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

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----- Original Message -----
From: "Khosrownia, Ghassem SPK" <GKhosrownia(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Friday, June 02, 2000 12:12 PM
Subject: RE: Increases in Foundation Bearing Pressure For Wind and Seismic

> 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
> 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
> of the most expensive to fix later.
> Who is that Author that you are referring to?
> [Ghassem]
>  -----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
> 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
> 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.
> 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
> 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,
> have found no justification anywhere in the literature or in practice to
> increase strengths solely based on the nature of the load, all other
> notwithstanding. If the code allows these increases to be applied, we
> recommend that your footing designer use those allowances at its own risk.