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Thickness used in punching shear calculation

Thickness used in punching shear calculation

(OP)
I have a suspended 7" slab which has a 2" cement mortar finish. My question is if I can use the entire depth when calculating my effective depth d. The center of the bottom reinforcement is 7/16" from the bottom. So, my question is if my d in the punching shear equations can be 9" - 7/16", or must I use only the slab thickness (7" - 7/16")? Thank you in advance.

RE: Thickness used in punching shear calculation

(OP)
Edit: My apologies; the distance from the bottom to the center of the reinforcement is 1.06", not 7/16".

RE: Thickness used in punching shear calculation

I would tend to use only the slab unless the finish is adequately tied into the slab so it can act compositely. Would have to have proper controls/testing to verify strength of finish as well as slab if you're intending to use the strength of the finish as well.

RE: Thickness used in punching shear calculation

(OP)
Thank you for the reply Mark.

I have another question.

There is also 1" steel plate which covers the entire slab. In determining the strength for punching shear, I'm trying to figure out how much the plate will distribute the wheel loads (a single large vehicle). I can completely ignore the plate and follow AASHTO to get an E value, but I don't believe this applies to shear calculations. I can't use the method for columns and base plates, because it uses the value halfway between the column and the edge of the base plate; in my case, the base plate covers the entire floor.

Any ideas?

RE: Thickness used in punching shear calculation

I wouldn't imagine the 1" steel plate would spread the load out that much beyond what is already being done by the vehicle wheels. Assuming the 1" steel plate is bearing directly on the slab, I would assume the load transmits straight through the plate and into the slab below. Maybe flare out a little bit, but nothing significant in comparison to most wheel footprints (especially if the vehicle has pneumatic wheels).

As far as utilizing the plate for shear strength, I presume you would again need to ensure that the plate and slab are acting compositely. If there's an epoxy holding the plate down to the floor than I imagine you'd be fine to assume plate contributes. If not, then probably not.

RE: Thickness used in punching shear calculation

If the plate covers the entire slab, you have no punching shear in the slab from wheel loads. The steel plate will dissipate the shear since it has a much greater shear capacity than the slab and will attenuate it. The load will be transferred to the slab as a distributed load, not a point load.

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