Isolated footing with" lost of ground contact"

[mar2805]

Hi!
When there is an force and a moment acting on an isolated footing in such a way that the resultant force acts outisde of L/6 part.
How to determine the soil pressures?

1.) An standard way would be to calculate pressures so that
presure= N/A + M/W = "+ and - presure"

minus cannot occour in nature so there is a formula to increase the "+ stresse" in this way
Claculate c=B/2-e
c-distance from the footing edge to the resultant force
B- footing width
e- M/N

then you increase the "+ pressure" using this forumla:

pressure=2/3 * ( N / c*B)

Check if this stress is under allowable ground stress


2.) the other method I have seen is to calculate the effective area of the footing using this forumla
B"=B - 2*e

And then

pressure= N/ (L*B")

Check if this is under allowable ground stress.

Both give very different pressure results!

Wich method is correct?

 

[hokie66]

e = M/N

That gives you the location of the resultant of the triangular soil pressure. Just solve the triangle for the maximum pressure.

I think you are on the right track with method 1), but it looks like you have used B for both footing width and length. OK for a square footing, but you need two variables if the footing is rectangular. I would write your equation as "pressure = 2N/3c/footing length".

 

[Ron]

Usually neglected but just for clarity, "minus" condition can occur in saturated soils.

 

[mar2805]

Just to be sure.
Please see picture attached.
My resultant force is outside the kern, so "e" (eccenticity) is greater then B/6.
I calculate the pressures and since there is an tension effect, tension is neglectetd and the pressure is increased with the previous formula mentioned in post 1.

 

[msquared48]

Face it. Your retaining wall is working, really working...

Mike McCann, PE, SE (WA)

 

[mar2805]

What do you mean?
Its not designed, sized, placed properly?

 

[msquared48]

As long as the soil is not overstressed and sliding is OK, the wall design is just fine. is OK. It is not optimal, but sometimes design limitations dictate that.

Mike McCann, PE, SE (WA)

 

[hokie66]

Just to clarify a bit, I think the OP is talking about a column pad footing, with a bending moment applied by the column, rather than a retaining wall footing.

 

[mar2805]

Yes hookie66-
Its an pad footing that supports an RC column

 

[mar2805]

Anyone? :-(

 

[JStephen]

See attached pdf, 2 pages.

 

[mar2805]

Sorry for the delay.
JStephen thank you for the input but I was thinking of with a bending moment IN TWO DIRECTIONS applied by the column...
I cant find anything on that.