Firstly a sincere thanks to the worshipful company of fire fighters for inviting me to speak at their annual fire lecture event. So first I have to admit that up until a while ago, I was probably secretly one of the doomsday brigade about timber construction and very much a closed mind on the subject. And so of the strongly held view that timber was frankly far too scary to contemplate for serious construction projects. Now Timber is a fascinating construction material for very many reasons, and I hope that will shine through tonight, above all else. One of the fascinating things about it is the diametrically opposite positions people take when it comes to their belief regarding the safety of timber in fire.
What’s to know?
Image | theclymb.com Image | theguardian.com
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Firstly we have the “well we’ve all seen a good old campfire “ category, typically backed up with the latest construction stage fire imagery. And reminding us all about concerns from the insurance industry, the statutory authorities, the mortgage markets. And truthfully I am personally aware of projects switching from timber to another form of construction even as late as detailed design stage, because of such concerns. So it’s a valid position.
What’s to know?
Image | structuremag.org Image | energy.korea.com
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But then on the other side of the debate, we have the it’s a very safe material in fire, because it chars, protects itself, therefore has intrinsic fire resistance, and when is everyone else going to get over their conservatism in the UK and simply agree a 70 storey timber tower is the safe next step. � All of this can be true too. There is oodles of very high quality research and evidence to support the role of charring in the protection of timber in a fire. But its these extreme ends of the scale views, that make this material so fascinating. And almost unique in fact.
Great Fire of London
Image | Ben Sutherland
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Timber structures trigger such strong responses and demands when it comes to fire safety, and it always has. But I wish such an equal quest for knowledge, and risk quantification and mitigation, was equally stirred in the construction industry about other materials and forms of construction.
Because quite frankly it is my view that this is very much needed. As designers and builders we are responsible for life safety and for property protection. It is not acceptable to assume it is someone else’s problem. And so many times this someone else is the fire brigade, and that is not acceptable either. In fact under CDM our duties are very clearly set out that this is not acceptable – from designers or from builders.
Knowingly
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So isn’t it time we dealt with fire performance of materials, and the potential consequences, knowingly. To reject the ‘we have always done it this way’ argument as it is simply just not good enough. That we do not accept a lowering of standards because something is complex. So I am very inspired by all the conundrums associated with timber in fire, and so the enormous potential for exciting innovations that therefore exist, if we attempt to unlock these conundrums. This unlocking will enable exciting design, as well as efficient and robust construction, whilst also explicitly addressing safety. And that’s what I mean by Knowingly.
Content
Why timber? Traditional forms Current and future trends Fire as a barrier and an opportunity Fundamental behaviours A robust method for a robust future
So I will set out our understanding of the trends driving the increase in popularity of timber. Because it is important to understand, why this subject is not going away any time soon. I will briefly explain existing traditional methods for fire safe design. To set the context for how the rapid innovation in timber as a construction material, means there is a limited body of precedent to support its fire safe design without a change in current codes and standards. It is clear fire is the rightful barrier to further innovation, but what is also clear the fundamental issues surrounding the performance of timber in fire can be understood to a much better standard than now. And I will wrap up by suggesting a concept approach for fire safe timber. All of this falls under the umbrella of something we are really hoping to make the construction industry think about right now - a concept we term Total Fire engineering - and I will set this out for you too. This evening it is as we propose it may be applied to Timber, but as I said there are many other materials and systems that require similar levels of care and attention.
1\ The requirement for change
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To achieve the goals we aspire towards we must simplify the problem, understand the limits to our knowledge and adopt a thoughtful and extensive approach in regards to timber buildings, with a view to eliminate our knowledge gaps and, to then engineer.
There are many reasons for using timber as construction material, but three of the most important and common motivators in the last 5-10 years are - Sustainability architectural push and an increasing skill with and demand for pre-fabrication. Prefabrication can improve the quality in the construction process and brings advantages of speed, precision, and easy transportation. These advantages are ever becoming more apparent, causing wood to be used in increasingly innovative ways.
