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The modelling of common corridor smoke control systems in residential buildings is one of the most frequently undertaken performance-based assessments by fire engineers in the UK. However, there is limited understanding of how the deterministic assumptions provided in modelling guidance are derived, and to what extent these are representative of a range of possible outcomes. To place these assumptions into context, the Monte Carlo method is applied using the zone modelling software B-RISK for an exemplar residential arrangement. Distribution functions are adopted to inform inputs for fire parameters, sprinkler performance, and occupant behavioural characteristics. An output distribution for the corridor smoke clearance time is determined and compared to the clearance time of an equivalent set of deterministic simulations applying the assumptions of modelling guidance. The probabilistic study indicates that modelling guidance assumptions typically result in a scenario severity greater than the 99th percentile of possible outcomes. The work also considers the sensitivity of individual input parameters, ranking these based on their impact. These sensitivity analyses highlight the importance of occupant behavioural assumptions around the pre-evacuation time and door open time, and the significance of fire parameters including the elevation of the fuel bed, soot yield, and fire growth rate. Sprinkler parameters are shown to have a lesser impact on the corridor clearance time. The work therefore provides practitioners with an indication of which parameters are worth considering in greater detail when assessing scenario sensitivity.

Concerns about the environmental impact of building construction is leading to timber being more commonly used. However, it often faces scepticism regarding its safety in the event of fire. This article provides a point of reference on the fire performance of cross-laminated timber through a review of large-scale tests. Although adequately protecting CLT can make its contribution to fire insignificant, some of the internal surface of an enclosure can be exposed whilst still achieving adequate fire performance. Natural fire tests show that the charring rate and zero-strength layer thickness are higher than commonly used in guidance documents. The type of adhesive used to bond lamellae influences performance where delamination can lead to secondary flashovers, particularly in smaller enclosures. Structural elements can potentially collapse without self-extinction and/or suppression intervention. Tests to date have focussed on a residential context and knowledge gaps remain regarding larger enclosures, such as office-type buildings.

It is common for fire engineers in the UK to consider a time-dependent series of events when using the Fire Dynamics Simulator to assess the performance of residential corridor smoke control systems. A significant proportion of this established timeline focusses on producing the necessary buildup of smoke and heat in the apartment of origin, prior to any smoke spreading to the corridor or stairs. This paper proposes an alternative approach that enables a compressed modelling timeline by using uniform initial condition parameters in the apartment of origin for the soot mass fraction, room gas temperature, wall and ceiling temperatures, and slab temperature. These conditions can be applied from the outset for separate means of escape and firefighting phases, providing greater flexibility and reducing the wall-clock times for simulations. To achieve this, simulations for a 1000 kW medium growth rate fire in a single reasonable worst-case exemplar arrangement were undertaken using an established timeline approach. This was then used to estimate the approximate initial conditions needed to inform the compressed timeline input parameters. When comparing the established and compressed timeline approaches, close agreement was demonstrated for the estimated conditions of corridor visibility and temperature. The paper also suggests prospective correlations for the initial condition parameters needed to represent a given maximum heat release rate. The paper ultimately provides engineers with a method for residential corridor smoke control modelling that reduces simulation times, as well as providing greater consistency in the demonstrated performance, by proposing initial conditions independent of the dimensions of the apartment.

Kitchen hob fires present a potential threat to occupants escaping from dwellings and calculations may be needed to assess the hazard. Determination of the thermal heat flux from flames to a target can be achieved through the use of hand calculation methods or computational tools. This article compares point source, parallel plane and cylindrical view factor hand calculations and computational simulations using B-RISK and Fire Dynamics Simulator of thermal heat flux with kitchen hob fire experiments presented in the literature. Knowing the level of accuracy of each method provides useful information to designers. Although the point source model is influenced by whether the radial distance is measured perpendicular to the heat flux target or is offset relative to the centre of the flame, the article concludes that it provides an adequate approach for the calculation of thermal heat flux in the case of kitchen hob fires.

Probabilistic analysis is receiving increased attention from fire engineers, assessment bodies and researchers. It is however often unclear which probabilistic models are appropriate for the analysis. For example, in probabilistic structural fire engineering, the models used to describe the permanent and live loads differ widely between studies. Through a literature review, it is observed that these diverging load models are based on surveys conducted between 1893 and 1976 and that widely adopted assumptions, such as the rule for combining permanent and live loads into the total load effect, are commonly adopted based on precedent. The diverging current models however relate to mostly the same underlying datasets and basic methodologies. Differences can be attributed (largely) to specific assumptions in different background papers, which have become consolidated through repeated use in research papers and adoption in background documents to codes. By reviewing the studies underlying currently applied probabilistic load models in structural fire engineering, a consolidated probabilistic load model is proposed in this paper. It is concluded that the total load effect is ideally described by KE·(G + Q), with KE the model uncertainty for the load effect, G the permanent load, and Q the imposed load. The model uncertainty KE can be described by a lognormal distribution with mean equal to unity and coefficient of variation (COV) of 0.10. The permanent load is preferably modelled by a normal distribution with mean equal to the nominal permanent load, and a COV which can either be assessed on a project specific basis, or can be set to 0.10 for a first assessment. For common occupancies (office, residential), the live load is preferably modelled by a Gamma distribution. The mean live load can be taken as 0.2 times the nominal, and the live load COV can be taken as 0.60 for large load areas (> 200 m2) and 0.95 for smaller load areas (

