Trusses

• Consist entirely of straight two-force members, connected at joints
• Example on board: decompose truss into two-force members and joints, and show how forces meet at joints

Trusses: Method of Joints:-

• Typically used to find forces in all or several of the members
• Each joint is a particle
• Particle equilibrium in 2D:

• For each joint, we have 2 equations, therefore, we can solve for 2 unknowns
• Must start process at a joint with only 2 unknown forces
• Find a joint with only two unknown forces

-First, may need to draw an FBD of the entire truss and find support reactions

• Draw FBD of selected joint

-Draw each force along the direction of the member
-Draw in tension (away from joint)
-Find angle of force from truss geometry
-Resolve angled forces into x, y components

• Apply equilibrium, to solve for 2 unknown forces

A + sign: the force is tension (T)

A – sign: the force is compression (C)

• Find the next joint that has only 2 unknown forces and repeat the process

–Typically this is adjacent to the prior joint

• Repeat with additional joints until all member forces are known.
• Remember to specify (T) or (C) for each force!