Tacoma+Narrows+Collapse

** Tacoma Narrows Bridge Collapse (Nov. 7, 1940) ** toc  =Introduction    =

The design and construction of the Tacoma Narrows Bridge proved to be an engineering feat, adding innovations to the suspension bridge design. The mid-span of the bridge was 2800 feet and was the third longest span in the world (Petroski 1995, 294). Even with such a long span the suspension bridge was designed to have a longer, sleeker, lighter appearance. The implementation of the elastic distribution theory, allowed the main stiffeners of the bridge to be much shallower then previous bridges (University of Washington Special Collections).

From the first day of opening, the bridge experienced extreme oscillations, motorists would complain about sea-sickness while driving over the bridge. Attempts were being made to stiffen the bridge and minimize the oscillations. The force of the wind would cause the center span to rise several feet; a motion that gave the bridge the name “Galloping Gerti” (Petroski 1995, 295).

On the morning of November 7, 1940 sustained gusts of wind around 42 mph caused the bridge to oscillate. At an increasing rate, the oscillations progressively got worse, causing closure of the bridge. Motorist watched as the Galloping Gertie rotated about 45 degrees, and without warning, the oscillations of the bridge turned to a violent torsional twisting (Delatte 2009,27-28). After a few minutes of this movement, the stiffening girders buckled, initiating the collapse (Delatte 2009, 28). The mid-span fell 190 feet into the river, with only one fatality; Coatsworth’s cocker spaniel. Coatsworth was one of the two motorists caught on the bridge as it started to collapse (Big Tacoma Crashes... 1940). There are continuing debates on what caused the torsional movement that initiated the collapse from; wind induced oscillations, motion of the bridge was caused by air vortices, and resonance of the bridge and the air vortices caused the torsional oscillations (Billah 1991, 188-119).

=Keywords =
 * Bridge Collapse
 * Tacoma Narrows Bridge Collapse
 * Resonance
 * von Korman Street
 * Aerodynamic Flutter

=Design and Construction =  In 1937, the Washington State Legislature created the Washington State Toll Bridge Authority. This Authority studied the request for a bridge that would connect Tacoma and the Kitsap Peninsula (Tacoma Narrows Bridge (1940), 2). Clark Eldridge, a Washington State Engineer, proposed a conventional bridge design. The design included 25 feet deep girders that sat below the two lane bridge (Tacoma Narrows Brdige (1940), 2). Eldridge's design had a very high construction cost and the Washington Toll Bridge Authority sought advice from Leon Moisseiff. Moisseiff's design was far more economical than Eldrige's bridge design. This is primarily due to the shallower supports of 8 foot deep plate girders, instead of 25 feet (Tacoma Narrows Bridge (1940), 2). Leon Moisseiff was looking for a very sleek appearance for the Tacoma Narrows Bridge (Delatte 2009, 26).

Using the deflection theory, Moisseiff improved the efficiency of suspension bridges under gravity loads (Delatte 2009, 26). Moisseiff's and Lienhard's Theory of Elastic Distribution stated the wind force of long spans will primarily be transferred to the cables rather than the stiffening members. The higher the stress in the cables, the harder it will be to deflect the bridge laterally (Scott 2001, 43). This theory allowed Moisseiff to effectively design a much shallower bridge (Tacoma Narrows Bridge (1940), 3). The design incorporated two main piers located along the span of the bridge (University of Washington Special Collections). The center span was the third longest span in the world, reaching 2800 feet (Delatte 2009, 28). The Tacoma Narrows Bridge with its shallow plate girders and narrower width gave the bridge a low stiffness and made a very flexible bridge (Delatte 2009, 26). Engineers who reviewed the design of the Tacoma Narrows Bridge voiced concern that the innovations extended beyond previous practice and that it was not safe (Delatte 2009, 27). After reviewing the design, it was estimated the 80 percent of the static wind load would be taking through the cables, and the lateral displacements would be unnoticeable (Scott 2001, 43). All bridges move in the wind, the stiffer the bridge, the less the bridge will react to the wind force. Since the Tacoma Narrows Bridge is a very light and flexible bridge, it offers very little resistance to the wind.

