Tacoma narrow bridge also known as Galloping Gertie, was first open to the public on July 1, 1940. At the time of opening, Galloping Gertie was the third-longest suspension bridge in the world. The bridge initially became famous due to its deck’s unusual vertical movements. But its reputation didn’t last long, only after four months from the opening on November 7, 1940, at 11.00 a.m. Tacoma Narrow bridge main span collapsed to Peugeot Sound by making it one of the biggest Engineering and Design Failures in history.
History
Tacoma bridge project proposal started to discuss in early 1920. Before the bridge was built, people had to go all long way around the southern end of Pegut Sound to make their journey. The bridge was planned to cut this long journey and create a straight passage by connecting Tacoma City and Kitsap City Peninsula via State Highway 14. This bridge proposal is not only beneficial for locals. It also became crucial for the military because of the McChord Air Field and Kitnap Naval base.
In 1928 Tacoma Chamber of Commerce hired bridge architect David B.Steinman to conduct the first survey to build a bridge over the narrow. Steinman created a field report and the first architectural layout for the bridge. But authorities moved away from his design.
1933 National Industrial Recovery Act
In 1933 president Roosevelt commissioned National Industrial Recovery Act. This act heavily focused on reducing unemployment and building new public infrastructures. To administrate these projects Public Work Agency(PWA) was set up under Harold L.Ickes. PWA reserved a total of $400 million to build bridges, highways and subways. They identified the importance of the Tacoma bridge. But their budget for this project was very strict.
Clark Endringe Bridge Design
Until 1937 several bridge designs were submitted for consideration to the PWA. But none of these designs got approved because of the high building cost.
WSDOT lead engineer Clark Edridge proposed a bridge design in 1938 for the narrow. His idea was the suspension bridge with two lanes and two towers. The bridge features 1300ft 2side spans with 39ft centre to centre cables and 25ft deep stiffening trusses. Geographically bridge western end landside is located higher than the eastern side. So he designed West tower 463ft 6inch and East tower 476ft 6inch. Each tower included eight bracings.
Clark’s final design had a 66:1 span to width ratio. Golden gate bridge has a 47:1 s/w ratio compared to that Clark’s original design was a narrow bridge. This ratio concluded the sector, but Clark’s believed a 25ft deep stiffening truss could make enough energy to make the bridge strong. Clark Endringe originally designed this bridge against 125mph wind speed.
But the reason he designed this bridge narrower is PWA and WSDOT guidance. They didn’t expect heavy traffic or commercial transportation over the bridge. They only needed a 2-way motorway design that could fit into their budget. Clark’s estimated $11 million budget to make his design real.
Leone Moisseiff Design
PWA and WSDOT considered Clark’s design but also they wanted to know they could be able to cut the cost furthermore. So they hired engineer Leone Solomon Moisseif as a consultant. Moisseif had the experience of working in Manhattan and Benjamin Franklin bridge projects. He made some cost-effective changes to Clark’s original design and cut the cost $11 million down to the 6$ million.
Moisseif redesign proposal matches the government needs. PWA and WSDOT approved a total of $8 million to build the Tacoma Narrow bridge.
What Really Moisseiff Changed?
Leone Moisseiff designed the TNB mainly using deflection theory. He included a 2800ft main span and 1100ft long two side spans to connect the bridge. Moisseiff made significant changes to Clark’s design substructure. He replaced Clark’s 25ft deep open stiffening truss design with a shallow T-shape plate girder with a 375:1 higher span to depth ratio. These design changes make the bridge much lighter. But it also makes the bridge aerodynamically unstable and slenderness.
Moisseif design also had an even higher span to width ratio of 75:1. Finally, bridge piers were built under Clark’s design, and superstructure was built under Moisseif changes.
Starting Construction
Tacoma Narrow Bridge construction started on November 23, 1938. After 19 months from that workers completed the bridge. July 1, 1940, Tacoma Narrow bridge was open to the public.
Why TNB Nicknamed Galloping Gertie?
During the construction period of the Tacoma Narrow Bridge (TNB), many workers got sick due to the bridge dramatic vertical oscillation waves. Frequently TNB made 2-4ft waves. On some rare occasions, it went up to 5ft. Because of this reason workers on the site nicknamed the bridge “Galloping Gertie“.Sometimes these waves last up to 8-10hours.
