Scoping study on the significance of mesh resolution vs. scenario uncertainty in the CFD modelling of residential smoke control systems

Computational fluid dynamics (CFD) modelling is a commonly applied tool adopted to support the specification and design of common corridor ventilation systems in UK residential buildings. Inputs for the CFD modelling of common corridor ventilation systems are typically premised on a ‘reasonable worst case’, i.e. no specific uncertainty quantification process is undertaken to evaluate the safety level. As such, where the performance of a specific design sits on a probability spectrum is not defined. Furthermore, mesh cell sizes adopted are typically c. 100 – 200 mm. For a large eddy simulation (LES) based CFD code, this is considered coarse for this application and creates a further uncertainty in respect of capturing key behaviours in the CFD model. Both co-existing practices summarised above create uncertainty, either due to parameter choice or the (computational fire and smoke) model. What is not clear is the relative importance of these uncertainties. This paper summarises a scoping study that subjects the noted common corridor CFD application to a probabilistic risk assessment (PRA), using the MaxEnt method. The uncertainty associated with the performance of a reference design is considered at different grid scales (achieving different ‘a posteriori’ mesh quality indicators), with the aim of quantifying the relative importance of uncertainties associated with inputs and scenarios, vs. the fidelity of the CFD model. For the specific case considered herein, it is found that parameter uncertainty has a more significant impact on the confidence of a given design solution relative to that arising from grid resolution, for grid sizes of 100 mm or less. Above this grid resolution, it was found that uncertainty associated with the model dictates. Given the specific ventilation arrangement modelled in this work care should be undertaken in generalising such conclusions

The fire performance of engineered timber products and systems

Timber is an inherently sustainable material which is important for future construction in the UK. In recent years many developments have been made in relation to timber technology and construction products. As the industry continues to look to construct more efficient, cost effective and sustainable buildings a number of new engineered timber products have emerged which are principally manufactured off-site. In terms of light timber frame, products such as structural insulated panels (SIPs) and engineered floor joists have emerged. For heavy timber construction, systems such as glulam and cross laminated timber (CLT) are now increasingly common. Despite many of the obvious benefits of using wood as a construction material a number of concerns still exist relating to behaviour in fire. Current fire design procedures are still reliant upon fire resistance testing and ‘deemed to satisfy’ rules of thumb. Understanding of ‘true’ fire performance and thus rational design for fire resistance requires experience of real fires. Such experience, either gathered from real fire events or large fire tests, is increasingly used to provide the knowledge required to undertake ‘performance based designs’ which consider both fire behaviour and holistic structural response. At present performance based structural fire design is largely limited to steel structures and less frequently concrete buildings. Many of the designs undertaken are in accordance with relevant Eurocodes which give guidance on the structural fire design for different materials. For the same approaches to be adopted for timber buildings a number of barriers need to be overcome. Engineered timber products, such as SIPs and engineered joists, are innovative technologies. However, their uptake in the UK construction market is increasing year on year. Little is known about how such systems behave in real fires. As a result the development of design rules for fire is a difficult task as failure modes are not well understood. To overcome this..

Demonstrating adequate safety for a concrete column exposed to fire, using probabilistic methods

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. The recently published PD 7974-7:2019 provides five routes to demonstrating 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. 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

Exploratory study into a safety format for composite columns exposed to fire

Current performance based structural fire engineering approaches evaluate structural behaviour under prescribed fire scenarios. The mechanical properties of the materials, the load conditions and geometric parameters are all however fraught with uncertainty, and there is currently no clear safety format ensuring the reliability of the design solution. In this contribution, a safety format is explored for evaluating the fire resistance of composite columns, following results obtained in earlier studies on uncertainty quantification. Using the safety format, a single nonlinear finite element evaluation of the fire resistance time is combined with a global safety factor, defining its design value. Under the assumptions derived from earlier work, the safety format works well, but additional parameter studies indicate that good performance is limited to relatively low ambient design utilization ratios. The results thus highlight the importance of uncertainty quantification and the limitations of basing a safety format for structural fire design on limited studies. It is concluded that detailed studies into the probabilistic description of the response of composite columns exposed to fire are required to generalize the results to a broadly applicable design rule

Target safety levels for insulated steel beams exposed to fire, based on lifetime cost optimisation

