Timber in Fire

This short course focuses on the design of timber structures for fire. It is aimed at architects, engineers and other built environment professionals in South Africa who want to explore the use of wood in residential or commercial structures. We hope you will start your journey in wood here!

short course

How to complete this course

The course is open access and the content is available to anyone with internet access. This course should be completed in the sequence of the numbered sections. We make use of four components in our course (a) sections with text and illustrations, (b) video clips to explain some of the concepts, (c) a short online quiz after each section to test your acquired knowledge and (d) a test, available to learners who wish to obtain a certificate of completion. Please contact us at info@thewoodapp.com with the short course name to complete this test.

To make the course accessible to most people, we’ve tried to limit the time needed to complete it (an estimated 2.5 hours). This course will give you an overview of selected building materials impact and the environment, and design strategies to incorporate timber in a sustainable way. For in-depth knowledge on some topics you might require input from other sources such as national standards, material suppliers, selected textbooks, articles, etc.
If you have any questions or suggestions, please contact us at

Timber in Fire Video 1

2. Basics of fire engineering and structural design

2.1 What is fire safety?
2.2 Design approaches
2.3 Quiz 1
2.4 Fire safety concepts
2.5 Quiz 2
2.6 What is fire engineering

3. Fire resistance ratings

3.1 Fire testing methods
3.2 Fire resistance ratings
3.3 Quiz 3

4. Behaviour of timber in fire as a material

4.1 Heat transfer
4.2 Thermal degradation in fire
4.3 Quiz 4
4.4 Charring and mass loss rates

5. Element design concepts

5.1 Charring of solid timber and glulam
5.2 Quiz 5
5.3 Protective linings
5.4 Reduced cross-section method
5.5 Fire design of glued-up panels and cross-laminated timber (CLT)
5.5 Connections in fire

6. International Building Regulations

7. Recent considerations

8. Conclusions

9. References

The design of timber structures for fire

By: Prof Richard Walls and Darren Sulon

1. Introduction and overview

This section of the course gives an overview of fire safety considerations for timber buildings. The aim of these notes is to provide practitioners with a basic understanding of fire safety, structural fire design and how timber behaves in fire. Additional resources will be required before being able to produce fire safety designs and specifications of such structures, but this document should equip you to know what is possible and how to achieve fire ratings for buildings.

Timber is typically perceived as having no fire rating as wood can burn and is used as a fuel (i.e. – if we braai with wood, isn’t it unsafe?). However, extensive research in the past decades has shown that fire resistance can be achieved for timber structures and safe designs can be produced. Due to the dependable charring behaviour of timber fire, resistance of members can be achieved.

This has been an important factor that has allowed for the construction of many high-rise mass timber structures around the world. Nevertheless, significant knowledge is needed to address the many aspects of fire safety required to produce suitable designs. Also, research is still ongoing and there are some areas that are still not fully understood.

South Africa lacks detailed guidance for the use of timber structures in fire. Fire safety requirements for timber structures, in general, will be discussed in relation to existing building codes.

However, for the detailed design aspects international guidance will be required. Also, even when international guidelines are followed, designs need to be approved by local fire authorities, and the willingness of municipalities to permit multi-storey timber structures varies. Authorities should be engaged early in the process to ensure that they will be willing to support innovative timber buildings.

2. Basics of fire engineering and structural design

2.1 What is fire safety?

In South Africa, fire safety is governed by the National Building Regulations (NBR) Act (Act 103 of 1977) (Republic of South Africa, 1977), which states that the aim of the Act, in terms of fire safety, is:
“…to provide for the requirements with which buildings shall comply in so far as precautionary measures against fires or other emergencies are concerned, including the resistance of buildings against the outbreak and spreading of fires, the protection of the occupants or users of buildings or other persons against fires, the aids or other installations to be in buildings for the combating or prevention of fires and for the vacating of such buildings in cases of fires or other emergencies.”
To put the NBR requirements into practice, the main standard used is SANS 10400-T (SABS, 2020) which provides deemed-to-satisfy rules for how fire safety can be achieved. SANS 10400-T describes the goals of fire safety as follows:
“Any building shall be so designed, constructed and equipped that in case of fire:
a) the protection of occupants or users, including persons with disabilities, therein is ensured and that provision is made for the safe evacuation of such occupants or users;
b) the spread and intensity of such fire within such building and the spread of fire to any other building will be minimized;
c) sufficient stability will be retained to ensure that such building will not endanger any other building, provided that in the case of any multi-storey building no major failure of the structural system shall occur;
d) the generation and spread of smoke will be minimized or controlled to the greatest extent reasonably practicable; and
e) adequate means of access, and equipment for detecting, fighting, controlling and extinguishing such fire, is provided.

