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vertical component of seismic load in bridges

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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.