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RE: UBC CODE INTERPRETATION

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Remember that capacities and demands need to be compared on like bases.  To
use factored loads (demands) and compare to allowable passive pressures
(capacities), friction etc. is not comparing "apples to apples."  Few
geotechs I've worked with have made the leap to providing values
corresponding to factored loads yet as their standard practice.  When you
discuss it with them they typically scratch their heads and tell me, "Well,
I have used such and such factor of safety to obtain the allowable values,
you figure out how you want to convert to factored loads."  As a result, I
typically convert the factored loads back to allowable loads and make the
comparison on allowable stress basis.  
Regards,
Bill Cain S.E.
Oakland CA




	-----Original Message-----
	From:	Dan Goodrich [SMTP:dang(--nospam--at)karren.com]
	Sent:	Wednesday, December 06, 2000 8:44 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Re: UBC CODE INTERPRETATION

	> You should include seismic acceleration on the foundation when
checking
	> sliding of the overall structure (i.e., sliding force = 7.25 kip
per your
	> example). The earthquake motions are imparted to the structure at
the
	bottom
	> of the foundation (via friction). The inertia of the foundation
will try
	to
	> resist lateral movement, i.e. it doesn't know how to interpret the
code,
	it
	> just reacts to acceleration (F=Ma).
	>

	It would also seem to be appropriate to include passive pressure of
the soil
	against the foundation to help against sliding.  I would not use the
full
	passive
	pressure value though.  Apply an appropriate factor of safety based
on your
	knowledge of the soils.  Your geotech could help.  Also, you should
call for
	compaction of the soil around the outside of the foundation if you
intend to
	use passive pressure.  If sliding is that much of a problem, bury it
down
	deeper into the ground.

	Dan Goodrich, P.E.


	> > -----Original Message-----
	> > From: Vinod Sahni [mailto:SAHNIVK(--nospam--at)nv.doe.gov]
	> > Sent: Wednesday, December 06, 2000 9:29 AM
	> > To: JLUTZ(--nospam--at)earthtech.com; seaint(--nospam--at)seaint.org
	> > Subject: Ref: UBC CODE INTERPRETATION
	> >
	> >
	> > Thanks for your reply.
	> > But I probably got misunderstood or I did not clarify my
	> > question enough.
	> >
	> > Basically, the question is this:
	> >
	> > Let us assume the following -
	> >
	> > Equipment Wt = 5 kip,  Accel at CG of Equipment = 0.65g
	> > Foundation Wt = 10 kips, Max Ground Accel = 0.40g
	> >
	> > Let us calculate base shear for foundation sliding check:
	> >
	> > Sliding Force = 5 kip x 0.65 = 3.25 kip ( I believe this will
	> > meet code).
	> >
	> > OR,
	> >
	> > Force = 5 kip x 0.65  +  10 kip x 0.40  =  7.25 kip (I do not
	> > think code
	> > requires this method).
	> >
	> > I would appreciate your looking into this further and giving
	> > your opinion.
	> >
	> > Thanks again.
	> >
	> >
	>
	>