27 April 2020
When extinction is a good thing: fire in mass timber buildings
This is an extract from the Timber 2020 Industry Yearbook.
What comes to mind when you hear the term ‘extinction’? If you were a search engine, you’d probably summon up images of post-apocalyptic desert land, sun-dried bones or even poor artworks of dinosaurs. In no way does it appear as a desirable scenario. However, fire engineers not only wish for extinction, but also relentlessly search for methods to design for it.
This article draws on the ‘Fire Safety Challenges of Tall Wood Buildings’ research project conducted by National Fire Protection Agency (NFPA) in collaboration with National Research Council (NRC) Canada, Research Institute of Sweden (RISE) and National Institute of Standards and Technology (NIST).
One of the principal methods used in fire strategies to ensure safety in buildings is compartmentation. It is especially important in high-rise residential buildings, where a defend-in-place policy is often implemented. In this strategy, firefighters must enter the building and fight the fire from within, while the occupants are waiting in their flats for an evacuation cue once the fire is contained. Fire-fighting operations and limitations in simultaneous evacuation of the occupants are the reasons why many regulatory frameworks do not permit inclusion of combustible materials within the building structure or external envelope above a certain height.
If the effects of the fire can be confined to one compartment, and sufficient protection or redundancy is ensured to the affected structural members, the fire will not endanger occupants nor affect the integrity of the building. Based on this principle, the majority of analytical and numerical analysis in fire engineering is based on evaluation of compartment fire dynamics.
A typical compartment fire goes through three stages:
- fully developed
Most importantly, every real fire eventually runs out of fuel and will self-extinguish.
However, the use of mass timber changes many assumptions traditionally used in the assessment of compartment fire dynamics. Studies have shown that the use of exposed (or inadequately protected) mass timber panels results in a steeper growth phase, with flashover occurring earlier than the baseline, and in many cases continuing burning after all compartment fuel load has been consumed. Prolonged burning also increases the exposure time of the facade to the effects of the fire. Therefore the methods developed for conventional materials, and the assumption that they do not contribute to the fire load, no longer apply for mass timber buildings.
The UK Building Regulations set out a series of requirements for the fire safety of buildings. Unlike other countries, the UK requirements tend to set a performance goal, rather than defining a prescriptive route to achieving it. However, these requirements are often vague and inconclusive:
‘The building shall be designed and constructed so that, in the event of the fire, its stability will be maintained for the reasonable period.’ (Part B Fire Safety, Schedule 1, Building Regulations 2010)
The question that immediately follows is: what is ‘reasonable’? To answer this question, one must understand what the regulator is trying to enforce: a design for burn-out.
The modern fire rating system stems from ‘Post-war building studies no. 20’. The document recommends one-, two- and four-hour resistance periods for various types of buildings or occupancies. The period of required fire resistance was based on an anticipated fuel load and the longest possible duration of the fire event. Using this approach results in an indefinite fire resistance of the structure, as it can withstand the most severe fire anticipated until all fire load is burned-out.
The concept of burn-out is embedded within the current Building Regulations and Approved Document B (ADB). The required fire resistance period given in Appendix B to ADB (for example, 60 minutes or 90 minutes) are historically derived from estimates for the lengths of time it takes for all combustible content to be consumed, and the fire self-extinguishes. The idea of achieving a resistance to burn-out is very powerful, as this (among other conditions) establishes that the building is safe for various activities associated with a fire strategy: defend-in-place policy, prolonged evacuation or safe fire-fighting from the inside of the building.
To use the concept of burn-out in mass timber buildings, one must prove that a compartment does not behave inherently differently to those in conventional buildings. However, a fire in a compartment with exposed timber surfaces, either by design or failure in protection, can remain in a continuous, fully developed stage or re-intensify during a decay phase. Assessing the fire safety for mass timber compartments solely in terms of fire resistance periods in a direct sense (ie the compartment resists fire for X minutes) would be a sign of a faulty fire protection strategy.
