Triborough Bridge Project - Case Study Example

Triborough Bridge Project The Triborough Bridge Project is situated in New York The bridge, which is officially known as Robert F. Kennedy Bridge, is a complex project consisting of three long-span bridges, small bridges, overpasses, approach highways, and recreational facilities. The system of bridges connects the boroughs of Manhattan, Bronx, and the Queens. Although the construction plans for the project started in 1916, the real work was delayed until 1920s because of the city’s financial constraints. Indeed, the city started allocating funds to the project in 1925 for surveys, but the real construction began in 1929. Despite the city’s financial constraints, the project was completed in 1936 (Gandy 288). The combined length of the system of the bridges including the approach highways is 6 kilometers. The major sections are the suspension bridge, vertical lift bridge, truss bridge, and the viaduct. The suspension bridge across the East River has a span of 421 m. The vertical lift bridge connecting Manhattan and Randall’s Island has a span of 94m. The truss bridge connecting Bronx and Randall’s Island has a span of 117 m. The viaduct connecting Randall’s and Wards Islands has a span of 4, 115 m (New York Roads). The Triborough Bridge Project was constructed by the motorized American civilization. The industrial revolution had spread to America and companies such as Ford were already mass-producing cheap cars. American economy was also booming, and an increasing number of people could afford luxurious lifestyles. Since the 1920s-generation comprised people who were against the American traditional ways, luxury and leisure activities took new turns. Thus, the need for outstanding construction projects that could provide the desired leisure was increasing. Hence, the construction of the Triborough Bridges was a new sign of civilization (University of Houston). Much of the planning and initial construction of the Triborough Bridge Project was done during the roaring 1920s. At that time, a significant number of Americans had moved to the cities to find new jobs. Most of the blue-collar jobs had been replaced by white-collar jobs such as public service, law, and personal enterprises. Because of the economic boom, most Americans particularly the middle class could afford the Henry Ford’s model T vehicles. Leisure activities among this social class increased. Thus, a generation that could travel further and further was created. Hence, a culture that was obsessed in traveling using motor vehicles emerged (University of Houston). When the construction work started, the Great Depression occurred. The New York Stock Exchange collapsed triggering mass unemployment. Thus, a significant portion of the people who had migrated to cities suffered the consequences. Most of them lost their jobs. The project was initially financed through the sale of New York City bonds, but most people were unable to buy the bonds because of poverty. Some people never trusted the bonds because they were no longer stable. Since New York City was unable to sell its bonds, funds for the project became scarce. As a result, the city was forced to suspend the Triborough Bridge Project from 1930 to 1933. Nevertheless, new funds in the form of federal government loans became available, raising hopes of the completion of the project. Because the federal funds were limited, the project had to be scaled down from a double decker to single bridge (Schoolman & Magid 238-239). How the system of bridges was built The Triborough Bridge Project was built with different designs at various sections. The construction materials were mainly steel and concrete. The choice of materials highly depended on the section of the rivers to be crossed. For the longest span, a suspension steel bridge was constructed because it was virtually impossible to construct any other bridge. Although the suspension bridge construction is expensive, it is still the most suitable way of building long-span bridges. For the shorter spans, vertical lift bridge, truss bridge, and viaducts were constructed. Concrete and steel were the main materials needed in the construction. For the suspension bridge across the East River, the structure required cables, deck units, hangers, towers, and anchorages. The suspension bridge cables were hung over two towers. The deck units were suspended from the cables and joined together to form a strengthening girder. The ends of the cables were fastened to the earth. The girders were fitted with movable hinges at the towers and fixed ones at the piers. Thus, when a passing traffic load is on the center span, it causes the deflection of the deck, pylons, and the suspension cables. The purpose of the towers is to enable the cables to be draped over long distances. A significant portion of the weight of the bridge is supported by the cables, which are fastened to massive concrete blocks at both ends. The load is evenly distributed by spreading the cables over a large area inside the anchorages. Because the project was carried out in the early 20th century, advanced construction tools were available for use. The industrial revolution had reached America, and people had turned to advanced ways of production. For instance, cranes were already available and frequently used in construction works. The steel smelting industry was also working. Mining tools and equipment were available. The surveying equipment used was optical devices capable of mapping large areas. Thus, the tools and technology available was used to produce provide the materials needed. For example, the steel smelting factories were used to forge the steel to required shapes and sizes. Cranes were used to lift heavy materials such as the steel decks. The main methods of construction were suspension and arch bridges. Despite the availability of advanced technology, the construction