Civil Engineering Reference
In-Depth Information
Table 1.3
Combination factors
Factor
y
0
y
1
y
2
Imposed floor loading in an office area of a building
0.7
0.5
0.3
Wind loading on a building
0.6
0.2
0
.0
For serviceability limit states, three combinations are defined. The most
onerous, the 'characteristic' combination, corresponds to the fundamental
combination (above) with the
γ
factors reduced to 1.0. For the example in
Expressions 1.6 and 1.7, it is
G
k
+
Q
k,1
+
ψ
0,2
Q
k, 2
(1.8)
and
G
k
+
ψ
0,1
Q
k,1
+
Q
k,2
(1.9)
It is normally used for verifying irreversible limit states, for example,
deformations that result from the yielding of steel.
Assuming that
Q
1
is the leading variable action, the others are:
•
frequent combination,
G
k
+
ψ
1,1
Q
k,1
+
ψ
2,2
Q
k,2
(1.10)
•
quasi-permanent combination,
G
k
+
ψ
2,1
Q
k,1
+
ψ
2,2
Q
k,2
(1.11)
The frequent combination is used for reversible limit states, for example,
the elastic deflection of a floor under imposed loading. However, if that
deformation causes cracking of a brittle floor finish or damage to fragile
partitions, then the limit state is not reversible, and the check should
be done for the higher (less probable) loading of the characteristic
combination.
The quasi-permanent combination is used for long-term effects (e.g.,
deformations from creep of concrete) and for the appearance of the
structure.
Some combination factors used in this topic are given in Table 1.3.
1.3.2.4
Comments on limit state design philosophy
The use of limit states has superseded earlier methods, partly because
limit states provide identifiable criteria for satisfactory performance.