Traditional forms
Heavy timber
Image | oakmasters.co.uk
Image (all in row) | LifeCycle Tower
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Wood itself has been used as a construction material for many centuries, but until relatively recently it has been used in three predominant forms: The first of these are heavy timber beams and columns. In these types of building the wood sections tended to be of large cross-sectional area and the amount of exposed timber was therefore relatively low compared to total internal surface area. Charring of such elements has been well researched since the 1940s. So from a fire perspective this form of construction is relatively simple, the solid structural elements are likely to be ‘oversized’ from a fire perspective and the relatively low exposed surface area means that the burning wood itself will not contribute much to the fire. It will char and not continue to burn once the fire has burned out.
Traditional forms
Low Rise (Housing) Image |.gnacarpentry.com
Image | npic-hmit2009.org
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The second type are two to three storey, lightweight timber frame buildings, typically used in housing. Experience from real fires and testing shows that lightweight timber burns relatively easily and quickly and therefore has very little inherent fire resistance. The typical strategy for fire protection is to encapsulate this structural form in fire resisting construction, typically with plaster board or fireline board. This is not without its challenges and risk. Now the fire protection system inherently becomes reliant on the quality of the encapsulation, in terms of design detailing, quality of construction and any damage to the encapsulation over the life of the building. However if done correctly the timber is not likely to be exposed to the affects of fire and can continue to perform its function for the duration of a fire.
Finally and more recently, roofs and the rather more rare sculpture forms, increasingly utilise structural timber elements. In these types of construction, the consequence to life-safety and property from fire are traditionally seen as being relatively low, Due to no habitable spaces being supported by the timber structure. And consequently the requirements for fire resistance are not to date considered to be a governing factor of design.
However, relatively recent technological advances have meant large, structural timber components can be manufactured off-site to a very high standard. Around the early 1950s, advances in glue technology mean that small pieces of sawn timber could be glued together to form much bigger and longer sections than could be achieved from a single piece of timber (for example Glulam and Laminated Veneer Lumber). This meant that large strong sections could be produced from cheaper lower grade timber. Similarly, more recently, in the last five or so years, computer aided manufacturing has enabled large, panelised, glued sections (for example Cross Laminated Timber) to be manufactured to extremely high tolerances off-site. The advent of mass production CLT, which is both light and strong, has increased the viability and demand for timber as it can be manufactured off-site, transported and craned easily, assembled rapidly and has the structural capacity for 10 or so storeys. So it’s a most viable construction material. And is greatly in demand.
Structural forms are also getting more complex, and modern timber buildings can no longer be thought of as simple beam and column structures. Instead, they comprise hybrids of Glulam and CLT, sometimes with concrete and/or steel components, and structures are often continuous and have complicated moment connections. So this is driving the need for a review of what we know and understand now about such timber systems in fire.
There is a clear desire to express as much of the timber as possible. This is not only for aesthetic reasons, It also eliminates the need for wet, decorative trades (such as plastering and painting) on site, which has significant benefits in terms of construction program, site logistics and very importantly cost. It also hints at a potential future where we reduce reliance on encapsulation and so reliance on the quality of that encapsulation, should we be able to find a way of creating Safe, expressed timber for these more complex structural forms.
In fact, whilst at the time of writing, the tallest, completed timber building is 10 storey, 32m high residential tower in Melbourne But there are several, international concepts and proposals in the public domain to build timber or hybrid timber buildings up to 40 storeys or more. Many fire codes and standards around the world limit the use of wood as a construction material based on historical restrictions born out of single structural element behaviour in a small scale fire test. This is a means of accounting for uncertainty. An example is limiting the height of timber buildings. This might seem arbitrary and unfounded, but it is in fact a means of mitigating increased risk of uncertainty in the event of a fire, through reducing the consequences of failure. If we want to push the boundaries of those limitations, we need to eliminate over simplification and uncertainty. To facilitate this we must have a thorough and fundamental understanding of timber, its true performance in fire, and the risks associated with it during the construction and operational stages.
