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Re: Vertical Component of Seismic Loads in Bridges

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Mark,

The Portuguese code states that the vertical component should be accounted for
in structures (including bridges) whose natural circular frequency is < 10 Hz.
In these cases it should be taken as 2/3 of the horizontal spectra but this code
is a bit old.

A more updated reference book is "Seismic design and retrofit of bridges" by
Priestley, Seible and Calvi (ISBN 047157998X) pag 70:

"... Vertical accelerations recorded by accelerographs are generally lower than
corresponding horizontal components and frequently are richer in short period
components. It is often assumed that the peak vertical acceleration is 2/3 of
the peak horizontal value. Although this appears reasonable for accelerograms at
some distance of the epicenter, there is increasing evidence that it is
nonconservative for near-field records, where peak horizontal and vertical
components are often of similar magnitude. ..."

Hope this helps

Pedro Santos
Raiz - Gabinete de Engenharia, Lda.


"Sprague, Harold O." wrote:

> Mark,
>
> There is a good discussion of the nature of earthquake vertical ground
> motions relative to horizontal ground motions in the US Army Corps of
> Engineers TI 809-04.  The 2/3 rule may not be conservative.  My experience
> indicates that this is especially true if you are in close proximity to a
> thrust fault or a subduction zone.  It is best to get a response spectra for
> both the horizontal and the vertical earthquake ground motions if it is
> perceived as an issue.  The TI 809-04 doesn't say what to do once you have
> the vertical earthquake ground motions.
>
> The 1997 UBC and 1997 NEHRP give a bit of guidance in regard to vertical
> seismic forces for prestressed components, but that is about it.
>
> The Japanese code does give some guidance on what to do with vertical
> earthquake ground motions for certain types of structures, but I don't think
> that there is any consideration for vertical for bridges.
>
> Regards,
> Harold Sprague
>
> > -----Original Message-----
> > From: marfra(--nospam--at)opus.co.tt [SMTP:marfra(--nospam--at)opus.co.tt]
> > Sent: Monday, April 03, 2000 9:47 PM
> > To:   seaint(--nospam--at)seaint.org
> > Subject:      Vertical Component of Seismiv Loads in Bridges
> >
> > Our firm is involved in the design of a major highway overpass in an
> > area designated seismic zone 3. The overpass consists mainly of many
> > spans that consists 90ft long pre-stressed concrete beams supporting an
> > insitu concrete deck slab.
> >
> > The center span is 300 ft consisting of a steel box girder suspended
> > from a steel arch by cables. Our concern is the design for the vertical
> > component of the seismic load. The structure is being designed in
> > accordance with AASHTO and we cannot find any provisions for the
> > vertical component of seismic loads. One firm with experience in Japan
> > recommends the use of the acceleration response spectrum for horizontal
> > motions factored by two thirds. They also recommend directional
> > combinations for the three possible dominant directions of motion.
> >
> > Do we need to consider the vertical component for the 90ft.spans?
> > Where do we find recommendations to evaluate the vertical component of
> > the seismic load for the cable suspended span and if necessary for the
> > 90 ft.spans
> >
> > Thank you
> > Mark Francois.
> >
>
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