Demonstrating adequate safety for exceptional designs and new design applications requires an explicit evaluation of the safety level, considering the uncertainties associated with the design parameters. The recently published PD 7974‐7:2019 provides five routes to demonstrate adequate safety through probabilistic methods but does not include worked examples. The case study in this paper presents three state‐of‐the‐art approaches for demonstrating achievement of an absolute safety target (acceptance concept “AC3” in PD 7974‐7:2019) for a concrete column in an office building with stringent reliability requirements, specified on the level of individual members and with respect to a standardized heating regime. The case study shows how fragility curves listed by, for example, industry organizations can support probabilistic approaches and a more comprehensive understanding of design performance.

Fire engineering has developed into a mainstream engineering discipline within the building design process. Building fire codes are increasingly complex, comprising thousands of requirements regarding a wide range of topics that must be considered. Fire engineers are required to possess increasingly complex knowledge about a variety of subjects, along with expertise in their application. This has been magnified with the proliferation of performance-based methods using a range of computational tools. This coupled with increased project performance pressures, raises the potential for errors in judgment. Errors in judgment may be caused by limitations in a given resource (e.g. time, information available, knowledge, etc) and/or neglect/over-focus on specific information (at the expense of other and more relevant information) through cognitive biases. This paper initially provides a broad overview of general decision-making, including the use of heuristics and cognitive biases. Examples of cognitive biases are presented which may be linked to errors in fire engineer decision-making. This study considers several fire engineering decision contexts where cognitive biases may exist which are associated with fire code application, modeling/calculations, probabilistic risk assessments, general fire engineering practice, and perceptions based on experience. Potential measures to mitigate some of these biases and prompt better decision-making are discussed. Those that may benefit from awareness of such biases and mitigation measures include not only practicing fire engineers, but also building developers, fire code committees, evacuation/fire/structural fire modeling developers, approving authorities, and fire engineering researchers/students.

A review has been undertaken which focusses on the fire safety document ‘Open plan flat layouts’ (NF 19), where the review identifies that NF 19 comes with limitations in its representation of sprinkler performance and its adopted values for door closing behaviours, location of fires, heat release rate and number of occupants when compared to more recent literature. To assess the impact of assumptions for door closing behaviours and fire location specifically, an event tree analysis of exemplar apartments has been undertaken, comparing NF 19 assumptions to data from literature. This analysis indicates that the previous assumptions for these parameters would not be expected to alter the conclusions of NF 19. As a result of the review and analysis, recommendations are made for the design of open plan apartments. The paper ultimately lays the groundwork for future studies, where additional assessments are needed to consider other NF 19 assumptions, including sprinkler performance, as well as to reassess existing guidance limitations derived from NF 19.

It is common for agencies around the world to conduct some form of cost-benefit analysis (CBA) when proposing an investment in a safety measure. Since resources are limited, expenditure on one particular safety measure reduces the availability of those resources for other measures. Due to finite financial resources, an investment in a safety scheme thus requires a trade-off. As such, the objective of a CBA is to assess whether the proposed measure provides a net-benefit to society or whether resources would be better directed elsewhere. Fire safety measures, such as sprinkler systems, are one means to reduce the risk posed by fire in buildings. Sprinkler systems have a long record of property and life protection but there is a cost associated with their installation and ongoing maintenance. For many years various agencies have examined whether sprinkler systems should be mandated in some or all buildings within their jurisdiction, and as a result have conducted different forms of CBA. However, there are a number of challenges when carrying out such assessments particularly when it comes to placing a value on a preventable fatality. This paper revisits previous CBA studies available in the literature for the installation of residential (domestic) sprinkler systems in single family dwellings. This paper applies recent work to extend the J-value methodology which employs the life quality index (LQI) concept as a means of valuing the benefit of increasing life expectancy through the mitigation of consequences, or reduction in the occurrence rate of a hazard. Similar to the earlier work, the paper finds that the unilateral installation of sprinkler systems in single family homes mostly results in a net-disbenefit to society (depending upon jurisdiction). Widespread adoption of sprinklers is generally only found to be cost-beneficial where the installation and maintenance costs can be split across a number of properties per installation, e.g. apartment buildings. This conclusion is reached through the adoption of a novel and objective judgement scalar that is easy to implement and interpret.

