Tacoma Narrows Bridge collapse
What was the Tacoma Narrows Bridge? What events lead up to the collapse? Losses? What about who or what to blame? What was the physics behind the failure? What new regulations, designs, policies, etc have resulted from this disaster?
About Tacoma Narrows Bridge
The bridge had its opening day on July 1, 1940. It was 5,939 feet long and 443 feet tall. It stretched from the town of Tacoma, Washington all the way to the Kitsap Peninsula over the waters of Puget Sound and the Tacoma Narrows. After the workmen finished the floor system, engineers and others noticed the deck's vertical wave motions, or "bounce". In May of 1940, engineers tried to take the "bounce" out of the bridge. They installed four hydraulic jacks (or "buffers") at the towers to act as shock absorbers. But, the devices made no noticeable improvement. The state engineers felt pretty confident in what they had done and chose to ignore the believed-to-be "harmless" bounce. They even told local newspapers that the bounce was normal.
Early in the morning on November 7, 1940, strong and gusty winds blew through the area. The winds blew directly against the deck's solid plate girder. The bridge began to "gallop", making waves as low as 2 feet and as high as 7 feet! At 7:30, the winds reached 38 mph. Two hours later, they were higher than 42 mph. Several of the engineer's had driven across the bridge that morning, noticing the oscillating, but figured it was only doing what it normally did.
Professor F.B. Farquharson arrived at the bridge at about 9:40, and stopped to take video footage and photos of the bridge for his engineering studies. By 10:15, the bridge was twisting and swaying wildly to the point where the bottom of the bridge was visible. Around 10:30, a large chunk of concrete fell from a section on the Western side of the center span. Around 10:55, it looked like it was going to go. Farquharson, who had his dog named Tubby in the car, decided to go and try to save him. However, when he reached the frightened dog, Tubby had bit his finger. Farquharson had made it back to the East Tower just in time. He is pictured in the photo above.
At 11:02 AM, a 600-foot piece of the roadway on the Eastern half broke off and fell into the water. The section broke from its cables and in a large cloud of dust and debri, tumbled into the Puget Sound.
Fortunately, there were no losses of life except for Tubby the dog, nor were there any injuries. However, the collapse did bring about challenges in its aftermath.
Tacoma and Peninsulan residents lost their link connecting their rural areas to the cities, and the military lost its vital link between Bremerton Navy Yard and McChord Field and Camp Lewis for the rest of World War 2. Also, merchants on both sides of the bridge lost income from the retail trades between Pierce and Kitsap Counties.
As well as the traveling troubles, the collapse also caused financial burdens. They first had to re-install a ferry service. The chunk of the expenses was the insurance, which was spread among about 22 companies, that added up to be around $5.2 million, which was 80% of its value. The bridge was about $6.4 million, which was a loss on its own. Now they needed to think about building another bridge. Not to mention, figure out how to build the bridge differently so it didn't have the problems it did before, and to spend more on the parts and procedures needed to do so. Between the legal/insurance issues, it all took about 9 months to sort out. Yikes!
Who's to Blame?
The reporters from the Tacoma Times were desperate to find out how the collapse happened and who was to blame, so they talked to the lead project engineer Carl Eldridge. He was very angry, this is what he told the newspapers:
"The men who held the purse-strings were the whip-crackers on the entire project. We had a tried-and-true conventional bridge design. We were told we couldn't have the necessary money without using plans furnished by an eastern firm of engineers, chosen by the money-lenders." Eldridge and other state engineers also protested Leon Moisseiff's design with its 8-foot solid girders, which he called "sails." But, it was no use.
Two months later, there was a new scandal: it went public that a PWA (Public Works Administration) engineer had refused to approve the bridge back in July 1940.David L. Glenn, a PWA field engineer on the site of the bridge, submitted a report to the PWA warning them of faulty designs and recommended that no one should accept the design. However, the PWA accepted it anyway and so did the Washington State Toll Bridge Authority. The PWA later fired David Glenn in January 1941.
Physics Behind the Collapse
The wind that morning produced something called resonance; which means an object's natural vibrating frequency is amplified by an identical frequency. In the collapse, the identical frequency was caused by the gusts of wind blowing across the bridge, which created areas of high and low pressure above and below the bridge. This caused the bridge to start vibrating, which weakened the cables and made them loser, which caused the bridge to start waving up and down and twisting side to side, which eventually caused the collapse.
What new policies/Regulations/Designs, etc have resulted from this disaster? How did they make Sure This Wouldn't Happen Again?
Othmar Ammann, one of the leading bridge designers and members of the Federal Works Agency Commission investigating the collapse of the Tacoma Narrows Bridge, wrote:
"The Tacoma Narrows bridge failure has given us invaluable information...It has shown that every new structure that projects into new fields of magnitude involves new problems for the solution of which neither theory nor practical experience furnish an adequate guide. It is then that we must rely largely on judgment and if, as a result, errors, or failures occur, we must accept them as a price for human progress."
The Bronx Whitestone Bridge was a very similar bridge to the Tacoma Narrows. In 1943, 14-foot high steel trusses were installed on both sides of the deck to weigh down and stiffen the bridge in order to reduce the oscillation. However, in 2003, they were removed and replaced with aerodynamic fiberglass fairings.