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Fire Design of Mass Timber Members – Part 2

Fire Design of Mass Timber Members – Part 2

Code Applications, Construction Types and Fire Ratings

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

This is Part 2 of a three-part paper on fire design written to support architects and engineers exploring the use of mass timber for commercial and multi-family construction.

Comparing Construction Types

As noted in Part 1, selection of construction type for mass timber projects is one of the more significant design considerations. Table 4 summarizes the main differences between Types III, IV and V, as well as the different types of wood systems permitted in each. These allowances are shown in IBC Section 602, Table 601 and Section 2304.11.

When looking to maximize the code’s current allowances in terms of building size for mass timber structures, considering the differences between Type III-A and IV construction is important. For example:

  • Type IV does not allow concealed spaces in floor or roof assemblies (e.g., dropped ceilings, soffits, chases, etc.), but 1-hour fire resistance-rated interior partitions are permitted. All other construction types including III-A allow concealed spaces. Note that requirements for sprinklers and draft stopping/fire blocking apply within concealed spaces per IBC Section 718 and the applicable NFPA sprinkler standard.
  • Except for exterior bearing walls, Type IV does not require demonstration of fire-resistance ratings for structural elements. This is a requirement for all other construction types including III-A (but only when a fire-resistance rating is required).
  • Type IV construction allows the use of CLT in exterior walls; Type III does not.

Table 5 illustrates these differences and others for a group B occupancy building.

The requirements of Type IV construction to have no concealed spaces in floors or roofs and for all interior partition walls to be solid wood or 1-hour rated can significantly impact its utility for some applications. The alternative of using Type III construction (or Type V where building size permits) avoids this limitation; however, the processes for demonstrating fire-resistance ratings also vary between Type IV and Types III and V. Methods for meeting fire-resistance rating requirements for mass timber elements in buildings other than Type IV construction are the focus of the rest of this paper.

Methods to Demonstrate Fire-Resistance Ratings of Mass Timber

When a mass timber building element or assembly is required to have a fire-resistance rating, IBC Section 703.2 requires the rating to be determined by testing in accordance with ASTM E 119 (or UL 263) or via one of six alternatives listed in IBC Section 703.3:

The required fire resistance of a building element, component or assembly shall be permitted to be established by any of the following methods or procedures:

  1. Fire-resistance designs documented in approved sources
  2. Prescriptive designs of fire-resistance-rated building elements, components or assemblies as prescribed in Section 721
  3. Calculations in accordance with Section 722
  4. Engineering analysis based on a comparison of building element, component or assemblies designs having fire-resistance ratings as determined by the test procedures set forth in ASTM E119 or UL 263
  5. Alternative protection methods as allowed by Section 104.11
  6. Fire-resistance designs certified by an approved agency

These alternatives are options when the exact assembly has not been tested per ASTM E 119 and a test report is therefore not available. They are all founded on ASTM E 119 testing.

There are currently limited options for fire resistance-rated mass timber assemblies from approved sources (e.g., Gypsum Association GA-600, American Wood Council’s Design for Code Acceptance 3 – Fire Resistance-Rated Wood Floor and Wall Assemblies, [DCA 3]) or certification agencies (e.g., UL listings). However, an increasing number of assemblies have been tested according to the ASTM E119 standard and are available publicly or on request from manufacturers. The number of available tested assemblies can be expanded using comparative engineering analysis described in Item 4 of IBC Section 703.3. Such an analysis, which seeks to justify the fire-resistance rating of an assembly or component similar to one that has passed an E119 test, can be performed by a fire protection engineer.

Item 3 of IBC Section 703.3, which permits the use of calculations in accordance with Section 722, is also frequently used to demonstrate the fire-resistance rating of exposed mass timber. IBC Section 722.1 states: The calculated fire resistance of exposed wood members and wood decking shall be permitted in accordance with Chapter 16 of ANSI/AWC National Design Specification® for Wood Construction (NDS®). 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, SCL, and CLT.

ASTM E119 Testing Method

According to Section 4.2 of ASTM E119-18, the fire test procedure is intended to do the following:

The test exposes a test specimen to a standard fire controlled to achieve specified temperatures throughout a specified time period. When required, the fire exposure is followed by the application of a specified standard fire hose stream applied in accordance with Practice E2226. The test provides a relative measure of the fire-test-response of comparable building elements under these fire exposure conditions. The exposure is not representative of all fire conditions because conditions vary with changes in the amount, nature and distribution of fire loading, ventilation, compartment size and configuration, and heat sink characteristics of the compartment. Variation from the test conditions or test specimen construction, such as size, materials, method of assembly, also affects the fire-test-response. For these reasons, evaluation of the variation is required for application to construction in the field.

