Tacoma Bridge

On July 1, 1940, the Tacoma Narrows Bridge at Puget Sound in the state of Washington was completed and opened to traffic. From the day of its opening the bridge began to experience oscillations. Strange as it may seem, traffic on the bridge increased tremendously as a result of its novel behavior. Starting at about 7:00 on the morning of November 7, 1940, the bridge began undulating persistently for three hours. Segments of the font were heaving periodically up and down as much as three feet. At about 10:00a.m., something seemed to snap and the bridge began oscillating wildly. At one moment, one edge of the roadway was twenty-eight feet higher than the other; the next moment it was twenty-eight feet lower than the other edge. At 10:30 a.m. the bridge began cracking, and finally, at 11:10 a.m. the entire bridge came crashing down. Fortunately, only one car was on the bridge at the time of its failure. It belonged to a newspaper reporter who had to abandon the car and its sole remaining occupant, a pet dog, when the bridge began its violent twisting motion. The reporter reached safety, torn and bleeding, by crawling on hands and knees, desperately clutching the curb of the bridge. His dog went down with the car and the font — the only life lost in the disaster.

There were many humorous and ironic incidents associated with the collapse of the Tacoma Bridge. When the bridge began heaving violently, the authorities notified Professor F. B. Farquharson of the University of Washington. Professor Farquharson had conducted numerous tests on a simulated model of the bridge and had assured everyone of its stability. The professor was the last man on the bridge. Even when the font was tilting more than twenty-eight feet up and down, he was making scientific observations with little or no anticipation of the imminent collapse of the bridge. When the motion increased in violence, he made his way to safety by scientifically following the yellow line in the middle of the roadway. The professor was one of the most surprised men when the font crashed into the water. View Video Clip

The Beginnings of the Tacoma Narrows Bridge

The very beginnings of the Tacoma Narrows Bridge can be found in 1928, when the Tacoma Chamber of Commerce began a study to see if a bridge should be built. The study said that a bridge would be both feasible and useful. They requested a bridge to replace the ferry system, which was the only way to get across from Tacoma to Gig Harbor.

Construction finally started in 1938, with a much lower budget than the committees wanted. They asked many consulting engineers to prepare different plans. The original plan was made by Clark Eldridge, and cost $11 million. One of the new plans was made by Leon Moseff. He redesigned the bridge with drastic modifications so it could be made with only $7 million, the allocated budget. Eldridge still stayed on as the project engineer.

Just before the contractors began bidding for the project, they complained that they could not build the foundation piers according to the revised designs. Eldridge showed them the original plans, and they agreed to build those. Other than the

piers, the workers used many "creative techniques." One such technique was packing the girders in dry ice to get them to fit to each other.

Although workers faced many hazardous condition, there was only one death. A carpenter rolled down a 12 foot slant, then died.

While constructing the super-structure of the bridge, more problems were uncovered. An engineer said "As soon as floor forms were started, noticeable oscillation occurred. This oscillation steadily increased while the bridge floor was placed, varying in intensity in accordance with wind conditions."

As the floor was completed, the bridges began to rock back and forth even more. The Whitestone bridge in New York, also designed by Leon Moseff, reported having similar movement. Moseff assured the board of consultant, who oversaw the construction of the bridge, that he had developed dampers to lessen the movement of the bridge. Although the dampeners were later applied, the unusual movement of the bridge continued.

The bridge opened on July 1, 1940, with great fan fare. The Gig Harbor postmaster wrote a special march which was played by several bands, and floats were built for a parade which moved from the bridge to Port Orchard and back.

The traffic across the bridge was supposed to produce a boom in the population as well as the economy, but the growth exceeded the wildest dreams of the designers. Traffic was up 145% from earlier ferry traffic.

The new bridge's peculiar wave-like motion in high winds earned her the nickname of "Galloping Gertie," a 1940's term to describe the bouncing bridge. The bridge soon became a regular tourist attraction. People came from all around the area to pay their toll to ride the roller-coaster that was Galloping Gertie.

