24 October 2019
Yearbook article: Fire performance of contemporary timber buildings
This is an extract from the TRADA 2019 Timber Industry Yearbook. Image: Tallest timber – silhouette height comparison between the Tianning Pagoda, Hyperion tree and Big Ben – Entuitive.
In August 2006, two natural historians Chris Atkins and Michael Taylor discovered Hyperion – a giant coast redwood, towering a record-breaking 116m above the Redwood National and State Parks in California. The record was short-lived; in April 2007, it was surpassed by a 153m-tall man-made ‘tree’ – Tianning Pagoda in Changzhou. As of 2018, the temple still holds the title for the tallest timber structure ever created, but it is likely to lose its crown in the upcoming decades.
Architects and developers are becoming increasingly interested in using timber as a main structural material. The primary drivers are:
- speed of erection
- market demand.
Recent developments have proved the solution can also be economical and efficient. Notwithstanding the development in structural timber, the question often asked by the public is: how can tall timber structures be made safe for fire?
That is where fire engineers play a role.
Even trees can’t grow indefinitely. The maximum height a tree can reach is about 130m. Similarly, the height of mass timber buildings is also limited; not only by physics, but also by building regulations.
In fire safety design, there are two approaches: prescriptive and performance. The prescriptive path focuses on satisfying the prescriptive (code-based) requirements. There is no requirement to understand the principles of fire dynamics, smoke control or human behaviour – if the clauses of the building regulation are satisfied, the design is deemed to be safe.
The ‘magic numbers’ and ‘golden rules’ embodied in the building regulations are accepted without question, but even a small diversion from the code will require the design team to develop a disproportionately detailed proof that an adequate safety level is achieved. For example, if the distance from the nearest exit door is 10% longer than the maximum allowed by the regulations, it should not be difficult to prove that an improvement in fire safety measures will compensate for the increased travel distance. However, in most cases, the designer will be asked to perform a comprehensive fire safety analysis.
The prescriptive approach is limited, and if the project is of unusual shape or incorporates untraditional building materials, it is very difficult to prove code-compliance. That is where the performance-based approach emerges; proving that the structure is safe with a holistic fire engineering analysis.
Timber buildings: approval worldwide
In North America, the building codes state that tall structures shall be of non-combustible materials (which timber is not), and the exact storey limit depends on the building properties and local codes. Tall timber structures beyond the code limitations require an ‘alternative solution’ – a justification that the structure performs equally well to the ‘acceptable solution’.
This alternative solution path creates the opportunity for elements of the performance-based approach to be applied.
In the UK, the design needs to meet the performance requirements, for example a certain fire resistance. The path taken to satisfy the ‘functional objectives’ is of lesser importance. However, in the aftermath of Grenfell Tower fire and at the time of writing, the Building Regulations have recently changed and introduced a ban that prohibits the use of materials other than those classified A1 or A2-s0,d0 to BS EN 13501- 1:2007+A1:2009 in the external walls of buildings with a storey above 18m. Cross-laminated timber (CLT) does not meet this requirement and it is currently uncertain if CLT will be eventually be exempt from the ban, although there is a great amount of discussion within the industry on this matter.
Nevertheless, high-rise timber buildings must also feature conventional fire protection systems. This includes, but is not limited to:
- detection and alarm systems
- smoke extraction.
In the design of mass timber buildings, it is good practice to incorporate passive and active fire protection systems exceeding the regulatory requirements to further demonstrate fire safety.
The achieved level of safety can be demonstrated quantitatively with a variety of tools: from simple analytical relationships, through numerical models to full-scale laboratory testing.
Will it burn? Combustibility of timber depends on many factors, with one being the most important: size. Anyone who has prepared a campfire knows it intuitively – one cannot start a fire with a tree-trunk size log. No matter how long we try to ignite it, the log will not sustain burning once the applied heat is removed. However, if exposed to heat for long enough, eventually even the largest logs will ignite.
The reason for this behaviour is charring, a process characteristic of thermosetting polymers and most solid organic compounds. It is a process of an incomplete combustion, leaving a porous residue composed primarily of carbon. In heavy timber members subject to fire, a char layer insulates the virgin material underneath. The protection of timber can be further increased with an application of fire-retardant coatings or impregnated treatments for smaller timber elements. Currently available semi-transparent products allow for an increased fire resistance, while retaining the natural beauty of CLT.
Architects and clients often wish to expose large areas of structural timber in bespoke projects. The exposed timber surfaces may continue to burn after the entire fuel load (furniture, stacked paper, etc.) has been consumed.
A typical fire can be divided into three phases:
- developed fire
Each phase presents a unique challenge to be considered in design.
