Roof truss selection for factory spans of 6 m to 9 m: Which truss type is most appropriate to design and supervise?

Difficulty: Easy

Correct Answer: Queen-post truss

Explanation:


Introduction / Context:
Choosing a roof truss depends on span, loading, roof form, and economy. Traditional timber and steel trusses have practical span ranges beyond which alternative forms become more efficient. This question focuses on common light industrial spans of 6 m to 9 m.


Given Data / Assumptions:

  • Factory roof spans between 6 m and 9 m.
  • Conventional symmetrical truss forms considered.
  • Standard practice span ranges guide selection.


Concept / Approach:
King-post trusses are most economical for short spans (about 5 m to 8 m). Queen-post trusses economically cover medium spans (about 8 m to 12 m, often quoted 7 m to 10 m). For 6 m to 9 m spans, the range overlaps, but a queen-post truss offers greater headroom and better economy as the span approaches 9 m, while still being efficient at the lower end of the range when future load or roof slope considerations apply.


Step-by-Step Solution:
Identify the target span band: 6–9 m.Recall typical span suitability: King-post ~5–8 m; Queen-post ~8–12 m.For a span band that extends up to 9 m, prefer the truss that comfortably covers the upper limit (queen-post).Select queen-post truss as the appropriate choice.


Verification / Alternative check:
Design handbooks and building practice commonly recommend queen-post for spans near 9–10 m; king-post becomes material-inefficient as span increases toward 9 m under industrial loads.



Why Other Options Are Wrong:
Mansard truss: Specialized broken-pitch roof form, not chosen primarily for this span band.King-post truss: Better for shorter spans; may be marginal at 9 m.Collar truss: Typically for steep-pitched roofs and shorter spans in domestic settings.None of these: Incorrect because queen-post is suitable.


Common Pitfalls:
Assuming the smallest truss that can span is best; economy and deflection often favor the next truss type as span approaches the upper end of a range.



Final Answer:
Queen-post truss.

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