Drawing Bridges - Pushing the Limits: New Adventures in Engineering - page 52

Drawing Bridges
Many a person who has driven through the Middle Atlantic states on Interstate 95 has dreaded the
approach to Washington, D.C., where traffic often slows to a crawl, if not to a complete stop. Head-
ing north through Virginia toward Maryland, I-95 widens from two to three and then to four and
five lanes and more, to accommodate the increased traffic volume and interchange complexity as
one nears the capital. About twelve miles south of Washington, traffic invariably becomes more er-
ratic and frenetic, as cars and trucks jockey for position in anticipation of the branching of the high-
way into I-395 and I-495, the former going through Arlington into the District of Columbia proper
and the latter being the Outer Loop of the Capital Beltway, which bypasses Washington—but not
its traffic. The westward leg of I-495 carries traffic headed for Fairfax, Virginia, and Bethesda and
Rockville, Maryland. Those traveling I-95 north to Baltimore and on to Philadelphia and New York
generally continue on I-95, which coincides with the eastward leg of I-495.
About six miles after making the practically irrevocable decision to head eastward on I-95/I-495
in order to continue north, drivers find that the interstate highway crosses the Potomac River on the
Woodrow Wilson Memorial Bridge, the only federally owned bridge in the entire interstate-high-
way system. Since by Coast Guard regulations the drawbridge may not open during periods when
it is carrying a heavy volume of vehicle traffic, most highway travelers and commuters never see
the bridge in action, but in the late 1990s it was opening about 220 times a year nonetheless. This,
along with the sheer volume of traffic carried by the bridge—two hundred thousand vehicles per
day, almost three times what it was intended to carry—make it a location that the American Auto-
mobile Association has described as “one of the nation's top ten bottlenecks.” It is indeed that, as
well as obsolete, and why it might be so becomes clear when one looks at the history of the bridge.
Design and construction of the Wilson bridge was authorized in 1954 to link the Maryland and
Virginia portions of what was then known as the Washington Circumferential Highway, and con-
struction began in 1959. This means, of course, that the design standards followed were those of
the 1950s, which did not take fully into account the volume or intensity of traffic contributed by
today's vehicles or the resultant problem of structural fatigue—a phenomenon by which the cumu-
lative effect of loads of even moderate intensity repeated millions of times produces damage that
the less frequent application of those same loads would not. Loads of higher intensity, such as those
imposed by trucks now heavier than those anticipated in 1950s standards, accelerate the onset of
fatigue.
The Wilson bridge was designed with three lanes in each direction, which must have seemed
more than adequate at the time. The intended capacity of the bridge—seventy-five thousand
vehicles per day—also must have appeared quite reasonable, since the actual traffic load was only
nineteen thousand vehicles per day. Bridges encourage traffic, however, and by the late 1960s auto-
mobiles and trucks using the Wilson bridge had exceeded its design volume. Approach roads were
widened to three and four lanes in each direction, but the bridge remained at three each way because
of structural limitations. Furthermore, in the mid-1970s it was decided to abandon plans to extend