Civil Engineering Reference
In-Depth Information
PLANNING TO AVERT DISASTERS
As the table shows, flooding is the most common cause of failure and is
most numerous in years in which there are large regional floods, impacting
numerous bridges. That was certainly true of 1993, when Midwestern floods
ruined many.
It was true again in 2005, during the Katrina disaster and subsequent
hurricanes. In hurricanes, it is generally not the winds that damage the
bridge but the surge from the ocean, which in turn pushes waters upstream.
At the same time, hurricane rains drench entire watersheds, from which
floodwaters pour into rivers. In 2005, several bridges from New Orleans to
areas north of Lake Pontchartrain underwent catastrophic failure, as did
bridges in Mississippi and Alabama.
Floods are indeed a large danger, but they may still not be uppermost in
bridge planners' minds. River floods (but not flash floods on steep streams!)
occur with enough early warning that bridges can be closed before there is
collapse and loss of life, though with the accompanying harm that evacu-
ation from nearby affected communities may be hampered. Viewed over a
larger period of time than that examined by the researchers, earthquakes
also cause much bridge damage, in the rare years in which large-magnitude
quakes occur. Earthquakes (and collisions) cause damage essentially without
warning, and therefore pose greater danger to life.
For planners and engineers (and for the professional standards commit-
tees that guide engineers), the tough problem, therefore, is to decide which
events are likely enough that it is worth building the structure to resist
them. To decide, we should forecast the
probability
of the extreme event
and its
intensity,
which can be a measure of wave force, or ground shaking,
or collision force. Put differently, we have to forecast the probability that
the hazard intensity will exceed some limit during a time period, such as
the bridge's expected life span.
What is more, we have to weigh the cost of mitigating the hazard
against the benefits in safety to be gained. If a very intense vessel colli-
sion or earthquake has only a one-in-ten-thousand chance of occurring per
year, then perhaps it is not worth investing in countermeasures. This is not
because we are heartless, but because it may be much more cost-effective to
seek safety elsewhere, as by reducing traffic accidents, which kill far more
people than structural collapses do. As we shall see, these are tough deci-
sions. They pose engineering and planning dilemmas to this day.
EARTHQUAKE
The United States undergoes a large regional earthquake only once every
decade or two, but when it does occur, it can damage multiple bridges. As
