Civil Engineering Reference
In-Depth Information
1200.00
Heating
Cooling
1000.00
800.00
600.00
400.00
200.00
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
Time (hrs)
Figure 11.6
Parametric curve
models. The structural Eurocodes provide methods for deter-
mining temperature distributions subject to certain conditions.
The thermal modelling approaches set out in the Eurocodes
are summarised in
Table 11.2
.
Heat transfer methods for materials that incorporate free
moisture should consider the effect of moisture migration with
time through the member in order to provide an accurate predic-
tion of the temperature of the element with time. This is gener-
ally accomplished through the incorporation of mass transfer in
the model providing additional information on the pressure field
due to steam production which, in certain cases, may influence
the tendency of a material to spalling. For many simple models,
the influence of moisture is either implicitly included (empiri-
cal models and tabulated data) or conservatively ignored.
11.3.1 Concrete
For materials with a high thermal conductivity (such as
steel) it is generally possible to ignore thermal gradients
within the member and assume a uniform temperature.
However, for concrete members having a low thermal con-
ductivity and including free and chemically bound mois-
ture, the calculation of heat transfer to the structure can be
very complex. A number of different methods may be used
to derive the temperature distribution within the member.
Eurocode 2 includes a number of temperature profiles for
slabs, beams and columns with the temperature profile for
slabs also being applicable to walls subject to heating from
one side. The temperature profiles are presented for specific
fire resistance periods and are therefore applicable only to a
heating regime corresponding to a standard fire exposure. In
principal, the calculation methods for which the temperature
profile is input data could be used to determine performance
due to different thermal exposure but there are no validated
test data to support this.
11.3.2 Structural steel
Steel loses both strength and stiffness with increasing tem-
perature. It should be borne in mind that the determination
of strength reduction factors for hot rolled steel is dependent
not only on the material but also on the test method, the heat-
ing rate and the strain limit used to determine steel strength.
The differences between test data are significant. The British
Eurocode
Material
Tabular
data
Simple
model
Advanced
model
EN 1992-1-2
Concrete
Yes
No
Yes
EN 1993-1-2
Steel
No
Yes
Yes
EN 1994-1-2
Composite (steel
and concrete)
Yes
Yes
Yes
EN 1995-1-2
Timber
No
Yes
No
EN 1996-1-2
Masonry
Yes
Yes
Yes
EN 1999-1-2
Aluminium
No
Yes
Yes
Table 11.2
Thermal modelling options in the structural Eurocodes
