Advanced modelling of travelling fire for structural design : experimental, numerical and analytical investigations

• Main Author: Charlier, Marion
• Format: Dissertation
• Language: English
• Published: Ulster University 2022
• Subjects: Fire engineering; Performance-based approach; Steel structures

The response of structures exposed to fire is highly dependent on the type of fire that occurs, which is in turn very dependent on the building itself. Inspection of some recent fire events in modern buildings highlighted fires engulfing a limited portion of the compartment - moving across the floor plate with time. This phenomenon, which generates transient heating of the structure, is idealized as a "travelling fire". This situation raised the question of the applicability of the well-known post-flashover fire models to large and open space buildings. Structural fire engineers have to make a choice as to the fire scenarios to be considered when designing a structure, but there is at the moment a lack of scientific knowledge regarding the conditions that can lead to the development of a travelling fire. In other words, there is a strong need in assessing when the travelling fire models are relevant to apply and should be considered in building design. Travelling fire is a complex topic, involving numerous parameters linked to the combustion reaction but also to the building it takes places in, it is therefore important to comprehend the relevant parameters both in the research approach and in the resulting methodology. A preliminary analysis of the physical parameters influencing the development of a travelling fire based on CFD simulations was undertaken to initiate the study of the conditions leading to the establishment and the progression of travelling fires. A theoretical fire load represented by discrete wood cribs based on a regular arrangement was used to investigate deep and square compartments while varying relevant parameters such as the openings and the crib spacings. This first development supported the set-up of three large-scale experiments involving steel structure and a continuous fire load. The latter was engineered to be representative of an office building and was unchanged for the three tests, which were all conducted with no artificial control over the fire dynamics. The three experiments only differed in their opening layout: two of them led to a travelling fire while the last one led to a flashover, highlighting the influence of the boundary conditions on the type of fire that occurs. The measurements represent unique data which support a better understanding of fire dynamics and the improvement of thermal models. In a next step, numerical simulations were conducted to calibrate the model for the conducted travelling fire experiments. A deeper knowledge concerning the appropriate representation of the fire load for large compartments in numerical modelling is needed to be able to further analyse the influence of compartment geometry on the development of a travelling fire and to perform numerical analyses of the temperature development. The initial numerical investigation was further developed to provide a calibrated model for continuous fuel beds, still based on a simplified representation to allow for practical applications and real building geometries considerations. The calibrated numerical model was then used to conduct parametric analysis, covering several building geometries, fire loads and opening layouts. This development allows to extend the experimental dataset, thereby revealing sensitivities of fire exposures to parameters of interest to designers. In addition to providing information about conditions leading to the establishment and progression of travelling fires, this work contributes to numerical efforts in modelling behaviour of structures that considers comprehensively the travelling nature of the fire. Although it is of great interest to develop numerical models, it is equally important to improve the analytical models for structural design and pre-design purposes, to also allow them to match the complexity of the considered problem. An analytical procedure was developed, based on the virtual solid flame concept, to determine the heat fluxes to a structural element due to a travelling fire for structural (pre-)design purposes. The developed model can potentially be used in the design practice for structural fire engineering applications involving travelling fire scenarios. Finally, several practical examples were detailed following the developed analytical procedure. Such approach enables to better understand the capabilities and limitations of the model, assessing the differences which may be observed while varying relevant parameters. This PhD thesis follows an advanced approach for travelling fires in structural design, covering experimental, numerical and analytical developments. Large-scale experimental data and extensive numerical analyses allowed to help understanding which parameters influence the occurrence and development of a travelling fire, as well as improving the existing analytical models; evaluating the thermal impact such fires generate on the surrounding structure. This novel modelling permits both to achieve structural safety and to further optimize the design of structures.