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# FOS for Overturning

• To: seaoc(--nospam--at)seaoc.org
• Subject: FOS for Overturning
• From: <MSSROLLO(--nospam--at)aol.com>
• Date: Thu, 25 Jun 1998 08:50:27 EDT

```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..

```