Built-up beams: When sizing cover plates, what percentage allowance should be added to the calculated area of cover plates to account for rivet (or bolt) holes in a traditional riveted built-up beam?

Introduction / Context:
In classic riveted or bolted built-up girders, cover plates over the flanges are pierced by fastener holes. These holes reduce the net area available to carry stress. A practical allowance is therefore added to the calculated required area so that the gross plate supplied still delivers the needed net capacity after deducting holes.

Given Data / Assumptions:

• Traditional riveted/bolted construction with cover plates.
• Allowance sought as a percentage of the calculated required area.
• Hole deduction approach based on standard practice.

Concept / Approach:

The net area equals gross area minus area lost due to holes. Rather than detailing every hole in preliminary design, a rule-of-thumb percentage is added to the required area. A widely used allowance is 15% for cover plates, which adequately compensates for typical hole patterns without being excessively conservative.

Step-by-Step Solution:

Verification / Alternative check:

Older steel manuals and exam standards consistently quote 15% as a practical additive for cover plates in riveted/bolted built-up beams, streamlining preliminary sizing before detailing exact hole layouts.

Why Other Options Are Wrong:

• 10% or 13%: may under-account for typical hole lines.
• 18% or 20%: often overly conservative at preliminary stage.

Common Pitfalls:

• Forgetting to recheck net capacity after final hole layout.
• Using the same allowance for flange angles or webs without considering different hole densities.

Final Answer:

15%.