2.2 Design approaches

There are two main design approaches when satisfying fire requirements, viz.: (a) deemed-to-satisfy (prescriptive), or (b) rational (performance-based) design approaches.

A deemed-to-satisfy requirement is defined as a “non-mandatory requirement, the compliance with which ensures compliance with a functional regulation” in SANS 10400-T. If any element of a building falls outside of the scope of the deemed-to-satisfy regulations, a rational fire design should be carried out.
A rational design is defined as a “design by a competent person involving a process of reasoning and calculation and which may include a design based on the use of a standard or other suitable document”. A competent person (fire engineering) is defined as a person who:

“a) is registered in terms of the Engineering Profession Act, 2000 (Act No. 46 of 2000), as either a Professional Engineer or a Professional Engineering Technologist, and b) generally recognized as having the necessary experience and training to undertake rational assessments or rational designs in the field of fire” according to SANS 10400-T.

In South Africa, the quality of fire engineering consultancy services is sometimes low with few engineers having received formal safety training. Hence, the quality of rational fire designs varies significantly, especially when it comes to different building materials such as timber. For projects it is very important to appoint consulting engineers who have a strong technical background when dealing with timber so that suitable and economical structures can be produced. Furthermore, most structural engineers have limited fire knowledge and they also have to be involved with the design process on a project. Hence, structural fire engineering knowledge is required.

2.3 Quiz 1

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2.4 Fire safety concepts

To assist with a general understanding of fire safety, the following concepts are important to appreciate in the design process: compartmentation, evacuation, active protection, passive protection and flashover (amongst many other concepts).

For further details, refer to the available fire engineering textbooks (Buchanan and Abu, 2017; Hurley et al., 2016).
Compartmentation – This refers to dividing a building into smaller compartments. Walls between compartments must be fire rated. Furthermore, a very important aspect is that all openings/penetrations in such walls must be fire rated. This includes having fire rated doors (which should not be propped open as in the case with the SA Parliament fire) and all penetrations for HVAC, pipes, electrical conduits and wet services must be fire rated.

It is very important to ensure that suitable specifications are provided for such openings.

Watch the following video which will give you an overview of passive protection products:

Timber in Fire Video 2

Evacuation – For any building it is vital for people to be able to safely escape. Hence, evacuation routes should be carefully laid out and protected. For a building it must be ensured that the Available Safe Egress Time (ASET) is greater than the Required Safe Egress Time (RSET) such that nobody will become trapped.
Active protection – This refers to all the systems that must be triggered and requires a human, electrical or mechanical response. This includes detection systems, sprinklers and fire extinguishers. They are typically important for notifying people about the presence of a fire, and fighting the fire.
Passive protection – Passive protection refers to systems which are permanently in place and do not require any intervention to function and prevent the spread of smoke and fire. Examples of passive systems include boards (Calcium Silicate and Type X Gypsum), spray-on products (vermiculite and perlite), intumescent paints and concrete encasement. Intumescent paint is a product that expands when heated to provide a protective layer to underlaying materials.
Flashover – When a fire starts burning in a room, hot gases rise and accumulate at the roof level. These hot gases radiate energy down onto the floor below. When the hot gas layer reaches around 550-600°C there is typically enough energy being radiated to cause all materials in the room to ignite at around the same time. At such time the fire changes from a few items burning in the room to everything burning simultaneously. This phenomenon is called flashover.

