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Odd Appendix D Question

Odd Appendix D Question

Odd Appendix D Question

(OP)
I was asked to review a shop drawing for attaching a concrete barrier to a slab using post-installed anchors, 8" deep. Here's the kicker: It's two slabs; an unbonded 4" slab on top of a 16" slab.

In terms of tensile capacity of the dowel, would it be based on 8" embedment - one failure cone, assuming both slabs act as one - or is it based on double the 4" embedment capacity - two 4" deep failure cones - which is what I'm thinking? I attached a sketch.

I couldn't find anything in ACI 318.

Thanks

RE: Odd Appendix D Question

I would actually expect it to be somewhere in between. The four inch deep cone in the bottom slab would active a larger portion of the upper slab then just it's own 4" cone.

I would be tempted to tell them to use 6-8" of embedment into the lower slab and neglect any contribution of the upper slab.

RE: Odd Appendix D Question

So would I...

Mike McCann, PE, SE (WA)


RE: Odd Appendix D Question

It's a question of whether they're tight regardless of not being bonded. If they've shrunk apart and you have a gap you'll get two independent cones. If they're tight you'll get a combined cone because the lower cone can't move without forcing a larger truncated cone out of the top slab.




RE: Odd Appendix D Question

I second Jayrod's approach if the numbers can be made to work.

An alternate "in between" method might be to take the maximum of:

1) The lower slab on its own OR.

2) The upper slab on its own treating it like a through bolt with an anchor plate the same size as the expected failure cone of the slab below.

If you use the upper slab at all, some thought should be given to where that uplift goes once it's been delivered to the non-composite slab.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Odd Appendix D Question

I would ignore the unbonded topping.

RE: Odd Appendix D Question

Hokie 93? Are you Hokie66's progeny? Either way, nice find. Have you been able to look through it? The more common case that I've encountered is fastening into a vertical surface that has a cold joint in it. The example that comes to mind is a brick angle where the fastener failure cone crosses a horizontal wall / slab joint. Do you know if the article covers something like that?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Odd Appendix D Question

(OP)
Thank you for your responses. One of my coworkers is going to look through her back copies of CI for the article.

In the meantime I believe the prudent thing would have them drill 8" into the lower slab.

RE: Odd Appendix D Question

Koot, I don't think your second method would represent a realistic loading scenario.

You're basically assuming that the bottom cone will break out and then slam into the top concrete. This isn't really the case. You have the possibility of progressive failure.

Failure 1. Top cone fails
Failure 2. Bottom cone is now overstressed by a factor of 2, fails
Failure 3. Anchor moves upward, bottom cone hits top slab which is now weakened by the failure plane of the top cone and fails

or

Failure 1. Bottom cone fails
Failure 2. Top cone is now overstressed by a factor of 2, fails
Failure 3. Anchor moves upward, bottom cone hits top slab which is now weakened by the failure plane of the top cone and fails

RE: Odd Appendix D Question

The mechanism that I describedis a progressive failure system TLHS. Here's my rewrite of your two scenarios:

Failure 1. SMALL DIAMETER top cone fails.
Failure 2. Bottom cone fails and bumps into LARGE DIAMETER top cone.
Failure 3. LARGE DIAMETER top cone fails with a SMALL DIAMETER failure cone already present from step #1. It's a failure annulus.

OR

Failure 1. SMALL DIAMETER top cone remains in tact.
Failure 2. Bottom cone fails and bumps into LARGE DIAMETER top cone.
Failure 3. LARGE DIAMETER top cone fails without a hole in the centre.

Maybe the attached sketch will help win you to the dark side. Of course, this assumes that, if load ends up being resisted by the upper slab, that load has a competent place to go next.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Odd Appendix D Question

Kootk:

I would tend to think failure mode 2 and 3 would be additive ≡ two things are intact - 1. Bearing strength of bottom cone to the underside of the top slab is greater than the shear strength of the top slab and 2. the remaining adhesive-concrete bond strength in the lower cone is greater than the preceding. That sequence would allow a punching shear failure of the lower cone through the topping slab.

Although, with the tendency of poor adhesive anchoring installation by contractors, I would bet on adhesion bond failure after failure cone #1 occurred, which in design would render 4" of the 8" embedment to be useless. I would mark it up as requiring 12" of embedment.

Good post, bridge buster.

RE: Odd Appendix D Question

Agreed. My money says that the capacity is pretty close to the monolithic value. This is just theoretical fun though. Until I get my hands on that CI paper, I'll be using the value based in the lower slab alone, just like everyone else.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Odd Appendix D Question

(OP)
Thank you MacGruber22

RE: Odd Appendix D Question

KootK,

I worry because of the moment and other incidental stresses that exist because of the lack of continuity. Those really shouldn't be high enough to fail anything, though, given the span to depth ratio we'd be talking about. So while I feel uncomfortable, it likely approximates the failure mechanism.

The adhesion bond would need to be checked against the lower slab embedment alone, though.

This is obviously just a fun conversation point, though, since I don't think any of us would ever design something that would require a couple of non-ductile failures to occur before we reach design strength. The client would rightfully come and scream at you because of the first failure plane being visible at the top of slab :D

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