Short Headed Stud Anchors - Composite Beams

[Stenbrook]

I have run into a situation where the contractor has installed 400 headed stud anchors to be 4" long with a 3" composite metal deck and 2" of concrete over the deck. This gives 1" projection over the metal deck. They were supposed to install 4 1/2" length to get the minimum 1 1/2" projection that AISC calls for.

AISC Section I3.2c(1)(2) states: “Steel headed stud anchors, after installation, shall extend not less than 1½ in. above the top of the steel deck and there shall be at least ½ in. of specified concrete cover above the top of the steel headed stud anchors.”

However, Section 1.11.6 stated: “When composite construction does not conform to the requirements of Sects. 1.11.1 through 1.11.5, allowable load per shear connector must be established by a suitable test program.”

The question is, does anyone know of a suitable test program that could help me evaluate the capacity of these shear stud anchors that are too short? I would really prefer to tell the contractor that he only needs to add a few anchors to make up for the reduced capacity vs telling them to uninstall all of them and reinstall.

 

[XR250]

Would the existing studs have to be removed in order to install the correct anchors?
These go down pretty quickly. Seems like it would be easiest just to have the correct ones installed.

 

[Stenbrook]

I thought about that except that more than half of the available spaces between the deck flutes already have anchors installed in the center of the beam for most of the beams. In order to install enough of the new anchors, you would have to put more than one in most of the flutes. Most of the beam flanges aren't wide enough for two rows of anchors even if the first anchor wasn't centered on the beam. But with it centered, it makes it impossible to maintain the required spacing between the anchors and the edge distance to the edge of the flange.

We can definitely reduce the number of studs we have to remove but I would say it is probably only about 25% that we could keep in place without intefering with the new ones.

 

[Lomarandil]

I've heard somewhere that studs can be stacked... that might help your situation.

 

[Stenbrook]

I hadn't considered that but I'm not sure they would be able to effectively weld a 1/2" extension to the current anchors.

 

[Brad805]

4 1/2" studs do not exist. I suppose you could special order them, but unless you need a container full of studs I am not sure that is cost effective.

 

[KootK]

1/2" isn't much of a difference. And, generally, I'm a firm believer in similar things performing similarly.

Unless there is some jurisdictional issue preventing it, this is the direction that I'd go:

1) Generally assume that it's fine as is.

2) Ask for one beam to somehow be load tested to validate the capacity of that beam's studs. Maybe this is an offsite test beam if that's easier.

One successful test isn't particularly meaningful in the statistical sense. I'd be doing it more just to be able to say that I did "something" to justify my liberal approach.

If the contractor would be willing to do this in order to obviate the need for new studs, great. If not, bring on the new studs.

I tend not to get overly excited about liability stuff in these situations. The nature of the game is trading risk for profit in an equitable way. And, really, that's true of anything credibly termed "business". Here, I'd be happy to do a favor for the team on something that surely does not matter in the expectation that might garner me a return favor on something that does matter in the future.

 

[XR250]

Maybe just plug weld a 1/2" nut to the top of each one?

 

[271828]

2) Ask for one beam to somehow be load tested to validate the capacity of that beam's studs. Maybe this is an offsite test beam if that's easier.

I wonder if a testing company could set up a pushout test. That's how one would verify the capacity of the studs in a research program. These specimens are pretty small and the testing procedure isn't difficult.

Mentioned in the following, among many other places.

https://vtechworks.lib.vt.edu/bitstream/handle/10919/26989/Ch2.pdf

https://www.mdpi.com/2076-3417/11/1/105

https://www.researchgate.net/figure/Conventional-pushout-test-setup_fig1_242658961

 

[KootK]

Pushout tests, in some statistically meaningful quantity, surely would be the by the book way to do the testing. I just struggle to imagine a contractor being willing to do that mid-stream on a live project. I'm guessing they'd prefer new studs.

 

[271828]

I'm sure new studs would be preferred if that's feasible at all.

 

[BridgeSmith]

Just curious, does AISC perhaps make some kind of exception if the studs extend half the depth of the slab?

We use 4" studs on 8" bridge decks as pretty much a standard practice. AASHTO only requires the studs extend 2" into the deck, regardless of the thickness. While 8" is typical for us, 9" or 10" thick decks are not uncommon in other places. all that to say, I seriously doubt you'd see any measurable reduction in capacity because the studs **only** extend halfway through the slab.

Also, assuming you could find 4 1/2" studs, you'd be right on the edge of violating the 1/2" minimum cover. That would indicate that the 1-1/2" minimum was never meant to apply to a slab as thin as yours.

 

[human909]

Agreed. It seems a 'rule of thumb' that doesn't suitably scale with thick/thin slabs. On the other hand it does seem reasonable to question how readily that 2" of concrete will be suitably engaged in composite action. It probably would, but 2" isn't a whole lot of "meat" about those 3" pockets created by the decking.

 

[Italo01]

You said that more then half of the available space is used, but how much is left?

If the space left is at least close, maybe would be better to just add new studs.

Lets say that the new studs on the available space can give you 30% of the shear needed, the installed studs would need to give only 70% of the nominal shear and i doubt that this 1/2" difference would impact the shear strength so much.

In this case, you would have to provide only part of the studs, would avoid the labor of removing the existing ones and would need to guarante that the installed anchors provide only this partial of the nominal shear.

 

[Italo01]

Also, did you use it 3/4" studs?

If you use 7/8" studs, the shear needed for the existing ones will decrease even more.

 

[hokie66]

With a slab only 2” above the ribs, the heads in the centre of that sounds as good as you can do. My preference would have been for at least 3” of concrete above the ribs. What shrinkage reinforcement do you have?

 

[Stenbrook]

@BrdigeSmith - As far as I know, I could not locate any exceptions. They state a minimum of 1.5" above deck to stay in line with available research and data. It also goes on to say that the 1/2" is jsut meant to be a specified cover and that if this gets a little smaller due to imperfections in the field it is ok, but the 1.5" needs to be maintained.

@Italo01 - most of the beams have about 1 stud per foot which leaves every other flute empty. Some have more studs then that leaving less than half available. I agree with your assessment that the existing studs are likely around 70% or more of the intended capacity and I can't really see how 1/2" is going to make that huge of an impact however, I can't just abritrarily say that they can give me 70%. I prefer to be able to justify that assumption with a calculation. Additionally, AISC requires that the HSA be 3/4" diameter or less. They don't let you use 7/8". Not really sure why that requirement is there.

@hokie66 - I agree that 2" isn't ideal. This project had a big height restriction and was an extension to an existing building so every inch matterered in getting this thing to flush out.