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omega at foundations

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DSA is requiring footings to be designed for omega
level forces.  In the past we have used the allowable
stress loading combinations to check stability and
soil pressure.  Then we would take the moment induced
in the footing (for the seismic combination) and factor
it up by 1.4 (for strength design of concrete) and then by
omega (we call this the "fudge factor" method).
Recently a plan checker has pointed out that this
method shouldn't be used because it factors the
reducing dead load moment by omega.  However,
as we have found in the past, simply using the omega
level load combination from the beginning usually
doesn't work because the footing quickly becomes
statically unstable.  Without a statically balanced free
body diagram, it is impossible to determine internal
shears and moments.  We would be interested in
hearing how other companies have been addressing
this issue.

Mark Pemberton, P.E..

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