Isolated footing with" lost of ground contact"
Isolated footing with" lost of ground contact"
(OP)
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?
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?





RE: Isolated footing with" lost of ground contact"
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".
RE: Isolated footing with" lost of ground contact"
RE: Isolated footing with" lost of ground contact"
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.
RE: Isolated footing with" lost of ground contact"
Mike McCann, PE, SE (WA)
RE: Isolated footing with" lost of ground contact"
Its not designed, sized, placed properly?
RE: Isolated footing with" lost of ground contact"
Mike McCann, PE, SE (WA)
RE: Isolated footing with" lost of ground contact"
RE: Isolated footing with" lost of ground contact"
Its an pad footing that supports an RC column
RE: Isolated footing with" lost of ground contact"
RE: Isolated footing with" lost of ground contact"
RE: Isolated footing with" lost of ground contact"
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.