When conducting fire and smoke modelling it is important to adopt an appropriate design fire. Calculations may adopt a design fire that is represented in terms of a heat release rate per unit area (HRRPUA). This paper provides a historical review of recommended values that are predominantly used in the UK as well as values from other jurisdictions where appropriate. It has been determined that many of the HRRPUA values provided in UK guidance are derived from ten fire incidents for industrial fires in the 1960s and 1970s, and five wood crib experiments undertaken in the 1960s. The HRRPUA values originally calculated from these incidents and experiments ranged from 86 kW/m2 to 650 kW/m2. The data was gradually adapted in various forms to consider shops and retail buildings, offices, hotel rooms and residential buildings, where variations in recommended values have occurred over time due to adjustments and calculations from various authors. In light of the observations, this paper provides updated recommended values from literature which relate to specific occupancies and building types. These are summarised as a range of potential values, such as 270 kW/m2 to 1200 kW/m2 for shops and 150 kW/m2 to 650 kW/m2 for offices, where designers will need to take care in considering the relevant source material.

Probabilistic Risk Assessment methodologies are gaining traction in fire engineering practice as a (necessary) means to demonstrate adequate safety for uncommon buildings. This induces a need to apply uncertainty quantification to structural fire engineering problems. Yet, the combination of probabilistic methods and advanced numerical fire engineering tools has been limited due to the absence of a methodology which is both efficient (i.e. requires a limited number of model evaluations) and unbiased (i.e. without prior assumptions regarding the output distribution type). In this paper, the recently proposed MaxEnt method is combined with the dedicated structural fire engineering software SAFIR to evaluate the ability of the method to achieve efficient, unbiased assessments of structural fire performance. The case studies include the probability density function (PDF) of (i) the standard fire resistance of a composite column; (ii) the load bearing capacity of a composite floorplate exhibiting tensile membrane action, after 90 min of standard fire exposure; (iii) the load bearing capacity of the same composite floorplate, considering a parametric fire exposure including cooling phase; and (iv) the maximum temperature reached in a protected steel element under realistic fire exposure. In the first application, the MaxEnt PDF correctly and efficiently captures the distribution obtained using Monte Carlo Simulations. The floorplate example under parametric fire exposure shows the true strength of the MaxEnt method as an unbiased assessment, as different failure modes are observed in the cooling phase resulting in an irregular shape of the load-bearing capacity PDF. For this case, reliance on a traditional assumption of lognormality for the capacity would result in an overestimation of the capacity at lower quantiles. The last case study yields a bi-modal output due to the physics-based duality between localized (traveling) and post-flashover fire development. While the MaxEnt captures this bi-modality, it does not accurately reproduce the distribution obtained by Monte Carlo Simulation. Limitations of the MaxEnt method and needs for further research are discussed at the end of the paper.

A simple framework for setting risk tolerability limits is proposed following a literature review of risk acceptance in fire safety engineering and a feedback round with international fire safety professionals. The framework provides practical guidance for the application of international fire safety guidance, such as the recently published UK guidance document on probabilistic risk assessment (PRA) for the fire design of buildings, PD 7974-7:2019, which provides a state-of-the-art framework to assess adequate safety for complex fire engineering designs. However, the application of this guidance document is hindered by two main constraints: (i) PD7974-7:2019 requires risk tolerability limits to be set, but lacks guidance on defining them for a specific building project, and (ii) no reference case studies are given that demonstrate the application of PRA methods to fire engineering design. In order to help fill this gap, a risk tolerability framework is developed and proposed herein. Finally, it is applied to a reference case study of a UK office building. Considering the developed framework, further research needs are identified. It is expected that the proposed risk tolerability framework and the presented reference case study can support the application of PRA to fire engineering design and ease stakeholder communication on setting risk tolerability limits.

This chapter examines structural fire engineering considerations that are specific to steel, which is a common construction material. First, thermal and mechanical properties of steel at elevated temperatures are discussed. Second, failure modes specific to steel structures (e.g. connection brittleness) are examined. Lastly, pertinent analysis techniques for structural fire engineering applications involving steel structures are presented. Notably, tensile membrane action theory is covered in depth, which is a mechanism that provides thin slabs with large load-bearing capacity, resulting from large vertical displacements, where induced radial tension in the centre of the slab (due to the large deflection) is resisted by a peripheral compression ring. Also, the performance of structural steel connections under fire conditions is closely examined. In the event of a fire, the global frame response is closely linked to the behaviour of such connections. When excessive axial and rotational deformations occur at high temperature, the connections need to be robust enough to provide structural integrity.

This chapter addresses design fires in the context of structural fire safety. First, the design fuel load must be derived for the given space, which represents the potential energy that needs to be considered. Once the design fuel load is established, estimation of the fire exposure intensity on the structure is a key next step in the process. Specifically, thermal boundary conditions acting on structural and/or insulative exposed boundaries must be derived so that subsequent heat transfer analyses may be conducted to determine the temperature histories of the given structural system. Structural fire engineering designs typically only consider a subset of design fires, which are those that are uncontrolled (any cooling effects of active fire protection systems and/or manual intervention are neglected). Other design fires (e.g. those used for determining the available safe escape time) would not commonly be used for SFE applications. Accordingly, fire dynamics concepts pertaining to uncontrolled fire exposures are presented in this chapter.

