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RE: UBC CODE INTERPRETATION

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If you are looking at sliding or overturning of the structure including the
foundation, then the inertia of the foundation should be included. If it is
below grade, the soil pressures on each side of the  foundation should be
included in the analysis, as well as the friction force at the base (due to
compression loads). In UBC 1997, see Sections 1629.1 and 1630.8.3, which
refer to the "foundation soil interface". UBC Section 1809.4 allows some
reduction in overturning moment based on the Ft force. (ASCE 7-95 Section
9.2.3.6 also states to design building foundations for foundation
overturning "at the foundation-soil interface". It also allows a 0.75
reduction in overturning moment at the foundation level, but I haven't seen
this reduction factor in other codes. IBC 2000 Section 1617.4.5 addresses
overturning but is not as explicit about the foundation level as ASCE 7-95.)
If you can tolerate some movement under seismic loads, I would use active
earth pressure on one side and passive earth pressure on the other side. If
you cannot tolerate lateral movement, you could assume that at-rest soil
pressures remain on each side of the structure and only count on friction to
resist sliding. 

Although traditionally many people have assumed that buried structures move
"with the ground", research has shown that earthquakes can induce lateral
seismic soil pressures on the side walls of a buried structure. The Corps of
Engineers publication EM 1110-2-2502 (1989) for design of Retaining and
Flood Walls does provide some formulas for effects of earthquake forces on
lateral soil pressures. According to that publication for a structure which
can tolerate some movement,  the soil pressure increases relative to active
earth pressure on the driving side and decreases relative to passive earth
pressure on the resisting side. For buried nonbuilding structures, IBC 2000
says that seismic design shall be addressed in the geotechnical report and a
substantiated analysis be performed using approved standards - but what
"approved standards" are to be used is not defined. 

I don't believe that there are any code-prescribed safety factors against
sliding or overturning of foundations under seismic loading conditions. You
just need to do a rational design that meets allowable stresses (i.e., a
safety factor of 1.0). 



> -----Original Message-----
> From: Vinod Sahni [mailto:SAHNIVK(--nospam--at)nv.doe.gov]
> Sent: Thursday, December 07, 2000 6:32 PM
> To: Sherman, William; seaint(--nospam--at)seaint.org
> Subject: REF: UBC CODE INTERPRETATION
> 
> 
> William Sherman,
> 
> Thanks for your comments.  I need a clarification though.  
> You mentioned
> that the inertia of the foundation will try to resist lateral 
> movement.
> Therefore, seismic force generated by the weight of the foundation
> should be considered.
> 
> I agree with you in the case when an object is located above 
> the soil. 
> This object will resist motion of the ground because of inertia. 
> 
> But when the foundation is buried and surrounded by soil (soil is
> compacted of course), then the foundation is part of the 
> soil.  It will move
> with the soil and there is no relative motion between the 
> foundation and
> the interface soil below.  And if there is no relative motion 
> between the
> surrounding soil and the foundation, the foundation itself 
> does not exert
> seismic force. 
> 
> And based on working stress criteria (no load factors, etc), 
> what is the
> minimum code required FS against overturning at the bottom 
> corner of the
> foundation due to seismic?
> Thanks in advance.
> 
> Vinod Sahni, SE
> 
> 
>