Travelling Fires for Structural Design
Date
08/2011Author
Stern-Gottfried, Jamie
Metadata
Abstract
Traditional methods for specifying thermal inputs for the structural fire analysis of
buildings assume uniform burning and homogeneous temperature conditions
throughout a compartment, regardless of its size. This is in contrast to the
observation that accidental fires in large, open-plan compartments tend to travel
across floor plates, burning over a limited area at any one time.
This thesis reviews the assumptions inherent in the traditional methods and
addresses their limitations by proposing a methodology that considers travelling
fires for structural design. Central to this work is the need for strong collaboration
between fire safety engineers to define the fire environment and structural fire
engineers to assess the subsequent structural behaviour.
The traditional hypothesis of homogeneous temperature conditions in postflashover
fires is reviewed by analysis of existing experimental data from wellinstrumented
fire tests. It is found that this assumption does not hold well and that
a rational statistical approach to fire behaviour could be used instead.
The methodology developed in this thesis utilises travelling fires to produce more
realistic fire scenarios in large, open-plan compartments than the conventional
methods that assume uniform burning and homogeneous gas phase temperatures
which are only applicable to small compartments. The methodology considers a
family of travelling fires that includes the full range of physically possible fire sizes
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within a given compartment. The thermal environment is split into two regions: the
near field (flames) and the far field (smoke away from the flames). Smaller fires
travel across a floor plate for long periods of time with relatively cool far field
temperatures, while larger fires have hotter far field temperatures but burn for
shorter durations.
The methodology is applied to case studies showing the impact of travelling fires on
generic concrete and steel structures. It is found that travelling fires have a
considerable impact on the performance of these structures and that conventional
design approaches cannot automatically be assumed to be conservative. The results
indicate that medium sized fires between 10% and 25% of the floor area are the most
onerous for a structure. Detailed sensitivity analyses are presented, showing that the
structural design and fuel load have a larger impact on structural behaviour than
any numerical or physical parameter required for the methodology.
This thesis represents a foundation for using travelling fires for structural analysis
and design. The impact of travelling fires is critical for understanding true structural
response to fire in modern, open-plan buildings. It is recommended that travelling
fires be considered more widely for structural design and the structural mechanics
associated with them be studied in more detail. The methodology presented in this
thesis provides a key framework for collaboration between fire safety engineers and structural fire engineers to achieve these aims.
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