Civil Engineering Reference
In-Depth Information
Among countermeasures, it is best to altogether prevent the collision
by keeping the ship accurately in navigable lane (the
fairway),
as with
electronic guidance systems and training for tugboat operators. But weather
alters currents, sandbanks shift, wayward boats force other vessels to take
evasive moves, barges are sometimes improperly loaded, and every once in a
long while a pilot is less than sober. So, we could reinforce the bridge with
stronger piers and pier foundations, though that's an expensive solution and
one by no means foolproof against gargantuan ships.
It may be cheaper to add a fender to a pier. If we are speaking of a small
bridge threatened by small vessels, the fender could be nothing more than
a rubber skirt draped around the column. Larger vessels must be met with
massive fenders built of logs or concrete. Or again, ships could be deflected
with clusters of logs fixed into the riverbed, or even with artificial islands.
As always, the question is just how much it is worth investing in
such measures. The decision should depend on frequency of ship traffic,
ship sizes and draught, water depth, width of the fairway, and navigational
complexity (currents, winds, turns, underwater obstacles). The more criti-
cal the bridge—traffic, economic impact, availability or lack of alternative
crossings—the more to be spent.
How intense a blow should the bridge (and its protective fenders) be
designed to avert? How improbable should that event be before planners
decide that it is just not worth trying to avert it?
The 2009 AASHTO specifications provide a standard. For “critical
bridges,” the expected annual frequency of a collision severe enough to
cause collapse should be no greater than .0001 (once in 10,000 years). For
regular bridges, the expected annual frequency should be no greater than
.001 (once in 1000 years).
TERRORISM
It has been speculated since 9/11 that terrorists might target bridges. By
comparison to natural and accidental hazards, which hold no preference for
famous bridges, terrorists would likely focus on long-span bridges that are
national symbols and have great economic importance.
In one respect, an attack against a bridge can be less threatening to
life and limb than attack against a building: users of the bridge are travers-
ing the structure and not (as in a building) occupying it. If the bridge and
its approaches are not congested with traffic, then the mere suspension of
vehicle entry will quickly clear the bridge. Bridge landings provide easier
and faster egress than do the doors to a high-rise building. With proper
surveillance, bridges can be closed early in response to suspicious activity.
