Trusses

There is not much to know about trusses but that they are a bunch of two-force axial members that are pinned together to create a structure. The pin points are known as joints and they are the load application points used for any calculations. Structural cell is a loop of members and the structure is stable only if all the cells are in triangular shape. There are a lot of different types of trusses with names assigned to them.

Trestle is a usually indeterminate truss with multiple earth contact points. Here are some points about trusses:

- Truss loads are only considered on the plane of the truss.
- Trusses are analyzed as 2D structures.
- Forces in a truss are known as internal forces.
- The forces on joints are all concurrent forces (axial), therefore, for the equilibrium analyses only the sum of forces are used.
- A third member framing into a joint that already connects two collinear members has no internal force and is known as zero-force member.
- Truss is statically determinate if
- Number of members = 2 x Number of joints - 3

There are redundant members (indeterminate) if
- Number of members > 2 x Number of joints - 3
- Truss is unstable if
- Number of members < 2 x Number of joints - 3
- The methods of calculation for trusses are
  - Method of joints: used when all members should be calculated. First the support reactions are calculated, and the joint at the reaction point is calculated to find the forces of the members connected to it. And the story continues to adjacent joints until all is done!
  - Method of Sections: used to calculate a given member. If only a few members are unknown, this is the best method to use. First find the support reactions, then make a cut through not more than three members with unknown forces (b/c of only three equations) and start calculating.

Here is an example on truss calculation:

Next is to learn to calculate pulleys and cables.

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