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

The plastic neutral axis is located such that the elemental areas yielding in
compression and tension on either side of the plane must be equal, such that
A k f 0
7.9
i=
1 , n i y , T , i y
=
An alternative method designing members with a temperature distribution across
its cross section is to assume the member has a constant temperature profile, of the
maximum temperature, and then increase its capacity depending on the fire
exposure and member support conditions.
Sk
y,
T
f
y
M
f
= 7.10
κ κ
1
2
where κ 1 and κ 2 are adaptation factor for non-uniform temperatures across the
cross section and along the beam, respectively.
These have values of:
κ 1 = 1.0 for a beam exposed on four sides
κ 1 = 0.7 for a beam exposed on three sides, with a composite or concrete
slab on the fourth side
κ 2 = 0.85 at the supports of a statically indeterminate beam
κ 2 =1.0 in all other cases.
Lateral Torsional Buckling of Members:
Buckling must also be considered for beams. Slender beams with no lateral
restraint to the compression edge of the cross section are prone to buckling and can
fail before the bending capacity of the beam is reached. If design of beams is
limited to the flexural capacity of the member, then this can lead to the under
design of members.
If the non dimensional slenderness, which is the ratio of slenderness to Euler
Slenderness, for the maximum temperature in the compression flange at time t
does not exceed 0.4, no allowance needs to be made for lateral torsional buckling.
Where the slenderness is greater than 0.4, the design buckling resistance moment
at time t of a laterally unrestrained beam is determined by dividing the reduction
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