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physics

Truss

Truss is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Truss rather than just read about it. In short: A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure. In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object".

Truss — main illustration
Truss — illustration

Key takeaways

  • Truss belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Truss to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Truss from memory before moving on to harder problems.

Reference excerpt

A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure. In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object". A two-force member is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, architectural trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. In this typical context, external forces and reactions to those forces are considered to act only at the nodes and result in forces in the members that are either tensile or compressive. For straight members, moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes, as is necessary for the links to be two-force members. A planar truss is one where all members and nodes lie within a two-dimensional plane, while a space frame has members and nodes that extend into three dimensions. The top beams in a truss are called top chords and are typically in compression, and the bottom beams are called bottom chords and are typically in tension. The interior beams are called webs, and the areas inside the webs are called panels, or from graphic statics (see Cremona diagram) polygons.

Etymology Truss derives from the Old French word trousse, from around 1200 AD, which means "collection of things bound together". The term truss has often been used to describe any assembly of members such as a cruck frame or a couple of rafters.

Characteristics

A truss consists of typically (but not necessarily) straight members connected at joints, traditionally termed panel points. Trusses are typically (but not necessarily) 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. In comparison, both the angles and the lengths of a four-sided figure must be fixed for it to retain its shape.

Simple truss

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. However, a simple truss is often defined more restrictively by demanding that it can be constructed through successive addition of pairs of members, each connected to two existing joints and to each other to form a new joint, and this definition does not require a simple truss to comprise only triangles. The traditional diamond-shape bicycle frame, which uses two conjoined triangles, is an example of a simple truss.

Planar 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 that 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.

Types For more truss types, see truss types.

There are two basic types of truss:

The pitched truss, or common truss, is characterized by its triangular shape. It is most often used for roof construction. Some common trusses are named according to their web configuration. The chord size and web configuration are determined by span, load, and spacing. The parallel-chord truss, or flat truss, gets its name from its parallel top and bottom chords. It is often used for floor construction. A combination of the two is a truncated truss, used in hip roof construction. A metal-plate-connected wood truss is a roof or floor truss whose wood members are connected with metal connector plates.

Warren truss

Truss members form a series of equilateral triangles, alternating up and down.

Octet truss Truss members are made up of all equivalent equilateral triangles. The minimum composition is two regular tetrahedrons along with an octahedron. They fill up three-dimensional space in a variety of configurations.

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 diagonal members to respond to tension. The Pratt truss design remained popular as bridge designers switched from wood to iron, and from iron to steel. This 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 a 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-long (1,289 ft) truss bridge built in 1912. The structure, still in use today, consists of nine Pratt truss spans of varying lengths. The Wright Flyer used a Pratt truss in its wing construction, as the minimization of compression member lengths allowed for lower aerodynamic drag.

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, to form a lattice.

Bowstring truss

… excerpt ends here. Continue reading the full article.

Illustrations

Truss: Truss bridge for a single-track railway, converted to pedestrian use and pipeline support. In this example the truss is a group of triangular units supporting the bridge.
Truss bridge for a single-track railway, converted to pedestrian use and pipeline support. In this example the truss is a group of triangular units supporting the bridge.
Truss: Typical detail of a steel truss, which is considered as a revolute joint
Typical detail of a steel truss, which is considered as a revolute joint
Truss: Historical detail of a steel truss with an actual revolute joint
Historical detail of a steel truss with an actual revolute joint
Truss: An Egyptian ship with a rope truss, the oldest known use of trusses. Trusses did not come into common use until the Roman era.
An Egyptian ship with a rope truss, the oldest known use of trusses. Trusses did not come into common use until the Roman era.
Truss: Planar roof trusses
Planar roof trusses

Worked examples

Example 1 — a first encounter with Truss

Start with the simplest possible case. Write down what Truss claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Truss before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Truss ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Truss

In research
Truss appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Truss in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Truss is common in secondary-school and first-year university syllabi. It links to neighbouring topics Airship technology, Architectural elements, Bridge components, so understanding it makes those chapters shorter.
In everyday life
Look for Truss outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Truss in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Truss means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Truss out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Truss in simple terms?

A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure. In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object".

Why does Truss matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Truss?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Truss.

Tags

  • Airship technology
  • Architectural elements
  • Bridge components
  • Mechanics
  • Structural system
  • Trusses

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