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

factor for lateral torsional buckling, χ fi , by a factor of 1.2, as was also used for
compression members. In the New Zealand Steel Code, the lateral torsional
buckling factor has the notation, α s .
Shear:
Designing for shear for fire situations should be modified from that for normal
situations by the variation of yield stress of steel. The modified formula has the
following form:
k
y,
TVs
V
f
= 7.12
κ κ
1
2
where κ 1 and κ 2 are as stated earlier for bending member and V s is the design shear
resistance for normal temperatures.
As covered in Section 7.2.2, the Eurocode has detailed methods for designing
structural steel members for fire by considering fire as another ‘load’ on the
structure. These equations would increase the use of unprotected steel in buildings
as the members can be designed to withstand the maximum likely temperatures.
The equations stated in this section are also listed in Buchanan, (1999), and would
serve as an effective means as an alternative to protecting steel as the norm.
7.2.3 Thermal Analysis of Steel Members:
As stated in Section 3 of this report, the analysis methods to determine the time
temperature relationship for protected and unprotected steel members in Eurocode
3 are timestep spreadsheet methods, as has been used throughout this report for
comparisons with SAFIR. Refer to Section 2.1 for more details on the applications
and applicability of these methods.
The method to determine the time-temperature relationship for unprotected steel is
quite non conservative, as currently used in NZS 3404, as it gives times that are
too short or temperatures that are too low. Sections 4.2.3 and 4.3.3 show the
correlation between the lines and the curves as simulated from SAFIR and the
timestep method.
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