Extended travelling fire method framework with an OpenSees‐based integrated tool SIFBuilder
Abstract
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.