2.5 Quiz 2

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2.6 What is fire engineering

According to the Institute of Fire Engineering in the UK the definition of fire engineering is:
“The application of scientific and engineering principles, rules (codes), and expert judgement, based on an understanding of the phenomena and effects of fire and of the reaction and behaviour of people to fire, to protect people, property and the environment from the destructive effects of fire.” (IFE, 2014)

Thus, at the end of the day the main aim of fire engineering is to primarily ensure the safety of building occupants, with the protection of property and good as a secondary objective. However, with the increasing influence of insurance companies in building development the protection of assets is becoming more and more important.

Structural fire engineering is a profession that exists between traditional fire and structural design. It is normally undertaken by structural engineers with an understanding of fire safety, heat transfer and the response of buildings at elevated temperatures. Events such as the collapse of the World Trade Centre have increased the interest and rate of research and interest in structural fire engineering worldwide in recent years.

A report from the Federal Emergency Management Agency (FEMA, 2002) which followed the World Trade Centre disaster, stated that: “The behaviour of the structural system under fire conditions should be considered as an integral part of structural design” (italics added). Thus, it can be seen that the structural engineering industry is slowly moving from prescriptive based methods towards rational structural fire engineering solutions, whereby fire considerations are starting to become core issues rather than problems addressed as an addendum.

However, to consider all aspects of fire design is a complex and multi-disciplinary task, typically left for specialists (Bailey, 2004).

3. Fire resistance ratings

3.1 Fire testing methods

To assess the performance of materials in fire, there are a number of test methods available. These include combustibility tests, fire spread tests and fire resistance (furnace) tests. For structural resistance, the most important tests are fire resistance tests based on the standard fire time-temperature curve. The standard that governs such testing in SA, is SANS 10177-2 (SABS, 2005).

Figure 1 shows time-temperature curves that are typically used for testing structural elements. The main curves available are (a) standard fire curve (also known as ISO 834 curve), (b) external fire, and (c) the hydrocarbon fire curve. For most typical buildings the standard fire curve is used, and it is the governing design fire in South Africa.

Figure 2 shows an example of a test furnace with a CLT sample installed in it prior to testing. The burners of the test furnace are controlled such that the temperature of the gas inside the furnace follows the required time-temperature curve.

Figure 1: Time-temperature curves

Figure 2: Cross-laminated timber (CLT) sample in a fire testing furnace (Van der Westhuyzen et al., 2020)

Timber in Fire Video 3

This provides an overview of how to test materials in fire, considerations for fire doors and similar aspects.
Samples tested in a furnace must satisfy (a) insulation, (b) integrity, and (c) structural resistance requirements when tested. A column will only be required to satisfy structural resistance requirements, whilst a timber slab would be required to satisfy all these requirements. The requirements are measured as follows:
Insulation – The unexposed face of a wall or slab is not allowed to have the average temperature increase by more than 140°C above ambient or 180°C above ambient at any point.
Integrity – Smoke and flames are not allowed to pass through samples via cracks that open up.
Structural resistance – The elements must carry the applied load for the required length of furnace testing.
An important consideration for the testing of timber is that the time-temperature curve is kept the same irrespective of what is tested. Hence, if a large timber sample is tested and releases a lot of energy, then the burners are simply throttled back such that less external energy is needed to fire the test.

3.2 Fire resistance ratings

The standard fire resistance ratings for different occupancies and numbers of storeys are presented in Table 1. This shows the required rating that a structural element would need to achieve to be considered safe. Hence, a four-storeyd office building would be classified as a G1 occupancy and would require a 60-minute fire rating for structural elements.

Table 1: Fire resistance ratings according to SANS 10400-T

3.3 Quiz 3

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4  Behaviour of timber in fire as a material

4.1 Heat Transfer

When timber is exposed to an external heat source, heat energy is transferred to the timber resulting in an increase of temperature within the material. Heat energy is transferred by three mechanisms, viz.: conduction, convection, and radiation. Convection and radiation are the primary mechanisms which contribute to the transfer of energy from the fire onto the surface of the exposed timber, while conduction is the mechanism responsible for transporting heat energy from the surface through the timber itself. When the heat supply is increased from its source, thermo-mechanical and chemical changes begin to occur within the timber.

