W.H. Chan
Ove Arup Partnership, Hong
Kong
C.J. Burgoyne
Department of Engineering, University of Cambridge,
Cambridge, UK
Structures are traditionally
regarded as carrying their load in a fixed manner. The distribution of dead
load forces is controlled by the erection sequence with, in special cases,
redistribution being induced at that time by the application of prestressing
forces to stays or cables, or by jacking at the supports. The live load
distribution is simply that which results from the normal behaviour of the
structure; the only control that the designer exerts over the distribution
comes from the relative sizing of the members.
The implications of this are so
fundamental to normal structural design that they pass unremarked by most
engineers, but the effect is that structures have to be designed for the
maximum range of loads that can be applied throughout the life-time of the
structure. The penalty in terms of weight and cost can be large, especially in
structures in which the extreme live load forms a high proportion of the total
load on the structure.
The use of active, or reactive,
structures, which change their configuration in response to the applied loads,
offers a way of overcoming that inefficiency, but at the penalty that the fixed
structures becomes a machine, with all the attendant difficulties of control and
maintenance.
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