methods were limited to what the 1930s technology could solve. For instance, there were no pre-stretched beams for the decks. The construction of the overpasses also relied mostly on steel instead of concrete. Therefore, nearly the entire project involved construction of towers and anchorages, fixing the deck units, filling the decks with concrete, and fixing the cables (New York City). The residents of New York provided the labor. When the construction was underway, the US was facing an economic crisis following the 1929 crash of the stock exchange. Since the federal government was desperate to revive the economy, it provided loans to state governments under the New Deal. The funds were supposed to finance major or minor public works. Because New York City desperately needed to complete the stalled project, it used its share of the federal funds to finance the construction of the bridge. Thousands of New York residents worked on the construction site the entire construction phase. Most of these residents provided the unskilled labor. The skilled labor came from the city engineers, who supervised various sections of the construction (New York City). Personal analysis of rebuilding the structure Modern bridges are constructed using steel and concrete. When choosing between concrete and steel, factors such as strength, overall weight, and costs of the bridge are put into consideration. The use of steel significantly reduces the weight of the bridge, but raises the overall expenses. Thus, if the bridge is to be constructed over a long span of river, steel would be appropriate because it is lighter than concrete. On the other hand, the use of concrete reduces the expenses but it lowers the strength of the bridge. Concrete also increases the total weight of the bridge itself. However, if the bridge is short, concrete would be suitable than steel because it is cheap. Apart from the costs and weight of the bridge, other factors such as the nature of the ground are also considered. If the ground is soft, a deep hole will be drilled until the bedrock is exposed. Drilling deep foundations and subsequent construction of pillars requires long steel bars and significant quantities of concrete. Thus, the erection of pylons could be expensive. When solving such a problem, engineers consider increasing the distance between successive pylons. However, increasing the distance between the pylons also limits the selection of materials for constructing the bridge. If the Triborough Bridge Project were to be constructed today, the available materials and technology would significantly reduce the cost. For instance, some sections of the bridge could be built using the cantilever method. In this case, the bridge structure is erected piece by piece. Here, the whole framework is supported by the previously completed sections. Thus, the bridge can hold itself in its position throughout the construction process. While using the cantilever method, great care must be taken to minimize unbalancing the entire structure. Although it is complex in terms of technology, the cantilever method is cheap because it reduces the cost of materials (Chauhan, Beniwal, Tarun, & Mkonda 12). The construction of the cantilever requires the use of falsework. The falsework supports one section of the deck while the other section is reinforced by a counterbalance anchored from the ground at each pier. In order to ensure stability, temporary strutting is used on the counterbalancing side. While constructing successive cantilevers using this method, the counterbalance should always be maintained. A different method of constructing a cantilever exists. If the bridge level is very low, the two end spans can be cast on falsework. The method is cheaper because there is no need of a counterbalance. For instance, for monolithic bridge decks and piers, the temporary struts at the counterbalances can be eliminated. Alternatively, a simpler construction method can be used. It involves assembling a span away from the construction site before transporting to the bridge site. The span is mainly made of concrete reinforced with steel. At the construction site, the span is placed in its position as a single piece. In order to limit the stress caused by the extending cantilever, a movable temporary tower and stay system is placed on the freely-hanging side. For wider sections, very long bridges are necessary. The construction of such long bridges can be achieved in two ways. First, a bridge comprising concrete deck reinforced with steel is constructed. Second, steel box girders can be constructed to form the bridge. For the first case, piers should be constructed at specific intervals. The piers, which are mainly made of concrete, support the weight of the bridge together with the passing traffic load. The erection of piers requires solid, stable ground, which can be achieved by drilling the earth until the bedrock is exposed. A strong foundation is then laid by erecting steel cables and filling it with concrete. For the second case, the selection of steel box girders is preferred because of its strength. The boxes themselves are rigid; they can withstand both tensional and compressional forces. The surface of the deck is filled with a thin layer of concrete. When fabricating the steel girders, care should be taken to minimize distortions and locked-in stresses. Distortions and locked-in stresses can prevent the girders from fitting accurately into their positions. Thus, it is necessary to fabricate the girders under controlled conditions to minimize the occurrence of faults. While fitting the steel framework, temporary bolts can be used to locate the girders accurately in their position. The framework is then welded to strengthen the structure (Chauhan, Beniwal, Tarun, & Mkonda 14). The suspension bridges built in the 1930s can be replaced with cable-stayed bridges. Although suspension bridges and cable-stayed