Contribution to the fire
In full-scale compartment tests, it is found that, for large areas of exposed CLT surfaces, timber’s contribution to the total heat release rate can exceed that of a fuel load (10MW baseline value for tests presented in Figure 1 – download full Yearbook article to access). However, a higher heat release rate does not necessarily pose a great issue for fire engineers – the structure and separation measures can be designed to cope with a more severe fire. The real question is: ‘Will the timber structure self-extinguish or continue to burn?’
In isolation, timber typically undergoes self-extinction as the energy balance is tilted towards heat losses. However, the presence of an external heat flux can create a self-propelling feedback loop between engaged timber members. This can lead to continuous burning much longer than an anticipated duration of the fire indicated by Approved Document B.
The feedback loop and increased burning can also be created by a single panel. If delamination occurs, it becomes an external heat flux to the rest of the member. In fact, delamination was identified as one of the main causes of a secondary flashover in timber compartments during full-scale tests. More rigorous standards for the fire performance of adhesives used in engineered timber products are being introduced in the US. New adhesives and stricter regulation will likely solve the delamination problem in the future and allow designers to treat CLT as monolithic. However, these standards are not yet in place worldwide.
Certain orientations of exposed CLT panels are more susceptible to not reaching self-extinction: large exposed surfaces in small compartments or exposed corners (wall to wall or wall to ceiling). Furthermore, the compartment fire dynamics are heavily influenced by the size and position of the ventilation opening (and therefore the availability of oxygen necessary for combustion).
Ventilation opening and facade exposure
A smaller opening size results in an under-ventilated fire with lower temperatures inside the compartment. In contrast, a larger opening leads to an increased burning rate and earlier fire decay, inflicting a shorter yet more intense fire impact on structural timber. Increasing a ventilation opening eventually leads to a fully fuel-controlled fire and further increase in size does not change the compartment temperatures. A larger ventilation opening also results in a more intense exposure of the external envelope to fire due to increased convective heat losses.
Post-fire loadbearing capacity
The mechanical properties of timber change at elevated temperatures, long before pyrolysis or combustion occurs. At 200°C, the material has already lost 85% of its ambient loadbearing capacity (Figure 6 – download full Yearbook article to access). Because timber is a poor heat conductor, with prolonged heating in fire conditions, the temperature (and thus mechanical properties) will vary across the depth.
Post-fire loadbearing capacity of timber members must be accounted for in the design process. Timber members may still experience reduction in properties for some time after the fire has gone out, as heat penetrates to the previously unaffected core of the members.
When the duration and intensity of the fire (including the contribution from mass timber) is known, the temperature profile across the load-bearing member can be evaluated with a finite difference method.
A common design solution implemented in mass timber buildings is encapsulating the timber entirely in plasterboard, thus protecting the timber surface from reaching 200°C (the onset of thermal decomposition). The time to reach that threshold shall be longer than the duration of the fire.
Installation of plasterboard protection can be very effective in avoiding a significant change in the compartment fire dynamics, allowing the standard design procedures for concrete or steel buildings to be followed. However, as with any protection method, it can fail if the specification, material or installation method is faulty.
Stakeholders often seek to expose part of the timber structure, as it is visually pleasing and results in a warm, welcoming space. Issues arising with exposed structural timber in buildings do not mean the designer cannot seek to expose some timber. The question here remains ‘what is the safe threshold?’ – in other words, how much exposed timber is ‘too much’?
In large compartments with a relatively small area of timber exposed, the risk of a feedback loop created by an interaction between smouldering panels is decreased. However, a secondary flashover can still occur due to an external heat flux on the structural panel originating from delaminated layers of CLT.
The designer can establish a condition in which the entire fire load in the compartment has combusted and the char line has not yet reached the first glueline by increasing the thickness of the outer laminae.
Even though a design rule to calculate the required thickness of outermost laminate could be developed, a step-by-step method is unlikely to appear in design guides in the next couple of years. Every design would have to be solved from first principles based on the test results or numerical studies. As the standard fire curve does not take into account a decay phase, parametric or natural fire curves should be used.
For smaller compartments, thickening laminates alone will not guarantee safety. In isolation (which can be considered the case for some large compartments), the charring rate is relatively predictable, but self-extinction in small compartments is not.
