Civil Engineering Reference
In-Depth Information
With the 8-storey building featured in the example in the previous chapter, the shear
forces, the bending moments, the rotations and particularly the horizontal displacements
could be considered as characteristics of the performance of the bracing system. As
the efficiency of the bracing system was improved by rearranging the elements of the
bracing system, the maximum deformations of the building decreased. A similar tendency
was observed regarding the change in the value of the global critical loads—and the
global critical load ratios which originate from them—and the fundamental frequency
of the building: in parallel with the decrease of the maximum translations, the value of
the global critical loads and the fundamental frequencies increased and the value of
the global critical load ratios decreased (Table 7.1).
The question emerges, which characteristic is best used for assessing the performance
of the bracing system: the maximum horizontal displacement, the fundamental frequency
or the global critical ratio? It can be said against the fundamental frequency that it
plays a relatively minor role in structural engineering design: the designer of an ordinary
building rarely carries out a dynamic analysis. The advantage of using the maximum
displacement would be its practical importance: it is very useful to know the
displacements of a building. However, there are two disadvantages here: the maximum
displacement is not a nondimensional quantity and its value also depends on the
direction of the horizontal load. The first disadvantage can easily be eliminated by
introducing the maximum displacement ratio as the ratio of the maximum displacement
to the recommended maximum displacement
H
/500 (the fourth column in Table 7.1).
However, the second disadvantage is an inherent one and can lead to unfavourable
situations as a fatal weakness of the bracing system may be underestimated. The
building used for the numerical example reveals this shortcoming. If an external load in
direction
y
is applied, then, although the performance of the bracing system is correctly
characterized by the maximum displacements, the displacements do not reveal the
obvious when the horizontal load has a 50° angle with axis
x
(Table 7.1). The fatal
