Shear Friction at Monolithic Concrete Confusion

[StrEng007]

According to ACI 318-19, it is necessary to check shear friction at locations of potential cracks. This includes monolithic pours.

I typically check the ends of my monolithic beams for shear using Vn = Vc + Vs. If required, I provide shear steel in the form of vertical stirrups to resist diagonal tension.

I always extend my bottom bars into the column supports, but I don't go out of the way to check shear friction unless it's a 2nd pour at a cold joint. Does everyone do the shear friction check at monolithic pours?

 

[KootK]

Does everyone do the shear friction check at monolithic pours?

Based on my travels, this is where I have landed on this:

1) Any plane you might draw anywhere on any member probably has to satisfy some version of shear friction. It's just equilibrium. The relevant question is whether or not its a version of shear friction that warrants evaluation.

2) Practically speaking, I don't believe that it is possible to generate a shear friction failure across any plane that is simultaneously:

a) Monolithically poured and;

b) Properly detailed for flexure at the same location.

This makes proper rebar detailing all the more important. You know, as if it weren't already FFS. Mission critical.

3) You can generate direct shear failures on planes parallel to the shear being considered. This is a different phenomenon from shear friction, however, and comes with a grossly higher capacity (10*SRT(f'c) in ACI parlance).

4) Hokie66 is right about everything and always has been. It's super annoying.

 

[KootK]

With reference to the above, it's useful to consider how that plays out in relation to modern hanger bar requirements for beam to girder connections:

1) There is no code requirement to check shear friction at such joints and, in my experience, no one does.

2) You probably will get a crack at or near the support interface.

3) We're cool with the crack so long as the joint is properly reinforced for flexure which is done by providing the required STM hanger reinforcement.

 

[KootK]

And since it's October, a pic of my 2014 pumpkin.

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