From: "Ron O. Hamburger" <ROH(--nospam--at)eqe.com>
Date: Sat, 12 Feb 2000 15:03:53 -0800
On 2/11/00, Jake Watson noted the inherent difficulty of applying LRFD to
foundation design (i.e. deciding how large the footing or piles must be, rather
than how much reinforcing steel is needed in the concrete), because the effects
of seismic overturning are magnified by the load factors, relative to those
determined using ASD approaches.
Jakes point is well taken, but it is not the only factor currently limiting LRFD
design of foundations (i.e. structure-soil interface). The other big missing
link is that geotechnical engineers have been trained, for years, to give SE's
recommended allowable bearing pressures that are typically limited by
considerations of long term settlement rather than bearing failure, and to which
they routinely apply a 1/3 increase for transient loads. The geotechnical
community have not yet trained themselves to provide SEs with true "strength"
design pressures, as would be appropriate for an LRFD approach.
The overturning issue is relatively easy and is already addressed by the codes
(both the 97UBC and 2000IBC). The 97UBC just says design foundations for ASD
load combinations. The 2000IBC, based on the 97 NEHRP reduces foundation
overturning loads by a factor of 0.75 which brings the overturning demands to
The SEAOC Seismology Committee has been trying to come to grips with this
problem for some years, unsuccessfully. Much more work needs to be done.