=Opening Day =  After two years of construction Tacoma Narrows Bridge was completed. Connecting Tacoma with the Puget Sound Navy Yard, the Tacoma Narrows Bridge opened to traffic on July 1, 1940 (Delatte 2009, 26), (Figure 1). The bridge's movement in the wind did not alarm motorist, the opening ceremonies suggested that the bridge would last (University of Washington Special Collections). Driving over the bridge, the flexibility of the bridge became very apparent to the drivers. As the roadway would rise and fall from the wind force, some drivers complained of sea sickness (Petroski 1995, 295). The Tacoma Narrows Bridge is famouse for the movement in the wind, the traffic increased over the bridge, and the bridge was named 'The Galloping Gertie' (Petroski 1995, 295).



<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">A series of checking cables and devices were insalled along the spans of the bridge to hold down the deck and prevent it from rotating in the wind (Petroski 1995, 301). Figure 2 is a photo the checking cables installed on one of the side spans of the bridge. The Washington Toll Bridge Authority hired Professor F.B. Farquharson of the University of Washington. Farqharson used scale models of the Tacoma Narrows Bridge, to study the bridge's reaction in the wind (Tacoma Narrows Bridge (1940), 3). The movement of the bridge was evident and Farquharson proposed devices that would help stabalize the bridge (Petroski 1995, 3​01). However, on November 7, 1940 the clamps that held down the center span slipped, allowing the bridge to move unhindered in the wind (Petroski 1995, 301).

=<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Collapse = <span style="font-family: 'Times New Roman',Times,serif; font-size: 95%;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">On the morning of November 7, Farquharson arrived at the site to look at the bridge's motion, the<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> bridge was moving in it's usual manner and up to this point no alarm was concerned (Scott 1991, 49), (Figure 3). Kenneth Arkin was also present that morning when sustained gust of 42 miles per hours caused the bridge to oscillate, after the movement of the bridge was determined unsafe, Arkin closed the bridge to traffic (Delatte 2009, 27). At this point the bridge was rotating at 38 oscillations per minute (Delatte 2009, 28). The oscillation were the highest that have been noted up to this point, and to Arkin, Farquharson, and to eyewitnesses no alarm of the bridge's saftey was expressed (Scott 2001, 49). Due to the presence of Farquharson and a video camera, the collapse was so well documented (Delatte 2009, 30). The footage of the collapse allows engineers to study what happened to the bridge, and how it collapsed, the footage is provided in the following video clip.

media type="custom" key="4759445" <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 90%; text-align: left;">Video Clip: The footage of the collapse is public domain and was obtained from the Prelinger Archives.

<span style="font-family: 'Times New Roman',Times,serif; font-size: 122.47%;">Farquharson noted that a sudden change in the bridge's motion, the bridge began to twisting with extreme violence. There was no intermediate shift in motion, the nines waves that were visible shifted to only two waves (Scott 2001, 50). The stays that were installed on the bridge to minimize the movement had broken, and after the north center stay broke the bridge began to rotate more than 45 degrees, causing a rise of 28 feet (Delatte 2009, 28). During this torsional motion the deck would rise up violently at a quarter points, while at other points on the side the motion would be in the opposite direction (Scott 2001, 51), (Figure 4). There were two drivers on the bridge at the time of this extreme motion, one a newspaper man, Leonard Coatsworth and the other a logging truck driver. Both abandoned their vehicles and made their way to safety. The only fatality was Coatsworth's cocker spaniel which was trapped in the car (Big Tacoma Bridge Crashes... 1940, 1). The bridge had endured all this force up until now and it became obvious to Farquharson that the bridge couldn't take much more of the violent twisting (Scott 2001, 52). The collapse was initiated when the stiffening girders began to buckle and the concrete curb crumbled. When the suspension cables began to break under the stress, a section of the bridge's mid span fell into the water (Delatte 2009, 29), (Figure 5). "The Tacoma Narrows Bridge collapsed with a roar and plunged into the waters of the Puget Sound, 190 feet below" (Big Tacoma Bridge Crashes... 1940, 1). The Tacoma Narrows Bridge was like all other suspension bridges that moved in the wind, a sustained wind is all it took to start the motion (A Great Bridge Fall 1940). At 11:10 A.M. the rest of the main span collapse into the water and the two side spans finally came to rest with a sag of 30 feet (Scott 2001, 53), (Figure 6).