This chapter introduces structural fire engineering along with the specific contents of this handbook. The handbook has called upon the input of global experts to deliver a resource that brings together a significant volume of material on the topic of structural performance in the event of fire. The book is primarily written for practicing consulting engineers. However, it is foreseen that it can be a useful resource for students of structural engineering who wish to develop a deeper understanding of structural performance in the event of fire, as well as building authorities to assist with review of such alternative designs. Many would argue that whilst evidence exists which suggests that prescriptive methods for structural fire protection usually lead to adequate performance levels, few if any could quantify the level of safety that is provided within this framework. More likely, the actual level of structural fire safety is variable from building to building, subject to coincidental structural design decisions for other hazards (and then retrospectively protected/insulated). In contrast, structural fire engineering proactively designs the structure and applied fire protection to achieve targeted performance levels under fire conditions.

Structural fire safety requirements implicitly balance up-front investments in materials (protection or element sizing) with improved performance (loss reductions) in the unlikely event of a fire. For traditional prescriptive fire safety recommendations, the underlying target safety levels are not clear to the designer, nor is the associated balancing of risk and investment costs. While easy to apply, guidance / code-based approaches to the specification of fire protection / resistance have the severe disadvantage that the level of safety investment is not tailored to the specifics of the case, resulting in large overinvestments in some cases, and possibly insufficient structural fire safety in others. This observation is a major driver for the use of performance-based design (PBD) methodologies, where the fire safety design is tailored to the needs of the building. However, it is posited in this chapter that traditional PBD in a structural fire context is deterministic, with the safety foundation premised upon the collective experience of the profession. It is separately noted that building forms are increasingly uncommon in nature, due to material choice, height, failure consequences, etc., and, as such, collective experience is increasingly a weak safety foundation. This is where probabilistic methods add value and provide a quantified / explicit basis for demonstrating the adequacy of a design. This chapter, thus, focusses on uncertainties and uncertainty quantification in the context of structural performance in the event of fire, introducing reliability and risk concepts, with supporting data and applications.

Previous research has investigated and developed systematic probabilistic models for parameters involved in determining the reliability of a structure under fire. The established models have been summarized and applied in this paper to quantify and compare the reliability of steel columns protected based upon the US prescriptive approach. A set of columns with a range of section factors are selected to study the influence of utilization ratio, restraint conditions, and fuel load density on the probability of failure under fire. The results show a relatively large variation in the value of probability of failure for columns with similar fire protection rating but different section factors and utilization ratio. The influence of fuel load density is presented in the form of fragility functions, where the probabilities of failure for expected fuel load density values are discussed. In addition, the probability of failure for columns across different stories of a building is calculated, leading to the conclusion that further harmonization of safety levels can be achieved if reliability-based quantifications are introduced in the design process.

This paper incorporates a probabilistic fire loss assessment method for reinforced concrete structures into a cost-benefit analysis to optimize a structural fire design. Economic losses in case of failure and survival of the structure are both quantified with, in the latter case, an estimate of the damage and repairs costs. As a case study, a cost-benefit optimization of the position of rebars in a concrete column is investigated. The column response in fire is evaluated using finite element simulations in SAFIR. Variations in cover thickness result in variations in failure probabilities and, for cases where no failure occurs, variations in repair costs due to heat penetration and residual out-of-plane deformation of the column. The optimum cover thickness is the one that offers the best trade-off between the various repair costs across the range of likely fire intensity levels. This optimum is sensitive to repair decisions such as the tolerance on the acceptable residual out-of-plane deformation after a fire. For the studied cases, the optimum cover thickness is smaller in slender columns than in stocky columns due to greater out-of-plane deformations in the former.

Structural risk assessment against fire requires robust material models that take into account the uncertainty in material behavior over a range of elevated temperatures. Such probabilistic...

A probabilistic risk assessment (PRA) is commonly accepted as a tool for performance based design in fire safety engineering, but the position of PRA in the design process, the relationship between different acceptance concepts (absolute, comparative, ALARP), and the responsibilities of the designer remain unclear. Aiming to clarify these aspects, the safety foundation of fire safety solutions is investigated, indicating that PRA is necessary for demonstrating adequate safety when no appeal can be made to the collective experience of the profession. It is suggested that PRA is not a methodology for ‘future fire safety engineering’, but rather a necessary methodology to provide an objective safety foundation for uncommon fire safety designs. Acknowledging that what constitutes ‘acceptable safety’ is subjective and may change over time, an objective proxy of ‘adequate safety’ is defined and proposed as a benchmark against which to assess the adequacy of fire safety designs. In order to clarify the PRA process, a hierarchy of different acceptance concepts is presented. Finally, it is shown how, depending on the applied acceptance concepts, the designer takes responsibility for different implicit assumptions regarding the safety performance of the final design.