The absence of clear target safety levels for structural fire safety severely hampers probabilistic structural fire design. In support of a generalized definition of target safety levels for structural fire safety engineering, optimum target safety levels for insulated steel beams are determined as a function of the fire characteristics by applying lifetime cost optimization (LCO) techniques. Where the fire development characteristics support the prospect of flashover, the Eurocode parametric fire curve is considered, otherwise fires are assumed to roam in search of fuel, leading to spatial variations in temperature, with thermal exposure to structural elements described via travelling fire methods. Fragility curves are derived as a function of, amongst others, the insulation thickness and fire load density, and applied in the LCO evaluations. The LCO results in an assessment of the optimum investment level as a function of the fire, damage and investment cost parameters characterizing the building. It is intended that the current contribution can be a stepping stone towards rational and validated reliability targets for PBD in structural fire safety engineering

Transient reliability evaluation of a stochastic structural system in fire : application of a probability density evolution method supported by evacuation models

Structural fire resistance is a fundamental component of the overall fire safety strategy for buildings. Specifically, with respect to life safety, the structural fire resistance is intended to allow for the safe evacuation of the occupants and access for the fire & rescue service. With the proliferation of performance-based design (PBD) methodologies, the efficiency of fire safety measures is increasingly being challenged. For low-rise buildings, with limited travel distances to a place of ultimate safety, evacuation may be very efficient, and from the perspective of life safety only limited structural fire resistance needed. For high-rise buildings with long evacuation times the opposite may be true. However, such interactions between structural and human response in fire are currently not clearly quantified, nor by extension explicitly considered in guidance. In support of rational decision making and cost-optimisation for (fire) life safety investments, the current paper tentatively explores the relationship between evacuation times in model office buildings on the one hand, and the time-dependent failure probability of critical structural components on the other hand. As a case study, the timedependent failure probability of an insulated steel beam is evaluated, and the expected number of fatalities assessed for different model office building heights (i.e. affecting evacuation duration)

The MaxEnt method for probabilistic structural fire engineering : performance for multi-modal outputs

Probabilistic Risk Assessment (PRA) methodologies are gaining traction in fire engineering practice as a (necessary) means to demonstrate adequate safety for uncommon buildings. Further, an increasing number of applications of PRA based methodologies in structural fire engineering can be found in the contemporary literature. However, to date, 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). An uncertainty quantification methodology (termed herein as MaxEnt) has recently been presented targeted at an unbiased assessment of the model output probability density function (PDF), using only a limited number of model evaluations. The MaxEnt method has been applied to structural fire engineering problems, with some applications benchmarked against Monte Carlo Simulations (MCS) which showed excellent agreement for single-modal distributions. However, the power of the method is in application for those cases where ‘validation’ is not computationally practical, e.g. uncertainty quantification for problems reliant upon complex modes (such as FEA or CFD). A recent study by Gernay, et al., applied the MaxEnt method to determine the PDF of maximum permissible applied load supportable by a steel-composite slab panel undergoing tensile membrane action (TMA) when subject to realistic (parametric) fire exposures. The study incorporated uncertainties in both the manifestation of the fire and the mechanical material parameters. The output PDF of maximum permissible load was found to be bi-modal, highlighting different failure modes depending upon the combinations of stochastic parameters. Whilst this outcome highlighted the importance of an un-biased approximation of the output PDF, in the absence of a MCS benchmark the study concluded that some additional studies are warranted to give users confidence and guidelines in such situations when applying the MaxEnt method. This paper summarises one further study, building upon Case C as presented in Gernay, et al

Developing fragility curves and estimating failure probabilities for protected steel structural elements subject to fully developed fires

Reliability methods are at the core of ambient Eurocode design. Realising exceptional / complex buildings necessitates that an adequate level of safety be demonstrated. Rationally demonstrating adequate safety can only be achieved through the application of probabilistic risk assessment (PRA). This paper presents a novel application of PRA in a structural fire engineering context. It first proposes a generalised limit state for protected steel members undergoing failure modes dictated by yielding. Subsequently, fragility curves describing failure likelihood in function of protection specification and mean fire load are presented for a 1,000 m2 compartment, subject to fully developed fires (parametric and travelling fires). The presented fragility curves have subsequently proven to be of value for further life-time-cost-optimisation applications, with the intent of arriving at explicit safety targets