A compound truss may be formed by connecting two simple rigid frames by:
Try it first: pick an option, then read the explanation.
Correct answer: B. three bars
Asked in SSC JE 27/01/2018 (Morning)
Detailed Explanation
Joining two rigid bodies in a plane needs three independent constraints — the same as supporting one rigid body on the ground.
Why “three bars” Is Correct
Connecting two simple trusses with three bars (or with one common pin and one bar) gives exactly three independent constraints, so the combination is rigid and determinate — a compound truss. The bars must not all be parallel or all meet at one point; otherwise the connection is improper (options c and d).
Why the Other Options Are Wrong
- A. two bars: Two bars give only two constraints, so the trusses can still move relative to each other.
- C. three parallel bars: Three parallel bars cannot stop movement at right angles to them.
- D. three bars intersecting at a point: Three bars meeting at one point cannot stop rotation about that point.
Key Point
Compound truss: two simple trusses + three non-parallel, non-concurrent bars (or one pin + one bar).
Key Concepts for Students
- Same logic as supporting a body with three reactions.
- Fink roof trusses are typical compound trusses.
- Method of sections is usually used first to find the connecting bar forces.
Practice more: this is question 9 in Beams and Trusses MCQs – Part 1. Try the whole set and check every answer instantly.