4.2 Thermal Degradation in Fire

When temperatures reach around 100 °C, free moisture stored within the pores of the timber starts to vaporise resulting in a process called dehydration. A consequence of the dehydration process is the dispersion of free moisture. Some of the moisture is transported out of the timber specimen at the exposed surface. However, heat exposure provides some degree of resistance for moisture to exit at the exposed surface and also forces moisture deeper into sections of timber. The vaporisation and transportation of moisture require a significant portion of energy. This has a heat sink effect within the timber and slows the increase in temperature. The dehydration also results in a permanent reduction in mass which is dependent on the mass of moisture available (i.e. moisture content). Dehydration also results in reductions in the wood’s strength – depending on heating medium, exposure period and the species of wood.

Following dehydration, at temperatures above 200 °C, the main mechanisms responsible for the ignition of timber take place: pyrolysis and combustion. Pyrolysis is the physical decomposition of timber from exposure to heat, and combustion is the highly exothermic reaction between oxygen and the decomposed (pyrolyzed) material. The pyrolysis process forms a variety of volatile gases, combustible and non-combustible, as well as solid charcoal and liquid tar residues on the exposed surface. The combustible gases formed during pyrolysis are responsible for flaming combustion of the timber. Since pyrolysis is activated only through the fact that the timber has been heated, the rate of decomposition of timber is dependent on the heat energy available. A useful description of timber’s decomposition rate is the mass loss rate. The mass loss rate is measured in g/m2/s and describes the amount of timber mass which is being converted into volatile gases. Quantifying the mass loss rate is important for design because it not only provides an indication of the decay in material performance, but also an indication of how much fuel is being added to the fire due to timbers decomposition.

Although it is generally considered that timber is a combustible material, it is indeed only the combustible gases resulting from pyrolysis which result in flames, not the solid timber itself. Hence, another useful description of timber in fire is a measure of the accumulation of residue of the exposure surface, called the charring rate. For engineering purposes, it is generally assumed that timber has decomposed to char when temperatures in the timber exceed 300 °C (or 550 °F in North America). Charring rates are, in essence, a predicter of the thickness of the charcoal layer on the exposed surface and is typically measured in mm/min. The residue char layer on the exposed timber surface is less dense due to the significant mass loss from dehydration and pyrolysis. After prolonged periods of exposure at high temperatures (above 500 °C), the char layer also has noticeable cracking and fissures. Nevertheless, the char layer has good thermal properties which provides resistance against energy transporting deeper into the underlying material.

Figure 3: The decomposition layers and temperature zones within timber when exposed to heat (Friquin, 2011)

As the char layers grows, combustible and non-combustible gases accumulate and disperse themselves in a mixture with oxygen in the surrounding air. It is at this point that there is a great potential for the timber to ignite. However, ignition will only occur once the mixture has reached its flammability limit. In other words, sufficient combustible gases need to be concentrated within the mixture. A useful way to describe the possibility of ignition is through the critical mass loss rate (LaMalva and Hopkin, 2021).

4.3 Quiz 4

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4.4 Charring and Mass Loss Rates

Figure 4 shows an illustration of typical mass loss rate curve for CLT. Mass loss rates for solid and glued-up sections of timber also display similar trends in terms of the mass loss rate.

Initially, the mass loss rate peaks due to their being no char layer to provide resistance to pyrolysis moving deeper into the timber section. After a short period of time, the mass loss rate begins to slow until it eventually reaches a so-called steady state. At this point flaming combustion of the exposed surface slows or ceases entirely.

The slowing of the mass loss is ultimately due to the accumulation of char on the surface, slowing pyrolysis. If enough oxygen is available from the surrounding air, the char layer can oxidise at a rate proportional to the mass loss rate, in effect keeping a constant char layer thickness.

Figure 4: Mass Loss Rate of CLT (Drysdale, 2011)

For standard post-flashover fires, engineers conveniently use the above-mentioned behaviour to describe a constant charring rate which is proportional to the mass loss rate. The charring rate is known to be dependent on numerous factors such as the incident heat flux (radiation), orientation of the grain relative to the heat source, density, moisture content, wood species, and permeability (Bartlett et al., 2019). The charring rate can also be affected by the measurement method and the testing orientation (horizontal or vertical) (Bartlett et al., 2019; Friquin, 2011). Hence, any person using charring rates must be mindful that there may be high variability in performance given these internal and external factors.

