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RE: Vertical Component of Seismic Loads in Bridges

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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.  

Harold Sprague

> -----Original Message-----
> From:	marfra(--nospam--at) [SMTP:marfra(--nospam--at)]
> Sent:	Monday, April 03, 2000 9:47 PM
> To:	seaint(--nospam--at)
> 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.