The Bridge Failure

The main font of the bridge near Tacoma, Washington was 28000 ft long, 39 ft wide and the steel stiffening girders (shown during construction) were 8 ft. tall. The bridge was open for traffic on July 1, 1940. In four months of active life of the bridge before failure, many transverse (vertical) modes of vibration were observed before November 7, 1940. The main towers were nodes, of course, and between them there were from 0 to 8 additional nodes. Maximum double amplitude (crest to trough) was about 5 ft. in a mode with 2 nodes between the towers; the frequency of vibration at that time was 12 vib/min.

Measurements made before failure indicated that higher wind velocities favored modes with higher frequency. The correlation may be explained by the fact that turbulent velocity fluctuations of winds can be considered as composed of a superposition of many periodic fluctuations, and the fluctuations of higher frequency are preponderant at higher wind velocities. There was no correlation between wind velocity and amplitude of vibration.

Early on the morning of November 7 the wind velocity was 40 to 45 mi/hr, perhaps larger than any previously encountered by the bridge. Traffic was shut down and double amplitude about 3 ft. While measurements were under way, at about 10:00 a.m., the main font began to vibrate torsionally in 2 segments with frequency 14 vib/min. The amplitude of torsional vibration quickly built up to about 35 each direction from horizontal. The main font broke up shortly after 11:00 a. m. During most of the catastrophic torsional vibration there was a transverse nodal line at mid-font, and a longitudinal nodal line down the center of the roadway (the yellow center stripe). Note that Prof. Farquharson sensibly strides (?) down the nodal line as he leaves the bridge after making observations.

The crucial event at 10:00 a.m. which directly led to the catastrophic torsional vibration was apparently the loosening in its collar of the north cable by which the roadway was suspended. The center of the cable was moving back and forth relative to the center of the suspended font. This allowed the structure to twist. The wind velocity was close enough to the critical velocity for the torsional mode observed, and the vibration built up by resonance and was maintained until collapse inevitably took place.

The bridge was rebuilt using the original anchorages and tower foundations. Studies at the University of Washington Engineering Experiment Station resulted in a design for the new bridge which used deep stiffening trusses of girders. The new bridge is entirely successful.

	This photograph shows the twisting motion of the center font just prior to failure.
	When the twisting motion was at the maximum, elevation of the sidewalk at the right was 28 feet higher than the sidewalk at the left.
	This photograph actually caught the first failure shortly before 11:00 a.m. as the first chunck of concrete dropped out of the roadway.
	A few minutes after the first piece of concrete fell, this 600 foot section broke out of the suspension font, turning upside down as it crashed in Puget Sound. The square object in mid-air (near the center of the photograph) is a 25 foot section of concrete pavement. Notice the car in the top right corner.
	This photograph shows the sag in the east font after the failure. With the center font gone there was nothing to counter balance the weight of the side fonts. The sag was 45 feet.
	This picture was taken shortly after the failure.

