# Recent Advancement in Assessment and Control of Structures under Multi-Hazard
## 3 Vibration Control of Structures  
Most of the structural damage caused by natural forces can be attributed to excessive vibrations. For static loads, vulnerability reduction can be achieved at the design stage by increasing stiffness and/or strength of structural elements. For existing structures, retrofitting strategies also aim to improve strength and/or stiffness of the structural elements. Similar strategies can also be used for dynamic forces such as wind and earthquakes, but newer and potentially more cost-effective solutions emerging in the scientific research are percolating to practical applications. These new solutions are not necessarily about increasing structural stiffness and/or strength. They fundamentally rely on changing the dynamic properties of the structures to make them less vulnerable to natural forces. This can, contrary to retrofitting in the traditional sense, even make the overall structure more flexible. A notable example is the well-established base-isolation technology for reducing earthquake-induced vibrations of buildings and bridges. Vibration reduction, also known as vibration control, makes use of different types of devices installed on the structure to reduce vibrations caused by different types of forces. While base-isolation, supplemental damping, and bracing systems to increase lateral stiffness and ductility have been researched and used for a long time, newer control strategies that rely mainly on dynamic devices installed on the primary structure are emerging. Depending on their mode of operation and need for external energy and/or internal feedback mechanism,vibration control devices can be broadly classified as passive, active, semi-active, or hybrid systems. The following list gives a few examples of these different types of control systems  
1. Passive: energy dissipation, base isolation, tuned mass dampers (TMD), and tuned liquid dampers. 2. Active: adaptive control, active bracing, and active mass damping. 3. Semi-active: semi-active energy dissipation, semi-active isolation, and semi-active mass damping. 4. Hybrid: hybrid bracing and hybrid mass damping. Detailed definitions and the basic theory behind these different classes of structural control devices can be found in the seminal work of [PERSON] et al. [50]. Most of these control concepts have been extensively investigated, and many of these systems are already installed in different types of structures. Passive control devices are the ones most frequently used, as they don't require external energy supply. Tuned mass dampers (TMDs), fluid viscous dampers (FVD), tuned liquid dampers (TLD), and seismic base isolation (BI) are the most popular passive control systems. An early state-of-the art review of seismically base isolated buildings is presented by [PERSON] [51], [PERSON] and [PERSON] [52], and [PERSON] and [PERSON] [17]. They discuss different types of base isolation systems and summarize findings of the contemporary literature about their performance in seismic response control in addition to presenting a parametric study of crucial design parameters for optimal reduction in seismic performance. [PERSON] and [PERSON] [19] present a state-of-the-art review of base isolation systems in seismic response mitigation. They focus on design code provisions for isolated structures and discuss effects of soft-soil and near-fault ground motions. [PERSON] and [PERSON] [18] present a state-of-the art review of seismically isolated bridges and identify some knowledge gaps in the contemporary literature. [PERSON] and [PERSON] [21] discuss different types of supplemental energy dissipation systems, including passive and active dampers for structural control. They provide an informative timeline of the development of these control technologies and describe the state-of-the-art review in the context of seismic-resistant design and the retrofitting of structures. A review on the behavior of structures with passive control systems exposed to seismic loads is presented by [PERSON] [52].