Uncommon structural fire engineering designs must demonstrate adequate safety, in principle, through a balancing of the uncertain future costs and benefits of safety investments. In structural design for normal load conditions (‘ambient design’) this level of detail is commonly avoided through the application of reliability targets. In order to inform the development of reliability targets for structural fire design, the background of the ambient reliability targets is discussed. It is found that different common ambient reliability targets are broadly comparable when taking into account differences in assumptions and applications. As recent reliability targets have been informed by simplified cost-optimizations, the derivation of such a model is presented. The derivation allows identification of possible pitfalls when extending ambient reliability targets to structural fire design. It is concluded that ambient safety targets cannot readily be scaled as a function of the fire occurrence rate for application to structural fire engineering problems. The underlying cost-optimization model is, however, applicable as a concept. A number of issues requiring further attention are identified.

In England, there are no fixed requirements on the parameters adopted when considering residential design fires, and analyses undertaken are often deterministic with limited consideration given to probabilistic assessments and the sensitivity of parameters. The Home Office dwelling fires dataset has been analysed, considering the fire damage area and the time from ignition to fire and rescue service arrival. From this, lognormal distributions for the maximum heat release rate (HRR) and fire growth rate of residential fires have been approximated. The mean maximum HRR ranges from 900 kW to 1900 kW, with a standard deviation ranging from 2000 kW to 3700 kW, depending on property type and room of fire origin. The mean growth rate, assuming a t2 relationship, ranges from 0.0022 kW/s2 to 0.0034 kW/s2, with a standard deviation ranging from 0.0071 kW/s2 to 0.0132 kW/s2. When considering incidents which result in immediate fire and rescue service call out following ignition, the mean growth rate increases to a range of 0.0058 kW/s2 to 0.0088 kW/s2. As a result of the analyses, design fire distributions are provided which can be adopted for probabilistic assessments. For deterministic analyses, it is proposed that an approximate 95th percentile fire may be adopted, aligning with a medium growth rate of 0.0117 kW/s2 and a maximum fuel-limited HRR in the region of 3800 kW to 4400 kW, depending on whether the dwelling is a house or an apartment. A 95th percentile design fire broadly aligns with values already specified in guidance, helping to substantiate the existing recommendations.

Assessing the evacuation of residential buildings in the event of a fire requires the number of occupants in the building to be determined. The English Housing Survey (EHS) provides a means of determining occupant density by including data on the number of residents and the total floor area of dwellings. From this it has been found that occupant density can be approximated as being lognormally distributed, with a mean for all dwellings of 44.6 m2/person and a standard deviation of 29.5 m2/person. When considering apartments separately from houses, it is found that apartments were in general more densely occupied with a mean of 38.7 m2/person and a standard deviation of 20.9 m2/person. The occupancy has also been determined with respect to the number of bedrooms per dwelling, with an overall mean of 0.97 occupants per bedroom and a standard deviation of 0.49 occupants. When considering the EHS occupant density distributions in the context of exemplar single-stair residential buildings, it has been found that the design capacity of the stair is unlikely to be exceeded, where for a typical 10-storey residential building the stair design capacity is 580 while the equivalent EHS 99.9th percentile is 165 or 528, depending on whether distributions are applied to each apartment independently or consistently across the full building.

Structural fire safety requirements implicitly balance up-front investments in materials (protection or element sizing) with improved performance (loss reductions) in the unlikely event of a fire. For traditional prescriptive fire safety recommendations, the underlying target safety levels are not clear to the designer, nor is the associated balancing of risk and investment costs. It is discussed that traditional (structural) fire safety design is deterministic, with the safety foundation premised upon the collective experience of the profession. However, it is noted that building forms are increasingly uncommon in nature, due to material choice, height, failure consequences, etc. Within the framework presented in PD 7974–7:2019 there is an expectation that probabilistic risk assessment (PRA) methods be employed to demonstrate adequate safety for cases where the collective experience of the profession cannot be called upon to guide design approaches. In doing so, any design must be demonstrated to be tolerable to society, and the residual risk as low as is reasonably practicable (ALARP). In support of the PD 7974–7:2019 framework, the paper assesses the failure probabilities of isolated steel elements in function of insulation thickness, utilization, load ratio and (mean) fire load. In support of a generalized definition of target safety levels for structural fire safety engineering, optimum target safety levels for insulated steel beams are then determined as a function of the fire characteristics by applying life-time cost optimization (LCO) techniques. The LCO results in an assessment of the optimum investment level as a function of the fire, and damage and investment cost parameters characterizing the building. It is intended that the current contribution can be a steppingstone towards rational and validated reliability targets for performance-based design (PBD) in structural fire safety engineering.