Successful fire tests have been completed on numerous mass timber elements and assemblies, achieving fire-resistance ratings of 3 hours or more. Additional tests by manufacturers and others are ongoing. Most tests are conducted according to ASTM E119 or its Canadian equivalent, ULC S101. Both utilize the same time-temperature curve and performance criteria and, as such, ULC S101 fire tests are usually acceptable to U.S. building officials. However, each project’s building official should be consulted if choosing this design route.

To help building designers compare options, WoodWorks has compiled a web-based inventory of completed mass timber fire tests. The Inventory of Fire Resistance-Tested Mass Timber Assemblies & Penetrations as new tests become available, and can be found at https://bit.ly/2FRwAPG.

Calculation-Based Method

As referenced in IBC Section 722.1, NDS Chapter 16 can be used to calculate the structural fire-resistance rating of various wood products, including solid sawn, glulam, SCL, and CLT.

As noted by Douglas and Smart in Structure magazine (July 2014), “The design procedure allows calculation of the capacity of exposed wood members using basic wood engineering mechanics. Actual mechanical and physical properties of the wood are used, and member capacity is directly calculated for a given period of time—up to 2 hours. Section properties are computed assuming an effective char depth, βeff, at a given time, t. Reductions of strength and stiffness of wood directly adjacent to the char layer are addressed by accelerating the char rate by 20 percent. Average member strength properties are approximated from existing accepted procedures used to calculate design properties. Finally, wood members are designed using accepted engineering procedures found in NDS for allowable stress design.”

The American Wood Council’s (AWC’s) Technical Report 10 – Calculating the Fire Resistance of Wood Members and Assemblies (TR 10) provides an in-depth explanation of the concepts and background associated with exposed wood fire design. This document also includes a number of design examples for exposed structural wood members utilizing the provisions of NDS Chapter 16.

Structural Design Calculations under Fire Conditions

When utilizing the char calculation option of NDS Chapter 16 to demonstrate fire-resistance ratings, a structural design check must also be done to determine structural adequacy of framing members under fire conditions. One of the main benefits of the char calculation method is that it accounts for the ability that heavy and mass timber have to form a char zone, which insulates the remaining wood cross-section, allowing it to retain structural capacity.

NDS Section 16.2.2 states that, under fire design conditions, the average member strength can be approximated by multiplying reference design values such as Fb by the adjustment factors specified in Table 16.2.2. As indicated in Table 7, an increase in allowable design stresses by a factor of 2.03 to 2.85 is allowed, depending on the stress under consideration.

For example, a 6-3/4-inch x 13-1/2-inch glulam beam with an unadjusted allowable bending stress of 2,400 psi would first be checked for all structural loading conditions and limit states (bending, shear deflection, vibration and others as applicable) using the full cross-sectional dimensions and adjustment factors per NDS Chapter 5. If this beam were required to have a 1-hour fire-resistance rating (perhaps as a floor beam in a Type V-A structure) then its effective char depth on all three exposed sides would be 1.8 inches (per NDS Table 16.2.1A). Its cross-sectional dimensions under fire conditions would be:

Width = 6.75″ – (2)(1.8″) = 3.15″

Depth = 13.5″ – 1.8″ = 11.7″

This reduced cross-section would then be checked under fire conditions, with allowable design stresses increased by the factors given in NDS Table 16.2.2. For example, a 2.85 increase factor could be applied to allowable bending stresses. AWC’s TR 10 provides design examples for a number of exposed timber applications under fire conditions.

The stress adjustment factor, K, to increase the reference design stress is for use when performing a structural capacity check under the fire load condition with allowable stress design (ASD) load combinations (e.g., D + L, etc.). This stress adjustment factor is not intended to be used with load and resistance factor design (LRFD) load combinations, including those intended for extraordinary events such as in ASCE 7-16 Section 2.5.

Appendix A of TR 10 provides design tools for beams and columns of solid sawn, glulam or SCL materials under fire design scenarios using the char calculation provisions of NDS Chapter 16. Table A1 provides a method to quickly check if a beam exposed on three sides passes the structural fire condition check provided the designer knows the beam’s size and maximum demand to capacity ratio (Rs) under the required non-fire condition using ASD load combinations. Table A2 provides a similar design table for columns exposed on all four sides.

For more information, the complete paper can be downloaded from 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. For assistance with a project, visit www.woodworks.org/project-assistance or email help@woodworks.org.