Alternative solutions for tall wood buildings
ABOVE: Brock Commons – Tallwood House. Photo credit: courtesy of Acton Ostry Architects Inc. © Michael Elkan Photography.
In recent years, tall mass timber buildings have become more prevalent, with dozens either erected or under construction around the world. Developers, architects, governments, and the wood products industry are increasingly interested in designing and constructing these unique and progressive buildings.
Tall Mass Timber Buildings
The term “mass timber” refers to large timber components such as glued-laminated timber (glulam) or other structural composite lumber (SCL) beams and columns, and cross-laminated timber (CLT) or laminated veneer lumber (LVL) panels. Due to their large cross-sectional area and the relatively slow rate of charring when exposed to fire, these structural wood products tend to have a significant amount of inherent fire resistance.
Although mass timber buildings taller than six stories are currently not permitted under the prescriptive building code requirements, building code committees in Canada are working on revisions to the code that would allow for the construction of tall wood buildings. However, until those changes occur, tall mass timber buildings can only be designed and approved using an alternative solutions approach.
In order to better understand how these products perform in fire, and how they impact the fire dynamics within a mass timber building, many fire research tests have been conducted in recent years – both in Canada and around the world.
In an effort to support Ontario’s wood industry and to contribute to the knowledge base for tall wood buildings, the Ontario Ministry of Natural Resources and Forestry recently published Ontario’s Tall Wood Building Reference. This reference is written primarily for the municipal Chief Building Official (CBO) who may receive an alternative solution proposal for a tall wood building. It’s also a valuable tool for developers and architects who are interested in having a better understanding of the various considerations that go into the development of an alternative solution when seeking approval from a CBO to construct a tall mass timber building.
Building Code Requirements
To understand how a building permit can be issued for a building that does not meet the prescriptive requirements of the building code, it is necessary to understand how building codes work in Canada. The regulation of building construction is a provincial responsibility under the Canadian Constitution. However, to support harmonization between provinces, the National Building Code of Canada (NBCC) is developed under the direction of the Canadian Commission on Building and Fire Codes as one of five national model codes. Some provinces adopt the NBCC as published, while other provinces and territories make minor or, in some cases, major changes. Enforcement of the provincial codes is typically undertaken by a municipality’s CBO.
In Ontario, a majority of the NBCC model is adopted, along with other Ontario-specific changes (e.g., to mid-rise combustible construction provisions), and is published as the Ontario Building Code (OBC). Compliance with the OBC can be demonstrated in one of two ways: by complying with the applicable acceptable solutions in Division B, or by using alternative solutions that achieve the levels of performance required of the applicable acceptable solutions. This means that the acceptable solutions in Division B of the OBC establish the required performance level, with respect to the applicable objectives and functional statements. Based on this, if a designer wishes to submit an “alternative solution,” the designer must first establish the level of performance required based on the Division B acceptable solution. This is the primary difference between an objective-based code and a performance-based code, the latter of which would explicitly establish the performance criteria. With an objective-based code, the designer must determine the level of required performance, quantitatively-based on the expected performance of the acceptable solution.
Once the performance levels are established based on the performance of a design compliant with the acceptable solution, as specified in Division B, the alternative solution can be evaluated to determine the performance level provided.The OBC requires that the performance level of the alternative solution be equal to or greater than the performance level of the acceptable solution.
Alternative Solution Submissions
Alternative solution submissions can take many forms, from simple trade-offs to complex fire safety engineering analyses. Alternative solutions for tall wood buildings are likely to be on the more complex end of the spectrum.
The primary strategy in developing an alternative solution for a mass timber building where noncombustible construction is otherwise required is to follow all the requirements for a noncombustible building, except for the use of mass timber structural members and assemblies. Then, mitigating features are introduced, as required, to meet the performance level provided by the acceptable solution of noncombustible structural elements. These mitigating features may include encapsulation of the mass timber and the addition of a sprinkler system component to increase the reliability of the system (e.g., onsite water tanks or multiple risers). The impact of these mitigating features can then be evaluated using a risk analysis to compare the acceptable solution to the alternative solution.
The alternative solution for a mass timber high-rise building would most likely need to address several aspects related to the combustible structure. These may include the structural fire resistance rating of the mass timber structure, which for a building over six storeys would be required to be a minimum of two hours. While testing is one way to demonstrate performance, tall mass timber buildings will, in many cases, be designed for fire resistance based on engineering principles related to the charring properties of the wood structure.
Should a fire occur, the combustible nature of the mass timber structure, and the impact the combustible structure may have on the fire’s growth, intensity, and duration, must also be considered. This contribution to the fire development will depend on the specific design of the building, and different scenarios must be considered, such as in the case where sprinklers fail to control the fire and the fire department is unable to respond.
When a CBO has the appropriate competence and experience, they may choose to review and evaluate the proposed alternative solution themselves. However, in many cases, an alternative solution for a tall wood building may be beyond the in-house expertise of a CBO. In these cases, a third-party review can become an important resource used by CBOs to evaluate complex alternative solutions.
In Canada, there are efforts being made to allow for tall mass timber buildings under the prescriptive requirements in the respective building codes. This would possibly reduce the need for alternative solutions for tall mass timber buildings in the near future. However, many of the buildings being designed and built today go beyond what is being considered in the current revisions to the building code. The buildings may be taller, or the architects would like more exposed wood, or the buildings may be designed for different occupancies than those permitted by the acceptable solutions in the code. In such cases, designers must continue to develop alternative solutions.
Download a PDF copy of Ontario’s Tall Wood Building Reference from the
Government of Ontario’s website: https://www.ontario.ca/page/building-with-wood.
Steve Craft, Ph.D., P.Eng., is a Principal Engineer with CHM Fire Consultants Ltd and an Adjunct Professor in the Fire Safety Engineering Program at Carleton University. He is the Chair of the ULC Fire Test Committee and a member of the CSA O86 Technical Committee on Wood Design where he chairs the TG on Fire Resistance.
as published in Municipal World, May 2018
