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New Bridge Construction Materials
Materials with improved characteristics will be used that will make the bridge construction safe, durable, and reliable. Materials like high-performance concretes, polymer concretes, and plastics will be utilized. As the fiber reinforced composites are becoming more tolerant towards temperature, they will be used extensively for bridge construction. Use of larger steel fibers will be used in the tensioned members. The economics of future bridge construction will implement a simple design, with an increased interface between design, erection, and maintenance. Progressive study in modern superior materials and management techniques will facilitate the construction of durable structures that do not require extensive maintenance.
The Stillwater Bridge, featuring a counterweighted, cable-and-tower design, embodies engineering significance as a rare surviving example of vertical-lift highway bridge construction of the Waddell and Harrington type. The significance of the Stillwater Bridge is best evaluated within the general context of Minnesota and Wisconsin movable highway bridges.
Movable bridges, also known as drawbridges, are constructed over navigable waterways when it is impractical or uneconomical to build fixed bridges of sufficient height to permit the passage of vessels. Human ingenuity has devised numerous systems for lifting, dropping, folding, rotating and retracting a span to provide temporary clearance. By the early 20th century, however, engineers had focused their attention on three, basic drawbridge categories: swing, bascule and vertical lift. Briefly defined, a swing span revolves in a horizontal plane around a vertical axis, a bascule span rotates in a vertical plane around a horizontal axis and a vertical-lift span rises and descends in a vertical plane.
In Minnesota and Wisconsin, as well as elsewhere in the nation, virtually all 19th century movable bridges were of the swing-span variety and the type continued to be constructed during the early 20th century. As late as 1935, a total of 51 highway swing spans were in operation in the Minnesota and Wisconsin. Not one of these structures survives. The demise of the highway swing span was nation-wide, reflecting its growing incompatibility with an urban setting. There were two basic problems with swing spans. First, the central pivot pier increasingly became an obstruction to navigation for the ever-larger vessels of the late 19th and early 20th centuries. Second, the swing span itself squandered valuable space. By requiring a clear turning radius, it prohibited the development of docking facilities adjacent to the bridge site. These shortcomings were especially onerous along highly industrialized urban waterways, where shipping channels tended to be narrow, highway crossings numerous and real estate prices high. For less crowded sites, the swing span remained a viable form of technology well into the 20th century, Most surviving swing spans, for example, are railroad bridges in rural regions or in relatively uncongested urban areas. But in the downtown waterfronts of late 20th century American cities, the swing span was marked for extinction. Its major adversary was the federal government.
No matter how loudly shipping and real-estate interests might denounce the swing span, there was no effective means of regulating movable-bridge design until the early 1890s, when Congress authorized the War Department to approve plans for all new bridges over navigable waterways and to seek the alteration of any existing bridge that interfered with "reasonably free, easy and unobstructed" navigation. In 1892, the War Department sent a clear message of future policy by way of Chicago, demanding the removal of a two-year-old swing span from one crossing of the Chicago River and denying permission to build a new swing span at another. The search for an alternate drawbridge technology began in earnest. Not surprisingly, Chicago was in the vanguard. In 1895, municipal authorities spanned the Chicago River at South Halsted Street with the world's first, modern vertical-lift bridge.
During the middle decades of the 19th century, an occasional vertical-lift span was constructed in Europe and the United States. Although their engineering was often ingenious, the bridges themselves were quite modest, designed mainly for canals and small navigable streams in cases where it was only necessary to lift the spans a few feet to clear traffic in the channels. The modern, long-span, high-rise vertical-lift bridge dates from the last decade of the 19th century. In 1892, Duluth, Minnesota, hosted a design competition for constructing a drawbridge over its harbor entrance on Lake Superior, which comprised a clear channel 250 feet in width. Under the rules of the competition, the successful design would leave the entire width of the canal free to passing vessels, which effectively eliminated traditional, center-pier swing spans.
Most responses to the Duluth competition employed some form of "sliding draw" mechanism, whereby the span moved back and forth on rollers. A striking exception was a design submitted, and later patented, by John Alexander Low Waddell (1854-1938). Waddell was a consulting engineer based in Kansas City, Missouri, who, during the next 40 years, would become one of the best-known bridge engineers in the United States. Waddell proposed to build a vertical lift bridge consisting of a simple truss span 260 feet long so constructed and supported as to allow of being raised vertically to a height of 140 feet above the surface of the canal. The Engineering News, October 27, 1892, reports on the Waddell entry in the design competition...
