Minimum distance from base plate edge to foundation edge (pressure)

[mikesg]

I am reviewing some drawings and there is a thing that worries me. Please excuse my bad technical english.

Rhere is a cantilevered column loaded with moment and shear and almost no axial force. The load is from one side only (no earthquake analysis). The moment couple is from forces of about 200kN (45 kip?) with distance of 42cm (16 1/2 in).
The column is to be installed almost at the edge of a big foundation and the tension is from the side farther from the edge of the foundation. The edge of the base plate that is bearing on the concrete however is shown to be aligned with the edge of foundation.
The engineer has shown a 'skeleton' of reinforcement that is 12.5x12.5 cm (5x5 in) square with rods N12 (1/2 in) with shear reinforcement 8mm at 20 cm (1/3 in at 8 in) distance. This skeleton lies in concrete under the bearing area.

Now - the question. Is this sufficient to prevent the concrete from crushing - shouldn't there be an analysis for slipping of a pyramid under that force? Is there a minimum edge distance for the bearing side of the plate?
I can provide whatever additional information is required to clarify the question.

Many thanks
Mike

 

[Lion06]

I don't think you would have to worry about the concrete crushing, it would be more of a shear failure with a triangular portion shearing off at the end.
I haven't ever really seen footings with shear reinforcement. If your footing has it then I would assume the engineer considered what you are concerned about.

 

[mikesg]

I guess I messed up the words By crushing I did not mean consuming the local compressive strength (which seems to be the term). I meant what you described - a triangular concrete prism breaking from the foundation (is breakout the term for this?) .

What made me curious was the very little distance from the edge on which this compression load is distributed - can I really rely on full compressive strength development when being only ~5cm (2 in) from the concrete edge?

Many thanks for your reply

Mike

 

[LOKSTR]

Mikesg,

You need to check the concrete pullout and side face splitting of concrete tor tensile and shear forces . If you have access to ACI-2005, follow the steps of appendix-D.


Lokstr

 

[mikesg]

Thanks for your reply.
I have checked pullout and breakout strengths of the tensile anchors and capacity is times larger than the demand. About side face splitting - as I mentioned the tensile group is on the farther side from the edge, about 50cm (20") from the side - i.e more than the minimum of 6d recommended in design guide 1, which is 18cm(6") in my case.

Mike

 

[csd72]

mikesg,

I think there is a little confusion regarding what you are concerned about. Are you concerned about the crushing of the concrete under vertical load or are you concerned about the lateral load on the anchors?

Crushing of concrete - unlikely for footings as long as the base plate is of sufficient size.

Lateral load - 1/3" at 3" ties look like they have been designed specifically for this.

regards
csd

 

[UcfSE]

The check for that is the two-way shear capacity, also known as punching shear. That check is in the ACI and should be in whatever code you are held to.

 

[JStephen]

If I remember right, in the ACI bearing allowables, it includes an increase in the allowable bearing when the bearing surface is larger than the load surface (the base plate).

You also get into an erection issue- just setting the column may tap a corner off there.

Also check if the corner is detailed with a chamfer or not.

 

[mikesg]

Thank you all for your answers.

csd72, I am concerned about the vertical load. Lateral load is relatively small: 55kN (12.5 kip) and there is a shear key welded to the base plate.

UcfSE, do you mean to check similar like for breakout of the concrete pyramid (or foundation under a heavy loaded column) or something else?

JStephen, you are right, this erection issue should be taken in consideration.

I will discuss with the owner and check on-site whether a larger distance can be provided - even 5cm (2") will make me feel much more comfortable.

Many thanks once again
Mike