A slender structure is very sensitive to dynamic loading. A high building, long span bridge, skyscraper and other similar structures may lose their function or in some serious cases may be destroyed under dynamic loading. In general, the kind of damage induced by the dynamic loading can be divided into two kinds, namely functional and safety loss.
In order to solve this kind of problem, it is conventional to increase the strength and ductility of load bearing structures. This is the usual practice in earthquake-resistance design for civil works. Another method is to employ some measure to reduce the response of the structure to such vibration.
The methods of reducing vibration of the structure generally can be divided into two classes, namely passive control and active control. The so-called passive control is usually carried out by increasing or decreasing a number of springs in the structure to change the stiffness of the structure, so that its natural frequency can be adjusted to depart from the frequency of dynamic loading.
Alternatively, a mass-spring damper may be installed on the structure to concentrate vibration energy in the mass-spring damper, so that the goal of reducing the vibration of main structure can be reached.
Another method is to arrange weak spots at some specific positions on the structure. In this way, strong vibrations will force these weak spots to produce plastic strains, by which energy consumption of the total structure can be increased. A common point of the passive control is that the dynamic characteristics of the structure are fixed after completing the structure, and the stress status of struts in the structure is completely determined by the time history of the loading. It is not necessary or even possible to take initiative action to change the characteristics. Therefore the designer must grasp in advance the complete characteristics of dynamic loading, such as magnitude and spectrum, so that an effect of the vibration reduction by these methods can be assured.
The so-called active control generally means installing a mechanism, such as an oil or air pressure mechanism, which can give additive force to the structure or change the status of inner forces in a part of the struts in the structure. Though theoretically the active control method can achieve a very good effect, it has a problem of stability in actual application, and also a large amount of energy has to be supplied. Furthermore, the mechanism itself is unreliable. Its range of applications, and especially applications for reducing the vibration of civil engineering and construction under dynamic loading, are much less than passive control cases.
As mentioned above passive control has the fault that its effect is poor, and active control has the faults that it is unreliable and requires a large amount of energy.