Flat single lacing — minimum thickness criterion Let l be the effective length of a flat lacing bar; choose the correct rule for thickness.

Introduction:
Lacing bars tie together main components of built-up compression members and must be sufficiently stiff and strong. Design codes specify minimum thickness and slenderness limits for lacing to prevent local buckling and to ensure adequate shear transfer between components.

Given Data / Assumptions:

• Flat single lacing bars are used.
• l denotes effective length of an individual lacing bar between node points.
• We seek the minimum thickness rule for flat single lacing.

Concept / Approach:
The minimum thickness requirement controls plate-like local buckling and ensures adequate stiffness. A practical empirical rule relates thickness to the effective length segment: t should not be too small compared to l. For flat single lacing, the conventional rule is t ≈ l/40 (minimum), together with separate limits on bar slenderness (L/r) and minimum width.

Step-by-Step Solution:
1) Identify that a flat, single lacing configuration is used.2) Apply the standard minimum thickness rule for such lacing.3) Take t(min) = l / 40.4) Check separately that L/r does not exceed the code limit for single lacing and that shear capacity is adequate.

Verification / Alternative check:
Design handbooks tabulate lacing proportioning rules; l/40 sits within the accepted range used in traditional steelwork practice for single lacing.

Why Other Options Are Wrong:
l/30 is unnecessarily thick; l/50 and l/60 may be too thin, risking local buckling and excessive flexibility.

Common Pitfalls:
Using the same rule for double lacing; ignoring end connections and rivet/bolt weld details that also control stiffness.

Final Answer:
t = l / 40 (minimum)