Trusses

• 1. Trusses
  

2. What are Trusses?
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 (revolute joint).
3. Shortly, a truss has the following three characteristics:
1-Axial loads only.
2-Loads are applied at end points only.
3-Elements are joined by pins only.
4. See some pictures of trusses:
5. 6. Planar trusses: planar trusses lie in a single plane and are often used to support roofs and bridges.
In roof trusses, the roof load is transmitted to the truss at the joints by means of a series of purlins.
7. Assumptions for design:
To design both the members and the connections of a truss, it is first necessary to determine the force developed in each member when the truss is subjected to a given loading. In this regard, these assumptions will be made:
1- Loads are applied at end points only.
2- Weight of the elements is neglected.
3- Elements are joined by pins only.
4- Axial loads only: if the force tends to elongate the member, it is a tensile force; whereas if it tends to shorten the member, it is a compressive force.
8. Truss problems
Some important notes:

• When a force (F) is applied at the center of a member of the truss, it must be divided into two equal forces (F1 & F2) where: F1= F2= F/2 and F1& F2 are applied at the ends of the member as shown in the following figure: