Civil Engineering Reference
In-Depth Information
Fig. 5.1(a), so that beam-column interaction causes major-axis bending in
both members.
For global analysis for gravity loads, each plane frame is assumed to
be independent of the others. For each storey-height column length, an
axial load
N
y
and end moments
M
1,y
and
M
2,y
are found for the major-axis
frame, and corresponding values
N
z
,
M
1,z
and
M
2,z
for the minor-axis
frame such as HAD in Fig. 5.1. The column length is then designed (or an
assumed design is checked) for axial load
N
y
+
N
z
and for the bi-axial
bending caused by the four end moments.
In the design of a multi-storey composite plane frame, allowance must
be made for imperfections. Global imperfections, such as out-of-plumb
columns, influence lateral buckling of the frame as a whole ('frame instab-
ility'). Member imperfections, such as bow of a column length between
floor levels, influence the buckling of these lengths ('member instability'),
and may even affect the stability of a frame.
Global analysis is usually linear-elastic, with allowance for creep, crack-
ing and the moment-rotation properties of the joints. First-order analysis
is used wherever possible, but checks must first be made that second-
order effects (additional action effects arising from displacement of nodes
or bowing of members) can be neglected. If not, second-order analysis
is used.
A set of flow charts for the design of such a frame, given elsewhere
[17], is too extensive to reproduce here. However, the sequence of these
charts will be followed for the frame shown in Fig. 5.1(b), which will be
found to be free from many of the complications referred to above.
Columns and joints are discussed separately in Sections 5.2 and 5.3.
The Eurocode methods for analysis of braced frames are explained in
Section 5.4, with a worked example. Details of the design method of EN
1994-1-1 for columns are then given, followed by calculations for two of
the columns in the frame.
5.2
Composite columns
Steel columns in multi-storey buildings need protection from fire. This is
often provided by encasement in concrete. Until the 1950s, it was normal
practice to use a wet mix of low strength, and to neglect the contribution
of the concrete to the strength and stability of the column. Tests by Faber
[44] and others then showed that savings could be made by using better-
quality concrete and designing the column as a composite member. This
led to the 'cased strut' method of design. This was originally (in BS 449)
a permissible-stress method for the steel member, which had to be of H-
or I-section. It then became available in limit-state form [19]. In this
