Structural Elements Supporting Fire Separations

By Steven Dannaway, PE | Principal, Operations Manager, Fire Protection Engineering | Los Angeles Office

This article exemplifies why it is critical for architects, structural engineers, and fire protection engineers to properly identify the types of fire resistance-rated separations within a building and coordinate the construction requirements.

Conditions for fire-resistance rated assemblies

The 2021 International Building Code (IBC) prescribes various conditions where fire-resistance rated assemblies must be provided to separate two areas of a building. Types of fire separations may include fire barriers, fire walls, horizontal assemblies, fire partitions, and smoke barriers. In buildings of Type II, III, and V construction, a commonly overlooked code requirement by designers and code officials is the supporting construction code requirement in IBC Chapter 7. The requirement is specific to fire barriers and horizontal assemblies, and includes the following:

• IBC 707.5.1 Supporting construction [fire barriers]. The supporting construction for a fire barrier shall be protected to afford the required fire-resistance rating of the fire barrier supported.
• IBC 711.2.3 Supporting construction [horizontal assemblies]. The supporting construction shall be protected to afford the required fire-resistance rating of the horizontal assembly supported.

The premise of the requirement is logical. Fire barriers and horizontal assemblies are fire separations intended to prevent the spread of fire from one side of the fire separation to the other. If a fire event threatens the structural elements that support the fire separation assembly and leads to premature failure of the structure, the fire barrier or horizontal assembly would not be expected to remain in place to serve its intended function. A 1-hour fire barrier separation protecting a shaft enclosure may not be effective if the steel beams and columns supporting the fire barrier are non-rated, and these supporting members fail earlier in the fire event.

Type II, III, and V Construction Considerations

In Type II-B, III-B, and V-B construction, IBC Table 601 permits structural elements to be non-rated. However, it is common in buildings, especially multi-story buildings, to have fire-resistance rated shaft enclosures, interior exit stairways, elevator hoistway enclosures, occupancy separations, and other types of fire separations that must be constructed as “fire barriers” or “horizontal assemblies” per IBC 707 or 711. The supporting construction code requirement is often overlooked as a result of this conflict between IBC Table 601 and IBC 707/711.

IBC Table 601 permits non-rated construction in these construction types, and designers may there. Yet, IBC 707.5.1 and 711.2.3 require the designer to identify any fire barriers or horizontal assemblies in the building, determine which structural elements support those assemblies, and provide fire-resistive protection to those structural elements equal to the hourly rating of the wall or assembly they support.

Example of Analyzing Supporting Construction

Let’s look at an example situation:

• Four-story university academic building.
• Mixture of Group B and Group A-3 occupancies.
• Project stakeholders desire the use of Type II-B construction, using noncombustible materials that are non-rated as permitted by IBC Table 601.

The IBC allowable height limits (Chapter 5) only allow Group A-3 occupancies up to Level 3 in Type II-B construction. Group B occupancies may be located on Level 4. Therefore, Group A-3 occupancies require an occupancy separation from the Group B occupancies on Level 4, which may be provided by a combination of 1-hour fire barriers and horizontal assemblies.

Structural elements that support the occupancy separation will require a minimum 1-hour fire-resistance rating as supporting construction. Assume the designer determined the Level 3 floor assembly would serve as the occupancy separation between Level 3 and Level 4. This approach requires all the supporting construction of the Level 3 floor assembly to have a 1-hour fire-resistance rating, potentially impacting a large portion of the building structure. A building once envisioned to be primarily composed of non-rated structural elements is now mostly fireproofed. A similar analysis would also be required for any fire-resistance rated stairway enclosures, shaft enclosures, or elevator hoistways in the building.