The Tacoma Narrows Bridge collapses due to high winds on this day in 1940. Fortunately, only a dog was killed.
The Tacoma Narrows Bridge was built in Washington during the 1930s
and opened to traffic on July 1, 1940. It spanned the Puget Sound from
Gig Harbor to Tacoma, which is 40 miles south of Seattle. The channel is
about a mile wide where the bridge crossed the sound. Sleek and
slender, it was the third longest suspension bridge in the world at the
time, covering 5,959 feet.
Leon Moisseiff
designed the bridge to be the most flexible ever constructed. Engineers
of the time believed that the design, even though it exceeded ratios of
length, depth and width that had previously been standard, was
completely safe. Following the collapse, it was revealed that the
engineers had not properly considered the aerodynamic forces that were
in play at the location during a period of strong winds. At the time of
construction, such forces were not commonly taken into consideration by
engineers and designers.
On November 7, high
winds buffeted the area and the bridge swayed considerably. The first
failure came at about 11 a.m., when concrete dropped from the road
surface. Just minutes later, a 600-foot section of the bridge broke
free. By this time, the bridge was being tossed back and forth wildly.
At one time, the elevation of the sidewalk on one side of the bridge was
28 feet above that of the sidewalk on the other side. Even though the
bridge towers were made of strong structural carbon steel, the bridge
proved no match for the violent movement, and collapsed.
Subsequent
investigations and testing revealed that the bridge was vulnerable to
vibrations generated by wind. When the bridge experienced strong winds
from a certain direction, the frequency oscillations built up to such an
extent that collapse was inevitable.
A replacement bridge opened on October 14, 1950, after more than two years of construction. It is the fifth longest suspension bridge in the United States, 40 feet longer than the original. Construction of the new bridge took into account the lessons learned in the collapse of the Tacoma Narrows Bridge, as did that of all subsequent suspension bridges.
Today,
the remains of the bridge are still at the bottom of Puget Sound, where
they form one of the largest man-made reefs in the world. The spot was
placed on the National Register of Historic Places in order to protect
it against salvagers.
taken from: http://www.history.com/this-day-in-history/tacoma-narrows-bridge-collapses [07.11.2012]
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