Civil Engineering Reference
In-Depth Information
over the whole height of the building. A part of it with a length equal to
a storey height is referred to as a 'column length', where this is necessary
to avoid ambiguity.
Imperfections within a storey-height column of length
L
are repres-
ented by an initial bow,
e
0
. This ranges from
L
/100 to
L
/300, depending
on the type of column and the axis of bending. For major-axis bending of
the columns shown in Fig. 5.1(a), EN 1994-1-1 specifies a bow of
L
/200,
or 20 mm in 4.0 m. This has to be allowed for in verification of the
member, but not in first-order global analysis. The condition of EN 1994-
1-1 for neglecting member imperfections in second-order analysis is,
essentially, that
N
Ed
0.25
N
cr
, where
N
cr
is the elastic critical axial normal
force for the member, allowing for creep, given by Equation 5.20.
A bow of 20 mm in 4 m exceeds the tolerance that would be acceptable
in construction because it allows also for other effects, such as residual
stresses in steel. The out-of-straightness
e
0
is assumed to occur at mid-
length. No assumption is made about the shape of the imperfection.
Imperfections in beams are allowed for in the classification system for
steel elements in compression, and in design for lateral buckling.
Frame imperfections are represented by an initial side-sway,
≤
, as shown
in Fig. 5.6(a) for a single column length of height
h
, subjected to an axial
load
N
. The action effects in the column are the same as if it were vertical
and subjected to horizontal forces
N
φ
φ
, as shown.
for a composite frame is the same as for
the corresponding steel frame. This is given in EN 1993-1-1 as a function
It is assumed that the angle
φ
Figure 5.6
Unbraced and braced frames