In the UK, prescriptive fire safety guidance offers a simple means of meeting statutory building regulation requirements, for most common building situations. In the context of prescriptive structural fire resistance, buildings are afforded a fire resistance period based upon their height and use. This fire resistance expectation is either to be achieved inherently by structural elements or via protection to them. The broad aim of such prescriptive guidance is the delivery of a consistent level of risk across all building types. For this to be achieved, as the frequency of fires and consequence of failure increases, the reliability of the fire resistance system must increase. This typically manifests in an increase in a building’s fire resistance expectation as height increases. Despite this common practice, this paper fundamentally reviews the concept of fire resistance as a height dependant metric for residential buildings, identifying the limitations of such an approach, utilising single stair apartment buildings as a basis for demonstration and further investigations. A risk correlation is proposed that seeks to explicitly define the structural fire resistance design goal (reliability of the fire resistance system) as a function of both height and occupancy. The correlation is calibrated against UK statistical data to determine what might constitute a common building. It is found that in single stair buildings, an appropriate benchmark case would constitute 7 apartments per level, ranging in size from 1 to 3 bedrooms (28 m2 to 101 m2). This benchmark case is used to determine a risk score which, for the purpose of achieving a consistent level of risk, becomes a constant in the proposed risk correlation. Four example building cases are chosen to demonstrate the application of the correlation. The cases are of the same notional height, but constitute differing occupancy (apartment) numbers. The proposed correlation indicates that the four cases, despite being of the same height, have significantly different fire resistance system reliability demands (96.3% to 98.5%) and, thus, fire resistance demands (154 min to 173 min). In light of the findings, it is concluded that, if conventional structural fire resistance thresholds are not to be exceeded (in the UK typically limited to 120 min), then, in tall residential buildings, the reliability/efficacy of the sprinkler system becomes increasingly important. Finally, a brief discussion is provided regarding the limitations of the approaches presented.

Fire safety engineers endeavour to ensure that a design achieves an adequate level of fire safety. For uncommon buildings, adequate safety cannot be based on precedent and an explicit evaluation of the adequacy of proposed safety features may be required. Commonly, this requires demonstration that the residual risk associated with the design is as low as is reasonably practicable. In those situations, a measure for a safety scheme’s benefit relative to its cost is required, as more efficient safety schemes should be preferred over less efficient ones to maximize the number of lives saved under societal resource constraints. To this end, the J-value has been introduced in other engineering fields as a decision support indicator for assessing the efficacy of safety features. The J-value has been derived from societal welfare considerations (the Life Quality Index) and is adopted in the current paper for applications in fire safety engineering. It is demonstrated herein how the J-value can inform decisions on fire safety, and how it can provide a basis for assessing whether or not a proposed fire safety scheme should be implemented. Future work will focus on its implementation as a tool for assessing the benefit of real life fire safety scheme implementations, such as sprinkler installations.

Large timber buildings, formed from both light and heavy timber construction, are becoming increasingly common in Europe. Many multiple-occupancy timber buildings, such as apartment blocks, are now constructed to greater heights and in densely populated urban locations. The fire-resistance performance of such timber buildings is generally related to the standard fire test. Alternatively, EN 1995-1-2 may be used to demonstrate fire resistance by means of calculation or numerical modelling. The latter is currently limited to standard fire exposure. In addition, modelling approaches are often avoided as many numerical codes do not normally offer the capability to model timber exposed to fire. The most obvious barrier is incorporating the different tensile and compressive strength/stiffness degradation with increasing temperature. Unlike many other structural materials, it is not possible to define a single relationship between timber Modulus of Elasticity (MoE) and temperature. When timber design is advanced to a ‘performance-based’ level further complexities will arise. For example, the definition of structure temperatures for non-standard fires is a difficult task, and assessment of strength/stiffness degradation on the basis of temperature alone is not sufficient due to char formation. As a result, when cooling is considered, material properties based upon stress state, temperature and temperature history are needed.To address the above limitations, a number of developments, which can be used with general FEA software, such as DIANA, to design timber structures for fire, are presented. The developments are incorporated via user-supplied subroutines written in FORTRAN code. The routines include code for determining MoE and strength based upon stress state, temperature and temperature history. They are implemented as part of a total strain-based constitutive model. The implementation of the routines is demonstrated using a simple continuous beam. The example is also used to demonstrate how compartmentation provisions and aspects of whole building behaviour can be used to better design large-section timber buildings. Comparisons are made with simple empirical approaches presented in EN 1995-1-2. Extensions to ‘performance-based design’ using parametric fires are also discussed.

