you look at this as a 7 inch slab with 5 inch stud rails properly installed and an extra 1 inch of cover?

PS - I know NOTHING about stud rails.

 

[KootK]

All this theory is nice, but does the actual construction serve the need? Can you look at this as a 7 inch slab with 5 inch stud rails properly installed and an extra 1 inch of cover?

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.

 

[IFRs]

Why not? How deleterious is the extra 1 inch cover, and why?

 

[KootK]

Why not? How deleterious is the extra 1 inch cover, and why?

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.

 

[sandman21]

While the program does not have the slab thickness in the project it does account for the stud depth in the calculation of the area of concrete and moment of inertia. The testing has the studs at the cover limits as that how it has been tested and is an easy to construct system. The sample problem you have would still develop a strut and tie to transfer forces and would still provide shear resistance, but with a reduced section. i think you can review the stud rail with the reduced length of stud and the reduced area and see if it works.

 

[KootK]

The sample problem you have would still develop a strut and tie to transfer forces and would still provide shear resistance, but with a reduced section

.

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:

Shear and bending are the same deal, you're progressively turning the corner with that shear to progressively build up your moment.

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.

 

[sandman21]

Yes, it's clear, I also don't think it matters. If the base cover was increased, you are also reducing the effective area the studs would increase your punching shear strength. If the anchor length is enough to engage the concrete outside of the inclined punching shear failure plane than decreasing the height of the studs in the calculation of shear stress, I think, would be acceptable. You may find that some studs no longer contribute based on the location and height. I do think you are in an area that will require the manf. to provide a full explanation with calculations or testing to justify the as-built condition.

 

[Trenno]

Chuck some more integrity bars in there to be sure!

 

[MacGruber22]

KootK - I hope you are billing them for your time on here. Seriously. Many would have already told them to pound sand.

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."

 

[BAretired]

Chuck some more integrity bars in there to be sure!

Not easy after the slab has been poured.

BA

 

[MacGruber22]

"post slab pour". oops

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

 

[Trenno]

FRP it is then!

 

[MacGruber22]

To me, FRP doesn't solve the failure plane issue with the studs. So...1. I don't see how the FRP and the stud capacities can be additive with the uncertainty of the studs, and 2. Can you practically get the FRP/and concrete to do all of the work. I would go with a post-installed column capital over the FRP in this situation......*if* you can't convince yourself that there really isn't a problem with the short studs.

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