Appeal 35050

2025 OSSC Table 601: Fire-Resistance Rating Requirements for Primary Structural Frame, Floor Construction and Associated Secondary Structural Members, and Roof Construction and Associated Secondary Structural Members in Type IV-HT.

Requires a fire-resistance rating of HT.

It is requested to allow noncombustible structural elements (columns, beams, floor and roof construction) at noted locations (east end below grade, circulation cores, and steel moment frames) in Type IV-HT construction when provided with a 1-hour fire-resistance-rating.

HT FIRE-RESISTANCE RATING FOR HEAVY TIMBER: Per §2304.11, structural elements that are provided as heavy timber will comply with the applicable provisions of §2304.11.1 through §2304.11.4. The minimum sizes outlined throughout this section provide the required fire-resistance rating to meet HT where heavy timber is specified.

1-HOUR FIRE-RESISTANCE RATING FOR NONCOMBUSTIBLE STRUCTURAL ELEMENTS: Per §602.4, building elements in Type IV construction are allowed to be constructed of noncombustible materials. Since no fire-resistance rating other than HT is specifically identified in Table 601 for Type IV-HT construction, it is proposed that the fire-resistance rating be provided as 1-hour in accordance with §703.2.

FIRE-RESISTANCE RATING AT CONNECTIONS: The following connections will be provided prescriptively in terms of the continuity and termination of the fire-resistance rating:
• Steel to steel (continuous 1-hour protection)
• Steel to concrete (transition of 1-hour protection to a 1-hour fire-resistance-rated assembly)
• Heavy timber to heavy timber (some exposed steel, but primarily concealed connectors or a wood beam bearing on a wood column)
• Steel to heavy timber (at limited conditions, since the steel is required to have a 1-hour fire-resistance rating, the heavy timber is therefore considered as supporting construction and is also provided with a 1-hour fire-resistance rating)

While Type IV-HT construction allows for exposed steel at the connections between heavy timber members, where heavy timber elements (beams) are framing into noncombustible structural elements (columns) requiring a 1-hour fire-resistance, §704.5 and §704.5.1 will be applied to the fire-resistance rating as it extends from the noncombustible structural element and overlaps the HT rating. This means that the 1-hour fire-resistance rating will overlap the HT rating (including the exposed steel connector) by 12 inches.

LOCATIONS OF NONCOMBUSTIBLE STRUCTURAL ELEMENTS:

East End Below Grade: The North Classroom Building is proposed as four stories of Type IV-HT construction. Due to the significant change in grade, sloping up across the site from west to east, there is a portion of the east end of the North Classroom Building that is below grade. Inside the concrete retaining wall, this area is proposed as steel columns supporting a concrete slab on metal deck. This floor system continues south to support the Connector as the steel allows for longer spans.

Circulation Cores and Steel Moment Frames: There are three, four-story bays containing the stair and elevator cores; they are all constructed of noncombustible structural elements, including a concrete slab on metal deck at each of the floors and at the roof, along with supporting construction. As well, steel moment frames are located in these three circulation cores to provide easier erection (the cores self-stabilize) and to simplify the stair design and connections. Additional steel moment frames are located parallel and slightly interior to the exterior walls (three bays) and perpendicular to the exterior walls at interior bays (three bays) for a total of six locations. The moment frames are required to provide adequate seismic resistance.

EQUIVALENT PROTECTION: If the mass timber elements were provided as 1-hour, with the fire-resistance rating determined by calculating the char depth as allowed per AWS 2018 National Design Specifications (NDS) for Wood Construction and outlined in Technical Report 10 (TR10), the resultant sizes would be dimensionally larger. Therefore, it is asserted that 1-hour provides an equivalent or better fire-resistance rating than that provided by HT.

For example, per Table 2304.11, after considering the associated load and span conditions, the minimum glulam beam at the classroom block would be 8 3/4 inches wide by 27 inches deep to achieve an HT fire-resistance rating. If this same member was provided as 1-hour, it would be 10 3/4 inches wide by 27 inches deep, which is a larger glulam beam size.

Also, per the 2021 IBC Handbook , it is stated that “Although Type IV-HT construction is not generally recognized as equivalent to 1-hour fire-resistance-rated construction, the code considers heavy timber to provide equivalent protection”. This supports the assertion above. It should also be noted that the interior bearing walls are allowed with a fire-resistance rating of 1-hour or HT per Table 601, further reiterating the equivalency.

CONCLUSION: Therefore, due to the documented support for 1-hour fire-resistance-rated construction as equivalent or better than a fire-resistance rating of HT as it applies to heavy timber, it is requested to allow noncombustible structural elements with a fire-resistance rating of 1-hour at the locations indicated in the North Classroom Building of Type IV-HT.

ATTACHMENTS:
Appeal Attachment #1 (3D of North Classroom Building, 1 sheet, dynamically viewable)
Appeal Attachment #2 (Structural Framing Plans of North Classroom Building and Details, 16 sheets)
Appeal Attachment #3 (Architectural Plans of North Classroom Building, 4 sheets)
Appeal Attachment #4 (Typical Fireproofing Details at Connections, 2 sheets)