To determine the force in BD of the truss shown (bottom chord A–C–E, top chord B–D, loads 10 t at B and 5 t at D), a section is passed through BD, CD and CE, and moments are taken about:
Try it first: pick an option, then read the explanation.
Correct answer: C. joint C
Asked in SSC JE 22/01/2018 (Evening)
Detailed Explanation
Choose the moment centre at the intersection of the two cut members you do not want in the equation.
Why “joint C” Is Correct
Members CD and CE meet at joint C. Taking moments about C removes both, leaving only FBD with the known reaction and loads. That is option (c).
Why the Other Options Are Wrong
- A. joint A: No cut member passes through A, so all three would remain.
- B. joint B: Only BD passes through B; CD and CE would remain.
- D. joint D: BD and CD meet at D; moments about D would give CE, not BD.
Key Point
Moment centre = intersection of the other two cut members.
Key Concepts for Students
- Top chord BD is in compression for gravity loads.
- With R_A = 8.75 t: moments of left part about C give F_BD.
- Method of sections is fastest for one or two members far from the supports.
Practice more: this is question 19 in Beams and Trusses MCQs – Part 2. Try the whole set and check every answer instantly.