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# Re: FOS forOvtg clarification

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: FOS forOvtg clarification
• Date: Fri, 26 Jun 1998 17:02:54 -0700

```>Charles Greenlaw wrote:
>
>. > Here, you can put your axis of rotation under the applied vertical load if
>. > you want and get rid of that load as an "overturning force".
>
>Actually, you'll never get rid of the applied vertical load, because you have
>to take into account the *reactions* which were eliminated when overturning
>about the toe was considered.  With tipping impending, the reactions of all
>of the vertical forces are considered concentrated at the toe.  By taking
>moments about the toe, *that* value is eliminated.
>
>A. Roger Turk, P.E.(Structural)
>Tucson, Arizona
>
Correct, all of the forces are still there. The question dealt with
what to call that vertical force. A semantic matter. The FOS issue concerns
"overturning" forces, and between the two choices the inquiry posed, it made
a major difference whether the loads with the big numbers were deemed
overturning forces or not.

My reply suggested that there were no rules as to where the
overturning pivot point had to be, and therefore one could choose it so as
to eliminate the effect of the big-ticket loads, and alter the apparent FOS.

If the pivot point for moment calculations is put at the bottom of
the footing and directly under the line of applied vertical loads, then the
OTM only involves the applied horizontal load. The Resisting Moment at
equilibrium comes from the upward soil pressure resultant, wherever it is,
times its distance from the pivot point. The maximum possible RM is when the
upward soil resultant gets as far from that pivot as it can --however far
that is. The FOS would be the ratio of the max possible RM to the
equilibrium RM....as I see it.

Of course, the upward resultant can always be "considered"
concentrated at the toe, where many put it, but I am not sure the soil would
comply.  Whoever invented the FOS provisions on footing overturning failed
to say that this improbable, high stress location is what they had in mind
for computational purposes.  As with sex, more than one position is viable,
and other people's custom may differ from that practiced by the code writer.

Charles O. Greenlaw, SE    Sacramento, CA

```