Civil Engineering Reference
In-Depth Information
should reinforce all susceptible bridges. But the agency may own thousands
of bridges, subjected to varying scour risks, and it is just too expensive to
reinforce all of them.
One recommendation is that the threshold scour should be set at the
highest intensity likely to be encountered over 75 years, the typical design life
span of a modern bridge. Like earthquake and flood hazard criteria, this one
too depends on accuracy of available data, this time data on stream behavior.
But stream scour is a slow and complex phenomenon, for which scientific
forecasts may be very uncertain. With new technologies, it is increasingly
affordable to install monitoring systems at bridge piers. These measure scour
depth and give warning when action must be taken against scour.
VESSEL COLLISION
Ocean-going vessels have become so large that, even at slow speeds, their
momentum makes it impossible for them to suddenly turn or stop. In Amer-
ica and abroad, collision with bridge piers has indeed caused catastrophic
collapse with loss of life.
The destructiveness of the impact depends on the ship's mass, speed,
and kind of impact, whether direct frontal hit by the bow, sideswipe by the
hull, or impact by the deckhouse against the span. It depends also on the
ship's structural deformation (absorption of energy) upon impact as well as
on the bridge components' capacity to deform or become displaced without
collapsing (figure 6.6).
B
A
C
Figure 6.6. Ways a vessel can collide with a bridge. Adapted from L. Gucma, “Meth-
ods of Ship-Bridge Collision Safety Evaluation,” 2009.
