A fundamental aspect of any research or solution development is to define and understand the problem. Where an absence of information or subjectivity exists, the void is filled with mitigation features driven by risk perception, precedence, & risk avoidance. Owing to the complexity of Mass Timber and design of the NBC, a methodical approach is required to narrow down the considered derogations, underpinning intent, and risk framework of the NBC. Fundamentally, the following questions inform and guide us to defining the problem. 

• To what extent does prescriptive guidance limit the use of Mass timber?
• Why does prescriptive guidance limit the use of Mass Timber? 
• What is the limitation intending to achieve?
• How does the risk change when Mass Timber is exposed and to what extent?
• Is the intent of prescriptive guidance able to be maintained with exposed Mass Timber with or without risk mitigation measures?
• How do we define the acceptance criteria / tolerable risk?

Figure 1 below illustrates the constituent steps for which one or more design solutions can be identified and proposed. The process follows that of a performance-based design approach that is adapted for the Canadian regulatory environment.  

Arguably, Step 1 is the most critical component of the process as identification of the acceptable solution(s) forms the lens to which an alternative solution(s) will be viewed. A logical starting point would be the relevant construction article to which the building will be designed to. 

For clarity, within the NBC the construction article provides the vehicle for the building code to assign an implicit ‘risk profile’ that assigns risk mitigation features based upon the aspirational building design. The structure of construction articles is as follows:

Note. Specific construction articles do also extend beyond the above provisions and contain additional limitations including firefighting access, use prohibitions, occupant load limits etc. For simplicity and brevity, these nuances are not contemplated further in this article.

Following the logic thread, the use of exposed mass timber could fall within:

• A building that exceeds height / area limitations of a construction article designated as requiring noncombustible construction, 
• EMTC construction article should the height exceed the permitted limit and / or if the exposed percentage or orientations exceed prescriptive limits.  

It should be noted that Building Code Consultants and design stakeholder’s adept in code language are able to interpret acceptable solutions in multiple ways to meet objectives. This can include considering that incorporating exposed Mass Timber is a derogation to Subsection 3.1.5 that permits limited amounts of combustible components (including wood components) within noncombustible buildings. It is considered that such approaches, although potentially valid, should be viewed in a more global context.   

Step 2 signifies the core of the challenge as with the acceptable solution(s) / prescriptive bounds outlined, we must define the intent of those provisions.  

Typically, noncombustible construction is the considered derogation thus the NBC assigns the below functional statements and objectives:

Functional statements

F02 – To limit the severity and effects of fire or explosions

Objectives

OS1.2 – An objective of this Code is to limit the probability that, as a result of the design or construction of the building, a person in or adjacent to the building will be exposed to an unacceptable risk of injury due to fire. The risks of injury due to fire addressed in this Code are those caused by fire or explosion impacting areas beyond its point of origin

OP1.2 – An objective of this Code is to limit the probability that, as a result of its design or construction, the building will be exposed to an unacceptable risk of damage due to fire. The risks of damage due to fire addressed in this Code are those caused by fire or explosion impacting areas beyond its point of origin

Should a variance of the EMTC provisions be identified, the lens changes. Unfortunately, no objectives or functional statements are assigned within the EMTC construction article for EMTC or noncombustible construction, nor are any prescribed for the sentences permitting exposed mass timber within a EMTC building. It could be considered that the below that are referenced directly to fire resistance rating (no direct connection). Subsequently the below provisions could be coupled with the objectives listed above:

F03 – To retard the effects of fire on areas beyond its point of origin

F04 – To retard failure or collapse due to the effects of fire

OS1.3 – An objective of this Code is to limit the probability that, as a result of the design or construction of the building, a person in or adjacent to the building will be exposed to an unacceptable risk of injury due to fire. The risks of injury due to fire addressed in this Code are those caused by collapse of physical elements due to a fire or explosion

OP1.3 – An objective of this Code is to limit the probability that, as a result of its design or construction, the building will be exposed to an unacceptable risk of damage due to fire. The risks of damage due to fire addressed in this Code are those caused by collapse of physical elements due to a fire or explosion 

A critical evaluation of the identified functional statements and objectives would indicate that the NBC restricts the use of Mass Timber (exposed or encapsulated) for the purpose of:

• Limiting fire severity
• Managing the risk exposure caused by fire spread (the compartment of origin is not a concern) to
  • people in or around the building
  • the building itself for fire damage

If we extend this to fire resistance, the risk exposure for people and property is to be managed for building collapse.

A fundamental aspect outlined within the objectives is that to achieve compliance within the NBC framework, we must assess probability and ensure risk exposure is not considered unacceptable. The NBC produces intent statements designed to provide clarity and substance to risk evaluation. The intent statements for noncombustible construction are outlined below.

OS1.2 – To limit the probability that combustible construction materials within a storey of a building will be involved in a fire, which could lead to the growth of fire, which could lead to the spread of fire within the storey during the time required to achieve occupant safety and for emergency responders to perform their duties, which could lead to harm to persons.

