Question

In a truss it is assumed that the members are joined by

• A. Rough pins
• B. Smooth pins
• C. Either of them
• D. Neither of them

Hint:

In ideal truss analysis, joints are assumed to be frictionless so that members carry only axial tension or compression.

Answer 1

The Correct Option is B

Step 1: Understanding the ideal truss assumption.
In the analysis of trusses, certain ideal assumptions are made to simplify calculations. One key assumption is related to the nature of joints between members.
Step 2: Nature of joints in a truss.
Truss members are assumed to be connected by smooth pin joints.
Smooth pins allow rotation but do not resist moments, ensuring that only axial forces (tension or compression) act in the members.
Step 3: Analysis of options.
(A) Rough pins: Incorrect, because rough pins would resist rotation and introduce moments.
(B) Smooth pins: Correct, as this assumption ensures members carry only axial forces.
(C) Either of them: Incorrect, since the assumption is specific to smooth pins.
(D) Neither of them: Incorrect, as pin joints are essential to truss behavior.
Step 4: Conclusion.
Hence, in a truss, members are assumed to be joined by smooth pins.

Answer 2

Rather than starting from the standard textbook assumption list, we can work backward from the basic requirement that every truss member behaves purely as a two-force member (axial tension or compression only, no bending).

For a member to carry only axial force with no internal bending moment, the joint connecting it to the rest of the structure must be incapable of transmitting a moment into the member; it must act as a hinge that allows free rotation.

1. Rough pins: A rough (frictional) pin resists relative rotation between the connected members, which means it can transmit a moment through friction at the joint. This would introduce bending in the members, violating the pure axial-force assumption, so this is inconsistent with truss behavior.
2. Smooth pins: A smooth, frictionless pin offers no resistance to rotation, so it cannot transmit any moment to the connected member — exactly the condition needed for every member to remain a pure two-force axial member. This matches the requirement.
3. Either of them: Since a rough pin does transmit moment (as shown above) while a smooth pin does not, the two are not interchangeable for the purposes of this idealization, so this option is inconsistent.
4. Neither of them: A joint must still allow the members to be physically connected and free to rotate relative to each other; some form of pin connection is still assumed, so this option is incorrect.

Only a frictionless, moment-free connection preserves the two-force-member assumption that underlies all truss analysis.

Therefore, the correct answer is smooth pins.