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RE: Collision Load for Highway Bridges

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I don't do bridge design, but from articles I have read I was under the
impression that modern bridge design is based on the possibility that
columns are destroyed by vehicle impact.

1)	Too much resistance in the columns increases the risk of injury to
occupants of the colliding vehicle.

2)	If the impact force exceeds the design force then the column will be
destroyed and the bridge deck will collapse on the colliding vehicle and/or
other vehicles.

So I believe the bridge is designed to behave in catenary with some static
traffic load. Which means the bridge deck has to be continuous over the
critical column, therefore cannot be precast concrete units simply supported
on the critical column. Whilst I know some bridges have been designed on
this basis (of catenary), it may not yet be in the codes: other than in
general terms of robust design.

As for trucks with excessive load heights, they shouldn't get on the freeway
in the first place. All slip roads leading onto the freeway, should have
over head chains which hit the trucks, or some similar physical control
feature which warns the truck drivers. If not practical on all slip roads
then some similar control feature at suitable distance from the bridge to
give adequate warning, and possibly a slip road/lane to allow safe exit.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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