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

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I think the question of whether or not you feel it is appropriate to have a lower factor of safety for a temporary load is really the issue here. Does the structure need to be built with a foundation that is governed by a load case that has a low probability of occurring, or is it valid to have a factor of safety of two instead of three for temporary loads?
Just some practical thoughts instead of technical.

Kirk

"L. Thomas Bayne" wrote:

Does anyone know by what technical rule, equation, etc.  This notion of increasing for wind and seismic loading originated oris 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.

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