To determine the force in AB of the cantilever truss shown by the method of sections, the section is passed through AB, AD and ED, and moments are taken about:
Try it first: pick an option, then read the explanation.
Correct answer: C. joint D
Asked in SSC JE 23/01/2018 (Evening)
Detailed Explanation
In the method of sections, we cut through at most three members with unknown forces and use equilibrium of one part. The trick is to choose a moment point that removes two of the unknowns.
Why “joint D” Is Correct
Of the three cut members, AD and ED both pass through joint D. Their moments about D are zero, so taking moments about D leaves only FAB (and the known loads) in the equation. That is option (c).
Why the Other Options Are Wrong
- A. joint C: Moments about C would still include AD and ED.
- B. joint B: Moments about B would include AD and ED (they do not pass through B).
- D. joint A: AB and AD pass through A; moments about A would give ED, not AB.
Key Point
Method of sections: take moments about the point where the other two cut members meet.
Key Concepts for Students
- To find a diagonal force in a parallel-chord truss, resolve vertically instead.
- Cut no more than three members with unknown forces.
- Ritter's method = method of moments with sections.
Practice more: this is question 16 in Beams and Trusses MCQs – Part 2. Try the whole set and check every answer instantly.