THE USE OF NUMERICAL OPTMISATION TECHNIQUES IN COMPUTATIONAL FIRE ENGINEERING
MODELS: A STUDY THROUGH EVACUATION MODELLING ANALYSES
Rodrigo Machado Tavares
Evacuation models have been playing an important function in the transition
process from prescriptive fire safety codes to performance-based ones over the
last three decades. In fact, such models became also useful tools in different
tasks within fire safety engineering field, such as fire risks assessment and
fire investigation. However, there are some difficulties in this process when
using these models. For instance, during the evacuation modeling analysis, a
common problem faced by fire safety engineers concerns the number of simulations
which needs to be performed. In other terms, which fire designs (i.e.,
scenarios) should be investigated using the evacuation models? This type of
question becomes more complex when specific issues such as the optimal
positioning of exits within an arbitrarily structure needs to be addressed.
In the other hand, numerical optimisation techniques have been applied to a
range of different fields such as structural analysis. These techniques have
shown to be a powerful tool for designers, saving their time and consequently
reducing costs during the process.
For this reason, the emphasis,
throughout this study, is to develop a methodology that enables the optimisation
of fire safety analysis of structural designs. In other words, the current
research was primarily intended to demonstrate and develop this combination of
fire engineering tools and techniques such as the Design of Experiments (DoE)
and numerical optimisation techniques. For this purpose, a Computational Fire
Engineering (CFE) tool combined with Numerical Optimisation Techniques and
associated statistical methods (i.e., Design of Experiments (DoE) and Response
Surface Models (RSM)) are used. The study is focused on evacuation modelling;
nevertheless the methodology proposed here could equally be applied to CFD-based
fire simulation tools. While the approach that has been developed is intended to
be generally applicable, the techniques have been explored and demonstrated
using the buildingEXODUS computational package. This fire engineering simulation
tool is used worldwide, to improve the fire safety in building designs.
This study therefore intended, besides to develop a numerical methodology to
allow the efficient optimisation of fire safety aspects of structural designs,
to understand how the core variables impact the evacuation efficiency.
For instance, a common problem faced by fire safety engineers, in the field of
evacuation analysis, is the optimal positioning of exits within an arbitrarily
complex structure. This problem is usually addressed through time consuming and
expensive trial and error. While a solution is usually found, to this problem,
it is seldom the optimal solution, resulting in a compromise in building
performance and safety.
The methodology explored in this thesis, as
applied to CFE, was initially based around a relatively small set of physical
variables. This approach evolved and was subsequently expanded to include more
complex behavioural, procedural and environmental parameters. The methodology
has also been further developed and applied to evacuation simulation.
This integrated approach is intended to help fire safety engineers and designers
to develop optimal designs (i.e., safe designs) in a optimized manner. In
reality, this was the motivation of this study: to introduce numerical
optimisation techniques and associated concepts, well known within the
operational research field, as an approach for a more efficient and systematic
procedure when developing and/or improving fire safety designs.
comparisons between the outputs obtained, using these different DoE techniques,
have been also performed in order to analyze which technique is most suitable
for the optimisation of structural designs.
This thesis describes a
number of analyses (of a variety of structural designs) that have been used to
calibrate the optimisation technique. This included the use of the
buildingEXODUS simulation tool, as mentioned previously, followed by the
application of a variety of optimisation techniques (both gradient and
non-gradient based numerical optimisation techniques) as well as different types
of DoE (such as Latin Hypercube, Central Composite Design (CCD) and also a
Random approach) in order to improve the designs according to a number of
different variables. These variables have initially included physical
modifications to the geometry.
The proposed methodological approach
developed in this thesis is demonstrated on a variety of practical problems.
These problems are represented by 4 case studies which vary from complexity to
the nature of the variables. These case studies involved both types of problems,
namely: unconstrained and constrained.
The results obtained have shown
to be satisfactory, i.e., global minima and local minima closest to the global
minima region were found. For all the cases, a gradient-based algorithm (i.e.,
the Fletcher-Reeves numerical optimisation technique) and non gradient-based
algorithm (i.e., the Particle Swarm Optimisation numerical optimisation
technique) were used to find the optimal solution. And as mentioned before,
different DoE techniques were also applied.
The analysis revealed that
this methodology seems to be a very powerful tool for evacuation modelling
This systematic methodology to efficiently optimise evacuation
safety aspects of structural designs should be also extended to more complex
designs, such as larger enclosures and open spaces.
This methodology is
also intended to be applied to problems found in the field of fire simulation,
such as: the sizing and positioning of smoke extraction vents and the modelling
of cable fires.