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Fire Design of Mass Timber Members

Fire Design of Mass Timber Members

Code Applications, Construction Types and Fire Ratings

By Richard McLain, PE, SE and Scott Breneman, PhD, PE, SE

For many years, exposed heavy timber framing elements have been permitted in U.S. buildings due to their inherent fire-resistance properties. The predictability of wood’s char rate has been well-established for decades and has long been recognized in building codes and standards.

Today, one of the exciting trends in building design is the growing use of mass timber—i.e., large solid wood panel products such as cross-laminated timber (CLT) and nail-laminated timber (NLT)—for floor, wall and roof construction. Like heavy timber, mass timber products have inherent fire resistance that allows them to be left exposed and still achieve a fire-resistance rating. Because of their strength and dimensional stability, these products also offer an alternative to steel, concrete, and masonry for many applications, but have a much lighter carbon footprint. It is this combination of exposed structure and strength that developers and designers across the country are leveraging to create innovative designs with a warm yet modern aesthetic, often for projects that go beyond traditional norms.

This excerpt is from a paper written to support architects and engineers exploring the use of mass timber for commercial and multi-family construction. It focuses on how to meet fire-resistance requirements in the International Building Code (IBC), including calculation and testing-based methods. Unless otherwise noted, refers to the 2018 IBC.

Mass Timber & Construction Type

Before demonstrating fire-resistance ratings of exposed mass timber elements, it’s important to understand under what circumstances the code currently allows the use of mass timber in commercial and multi-family construction.

A building’s assigned construction type is the main indicator of where and when all wood systems can be used. IBC Section 602 defines five main options (Type I through V) with all but Type IV having subcategories A and B. Types III and V permit the use of wood framing throughout much of the structure and both are used extensively for modern mass timber buildings.

Type III (IBC 602.3) – Timber elements can be used in floors, roofs and interior walls. Fire-retardant-treated wood (FRTW) framing is permitted in exterior walls with a fire-resistance rating of 2 hours or less.

Type V (IBC 602.5) – Timber elements can be used throughout the structure, including floors, roofs and both interior and exterior walls.

Type IV (IBC 602.4) – Commonly referred to as ‘Heavy Timber’ construction, this option has been in the building code for over a hundred years in one form or another, but its use has increased along with renewed interest in exposed wood buildings. This construction type is unique in that fire-resistive behavior is based in part on the inherent and long-demonstrated fire resistance of large solid wood framing. Structural wood components are permitted in floors, roofs and interior walls when they meet minimum cross-section sizes. Per IBC Sections 602.4.1 and 602.4.2, exterior walls required to have a fire-resistance rating of 2 hours or less are also permitted to use FRTW framing, or CLT when covered with FRTW sheathing or noncombustible materials. In general, heavy timber components used in Type IV construction (called Type IV-HT in the 2021 IBC) can be exposed.

Heavy Timber Requirements

The fire-resistance of heavy timber construction is based on the inherent size of large timber elements. In a fire, large members tend to retain their load-carrying ability longer than members with small cross-sections; this is because a char layer forms on the surface while the interior remains undamaged and structurally sound. IBC Section 2304.11 (IBC 2015 Section 602.4) provides minimum heavy timber sizes for columns, floor framing and decking, roof framing and decking, and walls.

These minimum nominal sizes are applicable to solid sawn wood members. However, in consideration of engineered wood products, IBC Table 2304.11 (IBC 2015 Table 602.4) provides equivalent glulam and SCL sizes that qualify as heavy timber.Fire-Resistance Rating Requirements

For most building elements other than heavy timber, passive fire-resistive requirements are in the form of a required fire-resistance rating (FRR). The IBC defines FRR as the period of time a building element, component or assembly maintains the ability to confine a fire, continues to perform a given structural function, or both, as determined by the tests, or the methods based on tests, prescribed in Section 703.

FRR based on Construction Type

The construction type of a building determines many of the minimum required fire-resistance ratings for different building components, as shown in IBC Table 601.

A couple features of this table are relevant to mass timber.Footnote ‘c’ allows for timber components meeting the requirements of heavy timber to be used in the construction of all roofs having a fire-resistance rating of 1 hour or less in lieu of the required fire-resistance rating. This means that a mass timber roof meeting the minimum size requirement of heavy timber can be used in construction Types I-B, II-A and II-B which otherwise prohibit the use of combustible framing.

