Civil Engineering Reference
In-Depth Information
load of pure torsional buckling is obtained from
The basic critical loads may interact. Once again, the interaction of the basic
critical loads can be taken into account by using the approximate formula (3.20) or
the exact cubic equation (3.22), where
F
cr,X
, F
cr,Y
and
F
cr,
ϕ
are to be substituted for
N
cr,X
, N
cr,Y
and
N
cr,
ϕ
. The smallest root of the equation is the global critical load
F
cr
.
When the concentrated top load and the UDL on the floors act simultaneously,
their effect should be combined. Dunkerley's summation theorem (cf. section
3.1.1) offers a simple formula for the combination of the two load cases:
3.1.4
Shear mode situations
Closed-form formulae for the critical load of building structures under uniformly
distributed floor load, developing predominantly bending deformations were
given in the previous sections. This section shows how the theory can be
extended and the formulae given can be used with or without modification for
different special cases.
Bracing systems which (also) develop shear deformation are not discussed in
detail as their practical importance is relatively small. However, simple approximate
solutions are given below for some special cases.
The two typical cases when shear modes have to be considered are as follows.
Shear type deformations are
a)
concentrated on one storey level—'local' shear (
Fig. 3.6/a
)
,
b)
'distributed' over the height of the building—'global' shear (
Fig. 3.6/b
)
.
