FIG 11.0 (chapter opener) Services, including sprinklers and smoke detectors, can be coordinated and easily fixed, or modified, on a CLT soffit as illustrated at International House Sydney (by Tzannes Associates, 2017). Construction site safety and wellbeing remains a key concern and the use of smarter materials can displace old fashioned, and more hazardous, ways of working. It is also very obvious that building with a combustible material will pose specific challenges and this chapter introduces a number of fire safety issues specific to CLT use that must be considered and addressed by competent teams during design, construction and use. Across UK construction, the fatal injury rate on-site remains stubbornly steady at around 40 deaths per annum (roughly four times the all-industry rate).1 Around 82,000 workers a year suffer from work-related ill health and after asbestosis, silica dust (typically from working with concrete, sand, bricks, blocks and mortar) is the second greatest risk to health resulting in the additional deaths of over 500 construction workers in 2005 alone.2 Aside from the tragedy and human costs of such, annual costs to the industry for ill health and injury are estimated at £1.06 billion with 2.4 million lost days of work per annum.3,4 The situation can be quite different when building with CLT elements. Fabrication work is displaced from sites to a controlled factory environment and construction is much quicker with significantly reduced site programmes. Site logistics, sequencing and panel assembly can be planned, refined and communicated using 4D tools and existing BIM data with no need for on-site ‘fitting’ so risks can typically be considered in advance and designed out or better managed. Vehicle movements are hugely reduced with a fraction of the number of deliveries (benefitting site operatives and the general population). Panels are typically installed by small specialist teams trained in working at height requiring far fewer workers on site. Manual handling of heavy materials is virtually eliminated and lifting directly from a trailer minimises the need for double handling panels (a panel lift of up to 50m2 might be completed in 20 minutes). Teams typically work in isolation from other trades with few interface issues, hot works are eliminated and little cutting is required – limited to the smallest openings or possible bracing left in situ for transportation. Fixing processes are relatively simple with less equipment and fewer tools required. As such, projects can be managed and overseen more effectively which is particularly beneficial for dense urban sites or multi-phased and complex projects. The use of CLT limits, or offsets, many other significantly more hazardous operations, whether they involve materials of higher mass or those harmful to health (such as cement and concrete or gypsum board) and reducing working at height (itself responsible for around half of all annual site fatalities). There are well understood risks from wood dust,6 but these, like other risks, are best controlled in a factory environment where all processing and finishing is undertaken by machine, avoiding on site working and modification of panels. The risk of workforce harm across the project life cycle is also reduced. Follow-on trades can usually begin work straight after installation of panels within a significantly improved working environment. A weather-tight envelope can be achieved rapidly and CLT provides a degree of insulation before a shell is complete. Working with a CLT substrate involves screwing fixings directly into timber rather than drilling into steel or concrete and as a consequence sites are quieter with reduced dusting. End-of-use risks are also reduced: CLT can be readily designed for disassembly and if elements cannot be unscrewed, there will be no need to demolish or cut reinforced concrete or steel. Edge protection for upper floors may be fitted before panels are lifted above ground and scaffolding is not required for CLT installation. Panels forming outer walls can provide workforce protection and temporary protection can be easily fixed to panels and readily modified to suit changes on site. The challenge for teams remains communicating such benefits to decision makers when making comparisons with other forms of construction. The behaviour of any material in fire conditions needs to be carefully considered and addressed and timber is a combustible material that presents a potential fire hazard. There are however, well-established means of ensuring safety in fire situations for smaller, non-complex buildings whilst larger or taller buildings that must involve a more considered, holistic performance-based design by specialist fire consultants, being beyond the scope of guidance such as Approved Document B or Eurocode 5 (at the time of writing). Such buildings may represent greater risks due to height, scale, complexity, use or the amount of timber exposed. It remains the responsibility of team members to ensure that designs are safe irrespective of published guidance and teams who are limited in relevant experience or uncertain of the risks must not be shy of saying so and insisting upon appropriate input and support. Aspects considered within this section include: regulations and testing, CLT behaviour in a fire situation, design and construction stage and handover/use issues. The global upturn in CLT adoption has exposed inconsistencies in existing standards and test procedures between territories, few of which were developed with an awareness of mass or engineered timber. Around the world, new standards are being established that offer greater certainty to designers around the use of CLT.8 Extensive best practice guidance and test data is however already available that enables designers and teams to design safely.9 The current situation might be compared to