Structural Analysis Fundamentals / module 4
Pin-jointed trusses
Why triangles work, and how to find the force in every member.
What you will understand
- State the assumptions behind truss analysis and when they are reasonable
- Check whether a truss is statically determinate
- Find member forces by the method of joints
- Use the method of sections to reach a member directly
- Spot zero-force members and tell tension from compression
Lessons
Assumptions and determinacy
What we pretend about trusses, and whether equilibrium can solve them.
Method of joints and method of sections
Two ways to get member forces, and how to choose between them.
Trusses as beams, tension coefficients and space frames
Reading a truss as a deep beam, a faster hand method, and stepping into three dimensions.
Module summary
- Truss members carry axial force only, provided joints are pinned and loads act at joints
- m + r = 2j is a necessary check for determinacy, but not a guarantee
- Method of joints: start where only two member forces are unknown
- Method of sections: cut through the member you want and take moments to isolate it
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Check yourself
A stable plane truss has 13 members, 8 joints and 3 reaction components. What is it?
Practice
A truss member is calculated as −18 kN, having assumed tension. What is the magnitude of the force, and is it tension or compression? Enter the magnitude in kN.
Check yourself
Why can you not use a moment equation at a single truss joint?