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28 csengineermag.com MAY 2019 In January, the International Code Council (ICC) approved a set of proposals to allow tall wood buildings as part of the 2021 International Building Code (IBC). Based on these proposals, the 2021 IBC will include three new construction types—Type IV-A, IV-B and IV-C—allowing the use of mass timber or non- combustible materials. These new types are based on the pre- vious Heavy Timber construction type (renamed Type IV-HT) but with additional fire-resistance ratings and levels of required noncombustible protection. The code will include provisions for up to 18 stories of Type IV-A construction for Business and Residential Occupancies. This article is excerpted from a paper summarizing the propos- als, including their background, technical research that sup- ported their adoption, and resulting changes to the IBC and product-specific standards. IBC Definition of Mass Timber A relatively new category of wood products, mass timber can encompass well known and widely used products such as glue-laminated timber (glulam) framing as well as newer panelized products such as cross-laminated timber (CLT). The definition of mass timber adopted for the 2021 IBC is: Structural elements of Type IV construction primarily of solid, built-up, panelized or engineered wood products that meet minimum cross section dimensions of Type IV construction. In practice, mass timber as defined in the IBC has been an umbrella term that includes heavy timber elements, with heavy timber materials and sizes serving as the prerequisite to be considered mass timber. A subtle difference is that most mass timber utilized in Types IV-A, B and C have a minimum required Fire Resistance Rating (FRR) in addition to the intrinsic fire resistance due to the minimum prescriptive size require- ments found in Type IV-HT. Care should be taken to be clear when the minimum wood member size and detailing requirements of Type IV construction apply. When mass timber is used as one of the many current exceptions found in other construction types for "heavy timber" or "Type IV" construction, the heavy timber size and detailing requirements found in Section 2304.11 (IBC 2015 602.4) are applicable. Large format panelized wood products made from solid sawn laminations include: CLT, nail-laminated timber (NLT), dowel-laminated timber (DLT), and glue-laminated timber (GLT). Panel- ized products made from structural composite lumber include: LVL panels and a proprietary mass plywood panel (MPP) made from laminated plywood. While such products are sometimes called "mass timber," they will only meet the 2021 IBC definition of mass timber when they meet the specific size and detailing requirements. When such products are used in Type III or Type V construction, as "any material permitted by this code" in Sections 602.4 and 602.5, the Type IV dimension and detailing requirements do not automatically apply. Tall Wood Buildings in the 2021 IBC Up to 18 Stories of Mass Timber Scott Breneman, PhD, SE, WoodWorks – Wood Products Council, Matt Timmers, SE, John A. Martin & Associates, and Dennis Richardson, PE, CBO, CASp, American Wood Council.

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