Determining Vf, one-way slab shear

[MontrealEng]

I'm hoping someone can help me with this. The attached drawing will help get your mind around my situation.

I'm designing the base of a fix-free column (moment connection via base-plate on the existing slab).

Because I have a large moment acting on the base-plate, I am concerned with failure of one-way shear in the existing concrete slab. I am testing that the shear resistance of the concrete (Vc) against the one way shear acting on the slab (Vf), (ie: Vc needs to be > Vf). I know that the one way shear plane is located at d/2 from the edge of the baseplate.

Question is: How do I find what Vf is? Do I take some specified tributary area (as outlined by the outermost dashed line on my graphic), before calculating my stress distribution, just as I would when designing a spread footing? What do you think?

The small issues always take the most amount of time...

Thanks,

Carl, ing. jr.
Montreal, QC

 

[hokie66]

I think you are talking about punching shear instead of one way shear. In a 200 slab, why are you using d/2 as 50 to define the critical section? The slab would have to resist the vertical load and bending moment by nonuniform shear along the critical section.

 

[MontrealEng]

Punching shear would normally be my main concern, but the stress distribution resulting from the moment would cause a much greater shear force on one side than the other, hence my concern with one-way shear.

(I am assuming d = 100mm as I am being conservative and don't have the drawings of the existing slab available.)

I am using d/2 as the critical shear section because clause 13.3.3.1 of A23.3-04 (the concrete design handbook produced by the cement association of Canada) states:

"The critical section for two-way action shall be a section perpendicular to the plane of the slab and located so that its perimeter, bo, is a minimum, but the section need not approach closer than d/2 to the perimeter of the concentrated load or reaction area."

 

[hokie66]

The section you quoted for defining the critical section is for "two-way action", therefore punching shear, while one way shear is shear along a line parallel to a support. The moment, therefore the eccentricity, makes the punching shear stress along the critical perimeter nonuniform. I am sure your Canadian code defines a method for checking this stress.

 

[MontrealEng]

"...the punching shear stress along the critical perimeter nonuniform." I agree 100%.

This being the case, would it make sense to use the highest pressure of my pressure distribution for the entire affected area, then simply test two way shear?

 

[ron9876]

I believe you will find that punching shear will govern over the beam shear that you are considering.

 

[hokie66]

I am not sure what you mean by "test". Is this a research project? If so, a literature search for previous testing would be my first step.

If instead, you are simply analyzing an existing slab, the provisions of your code should tell you what to check. Different codes take a somewhat different approach, and I am not familiar with Canadian practice in this regard.

 

[MontrealEng]

I'm going to read up on the material and I'll let you know what I find.

This isn't a research project, this is a 11ton monorail frame set in an industrial plant.

 

[slickdeals]

Look at ACI Chapter 11, it has a nice figure showing your case of non-uniform shear stresses.

 

[rapt]

My 1994 copy of the Canadian code covers this perfectly adequately in clause 13.4 and specifically 13.4.5.5 for the combined effects of applied moment and axial effects.

Yes, You do have to check for 1way shear also (clause 13.4.2)for which 13.4.6 refers you back to the beam shear clauses.

I do not see the confusion.