DAMPED RESONANT APPENDAGES TO INCREASE INHERENT DAMPING IN BUILDINGS

It is demonstrated that the addition of a tuned mass spring dashpot sy stem with a relatively small mass and a high damping ratio can be an e ffective way to increase the inherent damping characteristics of build ings and reduce, thus, their response to earthquake excitations. The d emonstration is based on a theoretical formulation and on numerical an d experimental studies that confirm this formulation. In the theoretic al formulation, it is shown first that, if certain conditions are sati sfied, the damping ratios in two of the modes of the system that is fo rmed by a building and an appendage in resonance are approximately equ al to the average of the corresponding damping ratios of the building and the appendage. Based on this finding, it is then shown that an att ached appendage with a high damping ratio and tuned to the fundamental frequency of a building may increase the damping ratio in the fundame ntal mode of the building to a value close to half the damping ratio o f the appendage. In the numerical study, the response of a ten-storey shear building is analysed under two different earthquake ground motio ns with and without the proposed resonant appendages. Appendages with damping ratios of 20 and 30 per cent are considered. In this study, it is found that under one of the ground motions the maximum displacemen t of the building's roof is reduced 30 per cent with the appendage wit h 20 per cent damping and 39 per cent with the one with 30 per cent da mping. Similarly, with these two appendages the building's base shear is reduced 31 and 41 per cent, respectively. In the experimental study , a wooden three-storey structural model is tested in a shaking table with and without an appendage designed and constructed to have a dampi ng ratio of 53.5 per cent. The test is conducted under random and sinu soidal base excitations. In the shaking table test under random excita tion, the attached appendage reduces the response of the model 38.6 pe r cent, while in that under sinusoidal vibration 45.2 per cent.