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RE: fema 356 foundation acceptability

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Hello,
I have been going through FEMA 356 recently as well, and I may be able
to answer your question.

In the equation qc=3qallow, the 3 is to account for the geotechnical
engineer's original factor of safety.  Since you are using the
prescriptive procedure, make sure this factor of safety is realistic.
If possible, it would be a good idea to speak with the geotechnical
engineer about the factor of safety they used to get the allowable value
(see C4.4.1 for commentary on this).

The m factor is a ductility factor.  I'm not sure what codes you are
familiar with, but this is somewhat equivalent to the R factor in the
base shear calculation in the UBC.  The difference is that FEMA applies
the m factor to individual elements, based on whether they are
deformation or force controlled, and its value varies based on the
material and type of expected failure.  The UBC applies the R factor
globally to the entire structure based on the expected ductility of the
entire lateral system.

If I may restate your question to make sure I understand...how will an
existing foundation ever "fail" if the capacity is multiplied by a
"factor of safety" of 9?  Basically, the first factor of 3 is to bring
the allowable bearing pressures up to expected properties (what you
called upper-bound), to reflect a more realistic value.  The second
factor of 3 is just on the opposite side of the equation from most
building codes.  So, it's not really a factor of 9.  The only "bonus"
you get is the more "realistic" design capacity of the soil.

Make sure you watch out for the first 3.  There are a couple of
provisions that make you design your footings as force-controlled
elements, which would turn that 3 into a 1.

As for an example using Q,V,M, there's really no trick there.  They are
still based on statics.

HTH,
Lucas

-----Original Message-----
From: yilmaz yuva [mailto:yyburak(--nospam--at)hotmail.com] 
Sent: Saturday, November 08, 2003 1:53 AM
To: seaint(--nospam--at)seaint.org
Subject: fema 356 foundation acceptability


For a spread foundation modelled based on fixed base assumption and
using 
linear procedures acc. to FEMA 356 :

vertical load & moment capacities are calculated using qc=3qallow

foundation acceptability criteria requires m-factors for geotechnical 
components shall not exceed 3 and permits the use of upper bound
component 
capacities.

For an existing spread foundation, let the soil pressure is checked
against 
allowable soil pressure :

In practice, how is it possible to conclude the foundation unacceptable
with 
the use of qc=3 times allowable soil pressure and m-factor of 3.
(original 
design loads should be exceeded 3x3=9 times ?)

Please comment. If possible, please provide an example using Q,V,M.

Yilmaz

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