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

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You could just as easily ask why we use reduced load factors for load cases
with wind and seismic in factored load design. An increase in allowable
stress in service load design is comparable to a reduced load factor in
factored load design. It is considered reasonable to reduce the required
safety factor for such infrequent loads - refer to discussions on load
factors in such standards as ASCE 7. It is not directly related to rate of
loading effects on materials, although that could also come into play - for
soils, settlement is not likely due to transient loads vs long term loads.
Thus I would recommend permitting such stress increases for allowable
stresses which are based on a reasonable safety factor for "everyday" loads.


-----Original Message-----
From: L. Thomas Bayne [mailto:tom(--nospam--at)soilsengineering.com]
Sent: Friday, June 02, 2000 12:18 PM
To: seaint(--nospam--at)seaint.org
Subject: Increases in Foundation Bearing Pressure For Wind and Seismic
Loading


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.