Important Changes to the NBC 2020 and NFC 2020: Encapsulated Mass Timber Construction
(Kevin) Hsiao-Feng Wu, Technical Advisor, Codes Canada
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Video transcript
Introduction
Hi, my name is (Kevin) Hsiao-Feng Wu and I am a Technical Advisor supporting the Canadian Board for Harmonized Construction Codes—also referred to as the CBHCC.
This committee is the federal-provincial-territorial body responsible for developing and maintaining the National Model Codes in Canada.
I am a member of the Codes Canada team at the National Research Council of Canada.
Codes Canada acts as the secretariat to the CBHCC, providing administrative, technical and policy support, including publishing the National Model Codes.
To learn more about code changes and provisions in the National Model Codes, or about Canada’s national model code development system, please visit CBHCC’s website.
This presentation is part of a video series led by the CBHCC on the key technical changes incorporated in the 2020 editions of the National Model Codes.
This presentation will focus on important changes to encapsulated mass timber construction in the National Building Code of Canada 2020 and the National Fire Code of Canada 2020.
Topics
This presentation introduces the changes that have been published in the 2020 edition of the National Building Code of Canada (NBC) and National Fire Code of Canada (NFC), which introduce encapsulated mass timber as a new construction type.
I will describe what encapsulated mass timber construction (EMTC) is and what the acceptable solution in the NBC and NFC entails. Code references in this presentation are to the 2020 edition of the National Model Codes.
Why change the National Model Codes?
The construction industry, construction technology and methods and materials are constantly evolving.
Developers, architects, engineers, builders, and building owners across Canada are seeking additional options for a greater variety of heights and sizes of buildings that are permitted to be constructed of wood.
This quest is driven, in part, by interest in the proven, positive environmental attributes of wood.
Under the prescriptive provisions of the NBC 2015 that deal with fire safety, buildings are generally separated into two construction types: combustible and noncombustible construction. This classification system was introduced in the National Model Codes over 50 years ago.
The primary difference between combustible and noncombustible construction is that noncombustible materials do not fuel a fire.
Typical light-frame combustible construction has little inherent fire resistance unless protected.
Heavy timber construction is a special category of construction. Timber element is recognized as having some inherent fire-resistance capabilities, which are achieved by specifying minimum sizes for columns and beams, as well as minimum thicknesses for floor and roof components. These minimum dimensions provide this type of construction with a degree of fire safety because of the charring of the outside of the wood during a fire, which protects the inner material.
NBC 2020: Tall wood buildings
In addition to industry interest, tall wood buildings are also a priority for some provinces.
Constructing buildings from wood is not a new concept; tall wood buildings are already being built in Canada.
The 2020 Codes introduce EMTC as a new permitted type of construction.
NBC 2020: Combustible construction
Combustible construction is permitted for some smaller buildings. By adding suitable protection to combustible framing, various levels of fire-resistance rating can be achieved.
The 2015 provisions restricted the height and area of buildings of combustible construction, whereas they permitted unlimited height and area for noncombustible construction.
This restriction on the use of structural wood elements can affect building design in one of three ways:
- the height and area of the building must not exceed the limits placed on all forms of combustible construction, or
- the design needs to be altered to eliminate the inclusion of such wood elements, or
- the design must include a proposed alternative solution (on a specific project-by-project basis) for approval by the authority having jurisdiction, which can require significant resources and expertise, both for the designer to develop and for the authority having jurisdiction to evaluate.
In the NBC 2015, combustible buildings of residential occupancy (Group C) were restricted to six storeys in building height and 1 500 m2 in building area and combustible buildings of business and personal services occupancies (Group D) were restricted to six storeys and not more than 3 000 m2 of building area.
NBC 2020: What is encapsulated mass timber construction (EMTC)?
Let’s start at the beginning.
The definition of “encapsulated mass timber construction” for the purposes of the NBC 2020 is a type of construction for which a degree of fire safety is attained by the use of encapsulated mass timber elements with an encapsulation rating and minimum depth and cross-sectional dimensions for structural members and other building assemblies.
“Encapsulation rating” means the time in minutes that a material—or assemblage of materials— will delay the ignition and combustion of encapsulated mass timber elements when exposed to fire under specified conditions of test and performance criteria or as otherwise prescribed by the Code.
The definitions describe the essential characteristics of this type of construction, which include:
- requirements for minimum dimensions for the major structural wood (timber) elements, such as beams, columns, arches, wall, floor and roof assemblies; and
- a requirement for an encapsulation rating to be applied to these encapsulated mass timber elements.
