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PrEN1993-1-2 Annex E (informative) provides
the reduction factors for the design strength and elastic modulus
of Class 4 sections made of carbon steel at elevated temperatures
as shown in Table
1 and Figure
1. PrEN1993-1-1 defines Class 4 cross-sections as those in
which local buckling will occur before the attainment of yield
stress in one or more parts of the cross-section. Hence, light
gauge sections can be classified as Class 4 sections and the
design values provided can be applied.
For simplicity and conservative approach, prEN1993-1-2
adopts 0.2% proof stress as the effective yield strength for
the design of light gauge steel at elevated temperatures. The
reduction factor for the design strength kp0.2,θ for
different sections is given by:
 |
(1) |
where
| fp0.2, θ |
is the 0.2% proof strength at steel temperature θa taken
as effective yield value; |
| fy |
is the yield strength at 20°C; |
| fyb |
is the basic yield strength at 20°C as defined in prEN1993-1-3. |
The reduction factor for elastic modulus of
light gauge steel is assumed to be identical to that of carbon
steel.
BS5950-8 (2003) introduces the strength retention
(or reduction) factors for cold-formed steel up to 600°C
based on data obtained from tests performed by British Steel
(Lawson 1993; Sidey & Teague 1988).
In general, the British Steel tests showed that
the loss of strength of cold formed steel at elevated temperatures
exceeds that of hot rolled steel by between 10 and 20%, particularly
in the temperature range 400 to 600°C. Figure 1 shows the
strength reduction values corresponding to 0.5, 1.5 and 2% total
strains. It is noteworthy that the British Steel data are based
on a 95 % confidence limit with respect to the nominal material
properties, whereas that for hot-rolled sections is typical for
steel with ambient temperature properties at the specified minimum.
The confidence limit means that the actual strength of the sections
in fire conditions will generally exceed the design values, and
often by a significant margin (Lawson 1993; PD7974-3: 2003).
The strain limits specified in BS5950-8 allow
the strength reduction factor, at the higher strain, to be used
for flexural members failing in bending and the strength reduction
factor, at the lower strain, to be used for compression members
failing by buckling (see
Table 2).
BS5950-8 does not include the reduction factor
for elastic modulus of light gauge steel at elevated temperatures.
However, it was reported that the elastic modulus reduces broadly
at the same rate as the strength at 0.5% strain (Lawson 1993).
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