Increasing use of mass timber as a competitive building product in the construction marketplace requires sound design guidelines to ensure safe, efficient, and economic use of wood products. AWC fire research on mass timber conducted over the last year, with support from the Softwood Lumber Board and U.S. Endowment for Forestry and Communities, has highlighted the importance of the insulating effect provided by the outer char layer when wood burns, especially in larger wood elements such as used in mass timber.
As a result, AWC has released an updated “Technical Report 10 (TR10), Calculating the Fire Resistance of Wood Members and Assemblies.” TR10 shows how to design for the fire resistance contribution provided by this layer.
The updated TR10 provides designers with both new and revised procedures for designing unprotected and protected wood members and assemblies utilizing either mass timber elements or smaller light-frame members, and includes background information and verification on the design procedures and simplified pre-engineered design tables for different wood products and loading conditions. Expanded design examples in the Report demonstrate how to design both mass timber and lightweight wood members, assemblies, and connections for the required duration of code-required fire exposures.
The national building code process will be considering approval of mass timber buildings up to 18 stories in height in late October.
Primary revisions to TR10 include:
- Revision of terminology throughout the report to address changes to terminology in the 2018 National Design Specification® (NDS®) for Wood Construction.
- Addition of new Chapter 3, “Protection of Structural Members and Connections,” which summarizes the evaluation of test data and determination of the added fire resistance provided by wood, gypsum board, and insulation used to protect wood members and connections.
- Addition of design provisions for calculating the contribution of protection to the existing chapter on Design Procedures for Exposed Wood Members. Revised Chapter 4, titled “Design Procedures for Exposed and Protected Wood Members,” offers provisions for calculating both the increased structural fire resistance provided by various types of protection and the thermal separation provided by protective membranes. Connection design and details were modified to provide more guidance on how to address char contraction in connections at ends of members.
- Design examples were moved to a new Chapter 5. Calculation of thermal separation times were added to Example 5 (Exposed Cross-Laminated Timber floor design) and Example 6 (Exposed Cross-Laminated Timber wall design); a new Example 7 (Protected 2x10 joist floor) was added to provide example calculations of structural fire resistance and thermal separation times of gypsum-wallboard-protected floor assemblies; and new protected connection designs were added in Example 8 (Protection of Steel Ledger Connection) and Example 9 (Protection of Beam-Column Connection).
Next time a customer or building official asks you to provide documentation on the fire performance of a lightweight wood or mass timber assembly, just refer them to this new publication.