NBC 2020: What is mass timber?
As already mentioned, heavy timber is a form of combustible construction that is recognized as having inherent fire resistance because, in contrast to light-frame construction, heavy timber construction chars before losing its structural integrity when exposed to flames.
For reasons such as availability, cost and structural issues due to naturally occurring defects, we now tend to see engineered lumber products being used (such as glue-laminated timber products, cross-laminated timber or glue lam beams, nail-laminated timber) instead of conventional solid sawn lumbers. The premise from a fire-protection standpoint is similar: if a large enough cross-section of wood is assembled, then this “mass of timber” will have some inherent fire resistance before losing its structural capacity, owing to the predictability of the rate of charring.
NBC 2020: Minimum dimensions
The new requirements define the minimum dimensions of timber structural elements in EMTC. These have been developed to ensure the elements exhibit the fire-performance characteristics of heavy timber. The minimum size requirements are intended to ensure a charring rate and predictable fire behaviour when exposed to a standard two-hour fire.
NBC 2020: Encapsulation
The “encapsulation rating” refers to a material that will delay the ignition and combustion of the encapsulated mass timber element. So, encapsulation implies that the timber elements are protected, to some degree, from fire exposure should a fire occur.
The encapsulation serves two roles:
- reducing the contribution to fire severity
- contributing to the fire resistance of the structure
The required encapsulation rating is 50 minutes. It was determined that a delay in the involvement of the EMTC of 50 minutes was sufficient to allow the evacuation of the building of permitted size.
Acceptable solutions for achieving the required encapsulation rating are included in the provisions, based on the research conducted.
The exposed surfaces permitted for timber elements are restricted in order to ensure the exposed surface area is not sufficient to sustain a fire that would provide the radiation required to sustain flaming combustion.
NBC 2020: Occupancies (Mixed-use buildings: Group C and D occupancy restrictions)
Group C (residential) and Group D (business and personal services) are permitted to be constructed of EMTC.
For buildings constructed in accordance with the requirements applicable to Group C (residential) major occupancies, limitations apply to certain additional major occupancies.
The types of major occupancies permitted in mixed-use buildings are limited if the building is entirely constructed of EMTC.
There are fewer permitted occupancies in residential buildings because the occupants in these buildings may be sleeping. Therefore, the evacuation times of people in residential buildings will likely be higher in the event of a fire during the night.
Similarly, the location of major occupancies is restricted if the building is entirely constructed of EMTC.
For buildings constructed using the requirements for Group C major occupancies, the following major occupancies are restricted to the following locations:
- Group E: below the third storey
- Group A, Division 2: below the fourth storey
- storage garage: below the fifth storey
The building area of the major occupancies and storage garage is limited, and the risk to building occupants is reduced due to decreased overall building fire loads as a result of the reduced building areas.
NBC 2020: Exterior cladding
EMTC is not considered either combustible or noncombustible construction, even though timber is a combustible wood product. So, what types of cladding are permitted?
In general, cladding on an exterior wall assembly of a building, or part thereof, permitted to be of EMTC must be noncombustible, except under certain specific conditions.
The various compliance conditions for using combustible cladding can be grouped in four categories:
- combustible cladding that is contiguous over not more than four storeys,
- combustible cladding that is not contiguous across adjacent storeys,
- combustible cladding representing up to 100% of the cladding on exterior walls of the first storey
- an exterior wall with a combustible cladding that satisfies specific criteria when tested in accordance with a standardized fire test, such as CAN/ULC-S134, “Standard Method of Fire Test of Exterior Wall Assemblies,” which assesses the performance of the wall assembly with regard to its ability to resist flame propagation up the outside of a building.
The requirements are intended to reduce the potential for fire spread on the exterior cladding of buildings of EMTC through the use of noncombustible finishes on the exterior of the wall assembly or the use of a cladding or wall assembly that has been proven to resist flame propagation.
Combustible exterior cladding is permitted provided:
- it is not contiguous over more than four storeys,
- it represents not more than 10% of the cladding on each exterior wall of each storey,
- its elements are not more than 1.2 m wide,
- it has a flame-spread rating not more than 75 on any exposed surface, or any surface that would be exposed by cutting through the material in any direction,
- it is separated from other portions of combustible cladding on adjacent storeys horizontally by a distance of not less than 2.4 m,
- it is separated from other portions of combustible cladding horizontally by a distance of not less than 1.2 m.
