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
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 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
Mike McCann, PE, SE (WA)
RE: Odd Appendix D Question
RE: Odd Appendix D Question
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
RE: Odd Appendix D Question
RE: Odd Appendix D Question
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Odd Appendix D Question
In the meantime I believe the prudent thing would have them drill 8" into the lower slab.
RE: Odd Appendix D Question
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
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
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Odd Appendix D Question
RE: Odd Appendix D Question
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