Which of the following statements is true? A. If a truss has a non-triangular element, it will essentially be unstable. B. In such cases instability is the most probable result, but there are exceptions.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 25/01/2018 (Morning)
Detailed Explanation
A triangle is the only polygon whose shape cannot change without changing a side length. That is why trusses are built from triangles.
Why “Only B” Is Correct
A four-sided (or larger) panel without a diagonal can deform like a parallelogram, so instability is the most likely result. But it is not certain: in compound and complex trusses, a non-triangular panel can still be held rigid by the rest of the truss. So statement A (‘essentially unstable’) is too strong, and only statement B is true — option (b).
Why the Other Options Are Wrong
- A. Only A: A is too absolute; some trusses with non-triangular panels are stable.
- C. Both A and B: A is not true in every case.
- D. None of these: Statement B is true.
Key Point
Triangles make trusses rigid; non-triangular panels usually (not always) cause instability.
Key Concepts for Students
- A rectangle with one diagonal becomes rigid.
- Compound trusses can contain non-triangular regions and still be stable.
- Count members (m = 2j − 3) and check the arrangement.
Practice more: this is question 13 in Beams and Trusses MCQs – Part 1. Try the whole set and check every answer instantly.