Civil Engineering Reference
In-Depth Information
η
f
N
d
Combination factor
Design loading in fire: N
f.d
=
ψ
1.1
= 0.5
G
k.1
/Q
k
= 2.0
Load reduction factor
η
f
= 0.46
N
f.d
= 0.46
×
247.95 = 114 kN
Design resistance at 20
C, using fire safety factors:
N
f.20.Rd
= k
y.20
N
Rd
(
°
γ
M.f
)
Strength reduction factor k
y.20
= 1.0 at temperature 20
γ
M.1
/
°
C(see
Table 7.11
)
N
f.20.Rd
= 1.0
×
257.5
×
([1.1]/[1.0]) = 283.25 kN
Critical temperature: Degree of utilization
μ
0
= N
f.d
/N
f.20.Rd
= 114/283.25 = 0.40
Therefore, the critical temperature
θ
c
= 619
°
C (see
Table 7.12
)
Note that, from
Table 7.11
, the intermediate value of the linear interpolation
can be used.
The relative thermal elongation of steel
Δ
l/l should be determined from the
following:
For 20
°
C
≤ θ
a
<
750
°
C
10
−
5
10
−
8
2
10
−
4
Δ
l
=
l
=
1
:
2
×
θ
a
+
0
:
4
×
θ
a
−
2
:
416
×
For 750
°
C
≤ θ
a
≤
860
°
C
10
−
2
Δ
=
=
:
×
l
l
1
1
For 860
°
C
< θ
a
≤
1200
°
C
10
−
5
10
−
3
Δ
=
=
×
θ
a
−
:
×
l
l
2
6
2
where l is the length at temperature 20
°
C,
Δ
l is the temperature-induced
elongation and
θ
a
is the steel temperature in degrees C.
7.7.3 Unrestrained Beams
In the load resistance domain, lateral-torsional buckling capacity at compression
flange maximum temperature
θ
a.com
is presented by the following equation:
1
γ
M
:
f
where reduced yield strength of compression flange = k
y.q.com
f
y
at
W
pl
:
y
K
y
:θ:
com
f
y
χ
LT
:
f
1
M
b
:
f
:
t
:
Rd
=
:
2
θ
a.com
and
reduction factor
χ
LT.fi
for flexural buckling is based on normalized slenderness:
λ
LT
:θ:
com
= λ
LT
p
k
y
:θ:
com
/k
E
:θ:
com
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