Civil Engineering Reference
In-Depth Information
The special case of doubly symmetrical bracing systems (
Fig. 2.4/c
) is automatically
covered in
Table 3.3
by the rows defined by
τ
X
= 0.0. Values in Table 3.3 can also be
used to show the effect of neglecting the coupling of the basic modes in the
monosymmetrical case. According to the values, it is always unconservative to neglect
coupling. The maximum error made by neglecting coupling is 100%; when the two
basic critical loads
N
cr,Y
and
N
cr,
ϕ
are equal and eccentricity is maximum (
τ
X
=1.0), coupling
reduces the combined critical load to half of the basic critical load.
3.1.3
Concentrated top load; single-storey buildings
The formulae presented for the global critical loads were derived on the
assumption that the load of the structures was uniformly distributed over the
floors, being the practical case with multistorey buildings. In certain cases,
however, concentrated load on top of the building may need be considered. A
panorama restaurant, a swimming pool or water tanks may represent some extra
load on the top of the building which may not be covered by the uniformly
distributed floor load, considered to be the same at each floor level. Even a
relatively small amount of extra concentrated load on top of the building should
be taken into account as it represents a more dangerous load case than the UDL
case.
The concentrated top load case is also of practical importance when single-
storey buildings are investigated as the majority of the vertical load is on the
(top) floor, which is represented by a concentrated force on top of the
structure.
When the structure is under concentrated top load and the system develops
predominantly bending type deformation, the basic critical loads are as follows.
The sway critical loads in the principal directions are:
and the critical load for pure torsional buckling is given by
where
H
is the height of the building.
When the warping stiffness of the bracing system is zero, the critical
