A bridge is a structure built to span a distance in order to get from one side to another e.g. a valley, open body of water or other physical obstacles. Designs of bridges vary in accordance to their function and the type of terrain that they span across. They were first formed in nature by the means of a fallen tree or log across a river. Since then humans have used different materials as we have evolved giving rise to different means of bridge construction. There have been many engineering failures and one of the most significant and outstanding is the collapse of the original Tacoma Narrows Bridge.
The first Tacoma Narrows Bridge opened on 1st July 1940 and collapsed on 7th November 1940 just 4 months after completion. It was located in the United States Peninsula in North West Washington connecting Tacoma and Kitsap Peninsula where the Puget Sound narrowed to about 1400m. Soon after completion, the bridge obtained the nickname of Galloping Gertie due to its instability under windy conditions. Galloping Gertie was the third longest suspension bridge in the world behind the Golden Gate Bridge and the George Washington Bridge. Whilst the bridge was standing, its total length was 5939 feet (1810.2m) with the longest span at 2800 feet (853.4m). The clearance from the bridge to the Puget Sound River below was 195 feet (59.4m). The bridge was built to connect Tacoma and the Kitsnap Peninsula.
Although the bridge was built in 1940, the desire for a construction of a bridge dated back to 1989 with the proposal of a trestle. Now the bridge was backed up by the U.S Navy who needed improved access to a large naval yard on the Olympic Peninsula and by the U.S Army who ran McChord Field and Fort Lewis near Tacoma.
One of the first proposals for a new bridge was by David. B. Steinman in 1930. He suggested the design of a suspension bridge to the Tacoma Chamber of Commerce. The bridge consisted of a two lane, 12.2m wide decks with 18.3m wide floor trusses. There were to be 7.3m deep stiffening trusses to help resist the Narrows high winds. This suggestion was ruled out in 1931 due to reasons that showed that Steinman wasn't sufficiently active in working to obtain financing. Steinman later went on to design the Mackinac Bridge. Another problem with gaining finance for the first bridge was the trouble of buying out the ferry contract, which belonged to a private firm who ran service on the Narrows.
In 1933, E.M Chandler received war department blessing to design another bridge. This bridge was a two-lane cantilever bridge with a 366m span, which like Steinman's proposal went nowhere.
Local pressure led to the creation of the Washington State Toll Bridge Authority in 1937. At the time, the state highways department, working on behalf of the Toll Bridge Authority, prepared a new bridge design under the engineer Clark H. Eldridge. Due to the narrow deck of the bridge, this demanded a shorter span to resist static wind pressure, but due to the great depths of the Puget Sound, it demanded a longer span. A new design for the bridge arose in mid-1938 which consisted of a 792m span flanked by longer 396m side spans, 12m wide floor truss stiffened by a 6.7m deep stiffening truss. In June 1938, the Toll Bridge Authority received a grant of $2.7 million to help fund construction. The application requested for a $3.3 million loan from the RFC remained unacted on. A fundamental change then occurred to the design when the PWA requested the Toll Bridge Authority to obtain an independent engineer to review the designs. They called on Leon Moisseiff who was recognized as being one of the most prolific suspension bridge engineers of the era.
Leon Moisseiff along with Fred Leinhard proposed changes to the design that if accepted by the PWA and the Toll Bridge Authority, would radically change the original plans. The first change that was made was to the two towers that were at uneven heights due to unequal bank elevations. Moisseiff suggested that the towers should be of equal height and appearance for the aesthetics purposes. This resulted in the west end of the bridge being raised by 6m in order for the symmetrical appearance of the towers. Moisseiff also recommended that the span be lengthened to 853m with shorter side spans of 335m. Lower towers were also recommended at a height of 136m. Smaller bases of 15m were to be used instead of the original 18m. Moisseiffs new design permitted vertical and horizontal flexibility, which the bridge now proved to show remarkable economy and grace.
Even though the Tacoma Narrows Bridge was ranked third in length, its proportions were unprecedented. Its depth-span ratio was 1:350, which was almost a tenth of the 1:40 proportions of the Williamsburg Bridge. Its width-span ratio was 1:72 which was larger than the previous record of 1:47 of the Golden Gate Bridge. Although the ratios were very high, the span actually conformed to contemporary norms for static wind pressure and live loads. The PWA and the Toll Bridge Authority accepted Moisseiffs design changes stating that they assessed the Engineers reputation and not the design itself. Despite this, the Toll Bridge Authority Staff executed the detailed design in July and August of 1938 under the engineer Clark Eldridge with the work referred to Moiseiffs office in New York City for approval.
The Toll Bridge Authority advertised bids in August 1938 in a race to meet the PWA deadline for tendering construction contracts whilst still expecting RFC loan approval. The $3.3 million loan was finally approved in October and work began on the bridge in November 1938. It took just 19 months to complete the structure at a cost of $6.4 million, which was financed by the grant from the PWA and the loan from the RFC. The bridge lasted just 4 months before its collapse on 7th November 1940, which caused the extensive research into why it occurred. There was only one fatality due to the collapse of the bridge and this was 'Tubby' the dog who was too scared to exit an abandoned vehicle left on the bridge.
After the collapse many theories were discussed into what caused the failure. Ultimately, an investigative board for the Washington State Toll Bridge Authority declared that the collapse was due to the bridges design reacting to the wind. The speed of the wind at the time of collapse was approximately 42 mph even thought the bridge was built to withstand 120 mph winds. A cartoon from the Seattle Times, November 8th 1940, gives a simple explanation that the collapse was due to the improper shape of the structure causing extensive unnecessary resistance to wind which, could have been prevented, was there more money to spend on the bridge.
There were numerous investigations and explanations to why the bridge collapsed and this had a lasting effect on science and engineering. A commission formed by the Federal Works Agency studied collapse of the bridge. At first the reason for collapse seemed to be due to resonance. Slow steady harmless motions soon transformed to catastrophic motions under relatively light winds. Resonance seemed to answer the cause of collapse as it was thought that the Von Karman Vortex street frequency was the same as the torsional natural vibration frequency. This was actually found to be incorrect which led to further research into finding that the true failure was due to aeroelastic flutter.
Aeroelastic flutter occurs when aerodynamic forces on an object couple with a structures natural mode of vibration to produce violent periodic motion. The flutter velocity is the wind speed that causes the beginning of the flutter phenomenon, which is when the effective damping becomes zero. The forces insert energy onto the bridge during each cycle neutralizing the natural damping of the structure leading to an exponential growth response. This results in the oscillations of the bridge increasing in amplitude with each cycle due to the wind inserting more energy than the bridge can dissipate. Eventually the bridge will fail due to excessive deflection and stress.
What happened with the Tacoma Narrows Bridge failure on the morning of November 7th 1940 was the force of wind causing a small amount of twisting. The twisting caused the wind flow separation to increase creating a vortex, which further increased the lifting and torsional fluttering of the bridge. The bridge deck has a natural tendency to return to its original position, which coincidently matched the force of the wind. This produced a lock on effect, which exponentially increased the lifting motion until failure.
The Tacoma Narrows Bridge failure revealed the limitations of the deflection theory and globally increased engineers' awareness and practice in building flexible, light and slender suspension spans. It has given us invaluable information and led to the further research into aerodynamics and its effects on structures. Hopefully with our increased knowledge and awareness of our natural surroundings and the environment a failure like this would be unlikely to occur again.