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Vertical component of seismic load in bridges

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Our firm is designing an overpass in an area designated as Seismic Zone
3. Most of the over pass consists of prestressed concrete beams spanning
90 feet with an insitu concrete slab. The center span is 300 ft
suspended on cables supported by a steel arch.

AASHTO does not deal with the vertical component of seismic loads. We
are aware that recently the effects vertical component have been studied
but have any code recommendations been made?

One firm in the design team recommended using the acceleration response
spectrum factored by two-thirds with various directional combinations
for different directions of dominant motion.

Is this applicable for the short spans of 90ft.
Where can I get research or code recommendations on the above.

Thank you

mark francois