Interior nonbearing walls and partitions generally do not have a fire-resistance rating requirement, except for Type IV construction. In IBC 2015, note that the Table 601 reference to Section 602.4.6 should instead be to 602.4.8, which requires partitions in Type IV construction to be of solid wood construction or have a 1-hour fire-resistance rating.

Other FRR Requirements

In addition to requirements related to construction type, there are other requirements for fire-resistance ratings in the IBC. For multi-unit residential buildings, walls and floors between dwelling or sleeping units are required to have a fire-resistance rating of 1/2 hour in Type II-B, III-B and V-B construction when sprinklered throughout with an NFPA 13 system, and 1 hour for all other construction types (IBC 420, 708 and 711). Multiple separated occupancies (IBC 508.4), incidental uses (IBC 509), and special provisions (IBC 510) also require fire-resistance ratings of select components and assemblies.

Selecting a Construction Type

Selection of construction type for mass timber projects is one of the more significant design considerations. While it’s common to choose construction type based on structural material—i.e., to assume that steel and concrete structures should be Type II, light-frame wood should be Type V, and exposed heavy timber should be Type IV—this approach can lead to additional costs. While Type IV construction can be used for exposed mass timber projects, a full understanding of the allowable use of materials in all five construction types, as well as the unique allowances and limitations associated with each, will help to inform the most efficient design.To optimize a building design from a construction type perspective, it is best to start with Type V-B construction, and work up toward higher construction types. This avoids unnecessary defaults or assumptions—and unnecessary costs. The fact that certain materials are being used doesn’t mean there is only one option for construction type. Similarly, a mix of occupancy groups doesn’t dictate that certain materials, construction types or building configurations are required. For example, a mass timber building may have isolated steel, concrete or masonry structural elements, but this doesn’t mean that Type I or II construction is necessary, nor does it mean that some or all of the building can’t be framed with mass timber. Likewise, a building with mass timber elements has options other than Type IV construction. Note IBC Section 602.1.1:

602.1.1 Minimum requirements. A building or portion thereof shall not be required to conform to the details of a type of construction higher than that type which meets the minimum requirements based on occupancy even though certain features of such a building actually conform to a higher type of construction.

This section permits the use of elements commonly used in a higher construction type without requiring that the entire building meet all of the provisions of that construction type. For example, if a building’s size permitted the use of Type V-B construction, it could still be completely framed with noncombustible materials while being classified as V-B. Similarly, a Type III or V building could be framed with a combination of combustible and noncombustible materials.

As noted, from a cost efficiency perspective, it is usually best to start a building analysis with Type V-B construction as this provides the most flexibility in terms of allowable use of materials throughout the building while minimizing requirements for fire resistance-rated assemblies and structural elements. However, Type V-B is also the most restrictive in terms of allowable building size. All three of these factors—allowable building size, allowable use of structural materials, and required fire-resistance levels—are interconnected.

Construction type, which is a direct function of the types of materials used in a building (and vice versa) looks at limiting the size of the building based on fire and life safety risks. In other words, how much of a building’s contents and structure are combustible has an impact on presumed risk, which in turn affects the code’s allowances for building size. Fire-resistant measures required of the building’s structure dictate the length of passive fire endurance (i.e., 1 hour, 2 hours).

If Type V-B construction doesn’t allow as large a building as desired, the next step is to check Type V-A. The main differences between V-B and V-A are fire-resistance rating requirements and allowable building size. If Type V-A doesn’t allow the desired size, Type III-B is the next choice, with Type III-A following. Type IV construction has similar allowable building size limits as Type III-A; however, there are nuances to the selection of one or the other.

Read the full-length paper for a discussion of these nuances as well as methods of demonstrating fire-resistance ratings of mass timber; fire performance of glued-laminated timber (glulam), nail-laminated timber (NLT), tongue and groove decking, structural composite lumber, and cross-laminated timber (CLT); fire protection of connections; and interior finish requirements. It is available as a free download on the WoodWorks website, along with an Inventory of Fire Resistance-Tested Mass Timber Assemblies and Penetrations.

WoodWorks – Wood Products Council provides free technical support as well as education and resources related to the code-compliant design of commercial and multi-family wood buildings. A non-profit organization staffed with architects, structural engineers and construction experts, WoodWorks has the expertise to assist with all aspects of wood building design, including (but not limited to): allowable heights and areas/construction types, structural detailing of wood and hybrid systems, fire resistance and acoustical-rated assemblies, efficient and code-compliant lateral system design, alternate means of code compliance, and energy-efficient detailing. For assistance with a project, visit www.woodworks.org/project-assistance or email help@woodworks.org.