Civil Engineering Reference
In-Depth Information
TWO
COUNTING OUR BRIDGES
In this chapter, we ask the question: just how often must big decisions be
made about bridges? And to what extent is the United States facing a need
for new bridges, bridge reconstruction, and bridge rehabilitation?
The place to go for answers is the National Bridge Inventory (NBI),
a database maintained by the Federal Highway Administration to keep tabs
on bridge conditions in the states. It assembles data each year from reports
submitted by state transportation departments. As infrastructure is long-
lasting, the national inventory changes fairly slowly, so the 2011 data, which
we are using here, should remain a good indicator for years to come.
The fact that first strikes the eye is that there are over 600,000 bridges
in the fifty states plus the District of Columbia and Puerto Rico. This is
not even a full count, since the NBI counts only public bridges and leaves
out tens of thousands of privately owned railroad bridges. Of the total in
the NBI, 98 percent are road bridges, primarily meant to carry automobiles,
trucks, buses, etc., though some also have lanes for pedestrians and tracks
for trains or subways.
We classified the bridges according to length of the main span, so we
could begin assessing the nation's bridge infrastructure challenge. We wanted
to know, for example, how many are long enough that they could not have
been built—and cannot be rebuilt—simply as girder (or beam) bridges.
To qualify for our classification, the span had to be greater than 20
feet, which is a short starting point since a span of that length barely crosses
two road lanes. A 20- to 99-foot main span we classified as “short.” If a
bridge has a dozen spans, of which the single longest is 60 feet, then we
still classified it as short-span even though the entire bridge is much longer.
We classified a span of 100 to 329 feet as “medium,” and 330 and over as
“long.” When a bridge exceeds 330 feet, it will almost always have to be
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