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Truss

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T R U S S
In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in forces in the members which are either tensile or compressive forces. Moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes.
Planar truss

Planar roof trusses The roof trusses of the basilica di Santa Croce (Florence)
The simplest form of a truss is one single triangle. This type of truss is seen in a framed roof consisting of rafters and a ceiling joist, and in other mechanical structures such as bicycles and aircraft. Because of the stability of this shape and the methods of analysis used to calculate the forces within it, a truss composed entirely of triangles is known as a simple truss. The traditional diamond-shape bicycle frame, which utilizes two conjoined triangles, is an example of a simple truss.
A planar truss lies in a single plane. Planar trusses are typically used in parallel to form roofs and bridges.
The depth of a truss, or the height between the upper and lower chords, is what makes it an efficient structural form. A solid girder or beam of equal strength would have substantial weight and material cost as compared to a truss. For a given span, a deeper truss will require less material in the chords and greater material in the verticals and diagonals. An optimum depth of the truss will maximize the efficiency.

Space frame truss
A space frame truss is a three-dimensional framework of members pinned at their ends. A tetrahedron shape is the simplest space truss, consisting of six members which meet at four joints. Large planar structures may be composed from tetrahedrons with common edges and they are also employed in the base structures of large free-standing power line pylons. |

Simple tetrahedron Three dimensionally trussed structure - Pylon Diagram of a planar space frame such as used for a roof

Pratt truss

The Pratt truss was patented in 1844 by two Boston railway engineers, Caleb Pratt and his son Thomas Willis Pratt. The design uses vertical members for compression and horizontal members to respond to tension. What is remarkable about this style is that it remained popular even as wood gave way to iron, and even still as iron gave way to steel. The continued popularity of the Pratt truss is probably due to the fact that the configuration of the members means that longer diagonal members are only in tension for gravity load effects. This allows these members to be used more efficiently, as slenderness effects related to buckling under compression loads (which are compounded by the length of the member) will typically not control the design. Therefore, for given planar truss with a fixed depth, the Pratt configuration is usually the most efficient under static, vertical loading.
The Southern Pacific Railroad bridge in Tempe, Arizona is a 393 meter (1,291 foot) long truss bridge built in 1912. The structure is composed of nine Pratt truss spans of varying lengths. The bridge is still in use today.
The Wright Flyer used a Pratt truss in its wing construction, as the minimization of compression member lengths allowed for lower aerodynamic drag.

Bowstring truss

Named for their shape, bowstring trusses were first used for arched truss bridges, often confused with tied-arch bridges.
Thousands of bowstring trusses were used during World War II for holding up the curved roofs of aircraft hangars and other military buildings. Many variations exist in the arrangements of the members connecting the nodes of the upper arc with those of the lower, straight sequence of members, from nearly isosceles triangles to a variant of the Pratt truss.

King post truss

One of the simplest truss styles to implement, the king post consists of two angled supports leaning into a common vertical support.

The queen post truss, sometimes queenpost 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.

Lenticular truss

Lenticular trusses, patented in 1878 by William Douglas, have the top and bottom chords of the truss arched, forming a lens shape. A lenticular pony truss bridge is a bridge design that involves a lenticular truss extending above and below the roadbed.

Town's lattice truss

American architect Ithiel Town designed Town's Lattice Truss as an alternative to heavy-timber bridges. His design, patented in 1820 and 1835, uses easy-to-handle planks arranged diagonally with short spaces in between them.

Vierendeel truss

A Vierendeel bridge; note the lack of diagonal elements in the primary structure
The Vierendeel truss is a structure where the members are not triangulated but form rectangular openings, and is a frame with fixed joints that are capable of transferring and resisting bending moments. As such, it does not fit the strict definition of a truss; regular trusses comprise members that are commonly assumed to have pinned joints, with the implication that no moments exist at the jointed ends. This style of structure was named after the Belgian engineer Arthur Vierendeel, who developed the design in 1896. Its use for bridges is rare due to higher costs compared to a triangulated truss.
The utility of this type of structure in buildings is that a large amount of the exterior envelope remains unobstructed and can be used for fenestration and door openings. This is preferable to a braced-frame system, which would leave some areas obstructed by the diagonal braces.

Allan truss

Allan Truss illustrated
The Allan Truss, designed by Percy Allan, is partly based on the Howe truss. The first Allan truss was completed on 13 August 1894 over Glennies Creek at Camberwell, New South Wales and the last Allan truss bridge was built over Mill Creek near Wisemans Ferry in 1929. Completed in March 1895, the Tharwa Bridge located at Tharwa, Australian Capital Territory, was the second Allan truss to be built, the oldest surviving bridge in theAustralian Capital Territory and the oldest, longest in continuous Allan truss bridge. Completed in November 1895, the Hampden Bridge in Wagga Wagga, New South Wales, Australia, the first of the Allan truss bridges with overhead bracing, was originally designed as a steel bridge but was constructed with timber to reduce cost. In his design, Allan used Australian ironbark for its strength. 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 still in use today for pedestrian and light traffic.

Bailey truss

Bailey truss illustrated.
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.

Baltimore truss

Baltimore truss illustrated.
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

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 (patented in 1852) 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. 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.

Brown truss

Brown truss illustrated. All interior vertical elements are under tension.
This type of truss is particularly suited for timber structures that use iron rods as tension members

Burr Arch Truss

This combines an arch with a truss to form a structure both strong and rigid

Cantilevered truss
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.

