Truss in Construction - Load Mechanism & Features

Truss is an engineering structure made of straight members joined together at their ends by pin-joints, where it is loaded. Each member of a truss configuration possesses members with a uniform cross-section along its length. But different members of the truss may have different dimensions, based on the load coming on it. 

Truss structures are structural elements that are used over large span airy structures, where it provides practical and economical solution over concrete or solid structures.

Some of the examples of truss structures are roofs of the buildings, aircraft frames, bridges, television towers, etc.

What is Truss Action?

When a beam is subjected to a load, internal shear and axial forces are created. As a result of shear force, an internal bending moment is created, and the beam bends. 

Load Action in a Wooden Truss

But when a truss is subjected to a similar load, instead of shear, the truss members are subjected to compression and tensile forces. As there is no shear created, there is no bending moment. So, all the members of a truss are axially loaded, the adjacent members are concurrent at the joint, and the joints do not resist bending moment. This is called truss action.

Features of Truss in Construction

  • Truss is a combined structure of connected triangles, that together act as a single unit.
  • The triangle configuration of the truss system is designed in such a way to evenly distribute the weight and transfer it to the column and foundation below. 
  • Any truss member cannot be continuous through a pin joint. Hence, the truss members are two-force members. If it was continuous, it would have been analyzed as framed structures.
  • Trusses are two-force members that are connected by pins and they do not resist the moment.
  • Each pin joint in a truss configuration is a point of concurrency of the forces, within the members attached to that pin.
  • Load-transfer mechanism in trusses is through tension and compression in the truss members. There is no bending or shear action in a truss. 
  • Trusses are formed by connecting straight members at joints, called nodes.
  • Trusses can be simple truss, plane truss, or space trusses. A simple truss is a simple triangular truss commonly used for roof trusses. A plane truss is a truss consisting of coplanar members such as trusses on both sides of bridges. Space trusses are made of non-coplanar members, for example,  a tower of an electric transmission system.
  • Trusses are mainly constructed using lumber, and metal. Compared to lumber truss, steel truss holds heavy loads.
  • Trusses are a cheap construction technique used to construct open spaces with fewer materials.
  • Trusses are available in several types and design that gives several choices for the designer. 
  • Trusses are not only used in building construction but also used for bridge construction.

Components of a Truss

The main components of a truss are:
Components of a Truss

  1. Top Cord : Provides resistance to compression and tension, and overall bending.
  2. Bottom Cord : Provides resistance to compression and tension, and overall bending.
  3. Web Member or Bracing ( between the top and bottom cords)

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