Short Concrete Cantilever

[pbw]

I've got a short concrete cantilever 3" lg. x 1' deep. The cantilever is from a slab on grade that will extend over a conveyor pit and support a traffic plate. What is the appropriate way to check the cantilever? Should I check shear from a shear friction standpoint (corbel) Vu=Av*Fy*u? or should I check it as beam shear Vc=2*(f'c)^.5*bw*d? Can I incorporate both for capacity?

Also what are your thoughts on the width of the "beam section (bw)", I am considering a 2' beam width since the traffic plate is 2'wide.

Thanks
PBW

 

[dik]

I wouldn't normally worry about the shear on such a short cantilever. Test results for short plain concrete cantilevers indicate a concrete shear capacity of several hundred psi (600-800).

I would, however, try to have the load come down on a seat or confinement angle with 6" or 8" long studs going into the 'backspan', alternately, welded reinforcing can secure the seat angle.

The problem is more likely to be crushing of the concrete or lack of confinement of the concrete at the edge. If confinement reinforcing, in lieu of an angle is used, then take care that any bars parallel to the edge have sufficient cover and are adequately tied back. The hard deformed bar can generate high tensile stresses in the concrete if too close to the surface.

 

[Qshake]

I mostly agree with DIK but would also check for shear friction in addition to the unreinforced concrete strength.

Adding an armor angle is an excellent idea.

 

[pbw]

Thanks for the responses.

I failed to mention that the traffic plate bears on a seat angle; both are cast iron from Neenah Foundry. The seat angle is 3' long and has brackets with a 1/2" hole in the bracket @ 12" o/c. I am putting a #3 thru the holes; perpindicular to that I have #4 L bars @12" o/c.

I feel fairly certain that what I have will work for the truck loads...now just have to prove it to myself.

 

[dik]

Qshake:
should have added that the stud/rebar area be determined by shear friction... thanks.