Channel section orientation and radius of gyration When a large value of radius of gyration is not required, how are a pair of channel sections typically arranged?
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AChannels placed back-to-back
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BChannel flanges kept inward (toe-to-toe)
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CChannel flanges kept outward (heel-to-heel)
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DChannels placed with webs separated by spacers
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ENone of these
Answer
Correct Answer: Channel flanges kept inward (toe-to-toe)
Explanation
Introduction / Context:Built-up members using channel sections can be oriented to tailor the principal radii of gyration. The flange orientation affects stiffness about both principal axes and the torsional properties.
Given Data / Assumptions:
- Two identical channels are combined.
- Goal: not to maximize the radius of gyration; a more compact arrangement is acceptable.
Concept / Approach:Placing channels with flanges outward increases the radius of gyration about the minor axis, which is beneficial when buckling about that axis governs. Conversely, placing flanges inward reduces the spread of material, giving a smaller radius of gyration.
Step-by-Step Solution:
Assess objective: large r not required → choose compact orientation.Compact orientation corresponds to flanges inward (toe-to-toe).Therefore select “Channel flanges kept inward”.Verification / Alternative check:Section property calculations show reduced Iy (and hence ry) when flanges are brought closer together.
Why Other Options Are Wrong:Flanges outward increases r (contrary to intent); back-to-back without specifying flange direction is ambiguous; spacer-separated webs generally increase r.
Common Pitfalls:Assuming any built-up orientation always improves buckling; the layout must match the design objective.
Final Answer:Channel flanges kept inward (toe-to-toe)