Civil Engineering Design - Term Paper Example

Term Paper Introduction The Bay Bridge San Francisco is one of the master pieces of 19th century architecture. This bridge is extended over length of7.5km and is purposely built to join San Francisco with Oakland. This bridge was planned in early 19th century but due to technical complications it was not completed within a century. Eventually, the construction of bay bridge started on 9th of July in 1933 and was completed within 3.5 years. The initial design of the bridge was having two decks. One was dedicated to the vehicular traffic whereas the lower one was dedicated to train tracks. With the increase in traffic both the decks were made available for the traffic. In 1989 a strong earthquake, of the intensity of 6.5 on Richter scale, hit the eastern coasts of the bridge. It caused great damage to the bridge which was then completely renewed in 2002. Past Practice in the Design of Bay Bridge With the advent of automobiles, the need for transportation between Oakland and San Francisco increased. Therefore, California Legislature developed a California Toll Bridge Authority in 1929 which was responsible for the construction of bridge, joining San Francisco and Oakland. The route that was chosen for the construction of bridge passes through Yerba Buena Island which was, at that time, being used as US naval base. This route was chosen as it significantly reduced bridge length, construction cost and labor. But before starting the construction of the bridge, permission from the US congress was require for the construction over Yerba Buena Island, which was finally obtained in1931. The realization of Bay Bridge Construction started with the establishment of California Toll Bridge Authority in 1929 and a number of unemployed people got employment. Initially fourteen designs were submitted for the Bay Bridge. The final design consisted of a cantilever bridge from Oakland to Yerba Buena and a suspension bridge from Yerba Buena to San Francisco. Both the bridges were joined by boring a tunnel on Yerba Buena Island. The Bay Bridge was first designed by Charles H. Purcell who was a civil engineer by profession. This was the longest bridge designed at that time and designing and analysis of such a mega project was a big deal in that times. The bay bridge was designed in two sections which were then connected by making use of tunnel. Both the sections were designed differently according to the climatic conditions and land of both the regions. Also water depth was not same in the two areas which were joined by the bridge. The San Francisco region was covered in western span of the bridge and that portion of bridge was designed on the principle of suspension bridge. This suspension bridge was having heighted towers with long span to provide flexibility and strength to the design; whereas, the eastern section is having a blend of truss and cantilever design. In the picture below, Charles H. Purcell is having deep insight into the model of Bay Bridge Design. This project was not only limited to design and construction of Bay Bridge but it was also including the Transbay terminal design, toll booths and other administrative buildings. The cross sectional view of initial design of Bay Bridge is shown below: This design consisted of 6 rows of traffic on the upper deck, whereas, the lower deck was provided with two railway tracks and three lanes for heavy traffic. The train service on lower deck of Bay Bridge was started in 1938, just two years after the functioning of Bay Bridge, but it lasted till 1958 only. It was continued for a very short period of time due to overloading of traffic on this bridge. This railway system was handled by a very sophisticated control system which was provided at the railway control room. In 1940 this system was rushed to its peak and afterwards it became difficult to handle such huge rail traffic on this bridge which resulted in discontinuation of rail tracks and two more lanes for traffic were introduced at its place. After that the upper deck was also made five lane and was being used a track for traffic moving back from San Francisco to the other end, whereas, the traffic for San Francisco moved on the lower deck. During the same time, the speed of vehicles moving on Bay Bridge was changed. The reason behind this change was the heavy traffic on the road and the initial design of Bay Bridge was not strong enough to accommodate such heavy traffic at the speed of 45miles per hour and was reduced to 40 miles per hour. During the construction of Bay Bridge, ferries were extensively used to carry the commuters to the workplace and for the transportation of construction blocks at the sight of construction. This is the aerial of the bridge under construction and ferries are working to transport worker and material. Engineers really worked hard to construct a master piece of that time in limited resources and limited technological resources. Here the engineers are busy in ensuring constructional quality of the bridge. In 1989, an earthquake of magnitude 7.1 hit the bay