Steel design — thickness requirement for flat double lacing (compression members) For a laced compression member using flat bars in a double-lacing system, the minimum thickness t of each flat lacing bar is taken as what fraction of its effective length l (where l is the effective length of the lacing bar)?

Introduction / Context:
Laced compression members in steel structures use slender flat bars or angles to connect the main components and prevent out-of-plane buckling. Codes and classic design texts specify minimum size rules for these lacing bars to control local buckling and ensure adequate stiffness during fabrication and service, particularly distinguishing between single and double lacing.

Given Data / Assumptions:

• Member is a compression member with double lacing using flat bars.
• t denotes lacing bar thickness; l denotes the effective length of that lacing bar.
• Standard IS/handbook-style sizing rules apply (detailing requirements).

Concept / Approach:
Double-laced systems have shorter effective length per diagonal because two systems share shear and stabilize the main member more effectively. Hence, codes permit a lower t/l ratio than for single lacing. A common rule is: single lacing t ≥ l/40, double lacing t ≥ l/60. Effective length l is measured as the distance between inner end fasteners for single lacing and taken as 0.7 times that distance for double lacing in riveted/bolted work.

Step-by-Step Solution:
Step 1: Identify l as the effective length of a lacing bar for double lacing.Step 2: Apply detailing rule: thickness of flat for double lacing ≥ l/60.Step 3: Compare with options; select l/60.

Verification / Alternative check:
Reference tables and practice problems for laced columns repeatedly list single lacing t ≥ l/40 and double lacing t ≥ l/60, matching workshop-fabrication experience and preventing thin-plate local buckling.

Why Other Options Are Wrong:
l/20, l/30, l/40: These are too thick for double lacing and correspond to more conservative or single-lacing rules.
l/80: Too thin; risks local buckling and handling damage.

Common Pitfalls:

• Confusing single- and double-lacing thickness rules.
• Using panel length instead of the lacing bar's effective length.
• Ignoring that double lacing reduces effective length by about 0.7 in riveted/bolted connections.

Final Answer:
l/60