Steel design – lacing bars: In a built-up compression member with double lacing, what is the minimum required thickness t of flat lacing in relation to the length between inner end rivets L? (Assume guidance as per standard steel design practice; express your answer as a limiting ratio that ensures stiffness and resistance to buckling of the lacing.)

Introduction / Context:
Lacing bars connect the main components of a built-up compression member so that the parts act as a single column. Correct detailing prevents premature buckling of the lacing system and ensures that the whole member carries load efficiently. Double lacing requires specific minimum thickness to resist in-plane and out-of-plane forces and to provide adequate stiffness.

Given Data / Assumptions:

• A built-up steel compression member employing double lacing.
• L is the length between the inner end fasteners (rivet/bolt/weld) of a lacing bar.
• We seek the minimum thickness t of flat lacing as a ratio of L in standard practice.

Concept / Approach:

Design standards require minimum thickness limits to control slenderness of lacing bars, limit local buckling, and provide robustness during fabrication and erection. For double lacing, the usual prescription is less severe than for single lacing because load paths are shared. A commonly adopted rule is t not less than L/60 for double lacing (compared with t not less than L/40 for single lacing).

Step-by-Step Solution:

Verification / Alternative check:

Handbooks and teaching texts consistently list t ≥ L/60 for double lacing and t ≥ L/40 for single lacing to maintain an acceptable bar slenderness and prevent undue flexibility or damage during handling.

Why Other Options Are Wrong:

• t ≥ L/40 or L/50: appropriate or conservative for single lacing; unnecessarily thick for double lacing.
• t ≥ L/70 or L/80: too thin and prone to buckling; does not meet typical codal recommendations.

Common Pitfalls:

• Confusing single and double lacing requirements.
• Measuring L incorrectly (use the length between inner end fasteners along the bar).
• Ignoring corrosion allowance or fabrication tolerances that may require slightly thicker plates in practice.

Final Answer:

t ≥ L/60.