OP1.2 – To limit the probability that combustible construction materials within a storey of a building will be involved in a fire, which could lead to the growth of fire, which could lead to the spread of fire within the storey during the time required to achieve occupant safety and for emergency responders to perform their duties, which could lead to damage to the building.

Regretfully, the supplied information simply rewords the objective statements in an equally unhelpful format. The Intent statements are at best unhelpful, at worst unrefined and misleading. Thankfully recent research conducted into Mass timber and associated compartment fire dynamics provides the underpinning rationale essential to evaluating the acceptability of providing exposed Mass Timber construction.

Contemporary research has shown that in order to resolve exposed Mass Timber, we must first explore the origins of combustible construction and fire resistance as these concepts are inextricably linked. During the late 1800’s the risk appetite and safety expectations relating to fire within buildings were changing. Mostly driven by evolving societal risk tolerance, standards were being sought in relation to providing ‘fireproof’ buildings that exhibited a level of resistance to fire. Numerous standards and furnace tests existed during this period that sought to quantify a level of performance that a building, or building component, could withstand for a given fire exposure. [1]

Ira Woolson of the American Society of Testing of Materials (ASTM) oversaw the harmonization of the various tests into a single standard temperature / time curve that was finalized and adopted by 1918. [2] Over time, various fire stakeholders reviewed and evolved this curve into the ASTM E119, ISO 834, & CAN ULC-S101 standards in use today. Figure 2 below illustrates three curves and their relative harmonization. [3] [4]

Figure 2 Standard temperature time curves

Following the development and adoption of the standard temperature / time curve, debate ensued in relation to the applicability of the curve(s) to historical conflagrations and ‘real life’ fire scenarios. Further analysis was subsequently conducted that sought to establish ‘fire severity’ that effectively graded buildings by occupancy type and subsequent fuel loading. [2]  

Hazard categories were developed that assigned fire resistance durations on the basis that the assumed fuel loading would be consumed prior to the expiry of the fire resistance period. Ultimately that a building could survive ‘burn-out’ without suffering collapse. The predication of noncombustible construction underpinned this philosophy such that the structure would not contribute to the conflagration and inhibit burn out from occurring. 

As building design evolved, it was recognised that combustible construction could not be completely prohibited, and a risk-based approach could be adopted. This is manifested by low-risk buildings being permitted to contain combustible construction with low fire resistance periods designed as a utility to support means of escape and firefighting activities. Medium to high-risk buildings, however, were assigned higher fire resistance periods with combustible construction prohibited to ensure the building should be able to withstand burn out of contents without suffering collapse. [5]

Figure 3 illustrates the compartment fire behaviour predicted within a typical compartment and the potential that exposed Mass Timber will continue the burning process either through the contribution of the structure and / or delamination of Mass Timber panels. The representation of exposed mass timber being elongated burning caused by increase fuel load and / or cyclical burning caused by the introduction of fresh fuel by delaminating Mass Timber panels. 

Figure 3 Compartment fire behaviour

A common misconception amongst fire practitioners, and the wider construction community, is that the building needs only to stand for the period of assigned fire resistance after which full or partial collapse is entirely permissible. Research has disproven this theory with the principle that, for a medium to high risk building, a structure should be able to withstand burnout of the contents. 

It can be derived from the above that exposed Mass Timber presents deeper challenges than those suggested by the objectives and functional statements. Of note also is that the above does not delve deeper in the intricacies of timber as a fuel source or large compartment fire dynamics (travelling fires) that also present unique challenges and complications. 

With the problem defined, solutions and mitigation measures can be contemplated. From a first principles perspective, the below questions should form the basis of any solution(s):    

• Can / should the building structure be effectively separated from the contents such that the fire load assumptions remain valid?
• Will the proposed compartment design, including exposures, reflect the predicted compartment fire behaviour that underpins the fire resistance rating? i.e., will we still achieve fire decay and burn out or will exposed timber continue burning?
• How will the exposure of mass timber contribute to the compartment burn out? Will the orientation or percentage reradiate heat flux onto adjacent mass timber exposures thus elongating the combustion process?
• Does the assigned fire resistance period remain valid?

As a first step, we can look at the prescriptive provisions governing Encapsulated Mass Timber Construction (EMTC) as an exemplar of how exposed Mass Timber can be effectively mitigated. The rationale is presented as a hierarchy of risk measures as presented in Figure 4 that should form the basis of any risk-based solution.   