The future…
Past Present
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So in short, we have gone from a situation where timber construction comprised large ‘over-sized’ beams and columns, or it was totally encapsulated, or it was used in low risk buildings to a situation where technological advancements mean that exposed mass timber can be used in complex structural systems to construct buildings of 10 storeys and upwards. But this amount of exposed timber in a fire has the potential in itself to fundamentally change the fire , as it contributes to the effective fuel load within the enclosure.
FundamentalBehaviours
Timber structure as fuel
Temperature Response:
Combustible compartment
Non-combustible compartment
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This impact on the very behaviour of the fire itself, coupled with the demand for complex structural forms, means a fresh approach to the problem is needed. There will be an increasing reliance on the timber form for safety, and on the timber form for the prevention of fire spread, as the building scale increases. This fundamental change introduces both natural and healthy scepticism, as well as a clear need to review existing design methods for compatibility with the new design aspirations.
Barriers to innovation Timber structure as fuel Threshold for self-
extinguishment Charring rates in real
fire Preventing delamination
in fire Structural Frame
response
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But there are other fundamental behaviours that require our attention, apart from the timber structure itself contributing to the behaviour and severity of the fire itself.
Firstly there is a phenomenon where even if the fire has burnt out, but the timber construction surrounding the room is smouldering, the fire can transition back to a flashover fire event. This greatly impacts fire fighting.
FundamentalBehaviours
Timber structure as fuel Threshold for self-
extinguishment Charring rates in real
fire
High heat release fire
Low heat release fire
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Next, there is the issue of charring rates in real fires, as opposed to small scale tests upon which we currently rely. Test evidence shows the rates are altered as a function of the real fire severity.
FundamentalBehaviours
Timber structure as fuel Threshold for self-
extinguishment Charring rates in real
fire Preventing
delamination in fire
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Then for laminated timber elements, the glue layer can fail in a fire, and so layers can detach. and interestingly change the charring rate again, and can also contribute more fuel to the fire.
FundamentalBehaviours
Timber structure as fuel Threshold for self-
extinguishment Charring rates in real
fire Preventing delamination
in fire Structural frame
response
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Finally there is the matter of whole frame performance rather than single element performance in a fire. And this is why timber in fire is such a conundrum.
Risk +uncertainty
Construction
SteelLoses strength and stiffness
ConcreteUnpredictably spalls
TimberBurns & forms a predictable self-insulating char
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These fundamental behaviours require understanding during design, to limit consequences during the construction stages of a project but also the operational stages of a real building. It is worth noting that this situation is the same for other construction materials. All materials present risk, but we manage them in different ways. The role of the engineer is to identify risks and uncertainty and to mitigate them. This can be exemplified by looking at steel and concrete. Concrete too has unique behaviour in fire. It can spall explosively and in circumstances that are not unexpected in real building fires. The spalling can lead to exposure of the reinforcement and a consequential, brittle and sudden failure of the concrete element. Steel starts to lose its strength and stiffness at around 400degC, and by 700degC a simply supported steel beam or column would be very likely to collapse under the applied load. TURNOVER PAGE 700degC is not a particularly high temperature for a compartment fire, and so to mitigate the risk of failure, steel typically has to be encapsulated in fire resisting construction. It is equally reliant on the integrity of that encapsulation in a fire. After the Broadgate fire, the steel industry realised that not every structural element required protection, and so focused encapsulation is the approach in the industry today, supported by complex numerical analysis. For tall buildings there becomes increasing reliance then on suppression systems, and also physical measures to limit the area of a fire that can impact the structural systems. So it is a holistic approach to fire safety.
