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Engineers’ Eyes
Stay Steady on the Bridge
Built in 1939, the suspension bridge between Sedgwick and Deer
Isle meant travelers to the island no longer had to take the
ferry. |
That high,
half-mile-long suspension bridge spanning Eggemoggin Reach and
connecting Deer Isle to the rest of
Maine
is spectacular as both a sight and a feat of engineering.
Ever since its
construction in 1939, originally as a toll bridge, it has been a
welcome part of the local landscape. As the only auto route onto the
island, the two-lane bridge replaced the ferry as the main way to
reach the island. The building of the bridge is actually considered
the single most important event in the island’s history.
But it has national
significance, too, and not just for the imprint it leaves with
tourists who come through in summer. It happens to be a much-studied
bridge for the way it once oscillated in high winds.
Quite recently,
between 1981 and 1998, the bridge was the focus of a study,
“Aerodynamic Investigations of the Deer Isle-Sedgwick Bridge.” That
was by mandate of the Federal Highway Administration.
The bridge was an
example for engineers wanting to understand better how wind
interacts with long-span bridges.
Almost immediately
after the bridge’s construction in 1939, wind-induced vibrations
called for it to be stiffened by structural supports to reduce the
motions. Now, engineers talk about the Deer Isle-Sedgwick Bridge in
the same breath as two others with the poor aerodynamic
characteristics, the Bronx-Whitestone Bridge in New York (also a
1939 creation) and the Tacoma Narrows Bridge in Washington (1940).
The Deer
Isle-Sedgwick Bridge had been built during the Depression, and had a
relatively light and flexible stiffening structure. To prevent
severe damage, the bridge was re-fitted with diagonal stays and
cable restraining blocks.
But drivers today
take note of the bridge not for its sway, but its image across the
water and sky. It continues to be monitored to determine its
aerodynamic performance. |
