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Re: FOS for Overturning

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I always check overturning about each side of the footing and take the
worst case.  Depending on which side you are checking, the forces can be
overturning or resisting forces.  In this case overturning moment about the
left side would be 10x4=40 and resisting moment would be 3x2+15x4=66 with
FS=66/40=1.65.  Overturning moment about the right side would be
10x4+3x2=46 and resisting moment would be 15x4=60 with FS=60/46=1.3.  

Mark Florence, P.E.


At 08:50 AM 6/25/98 EDT, you wrote:
>I have a question regarding the correct factor of safety for overturning
for a
>foundation subjected to an uplift force.  For purposes of this discussion,
>assume I have a single footing with a horizontal force or 3 kips (to the
>right) and a upward vertical force of 10 kips, both applied at the center of
>the top of the footing.
>
>Assume the footing is 15 kips in weight and is 8'x 6' x 2' thick. Pardon the
>phony graphics, I am using AutoBadd.
>
>
>            ^ 10k
>            |
>            |
>     ------> 3k
>     ---------------     
>     |             |
>     |             | 2'
>     |             |
>     ---------------
>           8'  
>
> 
>1.	In calculating the FS for overturning, is it correct to call the 10k load
>an overturning load?  The load is at the center of the foundation and does
not
>actually induce any overturning except if you do statics about a toe.  The
>load itself actually does not cause overturning.  Even statics claims that is
>overturns about both sides simultaneously.  I realize that the footing weight
>is always a stabilzing force in both directions of rotation.
>
>2.	Assuming you do think it should be included as an overturning force, would
>the correct calcualtion be Movtg = 3x2 + 10x4 = 46 and Mstab= 15x4 = 60,
>therefore FS = 1.304   OR is the correct one: Movt = 3x2=6 and Mstab=
>(15-10)x4=20  FS = 20/6 = 3.333.  By definition, it would appear that the 1.3
>answer is correct and that algebraically adding common forces is not
>permitted.
>
>3.	Assuming you do not think the load is an overturning load, what if I move
>it off center to the right 1".  What is the magnitude of moment to attribute
>to overturning of the right side?
>
>Thanks for any help on this..
>
>
>