Tension member made of two channel sections: how should the allowance for rivet (or bolt) holes be made when calculating the net area?

Introduction / Context:
When computing the net effective area of a built-up tension member, deductions must be made for fastener holes along the critical (governing) section. For members composed of two channels, the deduction rule ensures the most critical potential failure path is considered.

Given Data / Assumptions:

• Member comprises two channel sections acting together in tension.
• Holes may lie in webs or flanges depending on connection detail to the gusset plate or splice plates.
• Objective: determine the correct allowance for hole deductions.

Concept / Approach:
The design convention is to deduct the greater of two possibilities because the net section must be safe for any plausible failure path: (1) two holes from each web (since each channel has one web, two channels give two web holes), or (2) one hole from each flange (two flanges per pair considered along the likely path). Taking the greater deduction gives a conservative net area.

Step-by-Step Solution:

Verification / Alternative check:
Trace potential zig-zag or straight failure paths across the section and ensure the chosen deduction covers the least net area (most critical path).

Why Other Options Are Wrong:
Deducting only webs or only flanges could underestimate the critical path; choosing “whichever is less” is unsafe.

Common Pitfalls:
Ignoring stagger corrections on zig-zag paths; using nominal hole size instead of standard hole diameter for deductions; missing the thickness appropriate to the fastened element.

Final Answer:
Two holes from each web or one hole from each flange, whichever is more