The thermo-physical response of timber structures in fire is complex. For this reason, debate still exists today as to the best approaches for simulating thermal response in fire using tools such as finite element analysis (FEA) modelling. Much of the debate is concerned with the thermal properties of timber, for example, conductivity, specific heat and density, at elevated temperature and how such properties should be implemented or interpreted in numerical calculations. For practitioners intending to use modelling as a fire design tool for timber buildings, guidance exists on the thermal properties of softwood in Annex B of EN 1995-1-2. These properties are limited for use under standard fire exposure conditions because of the way in which they were derived from calibration against focussed test data. As a result, they cannot be applied to non-standard fires, which are more representative of real fires due to a combination of varying heating rates and the decay phase of fire development. The limitations of the standard fire test (and associated curve) are widely understood. As a result, much recent structures in fire research has focussed on the ‘performance based design’ of buildings subject to increasingly realistic fire conditions. Such an approach allows engineers to quantify the level of safety that can be achieved in a building should a fire occur. In addition, the design of buildings to withstand fires proportionate to the risks foreseen and also the geometry present results in better value buildings that are inherently more robust. For the same approaches and associated benefits to be realised for timber buildings, then a number of barriers must be overcome. The most obvious of these is engineers' ability to determine timber structure temperatures as a result of fires other than the standard fire curve. This however presents a number of challenges. Upon heating, the moisture bound within begins to evaporate, volatiles begin to flow from the heated surface and char forms. The rate of which these behaviours occur and the nature of the char that forms depends on a number of factors, but most notably the rate of heating. Upon cooling, the timber member continues to generate heat energy as the surface oxidises. As a result, any models intended to simulate temperature development must consider the relationship not only between temperature and thermo-physical characteristics but also between heating rate and the process of heat generation. Many models have been developed for this purpose; however, they are extremely complex and are some way from being ready for implementation as design tools. This paper proposes implementing ‘heat of hydration’ routines, intended for the curing of concrete structures, to simulate the heating and cooling process in timber structures. Such routines are available in many commercial FEA software packages. The adoption of the hydration routines allows the heat generation process, as a result of oxidation, to be considered in parallel with solid phase heat transfer using apparent thermal properties. The approach is shown to be very effective in simulating temperature development in timber members subject to parametric design fires. The models developed are benchmarked against experiments conducted in the 1990s by SP Tratek. Predictably, a number of the heat generation parameters adopted are shown to depend on the fire dynamics considered. However, recommended parameters are given that provide an acceptable level of accuracy for most design purposes. Copyright © 2012 John Wiley & Sons, Ltd.

SIPS are formed from the lamination of two oriented strand board (OSB) facing plates and a highly insulating polymer-based foam such as expanded polystyrene (EPS) or polyurethane (PUR). The resulting lightweight panels are typically used as primary load-bearing compression elements for buildings such as domestic dwellings, apartment blocks, schools and hotels. The regulatory fire performance of SIPS, like many systems, is assessed via a standard fire test. However, it is widely accepted that this is merely a comparative method for determining the relative performance of one product when compared to another; hence, it gives little indication of a component's likely behaviour in a real fire. With this in mind BRE Global, with support from the UK Department for Communities and Local Government (CLG), have undertaken a research programme to determine the performance of SIPS subject to realistic fire conditions. The research programme exposed four two storey SIP buildings to natural fire scenarios using timber cribs. Two buildings were constructed with EPS core SIPS. The other two were constructed with PUR core SIPS. Each material set was subdivided by passive fire protection specification (PFP). These were specified on the basis of 30 and 60-min fire resistance. The experiments highlighted a number of weaknesses in the system performance of SIP structures with engineered floors. Firstly, where PFP is under specified or poorly installed, collapses of the engineered floor plate are very likely. Mechanisms for fire spread were also identified where fitting details were not appropriately sealed. In addition, there appeared to be little appreciable difference in the behaviour of buildings formed with EPS or PUR core SIPS. Finally, a number of system redundancies and alternative load paths were identified, which prevented total collapse of any of the test buildings.

SUMMARY Gypsum plasterboards are the most widely used passive fire protection for timber structures, especially in the case of light timber frame construction. Understanding the complex thermo-physical behaviour of plasterboard at elevated temperature is vital in the performance-based design of any structure adopting gypsum as passive fire protection (PFP). Numerous heat transfer studies have been conducted over the years where attempts have been made to simulate the fire performance of gypsum-protected assemblies, subject to standard fire exposure. However, contradictory thermal properties for gypsum plasterboard are apparent throughout. As a result, it is unclear from a practitioner's perspective as to which studies represent reasonable properties for design purposes. In recognition of this the authors present a numerical study highlighting the consequences of adopting many of the differing property sets available in the literature, the sensitivity of temperature development resulting from deviations from the assumptions that underpin such properties, and the consequences of adopting plasterboard properties derived from standard fire tests, in natural fire situations. The study presents heat transfer simulations conducted using the finite element software TNO DIANA coupled with both laboratory and natural fire tests conducted on Structural Insulated Panels (SIPs) and Engineered Floor Joists (EFJs). It is found from this study that plasterboard properties are highly sensitive to the assumed free and chemically bound moisture contents. Minor percentage changes are shown to have a significant influence on the temperature development of SIPs exposed to standard furnace fires, while some of the most accepted plasterboard properties available in the literature are found, in some cases, to be non-conservative when adopted in simulations of SIPs. More interestingly, it is also found that the properties of plasterboard available in the literature, largely derived from standard fire tests, are not independent of the heating rate. As a result, when such properties are applied to natural fire problems significant inaccuracies can occur. Copyright © 2011 John Wiley & Sons, Ltd.

