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RE: what is the FOS against overturning of structures....

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The resistive overturning moment is only related to the maximum soil
bearing pressure in the fact that you may have to increase the size of
the footing to reduce the actual soil bearing pressure which will result
in an increased resistive dead load (assuming uplift on the wall as
opposed to substrate failure).  The FOS is calculated based upon the
ratio of Resistive Overturning Moment over the Actual Overturning
Moment.  The ROM and the AOM are calculated based strictly on the forces
applied to the structure; i.e. soil bearing is an effect of the ROM not
a causation,   The soil bearing is checked after the FOS calculations
have been completed without regards to the affect of the FOS (the FOS
will only increase upon the widening of the footing).

Therefore, when using working stress design, any FOS in the soil bearing
capacity as set by the soils engineer will not have a direct bearing on
the FOS for the ROM. 

Now when using the LRFD method for design, the factor of safety is
placed into the loads applied to the structure and these factored loads
are used to directly calculate the ROM and the AOM.  

The 1997 UBC requires retaining walls to have a FOS of 1.5 for
overturning and sliding.  General Seismic and Wind requirements specify
that the structure only resist the loads as calculated in their
appropriate sections.  I would interpret this as a ROM/AOM = 1.  This
would be more critical with the ASD method as it would be for the LRFD
as the second method does incorporate some load factors.  Personally I
maintain a strict across the board FOS of 1.5 using the dead load
resistive moment.  This may be slight overkill but the level of
engineering comfort is well worth the relative increase of the
foundation size (for the resistive dead load of the concrete).


-----Original Message-----
From: Ed Workman [mailto:eworkman(--nospam--at)fix.net]
Sent: Thursday, July 29, 1999 2:02 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: what is the FOS against overturning of structures....



-----Original Message-----
From: Khosrownia, Ghassem SPK <GKhosrownia(--nospam--at)spk.usace.army.mil>
To: 'seaint(--nospam--at)seaint.org' <seaint(--nospam--at)seaint.org>
Date: Thursday, July 29, 1999 10:24 AM
Subject: what is the FOS against overturning of structures....


>What kind of FOS have you been using for the overall overturning stability
>of the whole structure? I am not finding anything in UBC or Reinforced
>Masonry Engineering Handbook.
>
None, directly.  However,  the FOS is implied in the foundation design
process.
For example, to determine maximum soil bearing pressure appropriate
combinations of load are applied to a shearwall/foundation assembly. The
resulting eccentricities will lead to soil bearing pressure maxima
which
are compared to possibles.
Specifically 0.85 dead and 1.0 "service" overturning compared to 1.33
allowable soil bearing, AND  0.9 dead and 1997 UBC overturning (i.e.;
"factored" seismic) compared to ultimate soil strength. Outside zone 4
combinations which include live load are more likely to control,  and so
forth for wind etc. That's the idea without getting tedious.
Please note that the practice of finding "service" soil bearing stress
and
multiplying by 1.4 does not conform to UBC wording and is unconservative
and
statically incorrect.
In short, if statics shows equilibrium the fos is at least one. If
ultimate
soil pressure does not exceed ultimate soil strength fos is that
established
by the geotech.
Pile foundations will show a similar pattern.