BeginnerTopic 14 of 106Engineering Mechanics
Method of sections
Cut through a truss and use equilibrium of one part to find specific member forces.
In depth
Cut through the truss, draw an FBD of one part, and use ΣM about a point where two unknown forces intersect. At most three unknown members should be cut. Efficient for finding a few specific member forces.
Key points
- Choose cut through the member of interest
- Moment point eliminates unknowns
- Check with another section
ExampleCutting three members in the middle of a bridge truss yields chord forces directly.
What is Method of sections?
Cut through a truss and use equilibrium of one part to find specific member forces.
Why is it important to understand method of sections?
The method of sections finds a few member forces quickly without solving the whole truss, which is useful for checking a critical bar.
When should I choose sections over joints?
Use sections when you need one or a few specific members, because a single cut and three equilibrium equations give those forces directly.
Where should the cut go?
Cut through the target member and no more than three other members with unknown forces, so the equilibrium equations can be solved.
How is Method of sections used in practice?
Typical applications include finding the force in the top chord of a bridge truss at mid-span, checking the force in one diagonal of a crane boom and estimating the force in a bottom member before a detailed analysis.
What should I learn before Method of sections?
Study "Method of joints" first (topic 13 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Method of sections?
Choose cut through the member of interest. Moment point eliminates unknowns. Check with another section.
Can you give an example of Method of sections?
Cutting three members in the middle of a bridge truss yields chord forces directly.
What comes after Method of sections?
The next topic is "Centre of gravity and centroid", which builds directly on this one.
How difficult is Method of sections?
It is classed as Beginner in Engineering Mechanics (topic 14 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.