Timber, like other structural materials such as concrete and steel, has its own Eurocode (Eurocode 5 part 1.2) for the structural fire design of buildings. However unlike other fire parts of the Eurocodes it is not widely adopted due to its inherent limitations. With the exception of a single Annex, the timber Eurocode (EN 1995-1-2) is only applicable to standard fire exposure. Annex A gives guidance on the charring rates of initially un-protected timber members in parametric fires, however in the UK the use of the Annex is prohibited by the national Annex to the code. The concrete and steel industries have undoubtedly benefited from performance based design whereby the structural fire design strategy is centred on a design fire (typically a parametric fire), which is more credible than the standard fire curve. Such an approach has resulted in more flexible, innovative buildings which have been designed based upon fundamental structural mechanics at elevated temperature, using advanced numerical models. At present however the same principals cannot be applied to the advanced fire design of timber buildings due to current limitations in the timber Eurocode. Where advanced calculation procedures are considered by the code (Annex B), much like many of the methods contained therein, the procedures are only applicable to standard fire exposure. The scope of applicability of the code stems from a fundamental problem regarding a lack of understanding of the heat transfer characteristics of timber in natural fires. The thermo-physical properties contained in the code are ‘effective’ properties. This essentially means that they are calibrated against test results to account for a lack of understanding regarding mass transfer, cracking and ablation both within the timber and char layer. Such calibrations have only been performed on timber members exposed to standard furnace conditions. To attempt to overcome this barrier and extend the scope of thermo-physical properties in the code a study has been undertaken to establish how the conductivity properties of the char layer influence the depth of char in parametric fires. Through calibration of an effective conductivity of the char layer against the parametric charring method contained in Annex A of EN 1995-1-2, it has been possible to establish a relationship between ‘heating rate’ and the effective conductivity of the char layer, in the heating phase of parametric fires. The modified conductivity model is shown to be applicable to a range of densities and moisture contents of timber and also variations in heating rate and fire load density. The latter is a direct result of the method used in the adaptation of the properties. The modified model is objectively critiqued and proposed further work is discussed in detail. The applicability of the modified model in the cooling phase of fires is also discussed.

As part of an ongoing research project to investigate the performance in fire of specific types of innovative construction products and techniques (ICPT), BRE Global have carried out large-scale fire tests to determine the response of different floor systems to a realistic fire scenario. The principal objective was to determine the mode of failure of different floor systems to provide information to key stakeholders (particularly the Fire and Rescue Service), which can be taken into account in the dynamic risk assessments that underpin fire fighting operations. This paper presents the results and observations from those fire tests for three floor systems: (i) solid timber floor joists, (ii) I-section floor beams with solid timber top and bottom flanges and an oriented strand board (OSB) web, and (iii) a timber truss incorporating solid timber upper and lower chord members and a pressed steel web member. These reflect the two most common types of engineered floor systems used in the UK and allow for direct comparison with a more “traditional” form of construction.

The advent of the structural Eurocodes has allowed civil engineers to be more creative in the design of structures exposed to fire. Rather than rely upon regulatory guidance and prescriptive methods engineers are now able to use such codes to design buildings on the basis of credible design fires rather than accepted unrealistic standard-fire time-temperature curves. Through this process safer and more efficient structural designs are achievable. The key development in enabling performance-based fire design is the emergence of validated numerical models capable of predicting the mechanical response of a whole building or sub-assemblies at elevated temperature. In such a way, efficiency savings have been achieved in the design of steel, concrete and composite structures. However, at present, due to a combination of limited fundamental research and restrictions in the UK National Annex to the timber Eurocode, the design of fire-exposed timber structures using numerical modelling techniques is not generally undertaken. The 'fire design' of timber structures is covered in Eurocode 5 part 1.2 (EN 1995-1-2). In this code there is an advanced calculation annex (Annex B) intended to facilitate the implementation of numerical models in the design of fire-exposed timber structures. The properties contained in the code can, at present, only be applied to standard-fire exposure conditions. This is due to existing limitations related to the available thermal properties which are only valid for standard fire exposure. In an attempt to overcome this barrier the authors have proposed a 'modified conductivity model' (MCM) for determining the temperature of timber structural elements during the heating phase of non-standard fires. This is briefly outlined in this paper. In addition, in a further study, the MCM has been implemented in a coupled thermo-mechanical analysis of uniaxially loaded timber elements exposed to non-standard fires. The finite element package DIANA was adopted with plane-strain elements assuming two-dimensional heat flow. The resulting predictions of failure time for given levels of load are discussed and compared with the simplified 'effective cross section' method presented in EN 1995-1-2.