Moment on footing
Moment on footing
(OP)
We know that when there is moment and axial load on footing we find eccentricity "e" =M/P and the footing is at least e times 6 long in direction of moment to avoid any tension in soil.
In order to decrease e (by increasing p) can we add the dead weight of proposed footing also in "p".
In order to decrease e (by increasing p) can we add the dead weight of proposed footing also in "p".






RE: Moment on footing
RE: Moment on footing
This is one of the reasons jib crane foundations are typically on big blocks on the order of 8'x8'x8' or more to keep the resultant force in a safe area of the foundation. I don't know if this would be an issue with your foundation but I think the worst case is when the moment is about the diagonal of a square or rectangular foundation. It might be worth checking.
Regards,
-Mike
RE: Moment on footing
RE: Moment on footing
RE: Moment on footing
This is more of a question than an answer I realize, but this is not my area of expertise.
-Mike
RE: Moment on footing
RE: Moment on footing
When "e" is greater than 1/6 the length of the footing, the footing may still be okay. You get a triangular bearing pressure under the footing, instead of a trapezoidal bearing pressure.
But "e" can never be greater than 1/2 the length of the footing.
DaveAtkins
RE: Moment on footing
I agree with above posts that its OK to have the eccentricity exceed 1/6 of the length of the footing. In addition to the weight of the footing, I include any dead weight (soil/slab) above the footing.
RE: Moment on footing
RE: Moment on footing
Many times, our geotechs give us such low allowable bearing pressures that the footing dimensions are extremely inefficient. Somtimes, the soil pressure at the heel exceeds the soil pressure at the toe, i.e., e is negative.