The word “contiguous” is important to note: it means being in contact or touching along a boundary or at a point. “Contiguous” is markedly different than the word “continuous,” which implies that there is an uninterrupted continuity to the element along at least one of its borders.
This basically permits vertical strips of combustible cladding, as seen in the schematic.
Combustible cladding that is not contiguous across adjacent storeys is permitted, provided:
- the combustible cladding is not contiguous across adjacent storeys,
- it represents not more than 10% of the cladding on each exterior wall of each storey,
- it has a flame-spread rating not more than 75 on any exposed surface, or any surface that would be exposed by cutting through the material in any direction,
- it is separated from other portions of combustible cladding on adjacent storeys by a horizontal distance of not less than 2.4 m.
You’ll note that the difference between this group of conditions and the previous is that:
- there is no limit on cladding element,
- there is no requirement to horizontally separate the width of the combustible cladding elements on the same storey.
Therefore, you could have a single-storey combustible element that is up to 10% of the cladding area of the exterior wall on that storey, provided it is not contiguous across adjacent storeys and spaced at least 2.4 m horizontally from combustible elements on adjacent storeys, and the flame-spread rating is not more than 75. This basically permits horizontal strips of combustible cladding.
Combustible cladding representing up to 100% of the cladding on exterior walls of the first storey is permitted provided all portions of the cladding can be directly accessed and are located not more than 15 m from a street or access route, measured horizontally from the face of the building.
“Street” has a very specific meaning in the NBC. A street means any highway, road, boulevard, square or other improved thoroughfare 9 m or more in width, that has been dedicated or deeded for public use and is accessible to fire department vehicles and equipment. An “access route” is not defined, but specific requirements are provided in the NBC to describe what is required for an access route for fire department use.
This permission is based on the availability and ease of access of firefighting personnel and equipment to the exterior surface of the first storey. Providing access within close proximity allows emergency responders to take steps to limit the involvement of the combustible cladding in the fire as well as to aid in the evacuation of building occupants in a timely manner.
NBC 2020: Other enhancements
There are other fire safety enhancements related to the use of encapsulated mass timber as a construction type. In the interest of time, I will just highlight some of the other changes.
Based on statistical evidence of combustible buildings, fires that originate or that spread to an exterior balcony will often spread quickly to the floors above. Balconies and decks in buildings constructed of EMTC may be built using timber elements and should be protected by an automatic sprinkler system in conformance with NFPA 13, "Installation of Sprinkler Systems," in order to limit the fire spread.
In general, fire-resistance ratings for fire separations between certain major occupancy types in buildings permitted to be constructed of EMTC are increased in order to limit the probability of fire spread.
Buildings required to be of noncombustible construction, even those of unlimited height and area, are permitted to have a roof covering of Class A, B or C, whereas buildings permitted to be of EMTC will be required to have a Class A roof covering whenever the roof height is greater than 25 m, measured from the floor of the first storey to the highest point of the roof. The intent of the roof classification is to prevent flying brands (a “firebrand” is a piece of burning material or wood) from spreading a fire to adjacent buildings or facilities. A fire on a roof assembly under the 25 m limit can be adequately controlled by fire services using water from the ground. Controlling the fire and reducing the flying brands will limit the probability that a fire will spread to an adjacent building or facility.
Buildings of Group D occupancy that are constructed of EMTC in which the floor level of the highest storey is more than 18 m above grade are subject to the provisions for high buildings. This includes requirements to limit smoke movement, requirements addressing emergency operation of elevators, as well as elevators for use by firefighters, and provisions for a central alarm and control facility.
There is also an administrative requirement to indicate the source of encapsulation ratings for encapsulated mass timber elements specifically on large‐scale sections in order to facilitate the review of the proposed work by the authority having jurisdiction (AHJ) to confirm compliance with the Code.
NFC 2020: Construction and demolition sites
So far, we’ve discussed how EMTC is included in the NBC 2020, but changes are also introduced in the NFC 2020. Section 5.6. of the NFC, Construction and Demolition Sites, contains provisions related to the construction (or demolition) of a building as a “hazardous process or operation.”
NFC 2020: Vulnerability
We’ve just finished discussing all of the limitations and special protections that we must provide in the design of our building so that it is safe AFTER it’s built, but what about during construction, before all those protection and mitigation measures are in place?
During that time, we actually have several fire risk factors:
We will have a large amount of combustibles in a single area, in the form of parts of the building already erected but not yet protected, as well as materials that are stored on site waiting to be installed.
