moment on footing

huangyhg

moment on footing
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".
yes you would add the weight of the footing itself as part of the dead load to check stability.  you would also account for any bouyancy issues in the footing while doing this.
phdpe,
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
another handy method is to locate the footing off-center from the column in order to shift your wieghts around - but re  
in peck's book he states there's nothing wrong with no load under part of the footing, just don't overload the other side.  i wonder why some go to lengths to avoid this case.   
one thing that comes to mind is the possibility that once the point of zero pressure is achieved the continued loading and soil pressure is not linear.  in other words an additional 50% in load does not result in another 50% in soil pressure, but maybe more due to shifting neutral axis which could impact overturning and stability issues.  
this is more of a question than an answer i realize, but this is not my area of expertise.
-mike
the unbalanced load creates a condition that is often not checked....the reduced contact area to which the load is applied, thus increasing the bearing pressure.  this is what peck is referring to in his admonishment to not overload the other part.
phdpe,
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
phdpe,
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.
see "structure magazine", june 2004, "engineer's notebook" on pg. 17-18- discusses the situation and gives some design curves.
sometimes the agency for whom you are designing these foundations will dictate if the resultant of the loads is required to be within the kern.  many state dots require that the eccentricity always be less than l/6 and that the maximum allowable bearing pressure on the soil is not exceeded.
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.