There is no simple answer here. We have been struggling with this issue
ever since the Northridge earthquake. I would bet that most states have no
written criteria on this issue.
In California anyway, the latest Caltrans Seismic Design Criteria (available
on their web site) says that "Ordinary Standard Bridges" do not require a
vertical analysis by ARS methods (probably 99.9% of a state's bridges are
ordinary). If the peak rock acceleration is greater than 0.5g, an
equivalent static load can be applied to check the superstructure (the
substructure is assumed to be ok). The equivalent static load is 25% of the
dead load, applied upward or downward for the worst effect.
For "Important" bridges, special project specific criteria needs to be set
up. We have used ARS analysis with the 2/3rds combination as you describe.
We have also had the Geotech provide a special vertical ARS curve and
combined that with lateral ARS.
From: marfra(--nospam--at)opus.co.tt [mailto:marfra(--nospam--at)opus.co.tt]
Sent: Monday, April 03, 2000 7:47 PM
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