Civil Engineering Reference
In-Depth Information
Design is essentially as for encased H-sections, except that in calculat-
ing the squash load
N
pl,Rd
, account is taken of the higher resistance of the
concrete, caused by lateral restraint from the steel tube, as follows.
The factor 0.85 in Equations 5.24 and 5.26 is replaced by 1.0. Also, for
circular sections only,
f
cd
is increased to an extent that depends on the ratios
t
/
d
,
f
y
/
f
ck
, l and
M
Ed
/(
N
Ed
d
), provided that the relative slenderness l
0.5.
For a circular section, there is also a reduction in the effective yield
strength of the steel wall used in calculating
N
pl,Rd
, to take account of the
circumferential tensile stress in the wall. This stress provides restraint to
lateral expansion of the concrete caused by the axial load on the column.
These rules are based on extensive testing.
≤
5.7
Example: external column
5.7.1
Action effects
The use of nominally-pinned joints and a braced frame enables the design
of an external column to be completed without further global analysis of
the frame of Fig. 5.1. From Section 5.5, the design ultimate shear force
from a fully-loaded beam is 246 kN. The size of the external column is
governed by the length 0-1 in Fig. 5.13(a). This supports load from nine
floors, so from Equation 5.10 the reduction factor for imposed load, with
ψ
0
=
0.7 as before, is
α
n
=
(2
+
7
×
0.7)/9
=
0.767
Figure 5.13
Dimensions and action effects for external column
