Analysis of Footing with d/dv Outside Footing Physical Dimension

[canstruct12]

Hello All,

I am looking at an existing footing. In trying to determine the shear force acting on the footing the calculation based on CSA A23.3 shows the location where i would take the shear force to be outside of the physical dimensions of the footing. ab (distance from pier face to edge of footing) = 280mm, d = 515mm and dv = 464mm (see attached sketch). Based on this, I am unsure of how to determine the Vf (i.e. where would I take the location of one-way shear and the area to be used to determine the reaction force from the bearing surface). This footing was constructed in 1960, and would have been designed to the code at the time.

The original as-builts indicate the material below the footing to be rock with a 40000 psf (~1940 kpa) bearing capacity

If anyone has any insight or experience analyzing footings like this, any information, references or analysis techniques would be greatly appreciated.

Thanks

 

[jayrod12]

When that is the case, the shear capacity is not the failure mode you need to be concerned about. It would be strictly bearing capacity of the soil below the footing, assuming there's no moment on the footing.

 

[canstruct12]

Thanks jayrod12, I was thinking the same thing, but it's nice to be able to prove through calculation.

Moment is not a concern for this footing, as the baseplates are pinned (2 anchor bolts interior to the column flanges) and the building has a lateral force resisting system (bracing through-out the structure). There is a minimal horizontal load, ~9kN, but this has only a minor effect on changing/redistributing the base soil pressure compared to a completely concentric vertical load.

 

[centondollar]

If the foundation is rock and bending moments are minimal, the bearing pressure distribution will be constant. Furthermore, if the foundation is rock, the footing will not deflect significantly, and thus will not produce significant curvature in its various cross-sections, and thus not experience any significant bending moment or shear force. What you have on your hands is essentially a uniaxially and uniformly stressed block of concrete.

It's difficult to calculate something that does not exist. Based on engineering judgement, what jayrod said is correct - the allowable bearing pressure is your governing and (in practice, with the information you've given) the only design criteria.

 

[KootK]

For something like this, I would say that your calculated capacity should be based on one of the following when it comes to the contribution of the footing projecting beyond the pier:

1) Unreinforced concrete flexure which would probably work easily.

2) Strut and tie design which may necessitate bar hooks for anchorage.

3) Most likely just judgment as jayrod12 suggested.

 

[canstruct12]

@KootK @centondollar

Thank you for the responses, I agree with your statements.
I have seen this used in other foundations founded on bedrock, and I wanted make sure I didn't miss anything.