shear friction footing no dowel

[delagina]

can someone guide how to calculate the shear friction in this case

 

[WARose]

Not sure if I've ever used just concrete for shear transfer at a interface for reinforced concrete. Will this footing always just have the compression load on it? Never any lateral?

 

[DETstru]

From a theoretical standpoint, you just need to determine the shear flow in the joint plane then use ACI to determine if you have the capacity.
If you don't, you can epoxy rebar from the top section of the footing down into the bottom section.

From a practical standpoint, can you make the footing work with the (bottom) section that is remaining? The top bar does nothing for you for a single downward load.

 

[delagina]

yes, only vertical load.
what should I be checking at in this case to make sure this is ok.

actually the original picture is wrong. it's a square footing with 4 sonotube piles.

 

[EDub24]

I agree with DETstru. Figure out the shear flow across the joint and use ACI to determine your capacity. ACI 318-11 section 11.6.5 (section 22.9.4.4 in ACI 318-14) gives shear friction capacity for normalweight concrete placed against roughened concrete (without reinforcement). If you don't have enough capacity you can add vertical bars across the joint. To determine the shear flow you'll have to dust off your old Mechanics of Materials book or you can google "Shear Flow on Beam Element".

 

[WARose]

Under normal circumstances I would be advising you to figure the shear flow (from the flexural load) and then calculate (as per the shear friction requirements of the code) the amount of re-bar required across the plane of the crack.

Here though, I'm not sure this is allowable: you've got no friction steel (forgetting about the sonotube re-bar that crosses the crack). And a pile cap doesn't qualify as a plain reinforced concrete structure in my book.

I would advise you to use some re-bars (Hilti into the first pour and hooked into the second) to make this happen.

 

[delagina]

what would happen if I don't have enough shear across the joint?
I just can't picture the concrete sliding horizontally due to vertical force?

 

[delagina]

if I have hook at the end of the footing I can use that rebar, right for shear.

 

[WARose]

I just can't picture the concrete sliding horizontally due to vertical force?

And I can't picture a perfectly vertical force either. (No horizontal from a lateral event on top of that?) So to be on the safe side, I'd comply with code. I don't know what this is supporting.....but it's got lawsuit written all over it with this plan.

 

[delagina]

the issue also is both the 1st and 2nd pour has been done

 

[WARose]

if I have hook at the end of the footing I can use that rebar, right for shear.

Maybe. But watch out for that: ACI code (at least the latest one I use) still requires you to fully develop the shear re-bar on both sides of the interface. With the Hilti you will have it on one side......but tougher to get that (i.e. full development) on the other side.

 

[rapt]

I do not think you can justify it.

Sounds like some demolition is needed or some external strengthening to provide a connection. And it should extend to the bottom of the footing/slab.

 

[delagina]

the vertical load here is not really big. it has 4 piers mainly for to minimize deflection per client request. they've used this type of foundation in the past.
it's supporting a pipe.

 

[EDub24]

Maybe. But watch out for that: ACI code (at least the latest one I use) still requires you to fully develop the shear re-bar on both sides of the interface. With the Hilti you will have it on one side......but tougher to get that (i.e. full development) on the other side.

Hilti has a new rebar design guide where they give equations for shear friction capacity for adhesive dowels where you don't have full development.

I just can't picture the concrete sliding horizontally due to vertical force?

If you can't adequately transfer the vertical loads and their corresponding shear flow then essentially the lower portion of the footing will do all the work. This means you will have an increased weight (the weight of the upper portion of the footing) and the lower portion will have a reduced 'd' which will lower your moment capacity.

 

[dik]

Your maximum concrete shear will be approximately (1.5*P)/(2*Ac) where Ac = w * tc. This is the maximum elastic shear stress for a rectangular section at mid depth but does not consider the compression loading. If this is more that approx 60 psi, then you should add some ties to hold the two halves together (to be safe, more than about 30 or 40 psi; the actual value should be small) In addition, check the footing for punching shear and beam shear. I don't know if this is covered by code anywhere.

 

[KootK]

I don't feel that shear flow based on bernoulli flexure would be appropriate for something of these proportions. Plus it's kinda hard in 3d. Here's what I recommend:

1) Model the thing in your head as a four strut strut & tie system.

2) Rough out the forces in each of the four struts.

3) Check the horizontal joint across each strut using shear friction.

4) Consider the vertical component of each strut force to be a clamping force doing the job that shear friction reinforcement normally would.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Will it work, with your small load from a pipe, with only the bottom part of the slab acting? If so, no need to worry further.