Civil Engineering Reference
In-Depth Information
Chapter 11
Structural fire engineering design
Tom Lennon
Principal Consultant, BRE Global, UK
doi
:
10.1680/mosd.41448.0169
CONTENTS
11.1
Introduction
169
11.2
Compartment time-
temperature response 169
11.3
Heat transfer
174
The purpose of this chapter is to explain the methodology underpinning the structural
fire engineering design process. Structural fire engineering design consists of three basic
components: choosing an appropriate design fire, using this information to derive the
temperatures of the structural elements and assessing the structural behaviour with respect
to the temperatures derived. For each element of the structural fire engineering design
process there are a number of options available to the designer depending on the complexity
of the project, the state of knowledge with regard to the structural material chosen and
the objectives of the fire engineering design strategy. Detailed information on the design
methodology in this area is available in the Institution of Structural Engineers'
Guide to the
Advanced Fire Safety Engineering of Structures
(2007).
11.4
Mechanical
(structural) response
178
11.5
Conclusion
180
11.6
References
180
11.1 Introduction
The traditional means of ensuring compliance with the require-
ments of the Building Regulations for structural fire safety is
to rely on the results from standard fire tests on individual
elements or components. At the simplest level structural fire
engineering is based on simple prescriptive rules and guidance
which ensure sufficient passive fire protection is applied to
structural members or that minimum dimensions are satisfied
to ensure load-bearing capacity and/or the separating function
is maintained for a period corresponding to the recommended
fire resistance requirement from the regulatory guidance.
In this way, structural engineers have been involved in fire
engineering for many years without necessarily being aware of
it and most probably being unaware of the background to the
development of the regulations and the guidance that under-
pins them. For example, a structural engineer responsible for
designing a reinforced concrete framed building will specify
the overall dimensions, size and position of reinforcement
dependent on the ambient temperature design considerations
in terms of loading and environmental conditions. In the vast
majority of cases, the structural fire engineering will simply
consist of checking in the tables produced in BS 8110 Part 2
to ensure that the design meets the minimum dimensions and
minimum depth of cover to the reinforcement for the specified
fire resistance period.
Within this simple process there are a large number of impli-
cit considerations on the likelihood of a fire occurring: the con-
sequences in terms of life safety should a fully developed fire
occur, the thermal exposure within the fire compartment and
the consequent temperature distribution through the structural
member. To a large extent structural fire engineering design
simply consists of making explicit decisions rather than relying
on the implicit assumptions within the prescriptive approach.
The three-stage approach to structural fire engineering
design is illustrated schematically in
Figure 11.1
.
11.2 Compartment time-temperature response
The first step in a structural fire engineering design is to evalu-
ate an appropriate compartment time-temperature response
to be used for the subsequent heat transfer and structural
response calculations. This initial process can itself be further
subdivided into two important preliminary tasks: the choice of
appropriate design fire scenario(s) and the selection based on
the design fire scenarios adopted of an appropriate design fire.
11.2.1 Design fire scenario(s)
The appropriate design fire scenarios should be determined on
the basis of an overall fire risk assessment taking into account
the nature and distribution of fire load within the project and the
Fire analysis
(compartment time-
temperature)
Heat Transfer
Analysis
(determination of
material
temperatures)
Structural analysis
(determination of
mechanical response)
Figure 11.1
Three stages of structural fire engineering design
