Civil Engineering Reference
In-Depth Information
Whatever the countermeasure, whether increased structural strength
or energy dissipation with a damper, it adds to the cost of the bridge. The
more intense the anticipated ground shaking, the more to be paid for
countermeasures. But when earthquakes are studied around the world, it
turns out, as one would expect, that the most intense ones are also the rarest.
Against the largest but least likely, it may simply not be worth investing
in countermeasures. The decision on how much to invest must be made in
part according to the importance of the bridge.
A structure may carry so much traffic and be so essential to the econo-
mies it connects (especially in areas where the highway system has little
or no redundancy—no other nearby highway bridge can take up the traf-
fic pressure) that planners deem it to be a
critical bridge.
By professional
standards, critical bridges must be considerably more hazard-resistant than
ordinary bridges.
To know just how earthquake resistant to make the bridge, engineers
must be given the peak acceleration, such as .4
g
. One way to set this ground
acceleration value would be to estimate the greatest ground acceleration
the bridge may expect in its lifetime—say it is just 75 years. According to
AASHTO specifications for regular bridges, that number should reflect the
most intense earthquake that could likely recur every 375 years—five times
the actual 75-year expected life of the bridge. For important bridges, the
specifications call for a larger peak acceleration, one that would only occur
every 1000 years. For even more critical bridges, the bridge may be designed
to resist the most intense acceleration likely to occur in 2500 years.
HYDRAULIC FORCES FROM FLOOD
A large proportion of US bridges (listed in the National Bridge Inventory)
cross bodies of water, so water hazards must indeed occupy the bridge plan-
ner's attention. On steep and narrow rivers, a particular danger is flash flood,
in which upstream rains and snowmelts rapidly drench narrow and steep
channels, causing sudden rises in water level. These are the floods that can
damage bridges with little warning.
Under widespread heavy rainfall covering many Midwestern states,
the region's large rivers swell more slowly, giving time for warnings and
closures of bridges. In recent decades, the rivers appear to have become
more susceptible to floods because of changes in land use. As land is covered
with impervious surfaces (parking lots, roofs, roads), water no longer seeps
through the soil but rather flows rapidly across the land surface. Rainwater
also runs rapidly through cities because it is channeled through storm sew-
ers. On rivers, embankments and levies meant to protect upstream towns
increase water velocity toward downstream settlements. Taken together,
