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Steel Designers' Manual - 6th Edition (2003)
1016 Fire protection and fire engineering
Manufacturers of fire-protection products now give guidance on the required
thickness of fire protection depending on the section factor of the member. The
example of a table for a typical spray applied (profile protection) shown in the
Appendix Minimum thickness of spray protection is taken from Reference 3.
Some fire protection materials are assessed at the single limiting temperature of
550°C. However, there is an increasing trend for manufacturers to provide thickness
recommendations for a range of temperatures. This allows designers to minimize
cost by tailoring protection thickness in an individual project to the limiting
temperature derived from BS 5950 Part 8.
34.3 Structural performance in fire
34.3.1 Strength of steel at elevated temperatures
Steel begins to lose strength at about 200°C and continues to lose strength at an
increasing rate up to a temperature of about 750°C, when the rate of strength loss
flattens off. This relationship is shown in Fig. 34.3. An important parameter is the
strain at which the strength is assessed. It is reasonable to take a higher strain limit
than in normal design, because fire is an ultimate limit state and much higher deflections
are allowed in fire tests than in normal structural tests.
U
L
a)
4-,
V)
0.9
0.8
0.6
0.5
0.2
H \
BS 5950: Part 8
i1.5 strain
—0.5 strain
Eurocode 3 proposal
_______ ________________ strain)
200 400 600 800
teniperature (°C)
Fig. 34.3 Strength retention factor for grade 43 steel at elevated temperatures