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

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Without answering your question (because I do not know the answer), one
reason that I can think of for allowing higher short term soil loads is that
I have been told that soil bearing values may be settlement limited as
opposed to load limited.  When settlement limited, settlement can be a
function of time.  Would this not be reason for allowing higher short term
values even greater than 1.33?
George Richards, PE 

-----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.