Section 704.2 of the 2018 International Building Code (IBC) was revised from its origin which began in the legacy building codes and was primarily intended to address steel construction. In earlier editions of the IBC, omission of fireproofing on portions of steel columns or beams behind ceiling or wall membranes of fire-resistance-rated assemblies was permitted. During more recent code development cycles, it was agreed that membrane protection alone was inadequate, especially for members carrying the upper floors of a building, since they could be exposed to fire which originates in a concealed space or from fire in a room if the membrane protection fails. Because steel has no inherent fire resistance (steel can yield quickly at temperatures commonly occurring in fires), the method and location of the protection is considered critical.
The 2018 IBC was revised to clearly indicate that columns as well as studs and boundary elements in walls of light-frame construction and are located entirely between the top and bottom plates are permitted to have their required fire resistance ratings provided by the membrane protection for the wall. Elements within fire-resistance-rated walls of light-frame construction are addressed directly in IBC Section 704.4.1 (Light-frame Construction) and can be part of a fire-resistance-rated wall assembly without any additional fire protection. Many buildings are built out of typical light frame construction; the concentrated loads from trusses or beams must have a continuous load path to the foundation. Previously, some jurisdictions were interpreting that these construction elements were considered primary structural columns and requiring them to be provided with individual fire protection. It was never the intent to require individual fire protection of these elements as they are not considered a portion of the primary structural frame.
IBC Section 202 defines the Primary Structural Frame as the Columns; Structural Members, Floor Construction and Roof Construction (all having direct connections to the columns); and Bracing members essential to the vertical stability of the primary structural frame under gravity loading. IBC Section 704.2 requires all columns, that are required to be protected, to be protected with individual encasement protection throughout the entire column length. This section clarifies that columns extending up through the ceiling must extend the required encasement protection from the foundation to the beam above.
By definition, a light-frame construction wall is primarily built with repetitive studs. When braced by sheathing or gypsum board attached to the wall framing, the design of studs, multiple studs, or posts is similar based on the L/d for the member buckling out of the plane of the wall. Perpendicular to grain bearing of the studs on the top and bottom plates is also a consideration and is the same regardless of whether the studs are spaced, studpacks, or posts. The structural design of all of these members is considered column design. In previous editions of the IBC or legacy codes an issue sometimes occurred when studs, multiple studs, or solid sawn members were framed integrally within a fire-resistance-rated light frame wall and were treated as columns according to the definition of primary structural frame, and were then required to be provided with individual encasement protection.
The intent of Sections 704.2 and 704.4.1 have been clarified in the 2018 IBC to state studs, boundary elements, posts, multiple stud groups, built-up columns, and solid columns that are framed within the wall and do not penetrate the top or bottom plates are all designed to the same criteria and shall be considered integral elements. These elements that are integral within the confines of the load bearing wall, and do not penetrate the top or bottom plates, are permitted to be protected in light frame construction by the membrane protection of the fire-resistance-rated bearing wall.
What is the correct application of 2018 IBC Section 704.3 (Protection of the primary structural frame other than columns) to wood construction?
Definitions for “primary structural frame” and “light-frame construction” are included in IBC Chapter 2. IBC Section 704.3 (Protection of the primary structural frame other than columns) is for systems that meet the definition of “primary structural frame,” but not heavy timber or light-frame construction. Floor joists, ceiling joists, and rafters in light-frame construction do not fall within the definition of primary structural frame. Likewise, wood beams, if required to be rated, (Type IIIA or VA building) are typically part of a light-frame system. Their fire resistance would be established by normal means, whether calculating fire resistance as an exposed wood member or protecting with other materials. As for Type IV, Table 601 requires no fire resistance rating for structural elements, as long as they meet the minimum required dimensions for Type IV construction as specified in IBC Section 2304.11.sf.