Civil Engineering Reference
In-Depth Information
where
ψ
0
is the combination factor (e.g., as in Table 1.3). This allows for
the low probability that all
n
floors will be fully loaded at once.
In an office block, the location of partitions is unknown at the design
stage. Their weight is usually allowed for by increasing the imposed
loading,
q
k
,
by an amount that depends on the expected weight per unit
length of the partitions. The increases given in EN 1991-1-1 range from
0.5 to 1.2 kN/m
2
.
The principal horizontal variable load for a building is wind. These
loads are given in EN 1991-1-4. They usually consist of pressure or
suction on each external surface. On large flat areas, frictional drag may
also be significant. Wind loads rarely influence the design of composite
beams, but can be important in framed structures not braced against side-
sway and in tall buildings.
Methods of calculation for distributed and point loads are sufficient for
all types of direct action. Indirect actions such as subsidence or differen-
tial changes of temperature, which occasionally influence the design of
structures for buildings, are not considered in this topic.
1.6
Methods of analysis and design
The purpose of this section is to provide a preview of the principal methods
of analysis used for composite members and frames, and to show that
most of them are straightforward applications of methods in common use
for steel or reinforced concrete structures.
The steel designer will be familiar with the elementary elastic theory of
bending, and the simple plastic theory in which the whole cross-section of
a member is assumed to be at yield, in either tension or compression. Both
theories are used for composite members, the differences being as follows:
•
concrete in tension is usually neglected in elastic theory, and always
neglected in plastic theory;
•
in the elastic theory, concrete in compression is 'transformed' into an
equivalent area of steel by dividing its breadth by the modular ratio
E
a
/
E
c
;
•
in the plastic theory, the design 'yield stress' of concrete in compression
is taken as 0.85
f
cd
, where
f
cd
γ
C
. Transformed sections are not
used. Examples of this method are given in Sections 3.4.2 and 3.11.1.
=
f
ck
/
In the strength classes for concrete in EN 1992, the ratios
f
ck
/
f
cu
range
from 0.78 to 0.83, so for
1.5, the stress 0.85
f
cd
corresponds to a value
between 0.44
f
cu
and 0.47
f
cu
. This agrees closely with BS 5950 [19], which
uses 0.45
f
cu
for the plastic resistance of cross-sections.
γ
C
=
