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Re: FOS for Overturning

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I think the first option is the correct one.

Zacheus.


-----Original Message-----
From: MSSROLLO(--nospam--at)aol.com <MSSROLLO(--nospam--at)aol.com>
To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
Date: Thursday, 25 June, 1998 12:48 PM
Subject: FOS for Overturning


>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..
>
>