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Truss

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For other uses, seeTruss (disambiguation).

Inarchitecturea trussis astructurecomprising one or more triangular units constructed with

straight members whose ends are connected at joints referred to asnodes.External forces and

reactions to those forces are considered to act only at the nodes and result in forces in the

members which are eithertensileorcompressiveforces. Moments (torques) are explicitly

excluded because, and only because, all the joints in a truss are treated asrevolutes.

A planar truss is one where all the members and nodes lie within a two dimensional plane,

while a space truss has members and nodes extending into three dimensions. The top beams

in a truss are called top chords and are generally incompression,the bottom beams are called

bottom chords and are generally intension,the interior beams are called webs, and the areasinside the webs are called panels.

[1]

Truss bridgefor a single-track railway, converted to pedestrian use and pipeline support

Contents

[hide]

1 Etymology of truss 2 Characteristics of trusses

o 2.1 Planar trusso 2.2 Space frame truss

3 Truss typeso 3.1 Pratt trusso 3.2 Bowstring trusso 3.3 King post trusso 3.4 Lenticular trusso 3.5 Town's lattice trusso 3.6 Vierendeel truss

4 Statics of trusses 5 Analysis of trusses

o 5.1 Forces in memberso 5.2 Design of members

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o 5.3 Design of joints 6 Applications

o 6.1 Post frame structures 7 Gallery 8 See also 9 References 10 External links

Etymology of truss [edit]

Truss is derived from Old French trousse, around c.1200, which means "collection of things

bound together."[2][3]The term truss has often been used to describe any assembly of

members such as acruckframe[4][5]

or couple of rafters[6][7]

but often means the engineering

sense of "A truss is a single plane framework of individual structural member connected at

their ends of forms a series of triangle (sic) to span a large distance."[8]

Characteristics of trusses [edit]

A truss consists of straight members connected at joints, traditionally termed panel points.

Trusses are composed of triangles because of the structural stability of that shape and design.

A triangle is the simplest geometric figure that will not change shape when the lengths of the

sides are fixed.[9]In comparison, both the angles and the lengths of a four-sided figure must

be fixed for it to retain its shape.

Planar truss [edit]

Planar roof trusses

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The roof trusses of thebasilica di Santa Croce(Florence)

The simplest form of a truss is one single triangle. This type of truss is seen in a framedroof

consisting ofraftersand a ceilingjoist,[10]

and in other mechanical structures such asbicycles

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.[11]The traditional diamond-shape bicycle frame, which utilizes two conjoined triangles,

is an example of a simple truss.[12]

A planar truss lies in a singleplane.[11]

Planar trusses are typically used in parallel to form

roofs and bridges.

The depth of a truss, or the height between the upper and lowerchords,is what makes it an

efficient structural form. A solidgirderorbeamof equal strength would have substantial

weight and material cost as compared to a truss. For a givenspan,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.[13]

Space frame truss [edit]

Aspace frametruss is a three-dimensional framework of members pinned at their ends. Atetrahedronshape is the simplest space truss, consisting of six members which meet at four

joints.[11]

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

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Simple tetrahedron

Diagram of a planar space

frame such as used for a

roof

Three dimensionally

trussed structure -

Pylon

Truss types [edit]

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A large timberHowe trussin a commercial buildingFor more truss types, seeList of truss typesorTruss Bridge.

There are two basic types of truss:

Thepitched truss, or common truss, is characterized by its triangular shape. It is mostoften used for roof construction. Some common trusses are named according to their

web configuration. The chord size and web configuration are determined byspan,

loadandspacing.

Theparallel chord truss, orflat truss, gets its name from its parallel top and bottomchords. It is often used for floor construction.

A combination of the two is a truncated truss, used inhiproof construction. A metal plate-

connected wood truss is a roof or floor truss whose wood members are connected withmetal

connector plates.

Pratt truss [edit]

The Pratt trusswas patented in 1844 by twoBostonrailway engineers,[14]

Caleb Pratt and his sonThomas Willis Pratt.[15]The design uses vertical members for

compressionand horizontal members to respond totension.What is remarkable about thisstyle is that it remained popular even as wood gave way to iron, and even still as iron gave

way to steel.[16]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 tobucklingunder 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.

TheSouthern Pacific Railroadbridge inTempe,Arizonais a 393 meter (1,291 foot) long

truss bridge built in 1912.[17]The structure is composed of nine Pratt truss spans of varying

lengths. The bridge is still in use today.

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TheWright Flyerused a Pratt truss in its wing construction, as the minimization of

compression member lengths allowed for loweraerodynamic drag.[18]

Bowstring truss [edit]

Named for their shape, bowstring trusses were first used for archedtruss bridges,often

confused withtied-arch bridges.

Thousands of bowstring trusses were used duringWorld War IIfor 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 [edit]

Main article:King post

One of the simplest truss styles to implement, the king postconsists of two angled supports

leaning into a common vertical support.

The queen posttruss, sometimes queenpostor queenspost, is similar to a king post truss inthat the outer supports are angled towards the center of the structure. The primary difference

is the horizontal extension at the centre which relies onbeamaction to provide mechanical

stability. This truss style is only suitable for relatively short spans.[19]

Lenticular truss [edit]

TheWaterville BridgeinSwatara State Parkin Pennsylvania is a lenticular truss

Lenticular trusses, patented in 1878 by William Douglas, have the top and bottom chords of

the truss arched, forming a lens shape. Alenticular pony truss bridgeis a bridge design thatinvolves a lenticular truss extending above and below the roadbed.

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Town's lattice truss [edit]

Main article:Lattice truss bridge

AmericanarchitectIthiel Towndesigned Town's Lattice Trussas an alternative to heavy-

timber bridges. His design,patentedin 1820 and 1835, uses easy-to-handle planks arranged

diagonally with short spaces in between them.

Vierendeel truss [edit]

AVierendeel bridge;note the lack of diagonal elements in the primary structure

See also:Arthur VierendeelandVierendeel bridge

The Vierendeel trussis a structure where the members are not triangulated but formrectangular openings, and is aframewith fixed joints that are capable of transferring and

resistingbending 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 engineerArthur Vierendeel,[20]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 forfenestrationand door openings. This is preferable

to a braced-frame system, which would leave some areas obstructed by the diagonal braces.

Statics of trusses [edit]

A truss that is assumed to comprise members that are connected by means of pin joints, and

which is supported at both ends by means of hinged joints or rollers, is described as being

statically determinate.Newton's Laws apply to the structure as a whole, as well as to each

node or joint. In order for any node that may be subject to an external load or force to remain

static in space, the following conditions must hold: the sums of all (horizontal and vertical)

forces, as well as all moments acting about the node equal zero. Analysis of these conditions

at each node yields the magnitude of the compression or tension forces.

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Trusses that are supported at more than two positions are said to bestatically indeterminate,

and the application of Newton's Laws alone is not sufficient to determine the member forces.

In order for a truss with pin-connected members to be stable, it must be entirely composed of

triangles. In mathematical terms, we have the following necessary condition forstability:

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