Taking Steps To Stop Gertie Bounce
Engineers noticed bounce “vertical oscillation“ after workers finished the bridge deck. In May 1940 they installed four hydraulic buffers in the towers to reduce the bounce. But it didn’t change the situation much. So toll authority contracted Washington University professor F.Bert Farquharson to study Galloping Gertie.
Galloping Gertie deck vertical oscillation happens because of the bridge narrow and shallow design. Bridge T-shape shallow girder acted like an aerofoil and it made drag and lift on the deck.
“We knew from the night of the day the bridge opened that something was wrong. On that night, the bridge began to gallop.”
professor F.Bert Farquharson
In October, engineers placed temporary tie down cables on the side spans. Also, they installed diagonal wires from the main cables to the deck in the main span. Tie down cables reduced the bounce inside span, but it didn’t work on main span oscillation. Professor Farquharson proposed cutting holes in the solid girder to pass the wind or installing steel wind deflectors to solve the problem. But before these ideas got into action in November, Galloping Gertie collapsed on November 7.
1940 November 7 Day Of Collapsed
In the early morning hours of Thursday, November 7 1940, TNB faced 35-46 mph wind. The centre span of the bridge waved up to 5ft. At 10.00 a.m officials closed the bridge.
10.03 – Twisting motion started in the roadway
10.07 – Twisting motion grew stronger and, the condition became violent. Every 5seconds the roadway tilted 28ft with a 45-degree angle from one horizontal side to the other side. The cable band at mid-span on the north cable slipped. This allowed the cable to separate into two unequal segments. That contributed to the change from vertical (up-and-down) to torsional (twisting) bridge deck movement. Twisting happens around 30 minutes.
10.30 – TNB centre span floor panel dropped 195ft below water. The roadway started to break up. Large chunks of concrete rain off from the span.
11.00 – Waves started to rip the span. Witnesses reported this
“Huge chunks of concrete broke off like popcorn and fell into the chilly waters far below. Massive steel girders twisted like rubber. Bolts sheered and flew into the wind. Six light poles on the east end broke off like matchsticks. Steel suspender cables snapped with a sound like gunshots, flying into the air like fishing lines”.
11.02 – Bridge Eastern half from the centre span 600ft roadway section twisted free and fell off to Puget Sound.
11.09 – Remaining section ripped free, and 1100ft side span dropped 60ft
11.10 – Allover
No human casualties happened from the incident. But “Tubby” a dog died from the accident.
What Went Wrong?
Tacoma Narrow Bridge disaster shocked the engineering world. Everyone needed to know what went wrong. Federal Work Administration appointed engineer Glen B. Woodruff, Othmar Amman and Dr Theodore Von Karmen to lead the investigation. Their final report was published in March 1941. According to their report caused to the TNB collapsed was random action of turbulent wind. Also, they identified a T-shape girder and excessive flexibility that made the bridge aerodynamically unstable on that tragic day.
Today modern science explained why Tacoma Narrow Bridge failed in “Torisonal Flutter“. TNB is usually faced with some vertical oscillation due to its design flexibility. But that force doesn’t make enough to make the bridge fail. On the tragedic day morning, TNB north cable mid span cable band slipped and allowed the cable to separate into two unequal segments. This situation started the torisontal movements in the deck and made a vortex situation around the deck. Torisontal motion and vortex frequency were matched and synchronized with the time bridge absorbing more wind energy. This kind of situation is called Torsional Flutter.
Typically vortex happens at 25-35mph wind speed. Torsional Flutter occurs in over 100mph wind speed. But TNB highly flexible design creates an unstable vortex and directly converts it to Torrison Flutter.
Starting A New Era
Without a doubt, Tacoma Narrow Bridge “Galloping Gertie” was one of the worst engineering failures in history. But it truly helps engineers and physicians to think in different way. Principles found from this accident later became a great backbone to great engineering structures all around the world.
In early 1930, engineers didn’t consider wind force and aerodynamic design as much. They thought heavy traffic loads as the main reason to bridge failed. But after this accident, the industry understood the power of wind force and the importance of aerodynamic designs. Engineers started to do more experiments with their 3D design models in the wind tunnels before actually building them. Also, the Tacoma Narrow Bridge disaster helps engineers to understand the limitation of Deflection Theory.