the end of the 19th century and early 20th century when steel-framed multi-storey construction was commonly built in the US before being adopted in Europe. In time of course, standards ‘catch up’ and it has been interesting to follow how forthcoming standards in the US have undertaken a clear-eyed approach to consider CLT application (including much testing) unlike the UK where it has been prohibited in limited circumstances following lobbying by those representing alternative material suppliers.10 Readers should ensure that they are aware of relevant guidance and legislation as it affects their projects from the outset, wherever they may be located. It is likely that building control or fire authorities may call for project specific modeling or testing of particular details or interfaces unless adopting a guidance based approach for a relatively straightforward building. These aspects should be factored into project planning in terms of preparation, cost and time, including that of any necessary response to results. Teams must be aware that standards and regulations typically define what is deemed to be the lowest acceptable threshold for safety, and that buildings are not always delivered as intended. The so-called ‘performance gap’ is applicable to many aspects of construction and it is not appropriate to expect that all work will be executed or maintained as intended.11 As such, there are plenty of opportunities to incorporate additional fire safety measures and further layers of safety should always be considered. Fire safety legislation and guidance is undergoing significant and long overdue broad-ranging revision in the UK. Following the Grenfell Tower fire tragedy, subsequent changes to Building Regulations (applicable to England and Wales) in 2018 introduced a prescriptive ban on the use of combustible elements within the external wall build-up of relevant (taller) residential buildings. Notwithstanding any outcomes of the new building safety regulatory framework, such a limitation does not ban CLT use in general but specifically prohibits the use of combustible materials within external walls of tall relevant (residential) buildings. Subject to other aspects, a CLT structure may still be considered appropriate for tall residential buildings (above 18m in height) as long as the wall construction, including any embedded structural elements, is entirely non-combustible and other safety criteria can be satisfied. Compliant facade solutions have been developed – typically prefabricated unitised facades more akin to those found on commercial buildings. The case studies for Fenner Hall and Brock Commons illustrate how taller CLT structures might accommodate such steel framed external wall modules. If the timber surface is not exposed, boarded protection (frequently termed ‘encapsulation’), typically using gypsum board may provide a limited degree of protection but this will fail in time. CLT may not ignite as readily as smaller section timber (with greater surface area) but will burn when exposed to a sustained flame or higher temperatures, contributing to a fire load and influencing possible fire break-out, internal fire spread and the self extinguishing potential of compartment fires. Combustibility can be limited (delayed) by the specification, careful application and maintenance of appropriate surface-applied retardants and once alight, CLT chars at a rate that may in some cases be calculable (although fire circumstances will differ widely). Char layers form, protecting timber beyond from pyrolysis, slowing the rate of combustion. CLT panels are thus typically sized to accommodate loss of mass from anticipated periods of fire exposure whilst maintaining the necessary residual performance in the remaining (unaffected) section of panel. Beyond maintaining structural integrity, some adhesives might pose a challenge when the char line approaches a glue line. Delamination can occur when some types soften, with the risk of charred lamella falling away in parts and exposing fresh timber beyond, potentially fuelling fire regrowth and raising the risk of secondary flashover or extending a fires decay phase. Thermosetting or modified fire resistant adhesives have been proven to mitigate this issue with no such fall-off or degradation of integrity under fire conditions. As with other forms of construction, most projects beyond the simplest forms will require specialist structural fire engineering input. Such expertise and competent decision making must be involved, potentially undertaking a qualitative design review, to clearly identify, and then address the hazards introduced through the use of engineered timber for specific building types, scale and occupancy. Teams will need to consider and set out structural objectives, whether that be to allow adequate time and conditions for escape and external access for fire fighting operations (primarily life safety) or for other higher risk buildings, not so readily evacuated, the acceptable likelihood of surviving burnout. For commercial buildings, asset protection from a value point of view or business continuity aspect may be additional considerations. FIG 11.3 Protective charring, as shown on the face and in the cut section of a small sample, over remaining timber need not readily compromise overall panel integrity if sized accordingly. How a material will contribute to the development of fire and its spread will affect how internal conditions impact the escape of occupants over time.12 Relevant performance aspects include: ignitability, flame spread and heat release, smoke production and the propensity for producing flaming droplets and particles. A physical barrier provides the best protection, but this is often not essential and flame retardant surface treatments may satisfy local standards and the requirements of a fire strategy by reducing the rate of