<span style="font-family: 'Times New Roman',Times,serif; font-size: 388.8%;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 388.8%;"> = Cause of Failure =

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The Tacoma Narrows Bridge was design for a wind speed of 100 miles per hours and a static wind pressure of 30 psf, the question rises how can the Tacoma Narrows Bridge fail at a wind speed of less than half and a static wind pressure of one-sixth (Delatte 2009, 30). To understand how the bridge fail, it is important to understand how the wind loaded the cross section, and the stresses that the bridge and its supports were put under (Morse-Fortier 2005, 2). It was initially thought that resonance was the cause of the collapse (A Great Bridge Falls 1940, 1).

Three engineers were assigned to investigate the collapse and report to the Public Works Administration (PWA); Othmar H. Ammann, Theodore von Karman, and Glenn B. Woodruff (Scott 2001, 53). The investigation for the PWA reported the following (Delatte 2009, 30-31);
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The bridge was well designed and well built. Although it could safely resist all static forces, the wind caused extreme undulations, leading to the bridge's failure.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Efforts were made to control the amplitude of the bridge's oscillation.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">No one realized that the Tacoma Narrows Bridge's exceptional flexibility, coupled with its inability to absorb dynamic forces, would make the wild oscillations that destroyed the bridge possible.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Vertical oscillations were caused by the force of the wind and caused no structural damage.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The failure of the cable band on the north end, which was connected to the center ties, probably started the twisting motion of the bridge. The twisting motion caused high stresses throughout the bridge, which led to the failure of the suspenders throughout the bridge, which led to the failure of the suspenders and the collapse of the main span.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Rigidity against static forces and rigidity against dynamic forces cannot be determined using the same methods.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Subsequent studies and experiments were needed to determine the aerodynamic forces that act on suspension bridges.

Theory 1
<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The engineers that conducted the investigation concluded that the high flexibility, narrowness, and lightness, and the wind force that day caused the torsional oscillation that destroyed the bridge. The oscillations that were present that day from the wind force approached the natural frequency of the bridge, causing resonance (Delatte 2009, 31). Resonance is the process by which the frequency of an object matches its natural frequency, causing a dramatic increase in amplitude (Delatte 2009, 31). Since the central span oscillated until it collapsed, it is often referenced as a classic example of resonance (Billah 1991, 119).

Theory 2
The theory published through the Public Works Administration is not the only theory that has been proposed for the cause of the Tacoma Narrows Bridge Collapse. Von Karman, an aeronautical engineer, proposed that the motion that was seen on the day of the collapse was due to vortices. When a air flow passes a bluff body it created the periodic shedding of air vortices, which create the von Karman Street Wake. This wake reinforced the oscillations that were present and caused the center span to violent twist until the bridge collapse (Delatte 2009, 31).

Theory 3
A third theory that was studied by K Yusuf Billah and Robert H. Scanlan was the cause of the collapse was the aerodynamic flutter. Scanlan stated that the Karman Street was present but the significance to the collapse was minimal (Delatte 2009, 31). Scanlan and Billah proposed the cause of the collapse is the change with the lift and drag forces along the span of the bridge. A negative damping affect along with a torsional degree of freedom caused the torsional flutter (Morse-Fortier 2005, 3). As the bridge deck rotated the wind force acting on the surface changed, when the bridge rotated back the forces pushed the bridge in the opposite direction (Morse-Fortier 2005, 3). As the deck rotated more and more the forces acting on the deck increased, and after the center stay fail, the bridge was free to rotate even more. This negative damping effect and increase in rotation lead up to the torsinal oscillation that caused the collapse of the bridge (Morse-Fortier 2005, 3).

The three theories that were proposed to be the cause of the oscillation that caused collapse of the Tacoma Narrows Bridge are so very different. Each theory agrees that the flexibility, narrowness, and lightness of the bridge allowed the propagation of the oscillations (Delatte 2009, 31).