5 Element design concepts

The most modern design standards for fire design of timber construction stem from North America and Europe (American Wood Council, 2018; CEN, 2004). While there are some differences between the two regions, their fundamental principles are the same.

The design concepts explored here will generally refer to methods and research adopted in Europe, specifically CEN 1995-1-2:2004, also referred to as Eurocode 5. Note that most of this section covers design concepts relevant to heavy/mass timber structures. Char rate approaches should be avoided for light frame timber assemblies. Protection of light frame timber can be achieved in using the concepts discussed in Section 5.2.

The primary principle adopted for design is that the amount of material lost due to charring is calculated. Thereafter, there is some damaged material ahead of the char front. Inside of the damaged layer it is assumed that the timber material has full strength. Members are designed based on the size of the residual area.

Watch the following video which covers the fundamental principles of designing timber for fire:

Timber in Fire Video 4

5.1  Charring of Solid Timber and Glulam

For the standard post-flashover fire, the charring rate for one-dimensional charring, see Figure 5, is often assumed to be constant with respect to time such that

where  d_(char,0) is the depth of char, and  t is the time of exposure to fire in minutes. The one-dimensional charring rate assumes the char front moves in one direction (i.e. not exposed to fire on two or more sides).

Figure 5: One-dimensional charring of solid timber cross section for fire exposure on one side only

If it is necessary to consider effect of corner rounding’s and fissures in the char, for example when there is two-dimensional exposure (see Figure 6), then a notional design charring depth must be calculated as:

Figure 6: Comparison of charring depth d_(char,0) for one-dimensional charring and notional charring depth d_(char,n)

Notional charring depths accounting for two-dimensional charring behaviour are higher. Typical charring rates β_0 and β_n are given in the fire design standards relevant to the buildings’ jurisdiction. In South Africa, caution should be exercised when applying the charring from SANS 10163-1 (SABS, 2003).

These regulations do not distinguish between β_0 and β_n , nor do they consider the internal and external factors which affect the charring rate already noted (except for the density). Moreover, recent research suggests that charring rates in SANS 10163-1 may not necessarily provide conservative estimates of charring for locally sourced timbers (Van der Westhuyzen et al., 2020). Some examples of β_0 and β_n  for European timber and panelling products are given in Table 2.

Table 2: Design charring rates β_0 and β_n of timber, wood panelling and wood-based panels CEN 1995-1-2:2004

5.2 Quiz 5

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5.3 Protective linings

Timber structural elements can achieve suitable fire performance through the specification of protective linings (cladding) and insulative material (i.e. passive protection). The most common types of protection in this regard are gypsum plasterboard, or sacrificial timber linings such as oriented strand board or plywood.

One common approach for calculating the fire resistance of the protective layers in light frame assemblies is the Component Additive Method (CAM). As its name implies, the CAM accounts for the summation of the effective contribution of any fire exposed lining, the performance of the timber structural members (if necessary), and possibility of protection due to cavity insulation material. A commonly used variation of CAM in North America is given in the report by American Wood Council (2012).

Figure 7: An example of a floor assembly produced from CLT concrete composite deck and glulam beams protected with gypsum cladding

Protective cladding is not necessarily always 100 % effective in protecting underlying timber from charring. Here, there are several likely scenarios which should be considered in design, see Figure 8.

The start of charring of underlying timber may delayed until time t_ch due to the protective lining;
Charring of underlying timber may commence prior to failure of the fire protection. In this instance the timber will char but at a lower rate until failure time t_f of the fire protection.

After failure of the fire protection, the charring rate of freshly exposed timber is substantially increased due to preheating of the compartment. The timber chars at approximately double the normal charring rate to a depth of 25 mm where sufficient char has accumulated to slow pyrolysis. After 25 mm of char has accumulated, the char will revert to the normal charring rate.

