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

Alternatively, for a conservative estimate of design resistance for a tension
member with non-uniform temperature distribution, equation 7.4 may be used by
assuming the whole of the cross section is at the maximum steel temperature
reached at time, t.
Compression Members:
Compression members are prone to buckling so the resistance to buckling of a
steel member at elevated temperature must be evaluated. The design buckling
resistance, N b,fi,t , at time t of a compression member should be determined from:
N
χ
= Ak f
7.6
fi
b, fi,
t
1. 2
y,
T , max
y
where χ fi is the reduction factor for flexural buckling in the fire design situation,
and k y,T,max is the reduction factor for the yield strength of steel at the maximum
steel temperature, T, reached at time t.
The constant, 1.2, is an empirical correction factor that allows for a number of
effects including the strain at failure being different from the yield strain. The
value of χ fi is taken as the lesser of the values of χ fi in the y and z axes.
Bending:
As with compression and tension members, the design of bending members
depends on the temperature distribution across the cross section. For uniform
temperature distribution, the design resistance of bending elements, M f may be
determined by:
M
f
Sk
y,
T
= f
7.7
where S is the plastic section modulus
y
For members with a temperature gradient over the cross section, the moment
capacity of the member may be calculated from:
M
f
i =1, n
Ai
zik
y,
T , i
f
y,
i
= 7.8
where z i is the distance from the plastic neutral axis to the centroid of the elemental
area, A i.
134