Maximizing Building Size Limits

Designers need not avoid wood construction due to the tabular height or area limits provided in the code. There are at least two ways the code allows a design to exceed the area and height limits for construction types.

• Fire Walls – IBC Section 706 specifically addresses fire walls and permits portions of a building separated by one or more of them to be considered as separate, side-by-side buildings for purposes of allowable size and height. In this way, wood-frame buildings can be designed as separate but connected buildings for code-compliance purposes.
• Podium Design – For mid-rise wood-frame buildings, architects and engineers are increasingly turning to podium or pedestal design instead of building directly on a concrete slab on grade. This can provide for additional floor levels for the building compared to what is typically permitted. Section 510 of the IBC addresses Special Provisions and allows five stories of Type III-A wood-frame structures over one or more levels of Type I-A construction. These typically 5-over-1 and 5-over-2 buildings are treated in the code as two structures when they are separated by a 3-hour fire-resistance-rated horizontal assembly. The podium is considered as a separate and distinct building for the purpose of determining height and area limitations and vertical continuity of fire walls. The overall height of the two buildings together is measured from grade plane, and the height-above-grade limitations of Chapter 5 for the upper building apply.

Fire-Resistance-Rated Assemblies

1- and 2-hour fire-resistance requirements for light-frame construction are generally met by using assemblies of products and materials, often including gypsum sheathing over wood, which have been tested in accordance with ASTM E119 or UL 263, although there are other methods based on testing that can be used as indicated in Section 703.3. Lists of wood floor and wall assemblies with accepted ratings are available from the American Wood Council (DCA 3: Fire-Resistance-Rated Wood-Frame Wall and Floor/Ceiling Assemblies, www.awc.org ). Other sources include Underwriters Laboratories (UL)’ fire-resistance-rated systems and products, the UL Fire Resistance Directory, and the Gypsum Association’s Fire Resistance Design Manual.

Fire Resistance of Heavy Timber

Part of the reason heavy timber has effective fire resistance is that it takes a long time to burn through large wood members. That means that heavy timbers have an advantage in a fire because they char on the outside while retaining their strength and load-carrying capacity, slowing combustion and allowing time for occupants to evacuate the building and improving safety for first responders. Heavy timber sizing is addressed in the code to allow for structural safety under fire conditions to be built in. When heavy timber is required to be fire-resistance-rated and the calculation methods of Section 722 of the IBC and Chapter 16 of the AWC National Design Specification (NDS) for Wood Construction are used, a defined percentage of the wood member is provided as sacrificial protection (considered to burn or char in a fire). Wood fiber beneath the char layer retains much of its strength during the fire, so by oversizing the member, structural integrity remains despite the loss of cross-sectional area.

This has direct application to multifamily construction. According to IBC Table 601, Fire-Resistance Ratings Requirements for Building Elements, heavy timber is permitted in roof construction as an alternative to 1-hour (or less) fire-resistance-rated construction for all occupancies, and in all construction types except Type IA.

Mass timber is well suited for fire resistance in multifamily construction. This was reflected in a case study showing the general liability insurance risks of a mass timber building to be no different than that of a concrete or steel building.¹¹

Fire Retardant-Treated Wood (FRT)

IBC Section 2303.2 defines requirements for FRT wood. Building designers must be aware that design values should be adjusted when FRT wood and sheathing is used. Strength adjustments are provided by the treaters based on proprietary chemical formulations. Sections 602.3 and 602.4.1 of the 2021 IBC clarify that both wood framing and sheathing are permitted in exterior walls of Type III and IV buildings where FRT wood is used. CLT is also permitted within exterior wall assemblies of Type IV construction not less than 6 inches thick with a 2-hour rating or less when the exterior surface of the wood panel is protected by FRT wood sheathing, gypsum board, or a noncombustible material. There are also applications for FRT wood in Types I and II construction, such as nonbearing partitions and exterior walls, roof construction, and balconies, subject to the limitations in Section 603.1.

Designing Building Elements for Different Adjacent Fire Ratings

One design consideration for some construction types is that floors and walls may have different fire rating requirements. For example, in Type III-A construction, load-bearing exterior walls are required to have a 2-hour rating, but a floor may only require a 1-hour rating. If it is necessary to address the continuity of fire resistance for the wall at the intersection of the floor, detailing to achieve this smoothly and effectively is important to a good design. The American Wood Council has developed fire resistance details for these intersections in their Design for Code Acceptance (DCA) 3 Fire-Resistance-Rated Wood-Frame Wall and Floor/Ceiling Assemblies, available at https://awc.org/codes-standards/publications/dca3. Designers should work with their local building official to determine acceptable solutions to fire-resistance detailing at the floor-to-wall intersection.

Multifamily Development Case Study

Photo courtesy of Pollack Shores, Matrix Residential

Project: The Brooklyn Riverside
Location: Jacksonville, Florida
Architect: Dwell Design Studio

The Brooklyn Riverside development consists of seven Type V-A multifamily buildings, with a total of 310 units and 77 private tuck-under garages. Density was an important design objective, and the use of sprinklers, as well as significant open frontage and parking, allowed the design team to add an additional story to the buildings, increasing the number of units. The project also includes a 2,200-square-foot clubroom, leasing office, fitness center, gaming center, pool, and amenity deck, which features a covered bar and lounge. Moisture management was a key design consideration and designers used multiple strategies. For example, wood structural sheathing panels were used as an exterior applied air barrier, breezeway entries and garage slabs were dropped 6 inches from adjacent finished floors and sloped toward the exterior, and perimeter isolation strips were used within each unit, between the gypcrete, wall finish, and baseplate.

Design Considerations For Framing With Wood

Traditional practices and code provisions for wood framing are established and well-known such that wood framing options and details are readily available to designers. Chapters 16, 17, and 23 of the IBC cover structural wood design, construction, and inspection techniques in considerable detail. However, in addition to selecting the appropriate framing technique, designers of wood buildings must consider factors such as shrinkage, differential movement, and seismic requirements.

Framing Types

There are three common types of framing for wood construction. For Type V-A buildings, where walls require a 1-hour fire-resistance rating, designers usually use traditional platform framing, where the joists sit on top of the double top plates of the wall. Balloon framing is based on the joists hanging off a ledger that is attached to the structural studs which frame the building. In modified or semi-balloon framing, the floor framing hangs off double top plates; it is often used as an alternative to platform-framed structures for both Type V-A and Type III-A construction.

Wood Shrinkage

Regardless of the framing type, IBC Section 2304.3.3 requires that designs for buildings over three stories consider the fact that wood shrinks as it dries. Shrinkage continues until wood reaches its Equilibrium Moisture Content (EMC), which averages 8 to 12 percent moisture content for most structures in the U.S.

Shrinkage effects must be considered for horizontal framing members (width or thickness) in the wall (top/sill plates) and floor (joists) design. Wood is anisotropic, meaning the dimensional change in wood is unequal in different directions. In most softwoods, radial shrinkage (across growth rings) is approximately 4 percent and tangential shrinkage (parallel to growth rings) is approximately 8 percent from green (unseasoned) to typical EMC for structures in the U.S. Longitudinal shrinkage (parallel-to-grain) for vertical framing members is generally negligible and does not affect building performance. Therefore, the majority of shrinkage will occur in the top plates, sill plate, and sole plates, and possibly the floor joists—depending on how the floor framing members are framed to the wall. If the design uses balloon- or modified balloon-framing, then sawn lumber joists won’t play a huge role in overall movement from shrinkage because balloon framing, unlike platform framing, does not accumulate shrinkage over all floors.