Extended travelling fire method framework with an OpenSees‐based integrated tool SIFBuilder
Item statusRestricted Access
Embargo end date29/11/2019
Many studies of the fire induced thermal and structural behaviour in large compartments, carried out over the past two decades, show a great deal of non-uniformity, unlike the homogeneous compartment temperature assumption in the current fire safety engineering practice. Furthermore, some large compartment fires may burn locally and they tend to move across entire floor plates over a period of time as the fuel is consumed. This kind of fire scenario is beginning to be idealized as ‘travelling fires’ in the context of performance‐based structural and fire safety engineering. However, the previous research of travelling fires still relies on highly simplified travelling fire models (i.e. Clifton’s model and Rein’s model); and no equivalent numerical tools can perform such simulations, which involves analysis of realistic fire, heat transfer and thermo-mechanical response in one single software package with an automatic coupled manner. Both of these hinder the advance of the research on performance‐based structural fire engineering. The author develops an extended travelling fire method (ETFM) framework and an integrated comprehensive tool with high computational expediency in this research, to address the above‐mentioned issues. The experiments conducted for characterizing travelling fires over the past two decades are reviewed, in conjunction with the current available travelling fire models. It is found that no performed travelling fire experiment records both the structural response and the mass loss rate of the fuel (to estimate the fire heat release rate) in a single test, which further implies closer collaboration between the structural and the fire engineers’ teams are needed, especially for the travelling fire research topic. In addition, an overview of the development of OpenSees software framework for modelling structures in fire is presented, addressing its theoretical background, fundamental assumptions, and inherent limitations. After a decade of development, OpenSees has modules including fire, heat transfer, and thermo‐mechanical analysis. Meanwhile, it is one of the few structural fire modelling software which is open source and free to the entire community, allowing interested researchers to use and contribute with no expense. An OpenSees‐based integrated tool called SIFBuilder is developed by the author and co‐workers, which can perform fire modelling, heat transfer analysis, and thermo-mechanical analysis in one single software with an automatic coupled manner. This manner would facilitate structural engineers to apply fire loading on their design structures like other mechanical loading types (e.g. seismic loading, gravity loading, etc.), without transferring the fire and heat transfer modelling results to each structural element manually and further assemble them to the entire structure. This feature would largely free the structural engineers’ efforts to focus on the structural response for performance-based design under different fire scenarios, without investigating the modelling details of fire and heat transfer analysis. Moreover, the efficiency due to this automatic coupled manner would become more superior, for modelling larger structures under more realistic fire scenarios (e.g. travelling fires). This advantage has been confirmed by the studies carried out in this research, including 29 travelling fire scenarios containing total number of 696 heat transfer analysis for the structural members, which were undertaken at very modest computational costs. In addition, a set of benchmark problems for verification and validation of OpenSees/SIFBuilder are investigated, which demonstrates good agreement against analytical solutions, ABAQUS, SAFIR, and the experimental data. These benchmark problems can also be used for interested researchers to verify their own numerical or analytical models for other purposes, and can be also used as an induction guide of OpenSees/SIFBuilder. Significantly, an extended travelling fire method (ETFM) framework is put forward in this research, which can predict the fire severity considering a travelling fire concept with an upper bound. This framework considers the energy and mass conservation, rather than simply forcing other independent models to ‘travel’ in the compartment (i.e. modified parametric fire curves in Clifton’s model, 800°C‐1200°C temperature block and the Alpert’s ceiling jet in Rein’s model). It is developed based on combining Hasemi’s localized fire model for the fire plume, and a simple smoke layer calculation by utilising the FIRM zone model for the areas of the compartment away from the fire. Different from mainly investigating the thermal impact due to various ratios of the fire size to the compartment size (e.g. 5%, 10%, 25%, 75%, etc.), as in Rein’s model, this research investigates the travelling fire thermal impact through explicit representation of the various fire spread rates and fuel load densities, which are the key input parameters in the ETFM framework. To represent the far field thermal exposures, two zone models (i.e. ASET zone model & FIRM zone model) and the ETFM framework are implemented in SIFBuilder, in order to provide the community a ‘vehicle’ to try, test, and further improve this ETFM framework, and also the SIFBuilder itself. It is found that for ‘slow’ travelling fires (i.e. low fire spread rates), the near‐field fire plume brings more dominant thermal impact compared with the impact from far‐field smoke. In contrast, for ‘fast’ travelling fires (i.e. high fire spread rates), the far‐field smoke brings more dominant thermal impact. Furthermore, the through depth thermal gradients due to different travelling fire scenarios were explored, especially with regards to the ‘thermal gradient reversal’ due to the near‐field fire plume approaching and leaving the design structural member. This ‘thermal gradient reversal’ would fundamentally reverse the thermally‐induced bending moment from hogging to sagging. The modelling results suggest that the peak thermal gradient due to near‐field approaching is more sensitive to the fuel load density than fire spread rate, where larger peak values are captured with lower fuel load densities. Moreover, the reverse peak thermal gradient due to near‐field leaving is also sensitive to the fuel load density rather than the fire spread rate, but this reverse peak value is inversely proportional to the fuel load densities. Finally, the key assumptions of the ETFM framework are rationalised and its limitations are emphasized. Design instructions with relevant information which can be readily used by the structural fire engineers for the ETFM framework are also included. Hence more optimised and robust structural design under such fire threat can be generated and guaranteed, where we believe these efforts will advance the performance‐based structural and fire safety engineering.