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

2 FIRES AND THERMAL ANALYSIS COMPUTER
MODELS:
2.1 SPREADSHEET METHOD:
2.1.1 Introduction:
The spreadsheet method of predicting the temperature of a steel beam uses principles
from heat transfer theory to estimate the energy being transferred to the steel beam,
and therefore the rate of temperature rise. The methods for protected and unprotected
steel members are basically the same, with different formulas used to allow for the
effect the protection has on the rate of heating of the steel. The method is
recommended by the European Convention for Constructional Steelwork (ECCS,
1993), to establish a procedure of calculating the fire resistance of elements exposed to
the standard fire, while a lot of the research and calibration with test data was done in
Sweden, (Gamble, 1989).
The method is a one-dimensional heat flow analysis that accounts for the properties of
the insulation as well as the area to perimeter ratio of the section, (Gamble, 1989). The
temperature of the beam can be calculated at each time step, by considering the energy
that the beam is subjected to during the previous time interval. The duration of the
time step does not significantly effect the calculated temperatures. Petterson et al,
(1976), suggest a time step so that there are 10 to 20 time steps until the limiting
temperature of the steel is reached, and Gamble (1989), uses a time step of 10 minutes.
For the analyses in this report, a time step of 1 minute is used for the majority of the
calculations as the speed and capabilities of modern computers requires very little
more effort to decrease the time step period to this.
The spreadsheet method assumes the steel member is of constant thickness governed
by the H p /A value. This is called a lumped mass approach as no regard is given for the
actual geometry of the cross section. Constant values for the thermal properties of the
steel such as the specific heat and density, are generally used to simplify the method
and number of variables in the spreadsheet.
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