Figure 8: Variation of charring depth with time due to inclusion of fire protection by protective cladding

5.4 Reduced Cross-section Method

Just like protective linings, timber itself can be used as fire protection due to its low conductivity and the insulating properties of the char layer. The methods described in this section are intended for the design of heavy timber elements (but not CLT). The fundamental principal of the methods here is that timber sections can be bulked-up by providing additional sacrificial timber which is allowed to be consumed during fire exposure. The char layer which accumulates is intended to protect the underlying timber and materials required for structural purpose. The method involves the determination of an effective charring depth which:
1. Uses a constant charring rate to determine the timber regions which have become damaged; and
2. Accounts for regions of the timber which may not be charred yet, but are heated enough that their structural performance is significantly reduced.
The section of timber which remains uncharred and unaffected by the heat source is commonly called the “effective” or “residual” cross-section. An effective cross-section is calculated by reducing the initial cross-section by and effective charring depth as follows:

where d0 approximates the section of timber which is heated above a specified temperature threshold for safe structural purposes, and k0 is a factor accounting for the total exposure time of the timber. Typically, d0 is taken as 7 mm and k0 is taken as t/tch for exposure periods less than tch and 1.0 for exposure periods larger than tch. For unprotected members, tch=20 min.

Figure 9: Definition of residual cross-section and effective cross-section

5.5 Fire Design of Glued-up Panels and Cross-laminated Timber (CLT)

The charring model in CEN 1995-1-2:2004 in its current form does not include the use of glued-up panels such as CLT. This is mainly because the behaviour of CLT panels exposed to fire requires careful evaluation, due to additional fire phenomena which would lead to poor fire performance.

One of these is significant char fall-off, as illustrated in Figure 10. After prolonged exposure, there is a high possibility of large pieces of char falling off under their self-weight due to poor connection between inter-layers of boards. In some cases, entire board layers can fall off in a process called delamination. If delamination occurs, there are significant implications in terms of the fire fuel load.

Freshly exposed timber rapidly burns due to the accumulation of heat within the compartment, leading to reignition of timber and flaming combustion, and the potential for a second flashover in the compartment fire. Char fall-off and delamination are found to be more pronounced on horizontally oriented CLT elements than for vertical elements.

Figure 10: Fire Test on CLT floor showing large sections of char falling off under their own weight

To allow the use of CLT elements in buildings, Klippel et al. (2017) have proposed a new charring model based on CEN 1995-1-2:2004, to be adopted in the next generation of the Eurocode for timber. The proposed model is a modification of the relationship between one-dimensional charring rate β0 and notional charring rate βn. This approach recommends that new coefficients are included, flexible enough to adapt to a variety of timber structural elements and their applications. The newly proposed notional charring rate is defined as

The coefficients (in order) consider the increased charring behaviour due to:
ks: the width of the timber member;
kpr: protection during different phases of the fire;
kn: corner rounding;
kg: gaps between boards;
kcr: cracking of the char layer;
kj: joints; and
kco: connections with metal.

For the purposes of CLT panel design only, two of these proposed coefficients are considered, namely kpr and kg, while the rest are set to 1.0. Just like the phased charring model prescribed in CEN 1995-1-2:2004 for protective cladding, each panel layer in the CLT and charring phase can have its own individual notional charring rate if required, see Figure 11.

Figure 11: Charring rates for different CLT in horizontal and vertical applications

Using the updated charring model, Klippel et al propose that the effective cross-section method can be used as originally prescribed in CEN 1005-1-2:2004, with specific attention given to selecting the thickness of the d_0 layer given the presence of cross-layers (for exact procedure, see Klippel et al. (2017)).

5.6 Connections in Fire

Ultimately it is important to realise the joints are critical in structural capacity, and most building collapses occur as a result of connection failure rather than anything else. Hence, timber connections should be carefully considered from the start to ensure that sufficient fire resistance can be provided.

In heavy timber structures, the most common types of connections involve the use of metal fasters, dowels, plates, and hangers. Metal has a high thermal conductivity, which can result in rapid heat conduction into deeper sections of the connection and timber members. A consequence of this is the timber surrounding the metal can begin to char at a faster rate. Moreover, the connection is able to char internally leading to a swift decay in structural performance.

A logical solution in this regard is to leverage the insulating properties of the timber, concealing metal components and limiting their external exposure. By doing so, it is possible to maintain a cool internal temperature of the connection, extending the time over which the connection can maintain its load. Connections can achieve necessary fire rates through the specification of fire protective cladding or by demonstrating a sufficient effective cross section to sustain the fire loads.

