Civil Engineering Reference
In-Depth Information
Figure 5.20
Part cross-section of revised internal column length 0-1
bending moments
M
y,Ed,1
68 kN m (Fig. 5.19),
the
N
Ed
e
0
moments, and the second-order moments about both axes; and
checking uni-axial and bi-axial bending.
Major-axis bending was found to be the most critical, with
M
y,Ed
=
81 kN m and
M
y,Ed,2
=
=
216 kN m and 0.9
M
y,Rd
5401 kN. The margins
on bending resistance so found were assumed to be sufficient to cover the
increase in the end moments caused by the change in column cross-
section, and the small increase in
N
Ed
from the heavier column.
=
443 kN m, when
N
Ed
=
Comment on column design
It is evident, above, that response to the uncertainties of loading, cracking,
creep and inelastic behaviour involve some judgement and approximation.
A small increase in the cross-section of a column reduces its slenderness
(and hence, the secondary bending moments) as well as increasing all its
resistances (
N
Rd
,
M
y,Rd
, etc.); so there is little saving in cost from seeking
an 'only-just-adequate' design.
5.9
Example (continued): design for horizontal forces
As explained in Section 5.1, horizontal loads in the plane of a typical
frame, such as DEF in Fig. 5.1(a), are transferred by the floor slabs to a
central core and to two shear walls at the ends of the building, Fig. 5.21.
It will be shown by approximate calculation that the system is so stiff, and
the relevant stresses are so low, that rigorous verification is unnecessary.
It is shown in Section 5.4.1 that allowance for the frame imperfections
is made by applying at each floor level of each frame a notional horizontal
force
H
Ed
Q
)/366, where
G
and
Q
are the total design ultimate
dead and imposed loads for the relevant storey.
=
(
G
+