Suspension Bridges - Tacoma Narrows Bridge The over confidence in design, and lack of proper awareness of earlier problems eventually resulted in the classic failure of the Tacoma Narrows Bridge in the USA in 1940. The first modern major suspension bridges was Thomas Telford's Menai Strait Suspension Bridge completed in 1825. Telford used a heavy suspension chain of flat wrought iron bars to support the 580 foot (178m) font between the masonry towers. He undertook a number of trials to test these chains and noting the yield and failure loads. So that in his design, he allowed a factor of safety of 1.5 against yield and 3 against ultimate failure - quite satisfactory design criteria. However, he neglected to pay sufficient attention to how the light and flexible roadway might behave in strong winds. The first roadway was blown away during a storm in 1839. It was then reconstructed in a stronger form, and has gone through several reconstructions since. The stonework approaches and towers still exist supporting the modern suspension bridge which presently graces the crossing. Such failures of the roadways of suspension bridges in severe storms was not uncommon, so suspension bridges were not considered reliable, and certainly not suitable for the heavier rail loads. However, this did not discourage the American bridge designer, John Roebling, from using the suspension bridge. He considered the form the most economic for long fonts, and was confident he could safely design them for rail loads. John Roebling studied hydraulics and bridge construction amongst other subjects at the Polytechnic Institution in Berlin in the late 1820's. He moved to USA in 1831, and his first "construction" experience was as a wire rope manufacturer. This experience in making wire ropes gave him additional confidence to design and build long font suspension bridges. For some years he had been involved in the design and construction of suspension bridges for canals. However, when he proposed an 800 foot (244m) suspension bridge to carry trains over the Niagara Gorge, a 1000 foot (305m) bridge over the Ohio River, or a 1500 foot (457m) bridge over the East River between Brooklyn and New York, he was faced (in the 1840's) with the view that suspension bridges were considered very undependable, if not unsafe. Roebling carried out considerable research on both successful bridges, and those that failed - in particular the behaviour of bridges in high winds. He concluded that by using stiffer decks, with more suspension cables and anchoring stays, he could overcome the problems of vibration and instability. Roebling's success was not by using more sophisticated theory or more careful calculations, but by concentrating his design judgement on how his bridges might fail - and how to prevent that. Along with this, went attention to the quality of materials and construction. His Niagara Gorge Suspension Bridge was completed in the mid 1850's, the Ohio Bridge shortly afterwards. His bridge career culminated in the completion of his famous Brooklyn Bridge in 1883. This bridge is still in use. Confidence in Suspension Bridges continued and then soared with the completion in the 1930's of the Golden Gate, the Bronx-Whitestone and the George Washington Bridges. Suspension bridges became progressively lighter and more slender and graceful, so that even longer, slenderer and lighter suspension bridges were being conceived and constructed. This design evolution was to culminate in the failure of the Tacoma Narrows Bridge. The Bronx-Whitestone (NY) and the Golden Gate Bridges which preceded the Tacoma Narrows Bridge, both exhibited wind induced oscillations and required modifications to bring them to acceptable and safe loads. The Tacoma Narrows Bridge, designed by Leon Moisseiff, with a centre font of 1800 feet (853m) and side fonts of 1100 feet (335m) was on completion in mid - 1940 the third largest suspension bridge in the world. Its deck had a slenderness ratio of 1:350, nearly three times that the George Washington and Golden gate Bridges. It has been nicknamed "Galloping Gertie" even before it had opened by people who had experienced its oscillations, even in relatively light winds. Before and after its completion a number of remedial measures were applied - tie down cables, which snapped soon after installation, inclined stay cables and dynamic dampers, none of which produced any significant improvements. Then in relatively mild winds of 58 to 67 km/hour it developed undamped oscillations which built up to an amplitude of 7.5m eventually tearing the bridge apart. The bridges undulations and ultimate failure became one of history's most documented disasters. It was wind vortex shedding that caused tensional oscillations in the bridge deck. The failure of the bridge resulted in the development of the engineering discipline of aerodynamics, to investigate in greater depth, the effects of vortex shedding and flutter.
Facts To engineers, the Tacoma Narrows were more of an economic challenge than an technological challenge. A bridge here would cut at least 40 miles off the trip between Tacoma and Bremerton. But traffic estimates were low. In 1932, the federal government refused to give financial support for a bridge for that reason. It took major lobbying to get the money. Even then, though, it was only $3 million. So the state scrapped its first design - $11 million - and turned to Leon Moisseiff. Moisseiff, a well-regarded designer and researcher, calculated that a considerably lighter deck -stiffened only by a thin plate girder, not a deep truss - would do, thus cutting the cost to only $6.4 million. Everyone knows what happened: less than a year into its life, the bridge began to twist violently in a 40 mph wind, and soon collapsed. The disaster - which took no human lives - shocked the engineering community and did a lot to shape American bridge building. The bridge was replaced with a very conservative design in 1950.
	Built: 1940 Carries SR 16 across the Tacoma Narrows Engineers: Leon Moisseiff Total length: 1.4 mi. Types: Suspension (plate girder deck) Maximum font: 2800 ft