flame spread across a surface. A broad range of such specialist treatments are commonly available for mass timber use, including intumescents, clear or coloured finishes and are typically applied in situ following panel installation. Common to all is the absolutely critical need for appropriate quality control during application to ensure that potential performance is as intended at the design stage. This is the ability of an exposed element to maintain load-bearing capacity and resist the passage of fire, smoke and heat to adjacent areas for defined periods.13 Again, passive measures are often most effective and CLT elements are typically sized, by calculation using known rates, to allow for charring whilst maintaining appropriate protection over required time periods. Connections, joints and edges: Beyond considering the behaviour of a panel surface, fire resistance at edges and connections needs to be considered. Panel joints and edges may burn differently under sustained fire conditions with the risk of burn-through if continuity of material or protection is not maintained. Although timber has a relatively low thermal conductivity, providing good insulation, steel connections and fixings may conduct heat beyond any exposed or heated element resulting in charring beyond the surface which could compromise structural load-bearing capacity or integrity. Compartment design: Appropriate fire stopping products for service penetrations, joint filling and interfaces are available. Choices may be limited in some markets and should be investigated in good time. Installation quality is again of critical importance during site works. Manufacturers and suppliers: Typically publish test data, including the fire performance of different build-ups. Such information is particularly useful at early design stages to ensure that space allowed for elements is sufficient to provide the anticipated performance. As with all materials use, teams must review every aspect of product and test data in the finest detail to ensure that any reliance or assumptions made based upon previous modelling, testing or resultant certification are applicable to project proposals. Changing arrangements slightly or using alternative products (or slight variants) may invalidate such an approach. New building warranty providers and insurance companies: Such bodies are typically conservative and may challenge the use of CLT for more complex or taller projects and are seen by many as the greatest obstacle to wider scale adoption of mass timber construction. Limits to potential cover can prove very significant and early engagement is again key to avoiding surprises whether through inexperience, lack of education or uncertainties around basic data about performance or risks. The extensive use of timber is a high-risk hazard that must be addressed during the construction phase with risk assessments reflecting changing hazards as works progress. Issues for the project and contracting team to consider (from before tender) include: The Structural Timber Association’s ‘16 Steps to Fire Safety’ guidance is relevant to all sites involving CLT and defines best practice guidance covering aspects such as roles and responsibilities, inspections, detection and warnings and the planning and integration of fire protection throughout a build programme.14 In terms of handover strategy, the use of CLT and any associated protection (such as fire resisting linings or surface applied retardants) must be recorded in full, unequivocal detail. This may be part of a fire strategy and should in turn form part of any building manual, operations and maintenance file or handover information so it can be understood and used by owners, tenants, occupiers and all those managing buildings. Uncovering a timber soffit above a boarded ceiling may be considered an attractive ‘improvement’ by future occupants so it makes sense to highlight the function of any in-situ protection to mitigate such risks. Where board lining is integral to a designed fire strategy that may in time be modified, some contractors have taken to physically marking the concealed panels or including notes within deeds or legal documents to make it clear beyond doubt that the integrity of the underlying protection should not be compromised. For owner/occupiers, it’s worth repeating that occupants should use and manage the building as intended and any deviation from this could compromise safety – this may include structural alterations or any changes to building services. Insurance providers may have particular views regarding the management of a building, particularly for taller or larger buildings. Anecdotal evidence regarding integrity following a fire event suggests that of a limited number of small (contained) fires known to have taken place within CLT structures to date, few, if any have posed a risk to the structural integrity. Affected areas would always need to be assessed which may result in areas of panels being cut out and replaced locally or simply being covered over. Safety considerations, including fire safety, are critical to the development of CLT projects (and insufficiently developed related strategies remain one of the most commonly cited reasons for not progressing CLT proposals). Like other aspects of the materials application, there are particular challenges as well as advantages to CLT use and it is imperative that teams seek appropriate advice and competent input and consider the challenges and consequences of likely performance throughout the entire project life cycle.
CHAPTER 11
SAFETY
CONSTRUCTION SAFETY AND WELLBEING
FIRE SAFETY DESIGN
Regulations and testing
A note of caution re: standards and regulations
UK regulation
CLT behaviour in a fire situation
Design stage issues
Understanding the objectives
Reaction to fire
Fire resistance
Construction stage issues
Handover/in-use issues