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RE: Increases in Foundation Bearing Pressure For Wind and Seismic Loading

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I believe the increase in allowable stresses should only apply when two
transient loads are in the same combination. I thought the philosophy of
seismic design, for example, was that as the number of redundancies are
reduced in the load path the static seismic loads are increased (by some
factors) to allow for a better safety factor. Such as designing the anchor
bolts in a braced frame for 3 time the actual seismic load. In my practice I
never cut corners in foundation design because it is one of the least
expensive parts of the structure to be built during the construction and one
of the most expensive to fix later.
Who is that Author that you are referring to?

 -----Original Message-----
From: L. Thomas Bayne [mailto:tom(--nospam--at)]
Sent: Friday, June 02, 2000 11:18 AM
To: seaint(--nospam--at)
Subject: Increases in Foundation Bearing Pressure For Wind and Seismic

Does anyone know by what technical rule, equation, etc.  This notion of
increasing for wind and seismic loading originated or 
is founded.? I am considering adding the following authors note to my report
when I recommend foundation bearing pressures:

Authors's Note:

Although it is common to allow a one-third increase for wind and seismic
loading, we don't. In our opinion there is no authority or technical
validity for this policy. The soils supporting foundations, don't know
whether there is an earthquake occurring or whether the wind is blowing. The
rate-of-loading will be relatively rapid for seismic events and somewhat
less-rapid for wind. However, we do not have any information relating the
time-response-spectra of a footing supporting a structure being subjected to
a wind or seismic load. While we have noted in the laboratory that rapid
rates of loading tend to increase the strength of some materials such as
concrete and steel near the boundary between elastic and plastic states, we
have found no justification anywhere in the literature or in practice to
increase strengths solely based on the nature of the load, all other factors
notwithstanding. If the code allows these increases to be applied, we
recommend that your footing designer use those allowances at its own risk.