bridges use cables to support the deck, their difference lies in the way the cables are attached to the deck and towers. For instance, in a suspension bridge, the cables move freely across the towers. In the process, they transmit the load to the anchorages at both ends. However, the design of cables is much different in a cable-stayed bridge. The cables in this category of bridges are attached to specific towers, which support the load independently (Chauhan, Beniwal, Tarun, & Mkonda 15). In the cable-stayed bridges, there are two ways in which the cables can be attached to the deck. First, there is a radial pattern whereby the cables are attached at different sections of the road and anchored at a single point near the top of the tower. Second, there is the parallel pattern whereby the cables are attached at different sections of the roadway and anchored at different heights of the tower, parallel to one another. It is more economical to construct a cable-stayed bridge than a suspension bridge. The former uses fewer cables than the latter. They can also be built out of duplicate pre-cast concrete sections. In addition, they are faster to build. Moreover, cable-stayed arrangement provides compression in the deck using the weight of the bridge itself. In this case, the deck imitates a pre-stretched beam. Normally, a pre-stretched beam is created by filling stretched steel beams with concrete. When the structure dries up, the tension in the steel is released. The stretched steel then forces the concrete to compress. Since concrete is strong in compression, the structure can withstand its weight together with that of the traffic. The same effect can be achieved cheaply by using a cable-stayed bridge (Chauhan, Beniwal, Tarun, & Mkonda 15-16). In the cable-stayed arrangement, tensile forces in the flange can be eliminated by spacing the cables such that the horizontal components of the forces cancel out. If the deck is filled with concrete, the joints can be glued in order to resist any shear forces. Steel joints can also be welded. For very long bridges that require many cables, the flange can be pre-stressed in order to eliminate tensile stresses. In this way, the number of cables needed to anchor the deck to the poles can be reduced. Eventually, the vertical force components are transmitted through the cables to the foundation. Unlike the Triborough Bridge Project, modern bridges use cellular steel box sections instead of trussed girders. The 1930s trussed girders were erected in small pieces or assembled into large pieces and lifted into position. Their major disadvantage was the high costs of fabrication. Trussed girders were also inefficient in terms of steel usage. In addition, they required intensive labor to erect and maintain. Thus, cellular steel box sections were preferred over trussed girders because they are cheap to fabricate. They can also be delivered quickly to the construction site. In addition, they can be fastened easily with bolts or welding. If the Triborough Bridge Project were to be constructed today, its rock anchorages could be replaced with gravity anchorage system. Gravity anchorage is the most efficient way of fixing the ends of the cables. It comprises looped over strand shoes fixed to anchor bolts in the concrete. Thus, a combination of friction, dead weight, and overburden resists the cable forces. If the Triborough Bridge were to be rebuilt today, its steel arch bridge section could be replaced by a concrete one. Currently, arch bridges are constructed using concrete pre-reinforced with steel. Such bridges can span up to 800 feet. While erecting an arch bridge, it is necessary to consider the technique of supporting the voussoirs (Chauhan, Beniwal, Tarun, & Mkonda 6). A modern reconstruction of Triborough Bridge Project would replace rivets with welded joints and bolts. Some of the steel joints in the bridge were fixed with rivets. Although rivets are strong, they have a major disadvantage while fitting. They are supposed to be heated before they are fixed in joints. The problem with heating is that the steel structure can be damaged, leading to a weakened rivet. A hot rivet may not fit properly into the intended rivet hole. Thus, a reconstruction of the Triborough Bridge can be done better with welded joints. In 1920s, there were no much-developed welding technologies compared to the present day. The steel framework could only be fabricated at the factory. However, the modern advanced welding technologies enable flawless fixing of joints. In summary, the modern reconstruction of Triborough Bridge Project would focus on reducing costs, increasing stability, and prolonging the bridge’s life. Though the construction materials are similar, new techniques such as pre-stretching of beams could be used to improve the strength of the bridge. The availability of modern welding technologies also enables the fabrication of steel at the construction site instead of industry level alone. In addition, the suspension bridge could be replaced with cable-stayed bridge, which is of a similar strength but uses less steel. Moreover, trussed girders could be replaced with cellular steel boxes, which are not only light but also cheap to maintain. Finally, some steel sections of the bridge could be replaced with concrete reinforced with steel. Works Cited Chauhan, K.C., Beniwal, Tarun, and Mkonda, K.K. “Modern Practice on Bridge Construction on World Railways.” N.p. n.d., Web. 12 November 2014. . Gandy, Matthew. Concrete and Clay: Reworking Nature in New York City. New York: MIT Press, 2003. Print. “Overview of the 1920s.” University of Houston. 2014. Web. 12 November 2014. . “Robert F. Kennedy (Triborough) Bridge.” New York Roads. n.d. Web. 12 November 2014. . Schoolman, Morton and Magid, Alvin. Reindustrializing New York State: Strategies, Implications, Challenges. New York: University of New York Press, 1986. Print. Read More