Civil Engineering Reference
In-Depth Information
used in this topic. A welded stud shear connector is treated like a single
material, even though its design resistance to shear,
P
Rk
/
γ
V
, is influenced
by the properties of both steel and concrete. For resistance to fracture of a
steel cross-section in tension,
γ
A
=
1.25.
1.3.2.3
Combinations of actions
The Eurocodes treat systematically a subject for which many empirical
procedures have been used in the past. For ultimate limit states, the prin-
ciples are:
•
permanent actions are present in all combinations;
•
each variable action is chosen in turn to be the 'leading' action (i.e., to
have its full design value) and is combined with lower 'combination'
values of other variable actions that may co-exist with it;
•
the design action effect is the most unfavourable of those found by
this process.
The use of combination values allows for the limited correlation between
time-dependent variable actions.
As an example, it is assumed that a bending moment
M
Ed
in a member
is influenced by its own weight,
G
, by an imposed vertical load,
Q
1
, and
by wind loading,
Q
2
. The fundamental combinations for verification for
persistent design situations are:
γ
G
G
k
+
γ
Q1
Q
k,1
+
γ
Q2
ψ
0,2
Q
k,2
(1.6)
and
γ
G
G
k
+
γ
Q1
ψ
0,1
Q
k,1
+
γ
Q2
Q
k,2
(1.7)
Each term in these expressions gives the value of the action for which a
bending moment is calculated, and the
symbols apply to the bending
moments, not to the values of the actions. This is sometimes indicated by
placing each term between quotation marks.
In practice, it is usually obvious which combination will govern. For
low-rise buildings, wind is rarely critical for floors, so Expression 1.6
with imposed load leading would be used; but for a long-span lightweight
roof, Expression 1.7 would govern, and both positive and negative wind
pressure would be considered, with the negative pressure combined with
Q
k,1
+
0.
The combination for the accidental design situation of 'fire' is given in
Section 3.3.7.
=