• Isolation – Encapsulation of the Mass Timber components insulates the structure from a fire condition thus inhibiting contribution as a fuel source
• Reduce – Any exposed Mass Timber is minimized such that the risk of continued burning by the size and orientation of exposed surfaces is low
  • Exposed surfaces of Mass Timber beams, columns, & arches – aggregate area not to exceed 10% of the total wall area of the perimeter of the suite or fire compartment in which they are located.
  • Exposed surfaces of Mass Timber walls – may be up to 35% of the total wall area of the perimeter of the suite provided each exposed surface faces in the same direction 
  • Exposed surfaces of Mass Timber ceilings – aggregate area not to exceed 10% of the total ceiling area of the suite where the exposed surfaces have a flame spread rating not more than 150, or 25% of the total ceiling area of the suite where the suite contains no mass timber walls with exposed surfaces.
• Control – EMTC construction articles require the building to be fully sprinklered to mitigate fire development

To extend the above further into a more meaningful and refined context, exposed Mass Timber solutions may encompass:

• Assessing potential compartment fire behaviour (compartment boundaries, fuel loading, ventilation etc.)
• Providing additional resilience to key structural components such that any potential collapse could be mitigated
• Utilizing compartmentation to isolate fuel packages and / or limit compartment sizes.
• Using strategic noncombustible components in designated locations to support means of escape and firefighting operations
• Assessing reliability and design of fire protection systems to target specific risk and / or adding additional resilience
• Calculating burnout of the compartment through exposure limitation and orientation (prevent reradiation between fuel sources from maintaining the combustion process)
• Assessing firefighting features to ensure sufficient supplies / equipment are available to support firefighting tactics and strategy
• Rationalize appropriate methods to calculate fire resistance for Mass Timber elements including assumptions and limitations of charring rates and connections 

It is clear from the above that any solution involving exposed Mass Timber is heavily reliant on the competency of the designer / practitioner. A firm comprehension of the problem is required in conjunction with a deep understanding of compartment fire dynamics and the combustion properties of wood. Evaluation of the solutions may also require specialist knowledge with advanced analytical / computational tools or perhaps more appropriately, acknowledging the limitations and assumptions of evaluation tools is equally as important. As a fire community we need to be honest with ourselves of our own limitations and knowledge gaps where further research is required.   

The final step on the journey is to demonstrate that the evaluated solution(s) present a tolerable level of risk. Within the NBC, the benchmark for tolerable risk is stipulated as that of an acceptable solution. It is here where the demise of many an alternative solution occurs. The challenge presented is to extract the implicit risk from within one or more acceptable solutions to benchmark against the evaluated risk of an alternative solution. 

The NBC expresses risk tolerance in the following text:

The objectives describe, in very broad terms, the overall goals that the NBC’s requirements are intended to achieve. They serve to define the boundaries of the subject areas the Code addresses. However, the Code does not deal with all the issues that might be considered to fall within those boundaries. The objectives describe undesirable situations and their consequences, which the Code aims to avoid occurring in buildings. The wording of most of the definitions of the objectives includes two key phrases: “limit the probability” and “unacceptable risk.”

The phrase “limit the probability” is used to acknowledge that the NBC cannot entirely prevent those undesirable situations from happening. The phrase “unacceptable risk” acknowledges that the NBC cannot eliminate all risk: the “acceptable risk” is the risk remaining once compliance with the Code has been achieved. The objectives are entirely qualitative and are not intended to be used on their own in the design and approval processes.

The above states that it is incumbent upon the fire engineer to extract quantitative measures, from purely qualified objectives, to demonstrate that the risk presented by an alternative solution is tolerable. Conceptually speaking, the above approach does make logical sense as building codes are effectively risk documents that, at a simplistic level, outline societal risk tolerance. The tolerance manifesting as acceptable solutions. What this fails to capture is that there are numerous valid methods in which risk can be satisfactorily expressed / evaluated. [6] Three prominent approaches are illustrated in Figure 5. 

As a brief overview:

• Deterministic – A quantitative approach that establishes a defined set of conditions that are envisaged to occur
• Probabilistic – A quantitative approach that that establishes the probability of events or outcomes occurring 
• Qualitative – An approach reliant upon discussion and judgement 

Absolute and comparative approaches assert the rationale within each approach in isolation or using comparison with an acceptable solution as the benchmark for tolerability. As detailed above, the NBC requires that a deterministic comparative approach be adopted. Further, the implicit risk of the acceptable solution being derogated from must be extracted and demonstrated. 

When put in a practical sense, the use of comparative assessment can be very limiting as the approach fundamentally relies upon a limited number of variances between selected scenarios. A base scenario is established that aligns with the requirements of one or more acceptable solutions. Subsequent scenarios should be identified that ideally contain a very limited number or variants from the base solution. The more variants between the solutions, the less meaningful a comparison is. 

It is also appropriate to highlight that comparative assessment can potentially be used inappropriately to highlight the inadequacies of prescriptive guidance. A code compliant scenario could be established that contemplates undefined limits that creates skewed outcomes to compare against. The results ultimately conclude that the prescriptive approach can include undesirable scenarios as opposed to demonstrating that the proposed design solution is ‘safe’. 

When comparatively benchmarked against noncombustible construction, exposed Mass Timber will not perform as well as steel or concrete in relation to contribution to fire (it is acknowledged that Mass Timber does have comparatively superior performance characteristics in certain applications however in relation to contributing fuel load, it is inferior). Exposed Mass Timber however may present a tolerable level of risk following deterministic absolute criterion thus it is the failure of the method rather than the solution in determining the risk evaluation.  

It is considered that when evaluating exposed Mass Timber, a varied approach to benchmarking risk tolerance is required. If the focus remains solely on comparative benchmarking, then the filter to which the alternative solution(s) are presented may not effectively capture the risk.