London Planned Projects (2030)Image | archinect.com
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And this equally applies to some of the other pressing problems facing the construction industry. One of the more interesting and increasingly prevalent ones is around fire safe façade design and construction. There are 236 London tower buildings of at least 20 stories in the planning or construction stages, according to an audit carried out for think tank New London Architecture, which studies the capital’s architectural landscape and urban planning. Around 80% of these buildings are residential, and one in five are under construction. - See more at: http://thepositive.com/londons-transforming-skyline/#sthash.aesYdhE1.dpuf The use of combustible insulation when creating energy saving building envelopes is rife, yet such little feedback and such little critiquing of the risk this poses is occurring in our industry right now.
Review
The need for change
Image | shigerubanarchitects.com
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So there is a clear case to make change, in order to meet the commercial drivers for timber construction forms. And to innovate to create cost effective solutions during design, for fire safe timber during construction and operations.
Review
The need for consistency
Almost uniquely, profound questions about timber response to fire, are posed, yet entirely ignored for
other structural materials.
Yet it is our position that these questions, born out of the barriers and perceived risks, are entirely valid for
And to re-iterate the need for consistency in this approach – its applicable to any combustible material. Not just timber.
2\ The proposed method
Total Fire Engineering Prioritised Fire Safety Goals Design Execution
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So how to achieve our aspirations? We suggest that this can be achieved by the application of Total Fire Engineering. This means the identification and prioritisation of all relevant fire safety goals, a robust design and assessment method to match, An insistence for high quality construction, And a realistic reliance on fire safety management in real building operations. Neither component is fit for purpose in isolation.
Total Architecture
‘…all relevant fire design decisions have been considered together and have been integrated into a whole by a well organised team.’
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Ove Arup believed in truly good design resulting from combined work with different disciplines, and each thinking not only about what they themselves need, but how their decisions will impact on other aspects of the overall design. When these are all considered together you can get a design that works in every respect. Furthermore, he recognised the significance of the end product, and that not only is good design a must, it must be built and operated effectively if it is to be successful.
Total Fire Engineering
Prepare
Design
SpecifyConstruct
Use
Fire Safety Solution
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We have adapted his philosophy at Arup, to expressly set out the change we believe is necessary for safety improvement in the UK. We are determined to bring simplicity and increased quality into the implementation, on site, of our complex analytical work, as well as our typical design stage work.
Fire Safety Design Goals
Derivation
Category Low Rise Medium Rise High Rise Remain in Place
Evacuation Period
Fast Short Longer N/A
Fire Fighting Methods
External External / Internal
Internal Internal
Consequences of failure
Low (Complete loss acceptable)
Medium(Localised failure acceptable)
High (Potential forloss of life)
Very High (Certain loss of life)
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The first step is to identify all relevant fire safety goals and engineer the best possible fire safety solution within the constraints of the project and costs. As a minimum the fire safety goals must include an adequate standard of ‘life-safety’, but they are also likely to include economic (e.g. property protection and business continuity), environmental and societal goals (e.g. protection of community assets and livelihoods). This is important because the larger, taller or more valuable a building or its contents the more unlikely it is to be acceptable to achieve only the minimum standard required for life-safety.
Fire Safety Design Goals
Burnout
Category Low Rise Medium Rise High Rise Remain in Place
Load BearingElement
Collapse tolerable.
Collapse must be unlikely.
Collapse must be extremely unlikely.
Collapse unacceptable.
Separating Element
Fire spreadtolerable
Fire spread to escape routes unacceptable
Fire spread unacceptable
Fire spread unacceptable
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Examining the reliance that is placed on construction elements for each of these building conditions shown on this slide here Shows that to achieve an adequate standard of safety, for any building other than the smallest buildings, the construction must survive the burnout of a reasonable worst-case fire, regardless of its form of construction. This a fundamental requirement and demands a move away from the current sole reliance on standardised fire curves and standardised small scale fire tests.
