FIRE DESIGN OF STEEL MEMBERS - Civil and Natural Resources ...

The variation of the mechanical properties of steel as given in the Eurocode do not
vary significantly from that given in the New Zealand Code so an adoption of the
Eurocode methods would be acceptable. Alternatively a modification to the linear
equation given presently for the yield stress could be made to adjust the limiting
temperature to equal that of the yield stress.
Stress-strain relationships of different grades of steel at elevated temperatures are
included in an Annex of Eurocode 3, as are equations, which give the relationship
between stress and strain at elevated temperatures. Although these would not be
necessary in this detail in the New Zealand Steel Code, an addition of a graph such
as Figure 3.4 would assist designers in understanding the behaviour of steel at
elevated temperatures.
7.2.5 Thermal Properties:
Other material properties that are included in the Eurocode are equations giving
the variation with temperature of thermal elongation of steel, specific heat of steel
and thermal conductivity of steel. The equations for these properties are all stated
and commented on earlier in this report in Section 1.6.2. For each thermal
property of steel, the approximate variation or constant value to use for simplified
cases is given.
These are: Specific Heat – 600 J/kg K
Thermal Elongation – 14 x 10 -6 (T s – 20)
Thermal Conductivity – 45 W/mK
These values of mechanical and thermal properties of steel are used in the
calculations determining the temperature rise and the strength loss of the steel in a
fire situation. As outlined in Section 3.1.4 and 3.1.5, the temperature rise of
protected and unprotected steel beams is recommended to be found by time step
spreadsheet methods. These properties are used for the calculations of steel
temperature by this method.
The Eurocode 3 also has clauses to allow temperatures and strength calculations to
be made by advanced calculation models, and methods, such as computer models.
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