Steel roof trusses: What is the maximum permissible slenderness ratio for a member that normally acts as a tie (primarily in tension) in a roof truss, considering occasional stress reversal under wind?

Introduction / Context:Slenderness ratio (effective length divided by least radius of gyration) limits help ensure that steel members do not buckle under compressive effects. In roof trusses, some members normally act as ties (in tension), but under wind or erection conditions they may experience limited compression, so design standards allow higher slenderness than for primary compression struts.

Given Data / Assumptions:

• Member is a truss tie, primarily in tension under gravity loads.
• Occasional load cases (wind, temperature, erection) may cause minor compression.
• We seek the maximum permitted slenderness consistent with common practice and exam standards.

Concept / Approach:

Members always in compression typically have tighter limits (for example around 180). For ties in roof trusses, many traditional guidelines permit higher slenderness, recognizing that the member is usually in tension. A commonly cited maximum for “members normally acting as ties” is 350, which balances economy with stability under occasional reversal.

Step-by-Step Solution:

Verification / Alternative check:

Historic working-stress references and many exam-centric summaries list 350 for ties, while main compression members stay near 180. This differentiation reflects the lower buckling risk for predominantly tensioned elements.

Why Other Options Are Wrong:

• 180–300: typical for compression struts or more restrictive serviceability cases; unnecessarily low for ties.

Common Pitfalls:

• Applying the 180 limit meant for compression columns to truss ties.
• Ignoring end conditions when computing effective length.

Final Answer:

350.