Editing Truss bridge (section)

== Design ==
[[File:Parts of a truss bridge.svg|thumb|The components of a typical truss bridge<ref>[https://books.google.com/books?id=A3oSAAAAYAAJ Science and Industry] {{Webarchive|url=https://web.archive.org/web/20170215092202/https://books.google.com/books?id=A3oSAAAAYAAJ |date=2017-02-15 }}, Members of a Truss Bridge by Benj. F. La Rue, Home Study Magazine, Published by the Colliery Engineer Company, Vol 3, No. 2, March 1898, pages 67-68.</ref>]]

The nature of a [[truss]] allows the [[analysis]] of its structure using a few assumptions and the application of [[Newton's laws of motion]] according to the branch of [[physics]] known as [[statics]]. For purposes of analysis, trusses are assumed to be [[Pin joint|pin-jointed]] where the straight components meet, meaning that taken alone, every joint on the structure is functionally considered to be a flexible joint as opposed to a rigid joint with the strength to maintain its shape, and the resulting shape and strength of the structure are only maintained by the interlocking of the components. This assumption means that members of the truss (chords, verticals, and diagonals) will act only in tension or compression. A more complex analysis is required where rigid joints impose significant [[bending]] loads upon the elements, as in a [[Vierendeel bridge|Vierendeel truss]].

In the bridge illustrated in the [[infobox]] at the top, vertical members are in tension, lower horizontal members in tension, [[Shear stress|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 the 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 has 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.

=== Model bridges ===
A pure truss can be represented as a pin-jointed structure, one where the only forces on the truss members are tension or compression, not bending. This is used in the teaching of statics, by the building of [[spaghetti bridge|model bridges from spaghetti]]. Spaghetti is brittle and although it can carry a modest tension force, it breaks easily if bent. A model spaghetti bridge thus demonstrates the use of a truss structure to produce a usefully strong complete structure from individually weak elements.