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Tacoma Narrows Bridge
United States of America, Washington, Tacoma Narrows Bridge
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Description
The Tacoma Narrows Bridge is a suspension bridge in Washington State (USA), built across the Tacoma Narrows Strait, which is part of Puget Sound. This bridge became famous due to its tragic accident that occurred on November 7, 1940. At 11:00 local time, with winds of about 40 mph (65 km/h), the central span of the bridge collapsed. Traffic was light at the time, and the only driver on the bridge managed to abandon his car and escape.
The collapse was captured on 16-mm Kodachrome color film. Based on this footage, a documentary film, The Tacoma Narrows Bridge Collapse (1940), was created, which made it possible to study the collapse process in detail.
Causes of the accident
The accident left a significant mark on the history of science and technology. The collapse of the bridge contributed to research in the field of aerodynamics and aeroelasticity of structures, as well as a change in approaches to the design of all large-span bridges in the world. Textbooks often indicate that the cause of the accident was the phenomenon of forced mechanical resonance, when the frequency of the wind flow changes coincides with the natural frequency of the bridge oscillations. However, the true cause was aeroelastic flutter - dynamic torsional oscillations caused by underestimation of wind loads in the design.
Description of the destruction process
The process of destruction of the bridge is described as follows: the breakage of the suspensions of the central span led to sagging of the side spans and the tilt of the pylons. Strong vertical and torsional oscillations of the bridge were a consequence of excessive flexibility of the structure and insufficient ability of the bridge to absorb dynamic forces. The bridge was designed for static loads, including wind, but the aerodynamic effect of the wind was not taken into account. Torsional vibrations were caused by the wind acting on the roadway around a horizontal axis parallel to the longitudinal axis of the bridge, and were amplified by vertical vibrations of the cables.
Dismantling and construction of a new bridge
Dismantling of the pylons and side spans began shortly after the accident and continued until May 1943. The anchor abutments, pylon bases, and some other parts of the old bridge were used in the construction of the new bridge. The completely restored bridge, called the Westbound Bridge, was opened on October 14, 1950 and was at that time the third longest suspension bridge in the world. Its total length was 1,822 meters, and the length of the central span was 853 meters. To increase stability and reduce aerodynamic loads, open trusses, stiffeners, expansion joints, and vibration damping systems were introduced into the design of the new bridge. The throughput capacity of the bridge was 60 thousand cars per day.
Construction of the second bridge
In 2002-2007, to increase the capacity of the highway, another bridge was built next to the old one, called the Eastbound bridge. This bridge has a total length of 1,645.9 meters, a central span of 853.4 meters and a pylon height of 155.4 meters. On windy days, its sway amplitude was 1.5 meters, which was a consequence of the arrogance of the chief architect Leon Moisseeff. He assumed that a suspension bridge 11.9 meters wide with a span of 854 meters would not be subject to significant vibrations from the wind, which, as it turned out, was a mistake.
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