In the two-bay truss shown (top chord D–E–F, vertical EC, 5 t downward load at E, 4 t horizontal load at D), the force in EC is:
Try it first: pick an option, then read the explanation.
Correct answer: C. 5 t compression
Asked in SSC JE 23/01/2018 (Morning)
Detailed Explanation
Joint E has three members: DE and EF (both horizontal, collinear) and EC (vertical). A 5 t load acts downward at E.
Joint E: \(\Sigma F_y = 0\): \(F_{EC}\) must push up on E with 5 t.
Why “5 t compression” Is Correct
Only EC can balance a vertical force at E. It pushes up on the joint, so it is in compression, with 5 t — option (c).
Why the Other Options Are Wrong
- A. zero: EC is the only member that can carry the vertical load at E.
- B. 5 t tension: EC pushes on E, which is compression.
- D. 4 t tension: The 4 t load is horizontal at D; it does not set the EC force.
Key Point
Two collinear members + one other member at a loaded joint: the third member carries the load component across the line.
Key Concepts for Students
- With no load at E, EC would be a zero-force member.
- Compression in a vertical post carries load down to the bottom chord.
- Collinear chord members can only balance horizontal forces at E.
Practice more: this is question 18 in Beams and Trusses MCQs – Part 2. Try the whole set and check every answer instantly.