Courtesy of LEVER Architecture

Tall mass timber is an industry term used to identify mass timber buildings, constructed of mass timber elements, that exceed current height limits for wood buildings set by the 2018 International Building Code (IBC). Recently approved provisions allowing for the construction of tall mass timber buildings up to 18 stories will be published in the 2021 edition of the IBC. Mass timber includes any product currently permitted for use in Type IV construction, such as cross-laminated timber (CLT), structural composite lumber, glued-laminated timber, and large section sawn lumber.

Because of the unique structural and fire resistance characteristics of solid timber walls and floors (CLT) and mass timber structures, in 2016 the International Code Council appointed a balanced committee of building officials, fire officials, architects, fire protection engineers, and industry experts to examine and propose appropriate code requirements. The committee’s work can be seen at this link: ICC Ad Hoc Committee on Tall Wood Buildings

2021 IBC Approved Code Change Resources

Building Officials Guide to Tall Mass Timber Code Changes

17 Tall Mass Timber Code Changes Recommended for Approval

  • IBC Section 602.4 Type IV construction (G108-18)
  • IBC Section 703.8 (new) Tested noncombustible protection contribution (FS5-18)
  • IBC Section 722.7 (new) Calculated noncombustible protection contribution (FS81-18)
  • IBC Section 703.9 (new) Sealing of adjacent mass timber elements (FS6-18)
  • IBC Section 718.2.1 Fireblocking materials (FS73-18)
  • IBC Section 403.3.2 and IFC Section 914.3.1.2 High rise sprinkler water supply (G28-18)
  • IFC Section 701.6 Owner’s responsibility (F88-18)
  • IFC Section 3308.4 (new) Fire safety during construction (F266-18)
  • IBC Table 504.3 (G75-18) Allowable height in feet
  • IBC Table 504.4 (G80-18) Allowable number of stories
  • IBC Table 506.2 (G84-18) Allowable area
  • IBC Section 3102.3 Special construction (G146-18)
  • IBC Appendix D Section D102.2.5 Fire Districts (G152-18)
  • IBC Sections 508.4.4.1 and 509.4.1.1 (new) Fire barriers at separated occupancies and incidental uses (G89-18)
  • IBC Section 2304.10.1 (new) Fire resistance of connections (S170-19)
  • IBC Section 1705.5.3 (new) and Table 1705.5.3 (new) Special inspection provisions (S100-19)
  • IBC Section 110.3.5 (new) Inspection of protection for connections (ADM35-19)

Tall Mass Timber Courses

Available AWC eCourses

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eCourse for CEU
  • DES441-1 – Taking Wood to the Next Level - CLT as a Floor or Roof Element

  • DES600 – Tall Wood Structures: Current Trends and Related Code and Standard Changes

  • DES602-1 – Fire-Resistance Design Primer for Mass Timber Construction

  • DES603 – Fire Tests in Support of Tall Mass Timber Buildings

  • DES604 – CLT Adhesive Tests in Support of Tall Mass Timber Buildings

  • DES606 – Experiences of a Mass Timber Builder

  • DES607 – Outcomes of ICC Tall Wood AdHoc Committee: Mass Timber Provisions in the 2021 I-Codes

  • DES607-A – Tall Wood Buildings in the 2021 IBC: Up to 18 Stories of Mass Timber

Tall Mass Timber News

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Mass Timber Fire Testing

ATF Compartment Fire Tests

A team of fire experts from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) working alongside scientists from the U.S. Forest Products Laboratory put identically furnished, multistory, one-bedroom apartments constructed of exposed, partially exposed, and unexposed (protected) five-ply cross-laminated timber (CLT) through a series of rigorously monitored fire tests. The tests provided valuable data that was used in the development of code change proposals submitted by the ICC Ad Hoc Committee on Tall Wood Buildings (TWB) for the 2021 International Building Code. A series of five tests were conducted. Each test was designed to replicate real world conditions across five scenarios. Identical, furnished, one bedroom apartments were constructed in a multistory building. The door between the living and sleeping areas was left open in both apartments. A three minute video capturing the highlights of each test is included in this playlist.

Full-Scale Fire Tests of a Two-Story Cross-Laminated Timber Structure Report


NIST/NRC Compartment Fire Tests

Compartment fire research was conducted to evaluate the contribution of mass timber elements to building compartment fires with the types of structural systems that are expected to be found in tall buildings. The mass timber elements used this research is Cross Laminated Timber (CLT). Fire tests were conducted to simulate conditions where the automatic fire suppression system has failed to activate the fire service was unable to intervene.

The contribution of exposed timber elements to a compartment fire was evaluated for a full fire duration using metrics such as charring rate, visibility, and temperature. The generated data would allow regulators and building designers to quantify the contribution from exposed timber elements, validate design equations, and develop fire protection strategies to mitigate the level of risk to occupants, fire fighters, property and neighboring property. In addition, the benefits of adding noncombustible protection to timber elements as passive fire protection to prevent or delay involvement of timber elements in fire were characterized.

This CLT compartment fire research was conducted jointly by the National Institute of Standards and Technology, USA, and the National Research Council, Canada, and the Research Institutes of Sweden. The research program was coordinated by the National Fire Protection Association’s Fire Protection Research Foundation. The research was jointly funded by a grant from the US Forest Service’s Wood Innovation program, the Property Insurance Research Group, and the American Wood Council.


ASTM E119 Wall Test Summary



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