Truss bridge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Truss_bridge Truss bridge From Wikipedia, the free encyclopedia Truss bridge A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials. Truss bridge for a single track railway, converted to pedestrian use and pipeline support Ancestor Beam bridge Contents Related NONE 1 Design Descendant Cantilever bridge, truss arch 2 History in the United States bridge, transporter bridge, lattice 3 Roadbed types bridge 4 Truss types used in bridges Carries Pedestrians, pipelines, 4.1 Allan truss 4.2 Bailey bridge automobiles, trucks, light rail, 4.3 Baltimore truss heavy rail 4.4 Bollman truss Span range Short to medium - Not very long 4.5 Bowstring arch truss (Tied arch bridge) unless it's continuous 4.6 Brown truss 4.7 Brunel Truss Material Timber, iron, steel, reinforced 4.8 Burr Arch Truss concrete, prestressed concrete 4.9 Cantilevered truss Movable May be movable - see movable 4.10 Fink truss 4.11 Howe truss bridge 4.12 K truss Design effort Medium 4.13 Kingpost truss 4.14 Lattice truss (Town's lattice truss) Falsework Depends upon length, materials, 4.15 Lenticular truss required and degree of prefabrication 4.16 Long truss 4.17 Parker (Camelback) truss 4.18 Pegram truss 4.19 Pennsylvania (Petit) truss 4.20 Post truss 4.21 Pratt truss 4.22 Queenpost truss 4.23 Truss arch 4.24 Waddell truss 4.25 Warren (non-polar) truss 4.26 Whipple Pratt truss 4.27 Vierendeel truss 5 References 1 of 11 1/26/2010 4:09 PM Truss bridge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Truss_bridge 6 Footnotes 7 External links Design The nature of a truss allows for the analysis of the structure using a few assumptions and the application of Newton's laws of motion according to branch of physics known as statics. For purposes of analysis, truss are assumed to be pin jointed where the straight components meet. This assumption means that members of the truss (chords, verticals and diagonals) will only act in tension or compression. A more complex analysis is required where rigid joints impose significant bending loads upon the elements, as in a Vierendeel truss. The integral members of a truss bridge [1] In the bridge illustrated in the infobox at the top, vertical members are in tension, lower horizontal members in tension, shear, and bending, outer diagonal and top members are in compression, while the inner diagonals are in tension. The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord (a horizontal member of a truss) is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated, but with additional strength added to other members in compensation. The ability to distribute the forces in various ways has led to a large variety of truss bridge types. Some types may be more advantageous when wood is employed for compression elements while other types may be easier to erect in particular site conditions, or when the balance between labor, machinery and material costs have certain favorable proportions. The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Modern materials such as prestressed concrete and fabrication methods, such as automated welding, and the changing price of steel relative to that of labor have significantly influenced the design of modern bridges. History in the United States Because wood was so abundant, early truss bridges would typically use carefully fitted timbers for members taking compression and iron rods for tension members, usually constructed as a covered bridge to protect the structure. In 1820 a simple form of truss, Town's lattice truss, was patented, and had the advantage of not requiring high labor skills nor much metal. Few iron truss bridges were built in the United States before 1850. Truss bridges became a common type of bridge built from the 1870s through the 1930s. Examples of these bridges still remain across the United States, but their Warren-type through-truss of numbers are dropping rapidly, as they are demolished and replaced with new the former Seaboard Air Line structures. As metal slowly started to replace timber, wrought iron bridges in the Railway, located near the U.S. started being built on a large scale in the 1870s. Bowstring truss bridges were village of Willow, Florida; abandoned since the a common truss design seen during this time, with their arched top chords. mid-1980s Companies like the Wrought Iron Bridge Company of Canton, Ohio and the King 2 of 11 1/26/2010 4:09 PM Truss bridge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Truss_bridge Bridge Company of Cleveland, Ohio became well-known companies, as they marketed their designs to different cities and townships. The bowstring truss design (photo (http://bridgehunter.com/ks/cloud/county-line/) ) fell out of favor due to a lack of durability, and gave way to the Pratt truss design, which was stronger. Again, the bridge companies marketed their designs, with the Wrought Iron Bridge Company in the lead. As the 1880s and 1890s progressed, steel began to replace wrought iron as the preferred material. Other truss designs were used during this time, including the camel-back. By the 1910s, many states developed standard plan truss bridges, including steel Warren pony truss bridges. As the 1920s and 1930s progressed, some states, such as Pennsylvania, continued to build steel truss bridges, including massive steel through truss bridges for long spans. Other states, such as Michigan, used standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built. Roadbed types The truss may carry its roadbed on top, in the middle, or at the bottom of the truss. Bridges with the roadbed at the top or the bottom are the most common as this allows both the top and bottom to be stiffened, forming a box truss. When the roadbed is atop the truss it is called a deck truss (an example of this was the I-35W Mississippi River bridge), when the truss members are both above and below the roadbed, a through truss (an example of this application is the Pulaski Skyway), and where the sides extend above the roadbed but are not connected, a pony truss or half-through truss . Sometimes both the upper and lower chords support roadbeds, forming a double-decked truss . This can be used to separate rail from road traffic or to separate the two directions of automobile traffic and so avoiding the likelihood of head-on collisions. Sky Gate Bridge R at Kansai International The four span through truss Airport, Osaka, Japan, is the Deck truss railroad bridge General Hertzog Bridge longest double-decked truss Pony truss bridge of over the Erie Canal in over the Orange River at bridge in the world. It reinforced concrete Lockport, New York Aliwal North carries carries three lanes of vehicular traffic. automobile traffic on top and two of rail below over nine truss spans. Truss types used in bridges See also: List of truss types Bridges are many times the best visible examples of truss use to the common person. There are many types of designs, many dating back hundreds of years. Below are some of the more common types and designs. 3 of 11 1/26/2010 4:09 PM Truss bridge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Truss_bridge Allan truss The Allan Truss , designed by Percy Allan, is partly based on the Howe truss. The Hampden Bridge in Wagga Wagga, New South Wales, Australia, the first of the Allan truss bridges, was Allan Truss illustrated originally designed as a steel bridge. It was constructed with timber to reduce cost. In his design, Allan used Australian ironbark for its strength. [2] A similar bridge also designed by Percy Allen is the Victoria Bridge on Prince Street Picton, New South Wales. Also constructed of ironbark, the bridge is Hampden Bridge showing the Allan truss design still in use today for pedestrian and light traffic. Bailey bridge Main article: Bailey bridge Designed for military use, the prefabricated and standardized truss elements may be easily combined in various configurations to adapt to the needs at the site. In the image at right, note the use of doubled prefabrications to adapt to the span and load requirements. In other applications the trusses may be stacked vertically. Bailey bridge over the Meurthe River, France. Baltimore truss The Baltimore truss is a subclass of the Pratt truss. A Baltimore truss has additional bracing in the lower section of the truss to prevent buckling in the compression members and to control deflection. It is mainly used for train bridges, boasting a simple and very strong design. Bollman truss 4 of 11 1/26/2010 4:09 PM Truss bridge - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Truss_bridge Main article: Bollman Truss Railroad Bridge The Bollman Truss Railroad Bridge at Savage, Maryland is the only surviving example of a revolutionary design in the history of American bridge engineering.