Damping characteristics of mero-type double layer grids

Damping is a phenomenon in mechanical systems by which the vibrational energy is absorbed and dissipated during oscillation. Much research effort has gone into the investigation of damping since 250 years ago. However, the complexity of damping phenomenon has prevented a complete understanding of the mechanisms by which the vibrational energy is dissipated. It is important to be able to estimate the amount of damping in structural systems, since this plays a major role in their dynamic behaviour. The most reliable results regarding damping in structures are obtained from dynamic experiments on structures. A number of test methods can be employed for the measurement of damping capacity of structures. There are three different types of damping. These are, 'viscous damping', 'Coulomb damping' and 'hysteretic (material) damping'. Viscous damping is the resistance offered to a moving body in a fluid. Viscous damping is the most common type of damping. Coulomb damping is caused by friction between surfaces which slide with respect to each other. Material damping is due to friction between the internal planes of a material. The damping capacity of a structure varies due to the variation of structural conditions such as initial strains and stiffness as well as amplitude and frequency of vibration. The present research experimentally studies the variation of damping in a MERO-type double layer grid due to the variation in the bolt tightness of the connectors. To carry out this study a I Om xI Om MERO-type double layer grid and the necessary equipment including a loading system and a data acquisition system are used. In this study by carrying out more than 120 tests, the damping ratios of the grid for different levels of bolt tightness as well as different support conditions are obtained. The results show that the bolt tightness has a major effect on the damping characteristics of the grid. The nature of this effect is discussed in Chapters 6 and 7. The results also show that the increasing of the number of supports of the grid will cause an increase in the damping of the grid. In addition, an increase in the amplitude of vibration is found to increase the damping of the grid.