In the last three decades, structural response control against earthquake ground motion has seen a great development. Three categories can be broadly classified in structural response control: passive, active, and semi-active control. Passive control systems, also known as passive energy dissipation systems, have been considered to be an effective and inexpensive way to mitigate earthquake risks to structures. A substantial portion of the input energy supplied by an earthquake can be dissipated with installing designated supplementary energy dissipative devices (EDDs) in a structure. Therefore, damage to the main structure is reduced significantly. Unlike active control systems, passive devices do not require an external source of power. This is a big advantage of passive devices as the unreliability associated with power supply and computer control during strong ground shaking is diminished. After a strong ground motion, damaged devices can be replaced with minimum time and cost if appropriate device access is provided in advance. Passive energy devices also have some other advantages, such as simplicity of manufacturing, low cost, effortless maintenance, and versatility of their applications.
Passive energy dissipaters need to be designed properly to maximize dissipative energy and protect the structure during an earthquake. As these devices are part of the overall structural system, they should be designed accordingly. To retrofit the structures and mitigate the earthquake hazard effectively, an appropriate design method is a must.
The structural damage caused by earthquake ground motion results not only from the maximum response but also from accumulated plastic deformations. However current seismic design practice, which accounts only for the maximum earthquake load and maximum lateral displacement, does not provide enough information on the inelastic response of the structure. In this regard energy-based seismic design methods, which utilize hysteretic energy as the main design parameter and account for damage accumulation, have been considered as a potential alternative to the conventional force or displacement-based seismic design methods.
In this thesis a stepwise multi-mode energy-based design method for seismic retrofitting with passive energy dissipation systems is proposed as an alternative to strength and displacement-based methods. The method utilizes modal pushover analysis to determine modal yield force and ductility factor of an equivalent single-degree-of-freedom system. Two modes with the highest participation factors are incorporated. The energy contribution of each mode is determined using energy spectra and the required amount of energy dissipation is estimated and used to retrofit the original structure with appropriate energy dissipation system.
The effectiveness of the proposed method is verified using nonlinear time-history analysis. For this purpose three case studies (3, 9 and 20-storey building) are used. They are analysed and designed for four different ground motions. It is concluded that the proposed method is easy to implement and yields an effective retrofitting design in which damage is confined within the added dissipative system.