By Kenneth Bland
American Wood Council
Today, new structural systems available for tall building design with wood represent the first significant challenge to concrete and steel in more than a century. The introduction of these ideas is being driven by building designers and code officials who are seeing successful implementation in tall wood buildings across Canada, Australia and other parts of the world, coupled with the need to find safe, carbon-neutral and sustainable alternatives to incumbent structural materials of the urban world.
Heavy timber (aka Type IV in the building code) is one of the oldest wood construction methods recognized in codes to date. Within this category, new framing styles that use cutting-edge mass timber technologies are gaining broader acceptance. As a result, a change to the 2015 International Building Code (IBC) was approved to recognize a new mass timber product called cross-laminated timber (CLT) for use in Types III, IV and IV construction.
CLT will complement existing mass timber products, including structural composite lumber products (e.g., laminated strand lumber, laminated veneer lumber, oriented strand lumber, parallel strand lumber) and glued laminated timber, also known as glulam. Additionally, there are several other innovative wood building systems that are not yet recognized in the code but are of interest for tall buildings, including dowel laminated timber and wood-concrete composites.
In particular, CLT has been in use worldwide for more than 30 years, but now U.S. designers and engineers are increasingly showing interest in picking up the trend. CLT is made by gluing together layers of solid-sawn lumber or structural composite lumber in a perpendicular fashion, typically creating panels with three, five or seven layers. This cross-lamination process provides dimensional stability, strength and durability equal to or greater than competing materials in many applications. Mass timber, such as CLT, is prefabricated offsite which speeds up construction, improves safety and efficiency, and decreases construction noise and disruption onsite.
Prior to the 2015 code cycle, the American Wood Council (AWC) updated both the 2015 National Design Specification (NDS) for Wood Construction and Technical Report 10: Calculating the Fire Resistance of Exposed Wood Members to address technical aspects for the design of CLT and provide guidance for those looking to specify the material. The multi-disciplinary CLT Handbook includes additional information on topics not yet covered in the IBC or NDS, such as energy, sound, vibration, enclosures, handling and protection.
Moreover, significant progress has been made in recent years in terms of what is permissible using wood – but there is still work to do to demonstrate what is possible using wood.
To continue the momentum, the International Code Council (ICC) formed the Ad Hoc Committee on Tall Wood Buildings in 2015 to research the building science of tall wood buildings. Created with the end-goal of introducing code provisions for the 2021 code cycle, the committee is working to introduce the use of mass timber in taller or “beyond current code” buildings. Consistent with the ICC code development schedule for the 2021 codes, non-structural provisions will be submitted for consideration during 2018, and structural provisions will be submitted for review and approval during 2019.
Another moving piece to watch is the bipartisan Timber Innovation Act, which aims to establish a performance-based research and development program to accelerate tall wood building construction in the United States. Continuing efforts from last year when the legislation was introduced, AWC and members of the Wood Products Alliance have joined forces to ensure the bill is re-introduced in the U.S. House and Senate, as well as to increase the number of co-sponsors.
This legislation promotes research and technical assistance to help industry professionals understand the possibilities of building with mass timber and its wide-ranging benefits to the U.S. economy and environment.
Mass timber construction directly supports jobs in parts of rural America that have yet to recover from the recession, and it presents the opportunity to lessen U.S. dependence on fossil-fuel intensive alternatives. Having the federal government encourage further development of this emerging construction technology stands to benefit and enhance environmental conservation, infrastructure development and job creation. More research, testing and demonstration projects are needed to make this headway, but AWC is confident that the industry is heading in the right direction.
To learn more, visit www.awc.org/tallwood.
Kenneth Bland, P.E.,is vice president of codes and regulations at the American Wood Council (AWC), whichrepresents the interests of the North American wood products industry that employs approximately 400,000 men and women in the United States. The AWC is committed to ensuring a resilient, safe and sustainable built environment. To achieve these objectives, AWC contributes to the development of sound public policies, codes and regulations, which allow for the appropriate and responsible manufacture and use of wood products.