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Re: FOS for Overturning

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Both the answer and the question appear to turn initially on semantics.

By "overturning", do you mean overturning of the footing free-body only, as
a consequence of eccentricity of the forces imposed on it, as you show....

Or do you mean factor of safety for the footing's role in resisting uplift
that results from "overturning" effects of forces acting on the structure as
a whole? (such as wind, seismic, earth pressure, overhung live loads, etc.)
If so, the concern is still there if the footing is a deep drilled cylinder,
or a pile, where there isn't an edge to uplift.

Perhaps the section of code that you have in mind could be identified in a
follow-up posting. It often happens that a code writing committee has a
meaning well understood but their wordsmithing fails to convey it
unambiguously to anyone not in on the oral commentary in the meetings.
Interpreting their output can resemble that parlor game where the first
person tells a contrived short story to the next person, who tells their
version of it to the next, and so on around the room until it comes back to
the source, altered beyond recognition.   

The BRBATES reply hints briefly at the nature of source loads making a
difference, as does Alex Nacionales, but then sticks to the setup you gave.
I agree with Bates that the concentric up-pull load does not cause an
overturning effect in the footing, and thus needs no safety factor relating
to overturning of the footing itself. If there were no applied horiz load at
the top of the footing, there would be no overturning effect on the footing
at all, even if the uplift pulled the footing bodily out of the ground.  

If this last effect is of interest, there is ambiguity as to whether the
applied uplift to which a 1.5 safety factor applies includes direct wind
uplift for which no axis of overturning rotation is apparent (and for which
no particular overturning moment would exist), or only includes the
overturning effect of horizontal wind and eccentrically disposed direct
uplift wind.

I have been told that the 1.5 SF was intended for when an unfactored service
load is resisted by gravity, there being otherwise no "allowable stress"
discount on the value of gravity as there is for the materials of construction.

But I don't know how far around the room I was in the parlor game.

Charles O.Greenlaw, SE     Sacramento CA




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..
>