Residential Building Inspector B1Floor ConstructionMedium
A floor assembly utilizes steel C-channel joists. Which of the following statements is true regarding the spanning capabilities of cold-formed steel floor joists compared to similarly sized wood joists?
- ASteel joist spans are primarily limited by fire-resistance ratings, not structural capacity.
- BSteel joists typically have longer allowable spans due to their higher strength-to-weight ratio.
- CSteel joists generally have shorter allowable spans due to their lower stiffness.
- DSteel joist spans are identical to wood joist spans of the same nominal dimensions.
Show answer & explanationAnswer & explanation
Correct answer: B. Steel joists typically have longer allowable spans due to their higher strength-to-weight ratio.
Cold-formed steel joists generally have a higher strength-to-weight ratio and greater stiffness than similarly sized dimensional lumber. This typically results in longer allowable spans for steel joists compared to wood, for equivalent loading conditions.
Why the other options are wrong
- A. While fire resistance is a factor, it doesn't primarily limit span capacity; structural capacity and deflection are the main drivers.
- C. Steel has higher stiffness for its weight compared to wood.
- D. This is incorrect; steel and wood have different structural properties leading to different span capabilities.
Steel vs. Wood Joist Spans
Comparison of the maximum allowable clear span for floor joists made from cold-formed steel versus dimensional lumber, based on their material properties.
- Steel has higher strength-to-weight ratio.
- Steel properties are more consistent than wood.
- Steel typically allows longer spans for similar depths.
Memory trick: Steel's Strength Stretches Spans, Wood's Weight Wields Well.