Shear transfer on short pier

[CDLD]

I've got a small column pier (9" high) sitting on an 8" elevated slab (dowels post-installed into slab).
Curious how others justify shear transfer at the slab/pier joint?
In a normal pier/footing case, I would develop the bars above/below the pier/footing joint and use the shear friction clauses in ACI.

Since I don't have adequate development length into the slab, would it be acceptable to assume the bottom tie transfers shear equally to the 4 pier vertical bars and design them as dowels into the slab (using concrete breakout clauses)?

Can you design the 4 vertical pier bars as anchors (with shear and tension component) and check for breakout from the slab.

 

[KootK]

I may be off base here, but wouldn't breakout be precluded at both the top and bottom of the pier in this sketch?

1) That is a joint that rarely gets checked explicitly. It's mostly just assumed to work based on common detailing, for better or worse.

2) When that joint is checked, I've only ever seen it checked as classic shear friction. Never true dowel action.

3) No, I don't feel that it does as you have drawn it. What you've drawn suggests to me a scenario where the vertical bar:

a) assumes an S-curve shape and;

b) invites concrete crushing inside of the bends.

In my view, those things suggest true dowel action and, therefore, potential breakout failure modes.

This is different from the Widianto setup where the shear is passed from the vertical bars to the ties with the vertical bars assumed to act as a rigid-ish spreader bar. Ergo no significant concrete stresses developed in within the curves of the vertical bar. Obviously, that's not an entirely true assumption. And it would tend to become less true for smaller diameter bars relative to larger ones.

 

[CDLD]

None the less, there are holes in the Widianto method. I mention a few below for sport.

I agree with your assessment of this method. Certainly a few loose strings. Admittedly, I have used this method and certainly haven't done any bending checks on the "spreader" bars, nor considered any eccentricity on the tie.

This is different from the Widianto setup where the shear is passed from the vertical bars to the ties with the vertical bars assumed to act as a rigid-ish spreader bar. Ergo no significant concrete stresses developed in within the curves of the vertical bar. Obviously, that's not an entirely true assumption. And it would tend to become less true for smaller diameter bars relative to larger ones.

I think I understand your point of the leading bar bending/curving at the bottom of the pier. Would you feel the same way if visualized the shear being passed to the rear anchor and held by the low tie?


As you and TLHS mentioned earlier I think, we can always add true shear dowels in the mid section of the pier.
This kind of irks me though, because I don't see this commonly done.

 

[haynewp]

I had a similar height pier and shear load a couple of years ago. Next time, I am going to instead try something like this and treat the pier as a thick grout bed, especially as the pier height gets even shorter. I would still have rebar ties in the pier that I don't show.

 

[KootK]

Admittedly, I have used this method and certainly haven't done any bending checks on the "spreader" bars, nor considered any eccentricity on the tie.

The nature of our profession is that we're reactive rather than proactive. We basically only check things have have gone wrong for us in the past.

Would you feel the same way if visualized the shear being passed to the rear anchor and held by the low tie?

Mostly. It would identical within the slab but, hopefully restrained by the ties in the pier. Of course, the quality of that restraint would depend on how well positioned the vertical wound up being relative to the ties meant to restrain it.

This kind of irks me though, because I don't see this commonly done.

I've never seen it done.

 

[KootK]

Given that we seem to have misgivings about all possible solutions, I still think that shear friction is, at the least, one of the more robust stories to tell.

A frustrating thing is that no one seems to know why shear friction bars need to be developed for yield. We've certainly discussed that often enough here. Is it just because that's how the SF testing was done? Maybe. My theory is that the purpose is to provide some capacity for redistribution given that, often, shear dowels wind up being used as flexural tension bars concurrently, even if unintentionally.

As such, I feel that the requirement to develop for yield is less important in flexural situations like this. I would feel pretty comfortable estimating capacity based on the model shown below. It's still shear friction at the flexural compassion block. But I can fathom no reason why the tension bars would need to be developed for anything other than their breakout capacity. In this model, the tension bars simply are not doing any other job than flexural tension. It just so happens that the flexural tension winds up being reacted by flexural compression where the shear transfer occurs.