=<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Tacoma Narrows Bridge Today =

<span style="font-family: 'Times New Roman',Times,serif; font-size: 90%;">In 1992, in reponse to protect the remains of the Tacoma Narrows Bridge against salvagers, the sunken remains were placed on the National Register of Historic Places. The bridge is home to various sea creatures like; giant octopi, wolf eels, and sharks (Submerged Cultural Resources Exploration Team). Figures 7 and 8 are underwater photos taken by the Submerged Cultural Resources Exploration Team. <span style="font-family: 'Times New Roman',Times,serif; font-size: 129.6%;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 144%;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> =<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Conclusion = <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> When it comes to new innovations and design for structures, it is important to remember public safety are the advances in design in line with the advances with the materials used. Moiseiff proposed a new design that was very slender, and lighter (Delatte 2009, 26). The collapse of the Tacoma Narrows Bridge showed the importance in damping, vertical rigidity, and torsional resistance in suspension bridges (Delatte 2009, 32).

The collapse of the Tacoma Narrows Bridge could have been avoided, primarily during the design phase. However, the implementation of damping devices in the bridge, after opening were either installed or being planned. The following is a list of methods that could have been used to prevent the collapse of the bridge (Delatte 2009, 31).


 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">using open stiffening trusses, which would have allowed the wind free passage through the bridge
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">increase the width to span ration
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">increase the weight of the bridge
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">dampening the bridge
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">using an untuned dynamic damper to limit the motions of the bridge
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">increase the stiffness and depth of the trusses or girders
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">streamlining the deck of the bridge

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">An untuned dynamic was installed in the bridge, but failed shortly after construction from the sand blasting procedures used on the girders. Stays were installed along the span but after failing, provided no damping resistance (Delatte 2009, 27).

There have been problems with suspension bridges prior to the Tacoma Narrows Bridge collapse, between 1818 and 1889, ten suspension bridges were destroyed or dmamged by the wind forcrs. These bridges all had small width-to-span ratios that gave the bridge a high flexiblilty in the wind (Delatte 2009, 32-33). As the use of suspension bridge increased, engineers neglect the lessons of the past and continued to design bridges with a longer, sleeker designs (Delatte 2009, 33).

After the collapse of the Tacoma Narrows Bridge, the current suspension bridges reamined vulnerable to the wind forces. As well as new suppension bridges that were built experienced movement in the wind. The George Washington Bridge in New York City and the Golden Gate Bridge both experience movement in the wind despite the weight of teh bridges and the design of stiffening members in the bridges (Delatte 2009, 34). The Millennium Bridge in London that opened in June 2000, had the same stiffness problems as the Tacoma Narrows Bridge. The vibrations of the bridge during used, increased concern and the bridge was closed (Scott 2001, 169). It is evident that the lessons from the Tacoma Narrows Bridge have been learned and applied, but as inovations of designs and materials increase, the engineers that design the longer, sleeker, and lighter bridges are going to need understand the effects that the loads have on the bridge. All bridges move in the wind, but the stiffness of the bridge and relative ratios of width to span, determine the magnitude the bridge is going to move. = = =Biography= <span style="font-family: 'Times New Roman',Times,serif;"> “Big Tacoma Bridge Crashes 190 feet into Puget Sound,” The New York Times, November 8, 1940. (2).

“A Great Bridge Falls,” The New York Times, November 9, 1940. (1).

Morse-Fortier, Leonard J., “Professor Robert H. Scanlan and the Tacoma Narrows Bridge,” Structures 2005, ASCE 2005. (12).

Billah, K. Yusuf, Scanlan, Robert H., “Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks,” American Journal of Physics 59 (2), February 1991, 1991 American Association of Physics Teachers. (118-124).

Delatte, Norbert J., //Beyond Failure//, ASCE Press, 2009. (26-37)

Scott, Richard, //In the Wake of Tacoma//, ASCE Press, 2001. (41-67, 169).

Petroski, Henry, //Engineers of Dreams//, Alfred A. Knopf, New York, 1995. (294-302).

“Tacoma Narrows Bridge (1940),” < [] <span style="font-family: 'Times New Roman',Times,serif; font-size: 95%;"> > (September 16, 2009). (11).

University of Washington Special Collections, “Tacoma Narrows Bridge,” < [] > (September 16, 2009).

Submerged Cultural Resources Exploration Team. “Tacoma Narrows Bridge.” < [] > (October 10, 2009). (4).