Fire-Resistance Design

A requirement unique to the new construction types is noncombustible protection for some mass timber elements. This noncombustible material applied to the mass timber helps determine fire behavior by delaying the contribution of the mass timber structure in a fire and has an added benefit of increasing the fire-resistance rating of the overall assembly or member. A summary of the fire-resistance rating for the new construction types is shown in the table below. Allowances for Type I-A and I-B are included for comparison.

A provision commonly used in Type I construction for high-rise buildings that do not exceed 420 feet in height allows a reduced FRR if the building has sprinkler control valves equipped with supervisory initiating devices and water-flow initiating devices for each floor (per IBC Section 403.2.1.1). With this provision, the FRR requirements for elements in a Type I-A building may be reduced to the lesser requirements of Type I-B construction (with the exception of columns that support floors), and the requirements for a Type I-B building may be reduced to the lesser requirements of Type II-A construction. These reductions are not available for the new construction types, making them more conservative than Type I requirements.

In addition to meeting FRR requirements, all mass timber elements used in Types IV-A, IV-B, and IV-C construction must meet minimum size criteria prescribed in IBC Section 2304.11.

Definitions of the new construction types (found in IBC Sections 602.4.1, 602.4.2, and 602.4.3) dictate that only mass timber or noncombustible materials can be used for the structural systems. Where mass timber elements are used, the definitions also include guidelines for whether the wood may be exposed on the building’s interior or must be covered with noncombustible protection. General allowances for exposed timber are:

• Type IV-A: No exposed timber permitted
• Type IV-B: Limited exposed timber permitted as follows:
• Ceilings (including integral exposed beams) up to 20% of floor area in dwelling unit or fire area,* or
• Walls (including integral exposed columns) up to 40% of floor area in dwelling unit or fire area,* or
• A combination of each using sum of ratios (actual exposed/allowable exposed wood) not to exceed 1.0
• Type IV-C: All exposed timber permitted*
  *Exceptions: No exposed timber is allowed at shaft walls, within concealed spaces, or on the exterior side of exterior walls.

When noncombustible protection is required to cover timber elements, it must provide at least two-thirds of the FRR. For example, a beam that requires a 2-hour FRR and requires noncombustible protection must achieve at least 80 minutes of protection from the noncombustible coverings. Section 722.7, which is new to the 2021 IBC, has been introduced to codify the demonstration of FRR using a combination of time assigned to the noncombustible coverings and inherent fire resistance of the mass timber framing members.

Any noncombustible material used as protection of mass timber elements can be tested by following the procedure outlined in IBC Section 703.6 to determine its contribution to FRR. However, two prescriptive options are presented in IBC 2021 Section 722.7.1. These options are 25 minutes per layer of ½" Type X gypsum board or 40 minutes per layer of 5/8" Type X gypsum board. Using these options, where mass timber is required to have noncombustible protection, two layers of 5/8” Type X gypsum board covering would meet the noncombustible protection requirements for a 2-hour FRR, and three layers would meet the noncombustible protection requirements for a 3-hour FRR.

The remaining one-third of the mass timber member’s FRR must be achieved through inherent fire resistance of the mass timber element. For example, a mass timber floor assembly requiring a 2-hour FRR would require 80 minutes of noncombustible protection, with the remaining 40 minutes achieved from the mass timber. In applications where the timber is exposed, the full FRR must be achieved through inherent fire resistance of the mass timber element.

There are several options for demonstrating the contribution of various elements to the FRR. One method is to provide the results of testing undertaken in accordance with ASTM E119 (or UL 263). However, if the exact assembly has not been tested, IBC Section 703.2 provides a number of alternatives. These alternatives are all founded on ASTM E119 testing. Item 3, which permits the use of calculations in accordance with Section 722, is frequently used to demonstrate the fire-resistance rating of exposed mass timber. IBC Section 722.1 notes that the fire resistance of exposed wood members and wood decking shall be permitted to be calculated in accordance with Chapter 16 of the ANSI/AWC National Design Specification (NDS) for Wood Construction. Chapter 16 of the NDS can be used to calculate up to a 2-hour fire resistance rating for a variety of exposed wood members including solid-sawn, glulam, and CLT.

Hybrid Construction for Shafts & Lateral Resistance

Provisions addressing materials permitted in shaft wall construction can be found in both the shaft enclosures section (713.3) and fire barriers section (707.2) of the IBC. These sections state that shaft walls can be constructed of any material permitted by the building’s type of construction. As noted, construction Types IV-A, IV-B, and IV-C permit the use of mass timber or noncombustible materials (or a combination thereof). This would indicate that the use of mass timber shaft walls in tall buildings is also permitted. This is true with one exception. Section 602.4 of the 2021 IBC notes that shaft walls in buildings taller than 12 stories or 180 feet must be constructed of noncombustible materials. Tall mass timber buildings which do not exceed these limits may utilize mass timber shaft walls; however, those shaft walls must have noncombustible protection on both faces of the wall.

While the code permits the use of mass timber shaft walls in many instances, worth noting is that most tall timber buildings are hybrids, utilizing non-wood materials for shaft enclosures such as concrete cores or steel braced frames with infill light gauge steel stud walls. One of the primary reasons for this is to use these materials as the building’s vertical lateral force resistance system. Until recently, the IBC and referenced standards, such as the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS) and ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, did not prescriptively recognize mass timber as an allowable seismic force resisting system. While this did change in the 2021 version of SDPWS and the 2022 version of ASCE with new options for the use of CLT shearwalls, the current provisions limit the overall system height of CLT shearwalls to 65 feet in areas of high seismic hazard. Mass timber vertical lateral force resisting systems have not been implemented to date on tall mass timber buildings in the US, with project design teams choosing to use primarily either concrete cores as shearwalls or structural steel-braced frames.

Photo courtesy of Atelier Jones

Type IV-B building with 12 stories.

TRENDS FOR INCREASED DENSITY IN LIGHT WOOD-FRAME

Although not specific to a change within the 2021 IBC, the past several years have continued to see an increasing trend of light wood-frame projects using innovative approaches to achieve greater density, site utilization, and value. Several examples of how this has been done include more incorporation of amenity spaces (roof decks, fitness centers, retail) as well as on-site parking, all located within the same building. Below is a discussion of the code provisions and design methodologies implemented to achieve these innovative projects.