Civil Engineering Reference
In-Depth Information
Codes give limits to the temperature rise of non-exposed surfaces,
relevant to criterion I, and detailing rules relevant to criteria I and E.
Design calculations relate mainly to criterion R, and only these are further
discussed in this topic.
It will be seen that, for practicable design, it is necessary to simplify
the predictions of both the action effects and the resistances, to a greater
extent than for 'cold' design. This last term refers to the normal design for
persistent situations and ultimate limit states.
3.3.7.1
Partial safety factors for fire
It is recommended in EN 1990 that all factors
γ
F
for accidental actions
should be 1.0; i.e., that these actions should be so defined that
γ
F,fi
=
1.0 is
the appropriate factor. Subscript fi means 'fire'.
For materials, design is based essentially on characteristic or nominal
properties; so for most materials, and for shear connection,
γ
M,fi
=
1.0. The
reduction below
γ
M
for persistent design situations is then significant for
concrete (1.5
→
1.0) and zero for structural steel (1.0
→
1.0).
3.3.7.2
Design action effects for fire
For a structural member with one type only of permanent loading and no
prestress, the combination for accidental design situations given in EN
1990 [12] simplifies to:
∑
G
k
+
A
d
+
ψ
1,1
Q
k,1
+
(3.29)
ψ
2
Q
,i , i
i
1
where
A
d
is the design value of the accidental action, and other symbols
are as in Section 1.3.2.
A floor structure for a building is usually designed for distributed loads
g
k
and
q
k
and, for fire,
A
d
can be taken as zero. For beams and slabs, the
'simply-supported' moments and shears are proportional to the total load
per unit area. To avoid additional global analyses for fire, the action
effects
E
fi,d
are related to those for cold design for ultimate limit states by
E
fi,d
=
η
fi
E
d
(3.30)
where, in the simple case considered here,
η
fi
depends on the ratio
q
k
/
g
k
and the values of
ψ
1
as follows. From Equations 3.29 and 3.30, and
Expression 1.6 with
Q
k,2
γ
F
and
=
0, and
ψ
1
=
0.7 (Table 1.3):
γ
g
+
+
ψ
q
. .( / )
. .(/)
10
+
+
07
qg
qg
GA
k
1
k
kk
(3.31)
η
=
=
fi
γ
g
γ
q
135
15
G
k
Q
k
kk
