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

Truss analysis (method of joints/sections) only balances forces, never moments, at each joint. That is only valid if the joint truly cannot carry a moment.

1. Rough pins: friction lets the joint resist rotation, i.e., carry some moment, which the method-of-joints force-only balance does not account for, so ruled out.
2. Smooth pins: a frictionless pin transmits force only, never moment, exactly matching what the force-balance-only method of joints assumes.
3. Either of them: incorrect, since only the smooth-pin behavior is consistent with a force-only joint balance.
4. Neither of them: incorrect, since a pinned (hinge-type) connection is still assumed at every joint.

So truss members are assumed to be joined by smooth pins.

Answer 2

A third way to approach this is to think about why the idealization is made at all, by comparing it against how truss joints are actually built in practice (welded, riveted or bolted gusset plates) and asking which idealization best represents pure axial-load transfer.

1. Rough pins: real riveted or welded joints do have some rotational stiffness and can transfer a small secondary moment; if this were the assumption used, truss members would need to be analyzed as beam-columns with bending, which defeats the purpose of the simplified truss method, so this is not the assumption made.
2. Smooth pins: even though actual joints have some rigidity, engineers deliberately idealize them as frictionless pins so that any small secondary bending effects are ignored and the members can be treated as simple two-force axial members; this simplification is what makes standard truss analysis (method of joints/sections) tractable, so this is the assumption actually used.
3. Either of them: since only the smooth-pin idealization keeps the analysis reduced to axial forces only, treating the joint type as interchangeable is incorrect.
4. Neither of them: trusses are still modeled as pin-connected frameworks (as opposed to fully rigid frames), so some pin idealization is always present; this option is incorrect.

Comparing the idealization against real construction confirms that the simplifying assumption used is frictionless, moment-free pins.

Therefore, the correct answer is smooth pins.