One tiny design flaw in the bridge caused it to collapse, and that giving way is still taught in engineering, architecture, and physics classes today, writes Marcia Wendorf.

Practical Engineering/YouTube

80 years ago on November 7, 1940, the Tacoma Narrows bridge collapsed, and the reverberations of that collapse still echo today in engineering, architecture, and physics lectures worldwide.

Built to span a mile-long section of Washington state's Puget Sound, the Tacoma Narrows bridge was a suspension-type bridge where the deck – or load-bearing part – is hung beneath suspension cables strung between towers.

Suspension bridge design Source: Practical Engineering/YouTube modified by Marcia Wendorf

The Tacoma Narrows bridge was the first bridge that was built with girders made of carbon steel anchored in concrete blocks. At the time, it was the world's third-longest suspension bridge, behind only the Golden Gate bridge connecting San Francisco and Marin County, and the George Washington Bridge connecting New York City and New Jersey.

Support for building the bridge came from the US military, which operated three facilities in the area: the Puget Sound Naval Shipyard, McChord Field, and Fort Lewis. Support also came from the Northern Pacific Railway.

Joseph Strauss statue Source: Steven Pavlov/Wikimedia Commons

Tacoma's city fathers consulted several notable bridge designers, including Joseph B Strauss, who had been chief engineer of the Golden Gate bridge, and David B Steinman, who went on to design the Mackinac bridge which connects the Upper and Lower Peninsulas of Michigan state.

Mackinac bridge Source: Justin Billau/Wikimedia Commons

Neither Strauss nor Steinman was selected, and instead, a local engineer named Clark Eldridge was chosen. Eldridge proposed a conventional suspension bridge design along with the inclusion of 25ft-deep (7.6m) trusses that would sit below the deck and would stiffen it.

Tacoma Narrows Bridge poster Source: Wikimedia Commons

A money-saving proposal

However, back in New York, two bridge engineers, Leon Moisseiff and Frederick Lienhard, had just published a paper that described how the stiffness of the main cables would absorb up to 50% of the static wind pressure pushing a suspended structure laterally.

Moisseiff proposed stiffening the bridge with a set of 8ft-deep (2.4m) plate girders instead of the 25ft-deep (7.6m) trusses Eldridge had designed, and this would reduce construction costs considerably. The Tacoma city fathers went with Moisseiff's design, and $6 million (equivalent to $109 million today) was allocated for the bridge's construction.

Construction on the two-lane bridge began in September 1938. At just 39ft (12m) wide, the bridge was quite narrow compared to its length of 5,939ft (1,810m), with a main span of 2,800 feet (850m).

As soon as construction workers completed the deck, they noticed that during windy conditions, it would move vertically, and they nicknamed the bridge 'Galloping Gertie'.

The Tacoma Narrows bridge was opened to traffic on July 1, 1940, and drivers quickly noticed that the bridge would oscillate vertically up to several feet.

Authorities moved to reduce the vertical oscillations by adding tie-down cables anchored to 50-ton concrete blocks located on the shore. The tie-down cables snapped within days.

Next, engineers tried adding cable stays connecting the main cables to the bridge deck at mid-span, but that also didn't work. Finally, engineers added hydraulic buffers between the towers and the deck that were designed to damp the longitudinal motion of the bridge. These failed when the hydraulic seals were breached when the bridge was sandblasted prior to being painted.

Authorities next hired an engineering professor at the University of Washington to analyse the problem. He and his students built a 1:200-scale model of the bridge on which they conducted wind-tunnel tests.

They submitted their conclusions on November 2, 1940, and they suggested drilling holes in the lateral girders along the deck to allow the wind to flow through, and the addition of fairings or deflector vanes along the deck to aid its aerodynamic shape.

Neither option was ever implemented because just five days later, on November 7, 1940, the Tacoma Narrows bridge collapsed.

The bridge comes down

As luck would have it, at 11am on that Thursday morning, very few people were on the bridge. One of the few people on the bridge, Leonard Coatsworth, was an editor for the Tacoma News and he wrote of his experience of the bridge coming down:

"Around me, I could hear concrete cracking. I started back to the car to get the dog, but was thrown before I could reach it. The car itself began to slide from side to side on the roadway.

"I decided the bridge was breaking up and my only hope was to get back to shore. On hands and knees most of the time, I crawled 500 yards (460m) or more to the towers ...

"My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb ... Towards the last, I risked rising to my feet and running a few yards at a time… Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows."

Sadly, Coatsworth's cocker spaniel Tubby was in that car, and he was the only fatality of the disaster.

A subsequent inquiry into the collapse determined that what brought the bridge down was a twisting motion that occurred when winds reached 40mph (64km/h).

Torsional Oscillation Source: Practical Engineering/YouTube

Called a torsional vibration, it caused one side of the roadway to go up while the other side went down. The cause of the torsional vibration was something called vortex shedding.

This is where a fluid, or wind, flowing past an object oscillates as vortices are formed on the backside of the flow. When these alternating zones of low pressure occur at a frequency that is near to the natural frequency of a structure, even small amounts of wind can cause major oscillations.

Vortex shedding Source: Practical Engineering/YouTube

When the amplitude of the motion increased beyond the strength of some of the suspension cables, they snapped, and the adjacent cables couldn't carry the weight, causing them to snap as well.

The collapse of the bridge was recorded by a local camera shop owner named Barney Elliott, and in 1998, Elliott's film titled The Tacoma Narrows Bridge Collapse was selected by the US Library of Congress as being culturally and historically significant.

Aftermath of the collapse

When the State of Washington went to collect on its insurance claim for the bridge, it found out that its insurance agent had made off with the premium payment, and that the bridge was only partially insured.

Tacoma Narrows bridge closed Source: Practical Engineering/YouTube

The weather system that caused the Tacoma Narrows bridge to collapse continued its way across the country, eventually producing the Armistice Day Blizzard on November 12, 1940.

It included snowfalls of up to 27 inches (69cm), winds of between 50 and 80mph (80 to 130km/h), 20ft (6.1m) snowdrifts, and 50°F (28°C) temperature drops in parts of Nebraska, South Dakota, Iowa, Minnesota, Wisconsin, and Michigan.

Record low pressures were recorded in La Crosse, Wisconsin, and Duluth, Minnesota, and 146 people were killed. Hundreds of duck hunters were stranded on islands in the Mississippi River where they died of the cold. One survivor was saved by his two Labrador retrievers who sheltered him with their bodies.

In Watkins, Minnesota, the blinding snow caused a passenger train and a freight train to collide, and residents formed a human chain to lead the passengers to safety. On Lake Michigan, 66 sailors died when three freighters sank due to the storm.

Today, all over the world, Barney Elliott's film is shown to engineering, architecture, and physics students as a cautionary tale of how the power of Mother Nature needs to be respected.

This article was written by and first appeared in Interesting Engineering