Fink truss

Fink Truss (half span and cross section)
The Fink truss was designed by Albert Fink of Germany in the 1860s. This type of bridge was popular with the Baltimore and Ohio Railroad. TheAppomattox High Bridge on the Norfolk and Western Railroad included 21 Fink deck truss spans from 1869 until their replacement in 1886.

Howe truss

Howe truss illustrated - the diagonals are under compression under balanced loading
The relatively rare Howe truss, patented in 1840 by Massachusetts millwright William Howe, includes vertical members and diagonals that slope up towards the center, the opposite of the Pratt truss.[10] In contrast to the Pratt Truss, the diagonal web members are in compression and the vertical web members are in tension. Examples include Jay Bridge in Jay, New York, and Sandy Creek Covered Bridge in Jefferson County, Missouri.

K truss

A truss in the form of a K due to the orientation of the vertical member and two oblique members in each panel.

Long truss

Designed by Stephen H. Long in 1830. The design resembles a Howe truss, but is entirely made of wood instead of a combination of wood and metal. The longest surviving example is the Eldean Covered Bridge north of Troy, Ohio, spanning 224 feet. One of the earliest surviving examples is the Old Blenheim Bridge, which with a span of 210 feet and a total length of 232 feet long is the second-longest covered bridge in the United States.
The Busching bridge, unfortunately used as an example of a Long truss, is an example of a Howe truss, as the verticals are metal rods. A Long truss has timber verticals.

Parker (Camelback) truss

Parker truss illustrated.
A Parker truss bridge is a Pratt truss design with a polygonal upper chord. A "camelback" is a subset of the Parker type, where the upper chord consists of exactly five segments. An example of a Parker truss is the Traffic Bridge in Saskatoon, Canada. An example of a camelback truss is theWoolsey Bridge near Woolsey, Arkansas.

Pegram truss

Pegram Truss
The Pegram truss is a hybrid between the Warren and Parker trusses where the upper chords are all of equal length and the lower chords are longer than the corresponding upper chord. Because of the difference in upper and lower chord length, each panel was not square. The members which would be vertical in a Parker truss vary from near vertical in the center of the span to diagonal near each end (like a Warren truss). George H. Pegram, while the chief engineer of Edge Moor Iron Company in Wilmington, Delaware, patented this truss design in 1885.
The Pegram truss consists of a Parker type design with the vertical posts leaning towards the center at an angle between 60 and 75°. The variable post angle and constant chord length allowed steel in existing bridges to be recycled into a new span using the Pegram truss design. This design also facilitated reassembly and permitted a bridge to be adjusted to fit different span lengths. There are ten remaining Pegram span bridges in the United States with seven in Idaho.

Pennsylvania (Petit) truss

Pennsylvania or Petit truss illustrated.
The Pennsylvania (Petit) truss is a variation on the Pratt truss. The Pratt truss includes braced diagonal members in all panels; the Pennsylvania truss adds to this design half-length struts or ties in the top, bottom, or both parts of the panels. It is named after the Pennsylvania Railroad, which pioneered this design. It was once used for hundreds of bridges in the United States, but fell out of favor in the 1930s, and very few bridges of this design remain. Examples of this truss type include the Schell Bridge in Northfield, Massachusetts, the Inclined Plane Bridge in Johnstown, Pennsylvania, and the Healdsburg Memorial Bridge in Healdsburg, California.

Post truss

A Post truss
A Post truss is a hybrid between a Warren truss and a double-intersection Pratt truss. Invented in 1863 by Simeon S. Post, it is occasionally referred to as a Post patent truss although he never received a patent for it. The Ponakin Bridge and the Bell Ford Bridge are two examples of this truss.

Queen post truss

Queen Post Truss
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 center which relies on beam action to provide mechanical stability. This truss style is only suitable for relatively short spans.

Thatcher truss

Thatcher Truss illustrated
The Thatcher truss combines some of the characteristics of a Pratt truss with diagonals under tension and a Howe truss with diagonals under compression. It is quite rare.

Truss arch

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. The latter form is common when the bridge is constructed as cantilever segments from each side as in the Navajo Bridge.

Waddell truss

Patented 1894 (U.S. Patent 529,220) its simplicity eases erection at the site. It was intended to be used as a railroad bridge.

Warren truss

Warren truss illustrated – some of the diagonals are under compression and some under tension
The Warren truss was patented in 1848 by its designers James Warren and Willoughby Theobald Monzani, and consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. Loads on the diagonals alternate between compression and tension (approaching the center), with no vertical elements, while elements near the center must support both tension and compression in response to live loads. This configuration combines strength with economy of materials and can therefore be relatively light. It is an improvement over the Nevilletruss which uses a spacing configuration of isosceles triangles.
A preserved original Ansaldo SVAaircraft, showing the Warren truss-pattern interplane wing strut layout
Warren truss construction has also been used in airframe construction for aircraft since the 1920s, mostly for smaller aircraft fuselages, using chrome molybdenum alloy steel tubing, with popular aircraft such as the Piper J-3 Cub. One of the earliest uses for the Warren truss design in aircraft design was for the interplane wing strut layout, as seen in a nose-on view, on the Italian World War I Ansaldo SVA series of fast reconnaissance biplanes, which were among the fastest aircraft of the First World War era. Warren truss construction is still used today for some homebuilt aircraft fuselage designs, that essentially use the same 1920s-era design philosophies in the 21st century.

Whipple truss

A Whipple truss is usually considered a subclass of the Pratt truss because the diagonal members are designed to work in tension. The main characteristic of a whipple truss is that the tension members are elongated, usually thin, at a shallow angle and cross two or more bays (rectangular sections defined by the vertical members).…...

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