Previously in our Fire Walls, Part 1: What do they get me? post, we had outlined what fire walls do from a code perspective. In addition to what role these assemblies serve, questions commonly arise relative to the construction detailing of fire walls given the number of requirements to consider.
The building code allows two options for fire wall construction:
- The first option is a single fire wall assembly that is structurally independent from both buildings (NFPA 221, 6.3 & 6.4).
- Cantilevered Fire Walls are entirely self-supported and nonbearing. No connections are permitted to building(s) or contents on either side other than to the flashing and should be constructed where there is a break in the structural framework (NFPA 221, 6.3).
- Tied Fire Walls are centered on a single column line or constructed between a double column line. The structural framing on either side lines up horizontally and vertically, and supports the roof (NFPA 221, 6.4).
- The second option is the construction of a double fire wall assembly consisting of two back-to-back fire walls. Each fire wall is supported laterally by the building frame on its respective side and independent of the fire wall and framing of the opposite side (NFPA 221, 6.5).
All fire walls are required to be designed and constructed to remain after collapse of the structure due to fire on either side of the wall (NFPA 221, 6.2.1). Specifically, these assemblies are required to be continuous both vertically and horizontally and have specific termination requirements.
Fire Resistance Rating
The required ratings of fire walls are based on the occupancy classifications within a building (IBC Table 706.4). Double wall assemblies are considered to have a combined assembly fire-resistance rating; for instance the equivalent of a 3-hour, single fire wall is 2, 2-hour rated fire walls in a double assembly (NFPA 221 Table 4.5).
The aggregate width of protected openings at any floor level in a fire wall is not permitted to exceed 25% of the length of the wall (780 CMR 706.8). Egress door openings within double fire walls are required to be protected with a pair of doors and a vestibule (NFPA 221, 126.96.36.199).
Whether a single or double fire wall is being explored as part of the design, it is important to review the necessary fire-resistant rating, structural stability, continuity, and opening requirements applicable to both options, as well as detailed criteria from 780 CMR Section 706 and NFPA 221.