This, however, only provides adequate protection with regards to the timber members. Careful consideration should be given to holes and gaps arising from the connection’s constructability. Dowel, bolt, and fastener heads can be concealed as far as possible by fitting glued-in timber caps to provide additional protection. Gaps between adjoining members can be fitted with fire protective sealants (e.g. intumescent coating often).

Figure 12: A dovetail beam to column connection after a fire test showing increased charring near the metal components (Palma, 2016)

6  International Building Regulations

The South African building regulations in SANS 10400 presently do not allow for the construction of timber buildings above two storeys. Hence, to construct high-rise timber buildings in South Africa, it is most likely necessary to demonstrate sufficient fire safety using international standards and practice. Safe buildings, which adhere to the requirements of a rational design as required by SANS 10400-T, can be produced.
One of the few building regulations which currently allow for the design of high-rise buildings using heavy timber, is the International Building Code. In its most recent update, the International Building code allows the use of cross-laminated timber (CLT) as an approved construction material and also allows buildings to be constructed to a height of 25.8 m if they are solely built from mass timber (ICC, 2021).

Over and above being able to safely predict the fire behaviour listed previously, building regulations such as the International Building Code exist to ensure all functional requirements are met (e.g., means of escape, internal fire spread, external fire spread and provisions for the fire service). The regulations are there to make sure that a reasonable standard of health and safety for persons in or around the building is provided. They are not intended to limit damage to property other than what may pose a life risk. Furthermore, they are not intended to mitigate or minimise financial losses due to fire. This can have implications in terms of the overall fire design of the building as regulations may be insufficient for the client’s needs.

For example, there may be a requirement or expectation from the client that a significant portion of the timber must be left exposed for aesthetic purposes. In this instance, building regulations may impose strict rules regarding how much of the structure can be left exposed. This is because the exposed timber can have adverse effects on the fuel load, as well as creating a greater risk of fire spread, travelling fires and poor compartmentalisation. This is discussed further below.

Another potential issue is the concept of green facades. In its current form the International Building Code does not allow for the using of combustible facade for high-rise structures. This is ultimately due to the high possibility that combustible facades could assist fire spread between floors, or potentially cause the fire to travel to neighbouring sites.

7  Recent considerations

As much as there have been significant advances in ensuring the mass timber structures are fire safe, there are still a number of aspects that require further research. An aspect of importance is that of fuel loads (the amount of available combustible material) that has historically been based on the contents of a building, and that assume that the building skeleton cannot burn.

However, when significant amounts of exposed timber are present, it increases the amount of fuel available and the heat release rate. Flames emerging from burning compartments can reach further, igniting materials further away and fires will burn for much longer. Hence, safety distances and passive protection may need to be increased.

Based on such considerations, many codes limit the amount of timber that can be exposed to limit fire intensity, as discussed above. Various organisations have been conducting large-scale fire tests to understand how fire dynamics are affected, along with looking at when timber will self-extinguish after burning for a period (Bartlett et al., 2019). Recent testing sponsored by ARUP has highlighted such considerations through a series of large-scale experiments, with images from the experiments shown in Figure 13 (ARUP, 2021).

Full details from the testing are still being made public.

However, the work does indicate the following from the first experiment:
Analysis of the experimental results and camera footage showed that the presence of the timber structure approximately doubled the heat release rate (HRR) compared to the value expected from the wood crib alone. Smouldering combustion of the timber members continued for hours after the cessation of flaming, burning in several hotspots, resulting in holes through the CLT slab. This paper improves the understanding of fire dynamics for open-plan, exposed timber compartments and has shown that issues such as external flaming, smouldering and the increase in HRR need to be considered when deriving practical design solutions (Kotsovinos et al., 2022).

Hence, the fuel load and duration of burning may, for timber structures with exposed material, potentially need to be increased – meaning that a higher fire rating might be required to accommodate exposed timber. Furthermore, the presence of detection and suppression systems, along with evacuation routes, becomes much more important.

