1、Proceedings of Bridge Engineering 2 Conference 2007 27 April 2007, University of Bath, Bath, UK CRITICAL ANALYSIS OF THE MANHATTANBRIDGE Julian Staden Departement of Civil and Architectural Engineering, University of Bath Abstract: This paper provides detailed information about the different aspects
2、 of the design of the ManhattanSuspension Bridge. Analytical reasoning is given to why each design feature was designed in the manor it was in order to fulfil the engineers design criteria. An attempt is also made to illustrate any shortcoming in the design of the structure and any ways in which the
3、 engineers could have potentially improved the design of the bridge. Attention will also be paid to the ways in which the bridge would be designed and constructed if it were to be built in the 21st century. The reasons why design principals and construction methods have changed will also be outlined
4、. Keywords: truss, stiffness, tension, deck, torsion Figure 1: ManhattanBridge. View from Manhattan side. 1 Overview of The ManhattanBridge The ManhattanBridge, although unfinished, was opened in 1909 and was the third bridge to span the East River running between Brooklyn and Manhattan. It was buil
5、t to provide another transport link between the two boroughs.Before construction began there was a great deal of controversy over the proposed designs for thebridge.Eventually a fairly typical looking suspension bridge design was approved that was designed by engineer Leon Moisieff. The bridge was t
6、o span 448metres between piers, which was a somewhat shorter distance than the main spans of either the Brooklyn or Williamsburg bridges that neighbore it.Even though the ManhattanBridge made no advancement in the spanned length achievable by a suspension bridge, it did represent a significant stepf
7、orward in the progression of bridge building. The bridge was the first to be designed using deflection theory, which quickly replaced all previous methods of bridge design. The design also incorporated steel towers that were only braced in two dimensions rather than three. This was another aspect of
8、 bridge design that had not been seen prior to the building of the ManhattanBridge. The entire structure of the Manhattan suspension bridge was to be designed in steel. The deck structure is comprised of four steel stiffening trusses that were each supported by regularly spaced suspenders that hang
9、from one of the four main cables. The four cables are supported by the two towers and are held down by anchorages 224metres from each side of the main span. 2 Designing The Structure The Structure The Manhattan Bridge was the first suspension bridge to be designed using deflection theory in calculat
10、ing how the horizontal deck and curved cables worked together to carry loads. Until this point, suspension bridges could only be designed using elastic theory which meant assuming small deflections. However, whilst this was an incorrect approximation, as suspended structures were sometimes observed
11、to undergo significant deflections, this was the only mathematical modelling that had been applied up to that time. At the time the ManhattanBridge was designed, this was the first occasion there had been an appropriate opportunity to use Melans deflection theory. This new method of analysis allowed
12、 the engineers to design the bridge with a more accurate understanding of how it would actually perform, allowing a greater economy of material usage. Deflection theory meant that all suspension bridges were proved to be stronger than previously considereddue to the curve in the main cables being mo
13、re efficient acarrying loads than stiffer forms of bridge. The new theory allowed the ManhattanBridge to be designed to belighter, with smaller stiffening trusses, than it otherwise would have been using elastic theory. These smaller lighter trusses would have been acceptable in normal circumstances
14、 .for example,if the bridge was to carry onlyvehicular traffic, but in the case of the ManhattanBridge,it was to also carry subway trains. Despite the application of the new theory, after the initialise of the bridge, it became clear that there was a significant flaw in its design. 2.1 Inadequacy of
15、 Design Each time a heavy subway train passed over the bridge, it caused local deflection of the stiffening trusses on the side of the deck of the trains passage. The deflection on one side induced torsion in the deck. This problem was further accentuated when trains began to pass over each side of
16、the bridge in opposing directions atthe same time. It was reported that at this point, each sideof the deck deflected by up to 1.2 meters, meaning a total relative deflection of over 2 meters. This put significant stresses into the bridge deck and subsequently lead to extensive repairs and stiffenin
17、g work needing to becarried out in order to allow the bridge to remain serviceable. In my mind, the most obvious failing in the design of the Manhattan Bridge is the location of the tracks subway .They have been located at each edge, rather than placing them in the central section of the deck, with
18、road lanes separating them. Keeping the tracks close together would have reduced the length of the effective lever arm that the trainss live loading would have had, reducing the torsion moment induced. Although I am uncertain as to the design engineers reasoning behind locating the subway tracks in these positions, a possible reason becomes apparent when looking at a cross section of the deck.