Total Fire Engineering
Implementation process
Brief & Framework
1. Stakeholder Engagement
2. Performance Goals
3. Design Constraints
Design
1. Trial Solution
2. Design Review
3. Solution Testing / Assessment
Execution
1. Specification
2. Construction
3. Commission-ing
4. Handover
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Arup already has an assessment methodology that is used commonly for non-combustible construction (e.g. steel and concrete). It is based on a requirement to survive burnout, and can be readily amended to be appropriate for combustible construction as well. There are five additional steps which must be incorporated.
Assessment method for timber
The additional steps required
1) Charring:Can be predicted to ensure sufficient material remains.
2) Additional Fuel:Must be taken into account in
the design fire.
3) Self-extinguishment:The timber must not continue
to burn.
4) Delamination: The glue securing the timber
must not fail.
5) Structural Response:Does Structure react when
elements are heated?
Design fire
Heat flux at material boundary
Heat transfer analysis
Material response
Structural response (local / global)
Sensitivity analysis
Take design actions
Solution Testing:
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These are the additional steps that we foresee to be necessary to achieve the design goals Utilising Charring rates from realistic flashover fires rather than the standard fire test Incorporating the additional fuel available from the timber structure itself Determining the threshold for Self-extinguishment of the fire surrounded by exposed timber Quantifying the structural response induced by thermo-mechanical behaviour Quantifying the impact of delamination, where appropriate Some of these are within our current capabilities whilst others require further work before we are confident enough to implement them. In each case, we are working towards mitigating the associated uncertainty by either: Additional research and knowledge generation Ensuring conservatism Applying prevention measures
?Image | linkedin.com
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It means then that the construction products and their fire performance And importantly the integrated fire performance of construction products must be knowingly dealt with too. The style in the construction industry at this time is to either refuse to produce the fire data to support a product or almost rely on semantics to make it appear something it is not. I want to be very clear I am not attributing this style to the timber industry, who have made open steps for improvement such as their construction stage risk assessment with the HSE. This is something we are concerned about industry wide, and part of the reason we are raising this issue of Total Fire Engineering. It is not just design, it is construction, it is safe operations too.
Contribution of timber to fuel load Failure mode analysis
Connections and joints / composite behaviour
Future needs:Research
Self-extinguishment conditions Delamination
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There are undoubtedly gaps in our current knowledge of timber and so in the predictability of timber in fire. But these are gaps which can be addressed to ensure that fire is not a barrier to the safe use of mass timber as a construction material, and also so we can confidently achieve the fire safety goals we have identified. This is increasingly true as people strive to build larger multi-storey structures with lots of exposed wood (especially lots of CLT). The timber industry have a major and potentially ground breaking opportunity to innovate in this regard. We are already working with Edinburgh University to interrogate Charring rates Flame spread Failure mode analysis Existing design guidance As the demand from arup to engineer timber structures rises and rises. It really feels like the next big thing.
Sky Believe in Better Building
Health and Fitness Centre Building 2 ICE London Civil Engineering
Award 2015 nominee 2014 Wood in Architecture Award
winner
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and much sooner than my cynical self had certainly allowed me to imagine.
Total Fire Engineering
Great things can happen when, ‘all relevant fire design, construction and operation decisions have been considered together and have been integrated into a whole by a well organised team.’
Thank you for listeningThe Fire Lecture 2015
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There is a duty on us all to adopt a robust and competent approach, And so knowingly address and explain fire risk I hope this presentation has provided some contribution to moving away from traditional methods around charring rates and encapsulation And makes the case for knowingly investigating true timber response to fire which can enable the more innovative forms of design coming into our market at this time. Solving the conundrums of timber in fire can only enable safer structures. THANK YOU FOR YOUR ATTENTION.
Some of the images used in this presentation are found from different sources on the Internet, and are assumed to be in public domain and are displayed under the fair use principle. Information has been provided on the image's source whenever possible.
