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causes for cracking at embed plate 4

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struct_eeyore

struct_eeyore

Structural
Feb 21, 2017
255
I have 2 small embed plates, 5x8x1/2 w/ 2 closely spaced headed studs at the end of a stub concrete cantilever. The embeds a staggered to fit roughly opp. of each other. The entire load carried by these two plates is about 600lb when the small section of roof they carry is fully loaded, otherwise it only supports about 250 lb. This is still in construction, and I got some lovely photos earlier today with a big old crack right down the centerline of the headed studs (see image)

Let's for a second ignore that the top reinforcing was not installed and hooked to contain everything. Then, even with the eccentrically loaded condition, my truncated prism failure is on an order of magnitude higher than the demand, as is the plain concrete moment capacity and shear accounting for the reduction in cross section where the studs are located. So I'm really struggling to determine the cause of failure here. From one side, a 20'L channel frames in, from the other a 7' channel - could this be driven by thermal expansion? I've yet to go to the site, but it looks like there's an angle between the south beam and the embed - prying action maybe?

Open to any and all feedback.


crack_gdhcen.jpg
 
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Do you have any other photos you could post?

I'm assuming this photo is plan view. It's kind of difficult to get an understanding of the geometry.
 
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Just spitballing here, but I'd say the closely spaces anchors created a failure plane that split when it cured. The dead load then exacerbated the issue and the crack grew.

The top reinforcing not being installed is pretty terrible, and for the sake of people building and using this building I'm glad this failed now...
 
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I'm going with the plates expanding thermally under the heat input from welding. That's a lot of embed for an unreinforced member that small.
 
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Plates are massive considering the 600 lb load... means thicker fillet welds. could BAR 3/8 or 1/4 with 3/16 or 1/8 fillet welds have worked? What end cover does the rebar have? Is there any top reinforcing, or just bottom? There is no provision for any composite action. The concrete work looks a little rough. I just noticed that with the plate expanding being thick and being buttressed against the beam, the thermal movement would have forced the break. There is no easy fix, other than to redo, properly.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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That ain't a crack. That unreinforced end is broken off. The plate on one side took the end of the stub with it when it separated. Probably started with drying shrinkage, and with no reinforcement to tie it all together, it is broken.
 
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Mr. hokie66 (Structural) is right .. There is no reinforcement visible and the reason could be hammering , impact during erection..








Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN
 
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The plates expanded during welding, and pushed off the end of the concrete, via the cross bar.
 
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Overall design seems to be strange...
But, I wonder thermal expansion is that much serious due to welding.
Is there any method to reduce welding heat problem like in this case?
-JRW
 
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I agree with Hokie. This seemed inevitable, and not really much question what caused the failure.
 
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OP said:
I have 2 small embed plates, 5x8x1/2 w/ 2 closely spaced headed studs at the end of a stub concrete cantilever. The embeds a staggered to fit roughly opp. of each other.
Click to expand...

I would like to know more about the stud configuration. How accurate is my sketch below?

Something of my assumption must be off because, the way I show it, the low plate has moved but it's stud is still embedded in the concrete.

C01_o60g2a.png
 
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What is odd to me is the plate on the other side has the same gap on the opposite edge closer to the free end. I wonder if these were wet set down into the concrete a wiggled side to side to set them where they needed to go.
 
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Koot - your sketch is approximately correct. The headed ends wouldn't directly butt up against one another, as the studs are 6" long, and the beam is only 8" wide. I was actually assuming they would provide some self-confining, laterally, where a compression strut would form between the two head in the middle. I agree it's an overkill design, but it was a last minute revision and I just slapped something in there that was already on the plans - lesson learned (I'm not taking any credit for the contractor not reinforcing the beam, so don't bust me too much)

structSU - I wouldn't be surprised if the 'wriggled' them in, as there is no steel to get in the way.
 
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Do you have proper edge distance and cover for such a small beam? How do you plan on having them fix the missing rebar?
 
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Kootk, I believe the gaps b/t the steel plates and the concrete are explained by the fact that one of the plates (the one on the bottom of the photo) has moved with the broken off end of the concrete and the other plate (the one at the top of the photos) has stayed with the rest of the beam, right?

structee, do we know that nothing collided with the steel channel beam connected to the plate that has shifted, for example a manlift or a crane, possibly applying a horizontal reaction at the embed plate? Just a thought, although I guess damage to the steel channel would probably be pretty apparent. Might be worth a look, for any localized damage or indication of impact with the steel channel.

Other than that, I think the theory of thermal expansion from welding might be plausible.
 
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Aesur,

The top and bot bars are only #5. 5/8" embed plates in 8" wide beams are standard practice around my parts, even if account for clear cover and stirrups.
 
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gte - I don't really know anything about what has been happening in the field. Despite having 3 inspections as part of our contract docs, we've never got called out there. Not surprising since it appears they never read any of our structural notes either. I've spoken with the local building department chief inspector, whos familiar with this job and been to the site - they quietly mentioned something along the lines of " they need to have someone supervising the job " - leads me to think it's just a subcontractor free-for-all down there. Needless to say, today I get an email that they 'fixed' this crack by patching it up with grout and/or epoxy... They also fired our client, the architect, so not sure how we're getting paid for this.
On a side note, has anyone actually used the clause in their drawings that if the building is not built per plan the seal is null and void?
 
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Sounds like a fundamental failing of the whole system. Design, construction, inspection, regulation.
 
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hokie,

You're not entirely wrong and I don't take offence. We've had the term "race to the bottom" thrown around a lot during my years in the industry, and I think it's fair to say we've reached the bottom in - in my neck of the woods at least.
 
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