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

temperature is chosen because it is the limiting temperature of the modulus of
elasticity in the New Zealand Code. The original formula for yield stress is based
on a regression analysis, Proe et al 1 , (1986), of data at rather low temperatures, i.e.
below 600 °C. Adjusting the slope therefore to have two linear lines work better,
as this allows the formulae to give accurate estimates in the temperature range
where the equation was formulated from, and by varying the higher temperature
section of the relationship, allows the yield stress to drop to zero at the same
temperature as the modulus of elasticity.
The Eurocode relationship between yield stress and temperature is much more
varied than the linear relationship presently in the New Zealand Code, so an
‘elbow’ in the line of yield stress would not be an inaccurate concept.
The chosen deviation from the present equation in NZS 3404 starts when the
temperature is 850 °C and the yield stress ratio is 0.08. This point is chosen as this
is where the curve from the Eurocode intersects the NZS 3404 line, subsequently
making the NZS 3404 line comparatively non conservative at temperatures greater
than 850 °C. A straight line equation has then been formulated so that the yield
strength drops to a value of zero at a temperature of 1000 °C. No experimental or
other data, apart from the correlation with the Eurocode curve has been used to
validate this approach.
The relationship between yield strength and temperature then becomes:
f
f
y
y
( T )
(20)
= 1.0
0 < T < 215 °C 3.5a
905 − T
= 215 < T < 850 °C 3.5b
690
1000
− T
= 0.08
150
850 < T < 1000 °C 3.5c
40