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Beams and Trusses MCQs – Part 1

15 questions. Tap an option to check it instantly, then read the solution.

Practice SSC JE and other civil engineering exam questions on beams and plane trusses: truss assumptions, perfect, deficient and redundant trusses (m = 2j − 3), simple, compound and complex trusses, zero-force members, the method of joints, the method of sections, and how trusses differ from beams and frames. Tap an option to check it, then open the full explanation for each question.

Question 1Medium
In a triangular truss ABC with span AC = 6 m, pin at A, roller at C and a 10 kN downward load at B, find the force in member AB. (Take ∠BAC = 60° and ∠BCA = 30°.)
Full explanation

Answer: B. 5√3 kN compressive

At B the members AB and BC are at right angles. Resolving along each: FAB = 10 cos 30° = 5√3 kN, FBC = 10 sin 30° = 5 kN, both compressive.

Question 2Medium
In the cantilever truss shown (top chord A–B–C attached to a wall at A, vertical BD, members AD, ED and CD, with CD inclined at 30° at C, and 100 t load at B), the force in member BD is:
Full explanation

Answer: A. 100 t compressive

Joint C (unloaded, two non-collinear members) gives BC = CD = 0. Then at B, BD alone balances the 100 t load: 100 t compression.

Question 3Easy
In the cantilever truss shown (top chord A–B–C, 100 t load at B, vertical BD, member DC inclined at 30° to BC), the force in member DC is:
Full explanation

Answer: C. zero

Joint C has only two members (BC and DC), they are not collinear, and there is no load at C — so both forces are zero.

Question 4Medium
If j represents the number of joints and r the number of external reactions, a structure that has 2j − r members will be:
Full explanation

Answer: A. Stable

m = 2j − r (with r = 3) is the condition of a perfect truss, which is stable and statically determinate.

Question 5Easy
Generally, in a truss, compression members are thicker than tension members. Is this true or false?
Full explanation

Answer: A. True

True: compression members can buckle, so they need larger, stiffer sections than tension members carrying similar forces.

Question 6Medium
If a truss has two or more members crossing each other (and is neither simple nor compound), it is:
Full explanation

Answer: C. Complex

A truss that cannot be built up as a simple or compound truss — often with crossing members — is a complex truss.

Question 7Medium
If a structure has 10 joints, at how many joints (at minimum) must the equilibrium equations be satisfied for stability?
Full explanation

Answer: D. 10

For the structure to be in equilibrium, every joint must be in equilibrium: all 10. (This question was cancelled in the revised key; see the note.)

Question 8Easy
The gross section of the web of a beam is defined as:
Full explanation

Answer: A. depth of the beam multiplied by its web thickness

Gross web area = overall depth × web thickness (d × tw).

Question 9Medium
A compound truss may be formed by connecting two simple rigid frames by:
Full explanation

Answer: B. three bars

Two simple trusses are joined rigidly by three bars that are neither all parallel nor all concurrent.

Question 10Easy
A beam is a structural member predominantly subjected to:
Full explanation

Answer: A. Transverse loads

A beam carries loads acting across (perpendicular to) its axis, which cause bending and shear.

Question 11Medium
In the two-bay truss shown (bottom chord A–C–B with pin at A and roller at B; top chord D–E–F; 4 t horizontal load at D and 5 t vertical load at E), the force in member BC is:
Full explanation

Answer: D. None of these

At joint B only BC can act horizontally, and the roller gives no horizontal reaction, so FBC = 0 — not listed, hence (d).

Question 12Easy
Trusses and frames differ in that:
Full explanation

Answer: D. Trusses can't bend but frames can

Truss members are pin-jointed and loaded at joints, so they carry only axial force; frame members are rigidly jointed and carry bending.

Question 13Hard
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.
Full explanation

Answer: B. Only B

A non-triangular panel usually makes a truss unstable, but not always — other members can still make the whole truss rigid. So B is true, A (‘essentially’) is too strong.

Question 14Hard
In the truss shown (bottom chord A–C–E with AC = CE = 4 m, top chord B–D, end members inclined at 60°, pin at A, roller at E; loads 10 t at B and 5 t at D), the force in member AC is:
Full explanation

Answer: C. (8.75/√3) t tensile

RA = 8.75 t. At A: FAB sin 60° = 8.75 → AB compressive; then FAC = FAB cos 60° = 8.75/√3 t tensile.

Question 15Medium
In the symmetrical truss shown (bottom chord A–C–B, apex D, vertical CD, base angle 30°, pin at A, roller at B, 3 t load at D), the force in member BC is:
Full explanation

Answer: C. (3√3)/2 t tension

RB = 1.5 t. At B: FBD sin 30° = 1.5 → BD = 3 t compression; FBC = 3 cos 30° = 3√3/2 t tension.