Everyone was shocked by the collapse of the 1940 Tacoma Narrows Bridge, but the engineers were particularly shocked. How did the most “modern” suspension bridge with the most modern design succumb to disastrous failure in a light wind?
Expert committees were commissioned by the State of Washington, insurance companies, and the US government to investigate the collapse of the Narrows Bridge. Othmar Amman, Dr. Theodore Von Karmen and Glen B. Woodruff were selected by the Federal Business Administration (FWA) as members of a three-person panel of senior engineers. Their report, written by FWA's director, John Carmody, became known as the "Carmody Board" report.
The Carmody Board published its results in March 1941. In summary, the investigation claimed that the "random movement" of the turbulent wind was what ultimately destroyed the bridge. The beginning of attempts to understand the complex phenomenon of wind-induced motion in suspension bridges began with this vague explanation. Three important details stand out:
(1) The "extreme resilience" of the 1940 Narrows Bridge was the main factor in its collapse;
(2) Solid plate beam and deck produced “drag” and “lift” similar to an aerofoil;
(3) Because little was known about aerodynamic forces, engineers had to use models to evaluate suspension bridge designs in a wind tunnel.
According to a summary article in the Engineering News Record, the Tacoma Narrows Bridge's "great resilience in both vertical and torsion" was its "main weakness". The extreme flex was caused by several things: The deck was unstable. At 8 feet (1/350 to mid-span), the deck was very small. Compared to the length of the center opening, the side openings were excessively long. The cables were too far from the side openings when fixed. The ratio of 1 to 72 deck width to center span length was extremely small. This is an unheard of rate.
According to the Board, the transition from vertical waves to destructive bending, torsional motion was a major factor in the bridge's collapse. The movement of the cable band on the northern cable in the middle of the span was a contributing factor in the formation of this formation. Primary cables typically measure the same length that mid-span cable tape secures them to the deck. When the tape slipped, the northern cable split into two sections of different lengths. Thin, flexible plate beams, which bent easily from unbalance, were quickly taken over. Gradual failure came when the unsteady movement began.
The most important conclusion of the inquiry board was clear: in building long suspension bridges, the technical community needs to learn more about aerodynamics.
Professor FB Farquharson, meanwhile, conducted more wind tunnel studies. He concluded that the "intense resonance oscillation" was due to "the cumulative effects of unquenchable rhythmic energies." In other words, the 8-metre solid slab beam and deck of the bridge collapsed due to its small weight with the increase in wind pressure.
When contacted soon after the disaster, Leon Moisseiff stated that he was "completely at a loss to explain the collapse." A week later, Moisseiff visited the collapsed bridge and toured it while Clark Eldridge kept an eye on him. While Moisseiff's design went beyond the confines of engineering practice, it fully conformed to the standards of then-accepted theory.
Günceleme: 04/11/2022 22:17
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