From a security standpoint, construction sites and the combustibles we just described are generally easily accessible for someone with nefarious intentions.
Of course, some construction activities, such as cutting, torching and welding, inherently pose a fire risk.
We also have to consider the risk of construction workers smoking on site.
What about if a fire starts?
How do we notify the workers in the building without a functioning fire alarm?
How do we get all the workers out safely and quickly without egress routes completed or protected from fire exposure?
If the limited available exit routes are being used by the workers, if firefighters need to get in for a rescue operation, what ingress routes do they use?
What about the site?
The area around the building is usually crowded on a construction site, making firefighter access difficult.
Depending on the design of the building, and the stage of construction, firefighters may be faced with a lack of water on site for fighting a fire.
NFC 2020: Hazardous processes and operations
The NFC applies to fire safety for buildings, parts of buildings, facilities, adjacent buildings or facilities, and associated areas undergoing construction, alteration or demolition operations.
The degree of application of the requirements in this Section may vary depending on the specific project risks, to be determined in accordance with the local AHJ.
NFC 2020: Building up
We’ve just quickly gone over what general provisions are contained in the NFC for fire safety at construction sites which is applicable to all buildings.
In the NFC 2015, additional provisions were provided to address, the then new construction type known as mid-rise combustible construction. Those provisions formed a new Subsection in the NFC titled, Additional Requirements for Residential and Business and Personal Services Occupancies.
All of the existing protections contained in the NFC 2015 for mid-rise, “stick built,” construction apply to EMTC construction and demolition sites PLUS some additional augmentations.
A lot of work went into developing the provisions that were introduced in the NFC 2015, but let’s take a look at what has been introduced specific for the EMTC into the NFC 2020.
NFC 2020: Construction access
Buildings of EMTC can be as high as 12 storeys in building height. Protection and access to egress stairs for onsite workers and emergency responders is important.
For EMTC buildings under construction, at least two construction access stairways are required and they must extend upward as each floor of new construction is installed. The stairways are extended upward to the highest floor level that has a floor to provide emergency services with the minimum adequate access to the highest floor possible at each phase of the construction or demolition. This will also facilitate the timely evacuation of building occupants and workers from the upper floor levels in the event of an emergency.
The requirements include a prescribed fire-resistance rating for the stairways and the inclusion of doors to help to limit the probability of the spread of fire from the floor area that is under construction to the access stairway and also to facilitate access for emergency responders and to facilitate evacuation of workers should a fire occur.
NFC 2020: EMTC specific requirements
Due to the potential height of buildings of EMTC, water supply available for firefighters to undertake an internal attack is considered to be critical. If a standpipe system is required, the NFC requires it to be installed progressively. However, there is currently no obvious mechanism for firefighters to determine if the standpipe is usable for a building that is under construction. As such, additional requirements are provided for standpipe installations for buildings of EMTC that are under construction. Testing of the standpipe system is required to be undertaken for each new level of hose valves installed as the building is constructed.
Prior to the mass timber element structure being encapsulated, the risk of fire is at its highest. Due to construction activities, all timber elements cannot be practically encapsulated immediately after they are installed. There are requirements for the protection of some of the timber elements during construction in order to delay the spread of fire so that workers have sufficient time to evacuate and emergency responders will be able to undertake suppression activities.
An encapsulation rating of 25 minutes (permitted to be a single layer of 12.7 mm Type X gypsum board) is required during construction:
- such that not more than 20% of the area of the underside of each timber element floor assembly on each storey is exposed during construction,
- on the interior side of required stairways and vertical service spaces where the enclosures are constructed of timber elements,
- on each face of solid lumber or timber element partitions not less than 38 mm thick and of partitions containing wood framing, and
- such that not more than 35% of the total area of structural heavy timber walls within the storey is exposed during construction.
The four uppermost contiguous storeys are permitted to be unprotected, that is, not complying with the above, during construction.
Conclusion
This concludes the presentation about important changes related to encapsulated mass timber construction in the NBC 2020 and the NFC 2020.
How to get involved
To participate in the code development process, visit the CBHCC’s website to find information about upcoming events and meetings, to submit a code change request, to comment on proposed changes during an open public review, or to volunteer to participate on a code development committee.
How to access the Codes
The National Model Codes are published by the National Research Council of Canada.
Visit Codes Canada publications web page on the NRC’s website to purchase a paper copy of the Codes or to access them in free electronic format.
Thank you.