Our article in Structure Magazine outlines our holistic approach to designing 5-story wood-framed buildings utilizing Type III construction. Congrats to co-authors Jared Hudson, P.E., and Shaun Kreidel, S.E. Great job!
A Practicing Engineer’s Approach to Wood-Framed Type III Construction
Design considerations and common detailing strategies for Wood-Framed Type III construction are discussed.
Light frame wood construction is often a desired construction method for low-rise multifamily structures due to readily available labor and materials, speed of construction, sustainability, and relatively low construction costs. A Type V construction classification as defined by the International Building Code (IBC) is commonplace for these structures; however, this construction type is limited to four stories of stacking wood construction. A Type III construction classification allows conventional wood-framed structures to include an additional level, bringing the allowable height to five stories above grade; see Figure 1 for an example of this type of construction. This construction type may be attractive to developers looking to maximize the occupiable square footage of a defined footprint while taking advantage of the many benefits that come with light-frame wood construction. To facilitate a Type III classification, unique structural and architectural detailing is needed to maintain the strength, stability, and serviceability of the wood-framed structure, as well as to address the applicable fire design requirements. These details are multidisciplinary in nature and require a high level of collaboration between the structural engineer, architect, and builder/developer to ensure that the project meets the owner’s expectations and the building code requirements of the Authority Having Jurisdiction (AHJ).
Depending on the requirements of a given project, practicing engineers may need to investigate certain design aspects that become critical when meeting the requirements of Type III construction. These design considerations include material requirements, fire-resistance rating requirements, the importance of designing for wood shrinkage, and structural detailing strategies to accommodate fire-resistance ratings at the intersection of the floor/roof assemblies and exterior wall assemblies.
Material Requirements
While construction Types I, II and III all require the use of non-combustible materials at exterior walls, the IBC recognizes the use of fire-retardant-treated (FRT) sawn lumber and FRT wood structural panel (WSP) sheathing as acceptable materials to satisfy the requirement under Type III construction. Practicing engineers should account for FRT lumber and FRT sheathing strength reduction factors due to the treatment process. The strength reduction factors are manufacturer-specific, thus coordination with the architect and builder/developer is recommended if the intended product is unknown.
FRT treatment process results in sheathing strength reduction factors which can decrease both the allowable spans and the lateral strength/stiffness of diaphragms or shear walls. FRT lumber treatment process also affects the structural properties of sawn lumber; the designer may need to augment the wall/header designs to mitigate these effects. Table 1 illustrates the strength reduction factors from two manufacturers of FRT sawn lumber. Assumed in-service temperature of the lumber is an important consideration that may cause variation in structural property values between manufacturers. High in-service temperatures of more than 100 degrees Fahrenheit will correspond to a greater reduction in strength and stiffness when coupled with fire retardant treatment. The engineer should also account for any wood incising reduction factors that might be needed to treat the lumber and consider using lumber that does not require incising to mitigate the amount of strength reduction. All minimum assumed FRT properties should be listed as design assumptions in the contract drawings to ensure that suitable lumber and WSP products are utilized.
A designer may encounter situations where spans or loads require structural properties beyond what FRT lumber alone can provide. At this time, there are no fire-treated engineered wood products on the market (e.g., LVL, PSL, LSL) known to the author. One strategy available to designers is to utilize a flitch beam; a composite beam that consists of FRT wood laminations bolted to a continuous steel plate. The FRT laminations of the composite assembly will maintain the non-combustibility requirement; however, special attention to detailing to adequately conceal the heads of the bolts of the flitch beam assembly will be required. The designer should also consider the expansion of the longitudinal steel due to elevated service temperatures for longer-spanning flitch beams. Another strategy that the designer can employ is the use of rolled steel framing members within the exterior wall. These members may require additional fire protection in addition to meeting the noncombustible requirements of the code; the project architect should be consulted for additional fire protection requirements of these members.
Fire Rating Requirements
Type III construction requires that exterior loadbearing walls satisfy a 2-hour fire-resistance rating (FRR). If exterior walls can be classified as non-load bearing, the FRR can be reduced to 1-hour for certain occupancies. A 2-hour FRR is usually accomplished by having two interior layers of gypsum board. Over the full perimeter of the structure, the added cost of an additional layer of gypsum board can be substantial. A common industry interpretation of a non-load bearing exterior wall is one that does not support anything but its self-weight and the self-weight of the walls above. The structural designer can strategically run the framing parallel or introduce girder members parallel with the exterior wall to avoid a load bearing situation. In doing so, a FRR of 1 hour can be utilized and thus an extra layer of interior gypsum board can be avoided. This approach and interpretation should be discussed with the project architect and the AHJ during design to ensure compliance with the local building code.
The vertical continuity requirements of the rated exterior wall assembly have been a hotly debated topic between jurisdictions and design professionals, but the requirements have finally been clarified in the 2024 IBC. According to Section 705.6 of the 2024 IBC, the exterior wall FRR shall extend continuously from the top of the foundation/floor system below to the underside of the roof/floor sheathing above. However, if the fire separation distance (as defined in the IBC) is greater than 10 feet, the exterior wall FRR is permitted to terminate at the underside of a ceiling (floor or roof) assembly having an equal or greater FRR than the exterior wall. Detailing at the floor levels and the roof level will need to conform to these requirements. Some commonly used detailing strategies that meet these criteria are presented later in this article. Continue reading “M+K Article Published in STRUCTURE Magazine”
