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

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The author is myself.  It is probably the wrong way to present it.   If the
increases in bearing is to compensate for redundant transient loading,
shouldn't that be done at the time the foundation loads are computed by the
structural.  I don't think it should be handled by arbitrarily increasing
the allowable soil bearing pressures? I am wrong?
----- Original Message -----
From: "Khosrownia, Ghassem SPK" <GKhosrownia(--nospam--at)spk.usace.army.mil>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, June 02, 2000 12:12 PM
Subject: RE: Increases in Foundation Bearing Pressure For Wind and Seismic
Loading


> 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
I
> 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
one
> 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)soilsengineering.com]
> Sent: Friday, June 02, 2000 11:18 AM
> 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.
>
>