At each end of the movable span is a tower 170 ft. high, carrying at its top built steel pulleys about 15 ft. in diameter. Over these pulleys steel wire ropes, or chain cables, pass. One end of each cable is attached to the end piers of the trusses, and end to counter-weights which exactly balance the dead weight of the span. The only work left for the operating machinery is, therefore, to overcome the weight due to dirt, water, snow, etc. The power for operating the bridge is supplied by two electric motors placed at mid-span; the upward and downward motion being regulated by racks and pinions communicating with the power by means of steel shafting and spur and miter wheels.
Although the Duluth authorities selected Waddell's design, the War Department vetoed the construction of any drawbridge at the site at that time. Waddell, however, had devised a seemingly practical solution to the drawbridge problem. His vertical-lift navigation and dockage like a swing span, nor did it clutter up span-did not obstruct the shore approaches like a sliding-draw span. A few months after the cancellation of the Duluth project, the City of Chicago commissioned Waddell to modify his original design for a 130-foot span capable of 150-foot clearance over the Chicago River at South Halsted Street. This structure was completed in 1894.
The South Halsted Street Vertical-Lift Bridge remained the only example of its kind for over a decade. In later years, Waddell commented in the Journal of the Western Society of Engineers, May, 1924, that the long delay in constructing another vertical lift to the knavery of those in charge of subsequent bridge projects, who, as he put it, "demanded boodle...a condition with which [I] never did and never will comply." There were other reasons as well. During the period 1895 to 1905, engineers in Chicago and Milwaukee perfected several bascule designs, which were widely believed to be more economical for narrow waterways than Waddell's vertical lift. The new type received early and strong endorsement from the City of Milwaukee, which built 10 bascule spans between 1902 and 1910. It was subsequently adopted as the preferred movable-bridge type by the Wisconsin State Highway Commission, organized in 1911 to improve the state's roads and bridges. But the greatest obstacle to the initial acceptance of the vertical-lift span was the fact that the South Halstead Street Bridge contained certain mechanical flaws, which gave it the reputation for heavy first cost and maintenance and expensive operation.
In 1907, Waddell formed a partnership with John Lyle Harrington (1868-1942), a skilled civil and mechanical engineer who was largely responsible for reworking Waddell's invention into a rational, well-integrated design. In its essential form and dynamics, the "Waddell and Harrington version" remained true to the original 1892 design. Before the partnership dissolved in 1914, Waddell and Harrington designed about 30 vertical-lift spans for highway and railroad crossings. After they parted company, both men continued to work in the field, and Harrington's new office, Harrington, Howard, and Ash, became particularly well known, as was its successor, Ash, Howard, Needles and Tammen. Six vertical-lift highway bridges were constructed in Minnesota and Wisconsin before World War II. At least 5 were designed by Waddell and Harrington or successor firms. All were of the standard Waddell and Harrington type. The 1931 Stillwater Bridge was the last of this cohort to be completed. Its predecessor at the site was a timber, pontoon, Swing Bridge built in 1910. Owned and maintained by the City of Stillwater, the bridge was taken over by the Minnesota Department of Highways in 1925. By that time, the structure was fast deteriorating so as to be a source of apprehension for the safety of the loads it is obliged to carry. When the bridge was closed to heavy traffic in 1928, the Minnesota Department of Highways prepared preliminary plans for its replacement. These plans called for a series of fixed concrete-slab and steel-truss spans, which were to be designed by the Minnesota highway agency itself, and a single vertical-lift span, which was to be the responsibility of an engineering firm specializing in such work. In November, 1929, a design contract for $3,150 was awarded, on a competitive basis, to Ash, Howard, Needles and Tammen of Kansas City, Missouri. Construction on the bridge proper began the following summer, with the Minneapolis firm of Peppard and Fulton serving as general contractor and the American Bridge Company (Minneapolis and Gary plants) serving as fabricator. The project was completed in August, 1931, for a total cost of $460,174, shared on an approximately 50-50 basis by the states of Minnesota and Wisconsin.
At the time of the bridge's completion, the St. Croix River was only lightly used as a navigable waterway. Since most of the traffic was small craft, there was little occasion to operate the lift span, as the Minnesota Department of Highways noted in a 1938 letter, "for several years not a single request for its opening was received." Although the bridge was far more intensely involved in highway traffic, it was in the role of maintaining, rather than initiating, patterns of transportation, which, in fact, were already well established by the 1930s. The bridge does have significance, however, as a rare type of engineering construction. Only six vertical-lift highway bridges were built in Minnesota and Wisconsin prior to World War II, and the Stillwater Bridge is one of three that still survives.
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