Growth phase is the first stage of a fire and concerns life safety. The fire engineer must consider how the fire growth and smoke development affect the safety of evacuees. It is followed by the developed phase, which concerns the structural safety. The conditions are no longer tenable and are endangering the structural integrity of the building. The last phase, decay, is of special importance in timber buildings; a secondary fire development may occur if the wooden assembly continues to burn.
Growth phase: life safety
To ensure evacuation is possible, the Required Safe Egress Time (RSET) must be shorter than the Available Safe Egress Time (ASET). The RSET is the time required for all occupants to evacuate and includes detection, pre-movement and travel time. ASET is the time it takes to reach untenable conditions (toxin concentration, temperature and visibility) in egress paths.
Developed and decay phases: structural capacity
While the growth phase of a fire mainly influences life safety and evacuation, the developed and decay phases are those with most impact on the structural capacity of a timber structure.
During the growth phase, internally exposed timber can increase the rate of growth of the fire, thus reducing the ASET. It is worth noting that conventional buildings are permitted to have a certain amount of combustible internal finishes, which contribute to the growth phase of the fire in a similar fashion. Fire engineers quantify the time it takes to reach untenable conditions with empirical correlations or computer modelling.
Timber construction can lead to a higher uncertainty in ASET. For that reason, mitigation measures to reduce RSET such as improved signage, wider stairs and doors, active fire protection or reduced travel distances are available to fire engineers to design an egress strategy that ensures an adequate performance.
A concern during the decay phase for a timber building is whether the timber actually self-extinguishes after consuming all fuel in a compartment. In severe cases, an exposed timber structure may lead to a secondary flashover.
Fire engineers must ensure that the timber structure will self-extinguish after the fire load in the compartment has burnt out. Current research suggests ranges of heat flux and mass-loss rate (rate of fuel consumption) can be used to determine if timber will self-extinguish. However, the phenomenon is subject to variables such as:
- compartment geometry
- compartment construction.
Currently, the most definitive way to determine if the compartment will extinguish is to perform full-scale tests. In future, it may be possible to use numerical simulations to determine self-extinction without laboratory testing.
Structural capacity during the developed and decay phases is typically demonstrated by calculating the amount of timber charring that has occurred and the remaining load-carrying capacity of the timber elements. A typically assumed charring rate is 0.65mm/min, although it varies depending on:
- fire exposure
- fire protection treatment applied.
CLT is known to char non-linearly. As the char layer penetrates the material, the lamina may spall off, exposing the virgin material underneath sooner than anticipated.
Charring of longitudinal members (beams and columns) is slightly different. The heat transfer is no longer one-dimensional and can act on all sides of the member. This results in corner rounding.
Nail-laminated timber (NLT) chars linearly due to the vertical alignment of laminae. However, there is an opportunity for the sides of laminae to char if the connection is not robust and the cross-flow upwards is prevented.
Fire engineers must demonstrate that the timber structure has a sufficient loadbearing capacity for the duration of the fire, taking into account the portion of the material lost to charring. In long-span floors, the size of members is likely to be governed by deflection or vibration, rather than the loadbearing capacity; hence the fire resistance is already provided. In columns and walls, the size is often dictated by the axial stress capacity, thus the ‘protection by oversizing’ in size may be necessary.
Finally, a key consideration in timber buildings is the connections. Conventional connections in the timber assembly use steel as the connector material. Metal parts can conduct heat rapidly into the connections, where the mechanical stresses are the highest. Long fire exposure can lead to charring around the connector and separation at the interface of the two materials.
Researchers have investigated the use of non-metallic connections in the past, primarily replacing steel with glass- fibre-reinforced polymers (GFRPs), with good results.
Connection performance during a fire can be demonstrated using charring calculations, thermal analysis or laboratory testing. Typically, metal connections are protected with intumescent paint or are fully encapsulated within the timber member.
Although many tall and complex timber structures do not leave the drawing board, those that do often fascinate the industry and the public at large. For fire engineers, it means new, exciting challenges and solutions – not just checking the prescribed travel distances.
This is an extract from the TRADA 2019 Timber Industry Yearbook. Download the full article, including supporting images, references and further reading, here
Article written by Wojtek Serwatka, Fire Designer, and Matt Smith, Fire and Structural Engineer, of Entuitive. The authors are currently researching the fire resistance of all- timber connections (inspired by traditional woodworking joints). Both spoke on the Timber Focus Theatre at UKCW.
Interested in learning more about fire performance? This topic is being covered in detail by OFR Consultants' Danny Hopkin at the Better Timber Buildings conference, London, on 27 November 2019. He will give an overview of the changes to Building Regulations that affect timber, and discuss the work – both resulting and planned – necessary to address them. Book your space today