Figure 13: Photos from the large-scale fire tests being conducted on timber compartments (ARUP, 2021)

8  Conclusions

This brief course has sought to provide an overview of fire safety and how to attain fire safe timber structures. Due to the dependable charring behaviour of timber, fire resistance of members can be achieved. Design methods, such as EN 1995-1-2, provide procedures for calculating the resistance of members after certain periods of fire exposure.

South Africa lacks detailed guidance for rational structural fire design of timber systems, so international guidance is required. Joints are key in fire safety and their performance should be considered, as in most structural collapses joints play a key role.

In South Africa, there is limited experience amongst consulting engineers (fire and structural) in terms of fire safe timber. Engineers with suitable technical skills should be appointed on larger timber projects and the local fire approval authorities should be involved as early as possible.

Recent research has highlighted that the presence of significant quantities of exposed timber can increase fire risk, meaning that many codes limit the amount of timber that can be visible.

9  References

American Wood Council, 2018. National Design Specification for Wood Construction. American Wood Council, 2012. Component Additive Method (CAM) for Calculating and Demonstrating Assembly Fire Endurance. International Code Council.
ARUP, 2021. Mass timber fire safety – Update on experiments one to four.
Bailey, C.G. 2004. Structural fire design: Core or specialist subject? Structural Engineer 82, 32–38.
Bartlett, A.I., Hadden, R.M., Bisby, L.A. 2019. A Review of Factors Affecting the Burning Behaviour of Wood for Application to Tall Timber Construction. Fire Technology 55, 1–49. https://doi.org/10.1007/s10694-018-0787-y
Buchanan, A., Abu, A. 2017. Structural Design for Fire Safety. John Wiley & Sons Ltd, Chichester.
CEN, 2004. EN 1995-1-2:2004 – Eurocode 5: Design of timber structures – Part 1-2: General – Structural fire design. European Committee for Standardization.
Drysdale, D.D. 2011. An Introduction to Fire Dynamics, 3rd ed. ed. John Wiley & Sons, Chichester.
FEMA, 2002. World Trade Centre Building Performance Study: Data Collection, Preliminary Observations and Recommendations. Federal Emergency Management Agency, New York.
Friquin, K.L. 2011. Material properties and external factors influencing the charring rate of solid wood and glue-laminated timber. Fire and Materials 35, 303–327. https://doi.org/10.1002/fam.1055
Hurley, M.J., Gottuk, D., Jr., J.R.H., Harada, K., Kuligowski, E., Puchovsky, M., Torero, J.L., Jr., J.M.W., Wieczorek, C.J. 2016. SFPE Handbook of Fire Protection Engineering, 5th ed. Springer – Verlag New York.
ICC, 2021. 2021 International Building Code (IBC). International Code Council.
IFE, 2014. The Institution of Fire Engineers [WWW Document]. IFE Website. URL www.ife.org.uk/FAQs (accessed 3.18.14).
Klippel, M., Schmid, J. 2017. Design of Cross-Laminated Timber in Fire. Structural Engineering International 27, 224–230. https://doi.org/10.2749/101686617X14881932436096
Kotsovinos, P., Rackauskaite, E., Christensen, E., Glew, A., O’Loughlin, E., Mitchell, H., Amin, R., Robert, F., Heidari, M., Barber, D., Rein, G., Schulz, J. 2022. Fire dynamics inside a large and open‐plan compartment with exposed timber ceiling and columns: CodeRed #01. Fire and Materials. https://doi.org/10.1002/fam.3049
LaMalva, K., Hopkin, D. 2021. International Handbook of Structural Fire Engineering, The Society of Fire Protection Engineers Series. Springer International Publishing, Cham. https://doi.org/10.1007/978-3-030-77123-2
Palma, P. 2016. Fire behaviour of timber connections. Zurich.
Republic of South Africa, 1977. National Building Regulations and Building Standards Act 103. Government Gazette 145.
SABS, 2020. SANS 10400-T:2020 – The application of the National Building Regulations Part T: Fire Protection. South African Bureau of Standards.
SABS, 2005. SANS 10177-2:2005 – Fire testing of materials, components and elements used in buildings Part 2: Fire resistance test for building elements. SABS, Pretoria.

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