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

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My understanding is that the 1/3 increase is based on the 0.75 factor we traditionally applied to combined dead, live, and wind/seismic loadings (the reciprocal of 0.75 is 1.33). The best example is in Eq. (9-2) in ACI 318.  Obviously, these load factors evolved.  ASCE 7 equations no longer have this factor, but they still reflect the reduced probability of a combined loading event. 
-----Original Message-----
From: L. Thomas Bayne [mailto:tom(--nospam--at)soilsengineering.com]
Sent: Friday, June 02, 2000 1:18 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:
 
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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.