Base shear acting on spread foundations

[chrisrosebud2001]

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.

 

[BigH]

Going to use piles but say you are rock? I would say that shear keys in the rock are a good idea as well, as perhaps, embedding the foundation into the rock. For overturning - the rock won't "give" easily and if the footing is rigid enough, I doubt overturning would be a problem. If you wish, you could always install tie down dowels into rock that would have to be overcome for any movement - they can be passive - do not have to be tensioned.

 

[oneintheeye]

yes base shear will cause overturning moment

 

[BAretired]

Stabilizing Moment = P.a
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

 

[kslee1000]

While you can utilize shear key to addresses the sliding issue, it may benefit more by simple tie the indivisual footing to the ground floor, such as utilizing shear pin, or grade beam, to minimize/eliminate the shear and overturning at the footing base. Watchout for uplift due to transient loadings.

 

[StructuralJoe]

Hello again,

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

 

[BAretired]

I don't know anything about the Florida Building Code, but check out this link for some references:

http://www.google.com/search?q=Flor...id=gd&rls=IRFA,IRFA:2008-01,IRFA:en&aq=t&aq=t

BA

 

[slickdeals]

If you use the load combination of 0.6 D +/- W, then the safety factor of 1.5 is already included in it. For this load combination, the FOS = 1.0