BeginnerTopic 13 of 106Engineering Mechanics
Method of joints
Solve truss member forces by applying equilibrium at each pin.
In depth
Isolate each pin and apply ΣFx=0 and ΣFy=0, starting at a joint with at most two unknown member forces. Assume tension positive, and carry results to adjacent joints. Support reactions are found first.
Key points
- Ensures every joint is in equilibrium
- Good when all member forces are needed
- Negative results mean compression
ExampleAt a joint with a 10 kN load, a vertical and a diagonal member solve with two equations.
What is Method of joints?
Solve truss member forces by applying equilibrium at each pin.
Why is it important to understand method of joints?
The method of joints gives every member force in a truss by applying equilibrium at each pin, which is systematic and easy to check.
Where should I start a method of joints analysis?
Start at a joint with only two unknown member forces, because you can solve those with two equilibrium equations.
How do I know if a member is in tension or compression?
Assume tension, which means the force pulls away from the joint. A negative answer means compression.
How is Method of joints used in practice?
Typical applications include finding the forces in each bar of a small roof truss, checking the tension in a tie rod at a joint and preparing member forces for a steel connection design.
What should I learn before Method of joints?
Study "Trusses" first (topic 12 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Method of joints?
Ensures every joint is in equilibrium. Good when all member forces are needed. Negative results mean compression.
Can you give an example of Method of joints?
At a joint with a 10 kN load, a vertical and a diagonal member solve with two equations.
What comes after Method of joints?
The next topic is "Method of sections", which builds directly on this one.
How difficult is Method of joints?
It is classed as Beginner in Engineering Mechanics (topic 13 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.