An inspector is reviewing the design for a steel-framed floor system in a multi-story commercial building. The plans indicate that the steel beams are to be protected with spray-applied fire-resistive materials (SFRM). According to the IBC, what is the minimum thickness of SFRM required to achieve a 2-hour fire-resistance rating for a W12x50 steel beam?
- A7/8 inch (22.2 mm)
- B1-1/8 inch (28.6 mm)
- C3/8 inch (9.5 mm)
- D1/2 inch (12.7 mm)
Show answer & explanationAnswer & explanation
Correct answer: A. 7/8 inch (22.2 mm)
The required thickness of SFRM depends on the W/D ratio (heated perimeter divided by area of steel) of the beam and the desired fire-resistance rating. For a W12x50 beam, the W/D ratio is approximately 1.76. Consulting UL design guides (as referenced by IBC for specific assemblies), a 2-hour rating for a beam with a W/D ratio around 1.76 typically requires 7/8 inch of SFRM. This specific value is derived from UL design tables, not a simple IBC formula.
Why the other options are wrong
- B. 1-1/8 inch would typically provide a 3-hour or higher rating, or be for a beam with a significantly lower W/D ratio (more exposed surface area relative to mass).
- C. 3/8 inch typically provides a 1-hour rating for beams with higher W/D ratios or for lighter sections.
- D. 1/2 inch is common for 1-hour ratings on typical beams or 2-hour ratings on very heavy sections.
SFRM Thickness
The thickness of spray-applied fire-resistive materials (SFRM) is critical for achieving the specified fire-resistance rating of steel structural members, depending on the member's geometry (W/D ratio).
- Thickness depends on the W/D ratio (heated perimeter/area) of the steel member.
- Higher W/D ratio generally requires more SFRM thickness for the same rating.
- Specific thicknesses are found in approved fire-resistance rated assembly designs (e.g., UL listings).
- A 2-hour rating is a common requirement for floor beams in many construction types.
Memory trick: Steel's heat shield, W/D's yield, UL's field.