Base shear acting on spread foundations
Base shear acting on spread foundations
(OP)
Is it common practice to design for overturning moment due to base shear and the distance from the top of the pedestal to the bottom of footing? I have seen cases where the column and foundation were assumed to act as one and only the shear force considered. In this case, the column is not moment connected to the foundation. I am limited in space available for a foundation and am trying to avoid using piles. However, the foundation will be placed directly on bedrock and I was hoping I could get away with only using a shear key embedded in the rock, but obviously this will not help with resisting the potential overturning.





RE: Base shear acting on spread foundations
RE: Base shear acting on spread foundations
RE: Base shear acting on spread foundations
Overturning Moment = H.h
In the above:
P = vertical load; H = horizontal load
a = distance from P to edge of footing
h = distance from baseplate to bottom of footing
If H.h > P.a the footing will overturn unless you are counting on passive pressure or a tie force to remove the horizontal force at a higher level.
There should be a safety factor, F of at least 1.5 against overturning, so P.a >= F.H.h
BA
RE: Base shear acting on spread foundations
RE: Base shear acting on spread foundations
I have a similar concern. I have always used a S.F. of 1.5 for sliding and overturning of wall footing.
In this case I am Engineer of Record for a project and I will be designing foundations for "pre-engineered portables".
My concern is that am unable to find anywhere in the Florida Building Code that requires or even recommends the use of a S.F. of 1.5...
Can anyone reference code where this recommendation might be? Or maybe I have missed it somehow...
I intend to use the S.F. of 1.5 regardless, it would just be nice to have a code reference to discuss with the owner... as they have a "delegated engineer" who (when acting as the E.O.R.) in the past has failed to use a S.F. of 1.5... thus detailing smaller footings...
Thanks, Joe
RE: Base shear acting on spread foundations
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BA
RE: Base shear acting on spread foundations