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Truss bridge
From Wikipedia, the free encyclopedia
| Truss bridge | |
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| Truss bridge for a single track railway, converted to pedestrian use and pipeline support | |
| Ancestor: | Beam bridge |
| Related: | None |
| Descendant: | Cantilever bridge, truss arch bridge, transporter bridge, lattice bridge |
| Carries: | Pedestrians, pipelines, automobiles, trucks, light rail, heavy rail |
| Span range: | Short to medium |
| Material: | Timber, iron, steel, reinforced concrete, prestressed concrete |
| Movable: | May be movable - see movable bridge |
| Design effort: | Medium |
| Falsework required: | Depends upon length, materials, and degree of prefabrication |
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. This type of bridge structure has a fairly simple design and is particularly cheap to construct owing to its efficient use of materials. For purposes of analysis most truss bridges may be considered to be pin jointed where the straight components meet. A more complex analysis may be required where rigid joints impose significant bending loads upon the elements.
In the bridge illustrated in the infobox at right, 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.
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.
A few iron truss bridges were built in the United States before 1850. Bridges based on the Bollman truss (patented in 1852) were used successfully by the Baltimore and Ohio Railroad. Truss bridges became a common type of bridge to see built from the 1870s through the 1930s. Examples of these bridges still remain across the United States, but their numbers are dropping rapidly, as they are demolished and replaced with new structures. As metal slowly started to replace timber, wrought iron bridges in the U.S. started being built on a large scale in the 1870s. Bowstring truss bridges were a common truss design seen during this time, with their arched top chords. Companies like the Wrought Iron Bridge Company of Canton, Ohio and the King Bridge Company of Cleveland, Ohio became well-known companies, as they marketed their designs to different cities and townships. The bowstring truss design (photo) 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, like Pennsylvania continued to build steel truss bridges, including massive steel through truss bridges for long spans. Other states, like Michigan, utilized standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built.
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.
Some truss types are applicable to the construction of floor and roof structures and pylons as well as bridges.
- Some stub sections for various truss types employed in bridges follow - can you can assist by adding text ?
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.
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The Bollman Truss Railroad Bridge at Savage, Maryland is the only surviving example of a revolutionary design in the history of American bridge engineering. The type was named for its inventor, Wendel Bollman, a self-educated Baltimore engineer. It was the first successful all-metal bridge design to be adopted and consistently used on a railroad. The design employs wrought iron tension members and cast iron compression members. The use of multiple independent tension elements reduces the likelihood of catastrophic failure and the structure was also easy to assemble.
The Wells Creek Bollman Bridge is the only other bridge designed by Wendel Bollman still in existence, but it is a Warren truss configuration.
Thrust arches transform their vertical loads into a thrust along the arc of the arch. At the ends of the arch this thrust (at a downward angle away from the center of the bridge) may be resolved into two components, a vertical thrust equal to a proportion of the weight and load of the bridge section, and a horizontal thrust. In a typical arch this horizontal thrust is taken into the ground, while in a bowstring arch the thrust is taken horizontally by a chord member to the opposite side of the arch. This allows the footings to take only vertical forces, useful for bridge sections resting upon high pylons.
This type of truss is particularly suited for timber structures that use iron rods as tension members.
See Lenticular truss below
Most trusses have the lower chord under tension and the upper chord under compression. In a cantilever truss the situation is reversed, at least over a portion of the span. The typical cantilever truss bridge is a balanced cantilever, which enables the construction to proceed outward from a central vertical spar in each direction. Usually these are built in pairs until the outer sections may be anchored to footings. A central gap, if present, can then be filled by lifting a conventional truss into place or by building it in place using a traveling support.
The relatively rare Howe truss includes vertical members and diagonals that slope up towards the center, the opposite of the Pratt truss.[1]
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One of the simplest truss styles to implement, the king post consists of two angled supports leaning into a common vertical support.
This type of bridge uses a substantial number of lightweight elements, easing the task of construction. Truss elements are usually of wood, iron, or steel.
The Lenticular truss was developed by the famous 19th century engineer Isambard Kingdom Brunel for use in railway bridges. It consists of an arcuate tubular upper compression chord and lower eyebar chain tension links. As the horizontal tension and compression forces are balanced these horizontal forces are not transferred to the supporting pylons (as is the case with most arch types). This in turn enables the truss to be fabricated on the ground and then to be raised by jacking as supporting masonry pylons are constructed.
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An example of this truss type is the Schell Bridge in Northfield, Massachusetts.
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A Pratt truss includes vertical members and diagonals that slope down towards the center, the opposite of the Howe truss.[1] It can be subdivided, creating Y- and K-shaped patterns.
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The queenpost truss, sometimes queen post or queenspost, is similar to a king post truss in that the outer supports are angled towards the center of the structure. The primary difference is the horizontal extension at the centre which relies on beam action to provide mechanical stability. This truss style is only suitable for relatively short spans. [2]
A truss arch may contain all horizontal forces within the arch itself, or alternatively may be either a thrust arch consisting of a truss, or of two arcuate sections pinned at the apex.
Patented 1894 (U.S. Patent 529,220) its simplicity eases erection at the site. It was intended to be used as a railroad bridge.
The Warren truss consists of diagonals that alternate between compression and tension (approaching the center), with no vertical elements. Elements near the center must support both tension and compression in response to live loads.
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The Vierendeel truss, unlike common pin-jointed trusses, imposes significant bending forces upon its members — but this in turn allows the elimination of many diagonal elements. While rare as a bridge type this truss is commonly employed in modern building construction as it allows the resolution of gross shear forces against the frame elements while retaining rectangular openings between columns. This is advantageous both in allowing flexibility in the use of the building space and freedom in selection of the building's outer curtain wall, which affects both interior and exterior styling aspects.
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- Bridge Basics - A Spotter's Guide to Bridge Design - from Pghbridges.com - Illustrates many of the various types of truss arrangements used in bridges.
- Historic Bridges of Michigan and Elsewhere - Many photos of truss bridges are available on this informative and mainly truss-focused bridge website.
- Historic Bridges of the U.S. - An enormous database of historic bridges. Over 1000 truss bridges are listed here.
- Iron and Early Steel Bridges of Ohio - A comprehensive inventory of all remaining truss bridges in Ohio. Includes maps, photos, and invites visitor assistance in identifying extant or demolished bridges.
- Matsuo Bridge Company: Bridge Types - Truss
- Historic Bridges of Iowa - An illustrated list of different architectural bridge types found in Iowa, USA. Many of these are truss bridges.
- structurae.de The Structurae database on bridges.
- ^ a b Matsuo Bridge Company, Bridge Types - Truss, accessed September 2007
- ^ http://www.dot.state.oh.us/se/coveredbridges/truss_types.htm
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| Types of bridges | Moveable bridge · Beam bridge · Cantilever bridge · Arch bridge · Suspension bridge · Cable-stayed bridge · Truss bridge · Visual index to various types |
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| Lists of Bridges | Notable bridges · By length · Longest suspension bridge spans · Largest cable-stayed bridges · Longest cantilever bridges · Arch bridges · Bridge disasters | |