Solve heat balance equation
Based on the geometry of the compartment and orientation of exposed members, a condition for self-extinction can be evaluated. On a global, compartment scale the heat balance can be expressed with the following equation:
q̇ ”FB – feedback per unit area
q̇ c – energy released due to combustion
q̇ w – energy lost through walls
q̇ rad – energy lost through the opening by radiation
q̇ conv – energy lost through the opening by convection
Aw – area of the walls
The relationship above allows the average heat flux feedback on the walls to be established, if the properties of the compartment fire are known. Of course, the question arises: ‘What is the limiting value for q̇ ”FB for self-extinction to occur?’ This can be evaluated with an equation for mass burning rate ṁ”f :8
q̇ ”EXT – external heat flux
A threshold value of the burning rate ṁ”f for timber to reach self-extinction is between 2.5 g/m2s to 5 g/m2s. After all combustible content of the compartment is consumed, the only source of external heat flux on timber is the feedback between the panels (q̇ ”EXT= q̇ ”FB ). This concept allows for establishing the connection between micro- and macro- scale heat balance equations and evaluating if self- extinction is reached.
The equations shown above are difficult to solve in real engineering situations. However, understanding these relationships is crucial to understanding how a timber compartment can be designed to exhibit self-extinction, resulting in a safe design. The main principle is that the heat losses from the compartment must be high enough to limit the incident heat flux on the timber surface, leading to self- extinction. Heat losses can be altered either by the size and orientation of openings, or the build-up of the compartment walls.
The designers have a lot of control over these parameters. Therefore the compartment can be purposefully designed to include exposed timber surfaces but lead to self-extinction in all design scenarios.
Test every configuration
Another method of justifying the safety of a building with exposed timber surfaces is to test every compartment configuration present in the building. While at a first glance it may sound like a preposterous waste of a project budget, in some cases it may be a practical solution. The nature of and the incentives behind erecting CLT buildings, especially in the residential sector, dictate repetitive and modular units. It is possible that only a few configurations would need to be tested to establish the fire safety of an entire high-rise building.
Use known arrangements
It is useful to consider CLT compartments that have already been tested when designing a mass timber building. A variety of full-scale tests have already been conducted around the world. The designer could specify the compartments to be of similar size and panel arrangement to ones tested, thereby ensuring the availability of testing data from day one. In many cases, the researchers’ intent was to examine the behaviour of compartments that could be found in modern tall buildings, so such arrangements could potentially be readily used in design.
Such practice was incorporated in recent revisions of the International Building Code (USA) and the National Building Code of Canada, allowing for a limited amount of timber to be exposed internally in buildings up to 12 storeys in arrangements that have been tested in the past. There is a potential for the UK to follow with a similar revision, albeit placing more emphasis on a performance goal of such designs.
Extinction. Burn-out. These terms are typically used to convey a negative message. However, in the case of a fire event in a timber building, that is exactly what we want to achieve.
Mass timber structures, particularly mid- and high-rise buildings, are beyond the scope of typical building situations that Approved Document B is intended for. Considering the ambiguity in UK Building Regulations regarding the fire safety of mass timber buildings, it is the designers and engineers that must be ‘reasonable’ and provide safe and sustainable structures now and for future generations.*
Evaluation of self-extinction in compartments with exposed timber surfaces must form a part of a total fire safety strategy. Therefore, the co-operation between stakeholders and fire engineers is most effective when the latter are involved from the very early stages of the design.
As fire engineers, we can help the clients to evaluate the proposed design to ensure that it performs adequately and is safe to be occupied. This can be only done by applying fundamental knowledge and solving the problem with an academic approach, rather than relying on regulation-based solutions and step-by-step design guides.
* At the time of writing, timber cannot be used in the external walls of residential buildings in England and Wales that have a floor over 18m, and consultation about reducing this to 11m, among other things, is currently ongoing (Scotland also has restrictions). However, this is not a blanket prohibition of all-timber buildings or timber-hybrid buildings above these heights subject to the construction of the external walls.
This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here
Article written by Wojtek Serwatka, Engineer – Fire & Structural, Entuitive.
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