Civil Engineering Reference
In-Depth Information
newly built bridges since the 1960s is not in itself a sign of neglect. Despite
the decline in new completions, the bridge stock counted at (mostly) five-
year intervals since 1992 (table 2.3) shows steady growth, with a small
decline in the final half decade. The current stock of 605,086 represents
over a three percent increase in just under twenty years. Some slowing in
new bridge completions may be a good sign. It may well indicate that the
nation's number of bridges simply has approached the saturation point—by
the new century we had bridges at most of the places where we were ever
likely to build.
So it's important to draw the right lesson here. The lesson is
not
that
America has failed to build enough new bridges in the past three decades.
Rather, it is that the spurt of bridge building in the 1960s and 1970s is
coming due—these bridges are reaching an age at which they will pose
ever more problems.
ARE BRIDGES DEFICIENT?
Old age is just a broad indicator that a bridge may require attention.
Decisions on rehabilitation or replacement depend, of course, on actually
observed problems. The NBI keeps track of problems, which it divides into
two kinds, “structural deficiency” and “functional obsolescence.”
Let's start with the former. For each bridge in the inventory, a state
official fills out a form that evaluates the structural condition of the bridge
components on a nine-point scale, starting with 9 for excellent. A score
of 4 denotes deterioration, such as pieces falling off the structure. Skipping
3, we get to a 2, which indicates deterioration so severe that, subject to
close monitoring, the bridge may have to be closed. With a score of 1 the
bridge is in imminent danger of failing, so it should be closed to traffic,
but may still be repairable. At the bottom, a 0 means the bridge is out of
service and cannot be fixed. A bridge with a rating of 4 or below is labeled
structurally deficient.
The bridge may, however, be obsolete even if it is structurally sound.
For a particular type of road (say an interstate highway) and for a particu-
lar daily traffic load, engineers can consult national guidelines to decide
Table 2.3. Public Bridges in the United States, 1992-2011
1992
1997
2002
2007
2011
US stock of bridges
585,830
596,632
604,233
612,205
605,086
Source:
National Bridge Inventory
