Project Management
PM3110
Victor Nyankah
10/24/14
Tacoma Narrow Bridge

Executive Summary

The Tacoma Narrows Bridge was built in 1940 at the cost of $6.4 million funded mostly by the federal government’s Public Works Administration. The purpose of the brigde was to connect Seattle and Tacoma with the Puget Sound Navy at Bremerton, Washington. It was said to be the third largest of its kind. Though it was a long bridge, it was very narrow because it only had one way traffic from each direction. It had a center span of 2800 feet and a 1000 foot approaches on each side.

The bridge took nineteen months to be constructed and fell apart barely four months after its inauguration. It is however important to note that even before the inauguration, the bridge had started showing signs of incompetence and exhibiting strange characteristics which shouldn’t have gone unnoticed. The bridge started galloping and swaying and hence got the nick name, the “Galloping Gertile”.

In order to under the dynamics of suspension bridges, we need to understand the physics of bridges as propagated by Newton also known as Newton’s Laws.

All bridges are subject to Newton's three laws of motion.
Newton's First Law states that an object at rest will stay at rest unless acted upon by an Unequal force. For a bridge, this means that unless all of the forces on a bridge remain balanced, one or more components of the bridge will move. Left uncorrected, this instability can lead to a partial or total collapse of the bridge.
Newton's Second Law states that the force on an object is equal to its mass times acceleration. Gravity is an acceleration that causes a force upon a bridge and its parts. Wind also acts as acceleration on a bridge. For a bridge to remain standing, these forces must also be in balance.
Newton's Third Law states that every action has an opposite and equal reaction. As cars push down on the deck, something must push up with equal force. If not, the bridge will fail.
Problem

In reference to the above mentioned laws of motion, it is obvious that certain aspects of the laws were either neglected and or totally ignored. To begin with, it also worthy of note that the project managers responsible for the construction of the bridge tried to cut corners by way of saving money in the process, failed to realize the dangers of using certain materials. For example an Engineer by name of Eldridge initially designed a cost of $11 million for the construction of the bridge and called for 25-foot open-grid-work trusses that would stiffen the roadbed to ensure stability and safety but the eastern financier will not hear of it and kept finding cheaper ways to go about it. This in effect meant using the wrong type of materials that will be needed. They ended up selecting Moisseiff to the reevaluation of the budget and somehow come up with a cheaper suggestion. He proposed the use of solid-plate trusses that were only 8 feet high because that allowed him to design a more visually appealing bridge that will in turn use less steel and cost less. His theory and budget of 6.4 million was more appealing to the financiers and so it was accepted. The unfortunate part of his theory was that it was a tested one and therefore some of the risk associated with it the design. The Tacoma Bridge therefore was three time more flexible than the Golden Gate Bridge (which was the world’s flexible bridge up to that point).

There are four main components that make a good suspension bridge. The first one is the Abutment which is where the cables that stretch from shore to shore are anchored. Then we have the cables themselves stretching from shore to shore or extend down to the deck. The third component is the deck which is the actual travel surface of the bridge which in the instance was a little too narrow for the length of the bridge. Then finally we have the towers that are embedded beneath the water and extend up above the deck.

Causes

Clearly, there are some factored that might have eluded that project management team consequently resulting in the collapse of the bridge. As mentioned above, they were out of scope to a very large extent from the financial realms to the personnel to testing of theories and environmental factors. Financially they tried to cut cost by using inferior materials. The original budget that was presented to them would have cost $11 million but that eastern financiers figured it would be a good idea to cut the cost to $6.4 million. This would obviously use less steal and cost less. It is obvious that enough risk assessment did not go into the determination of the acceptable budge for the project. This is because RA would have pointed out that even though it’s good to save money in the short run, that amount of money saved could be costly in the long run. The engineers used an accepted design theory that incorporates both assumptions and experimental measurements in order to create estimated for the unknown.

Further studies shows that the side metal was also too thin and too solid. This means that if they had used a thicker metal with some spaces by way of a design in way would have allowed air to pass through easily. This would have avoided the wind from hitting the side so hard and raising it like a kite. It is important to note that each segment of the bridge must be able to sustain its own weight plus the weight of any load it might carry.

This size of the deck as opposed to the length of the bridge is another factor that was not diligently investigated. With the nature of the bridge in question and the length, having a size a small enough to only accommodate one way traffic was such a bad idea. Because the bridge span to over a mile in distance, it makes is vulnerable to the dictates of the wind if the deck is not wide enough to sustain it. The location of the bridge itself made it susceptible to the wind. It was located right next to the valley where the wind is most aggressive and uncontrollable at times. “Strong winds forced the Tacoma Narrows Bridge to oscillate in an unexpected, twisting fashion that ripped the bridge asunder and dumped the central span, a car, and a dog into the sea. Studies of the collapse led to the development of the science of bridge aerodynamics”.

Recommendations:

In view of that above mentioned issues related to the collapse to the Tacoma Narrow Bridge, there are few recommendations put together that should be considered in a few project. Since tension and compression play a very important role in the building of a suspension bridge, the long of cable that gives the required amount of tension should have been used. Compression is a factor like I mentioned before, therefore, as cables are pulled up and over the towers, they stretch from shore to shore which makes the towers press towards the ground. Basically enough tension with appropriate proportion of compression is what should have been considered.

Also since it’s a suspension bridge, they should make use of the strongest yet most light weight materials as possible. The weight of the material plays an important role because the bridge is all about weights: weight of towers, deck, the bridge itself, cables and of course the traffic passing through the bridge. It is therefore recommended that a good balance be drawn between the weight of the structure of the bridge and the weight of the bridge with the live load.

Finally a particular attention should be paid to the direction of the wind. This is because these bridges stretch across open expanses of water and because of that, there should be some allowance of movement for the bridge but at the same time still function. If the allowance is not enough then situations like the Galloping Gertie is likely to occur.

Executive Summary

Building bridges just like any other projects require pains taking and enough due diligence to ensure a successful venture. Budget allocation to project can play a big role in the success or failure of the said project. Over budging has its own repercussion as well as under budging. It will also be a good idea to consult with other people who may have some ideas of the said project, compare their recommendations and then make an informed decision afterwards. Social, environmental, political and technological factors should not be over looked either since they all have their important role to play.

References

Tacoma Narrows Bridge Collapses. By: Rogers, Charles W., Salem Press Encyclopedia, January, 2013

Abstract: The article focuses on how engineers fix the problem faced in Tacoma Narrows Bridge which includes pulling of the dangling deck upon the cables that hold it and compression. By: Hagler, Gina, Odyssey, May/Jun2014, Vol. 23 Issue 5, p28-31. 4p

Project Management, second edition, by: Pinto, Jeffery, K