area. This resulted in the destruction of Upper Deck at pier E9. This also resulted in the failure of lower deck and one person was murdered in this incident. This incident took place because of the rigid truss structure in the eastern part of the bridge. The other half of the bridge was remained safe as it was based on suspension system. After this destruction the bridge was kept closed for one month for the repair of the affected part and engineers started working for more reliable design of the bridge. Bay Bridge after being repaired: Modern Day Design of Bay Bridge After the earthquake of 1989, Civil engineers and architects were continuously busy in producing a long lasting, more reliable and flexible design for the bay bridge. Also they were trying to accommodate more traffic with the increasing population and reduced distances. Finally they reached at the conclusion of retrofitting the western span of the bridge and replacing the eastern span with self-anchored suspension bridge to overcome future earthquakes. The destruction in Prieta Loma earthquake of 1989 resulted in the realization of the fact that the part of the Bay Bridge designed as trusses was not flexible enough to resist any sort of unpleasant situation. This earthquake damaged the bridge as Oakland side of the bridge was moved 7inches towards east. This caused in excessive shear in the bolts of a section of the bridge which resulted in the damage of that part and hence, crushing of the lower deck as well. Therefore, it was needed to redesign the bridge or improve its design. The design engineers, after conducting a number of meetings, reached to a conclusion that development of current design to a flexible and more reliable one is much difficult. Therefore, it is easy to reconstruct the whole eastern span with new design by implementing more flexible technique. The engineers then came up with the most reliable designed which was never used before. This design was named as Self Anchored Single Tower Suspension bridge. Meanwhile the western span of the Bay Bridge was also retrofitted by making use of proper changes in the design of the suspension bridge to overcome seismic currents according to the latest standard. This was done by making use of excessive steel and concrete in the existing design. Entire western span was upgraded for facing more vigorous seismic activities. Therefore, steel was used in massive amount along with concrete to make suspensions tougher in seismic activities. Also the isolators and diffusers were incorporated in the bridge design to make it more reliable. The new Eastern Span of the bridge was designed as the world’s longest self-anchored suspension bridge. This design involved the design and production of specialized shear links and hinge pipes which act as shock absorbers and was designed to withstand more than 1500 years. Most of the task was based on retrofitting the existed nut and bolt design. Also the main focus was on developing an excellent twin suspension spans for holding the bridge in place. It was done by adding extensive number of steel plates to the spans in order to give strength and flexibility to the spans of the bridge. More than 17 million of steel was used in the construction of SAS. The deck of the bridge was braced with the latest design and also the diagonals connecting the lower and the upper deck were replaced with specially designed perforated steel plates. Concrete jackets and anchoring bolts were incorporated to fasten the pedestals with the tower legs. As far as bay bridge cables are concerned, they were also composed of 17000 individual strands. Pictorial view of the cross section of the bay bridge cables is shown below. The Future Although the current design of bay bridge is one of the master piece of current era but still it consists of a number of significant constructional drawbacks, inn which most common and threatening issue is the leakages which are being observed in the steel structure of the eastern span of the bridge. Currently it has been noticed that the newly designed bay bridge span is having internal leakages which may corrode the steel structure and can weaken the strength of bridge and reduce its life, which was actually structured to live for 1500 years(Derbeken, 2014). Conclusion With the advent in technology, civil engineering structures have improved a lot and it was only possible because of a strong teamwork. Current design of the bay bridge is the most fabulous piece of architecture and design engineers of this era. The new addition of SAS Bridge has not only facilitated San Francisco with an excellent design but also the most reliable design of the bridge. The new structure is capable is of withstanding more seismic activities as well as reliable enough to work as an emergency route in case of any unpleasant situation. Such mega project would have never been so successful without the coordination of public, high authorities and political personnel, hardworking and honest team and successful leaders. References DERBEKEN, J. V. 2014. Bay Bridges new problem: leaks. SFGate. Read More