Passive and Active Structural Vibration Control in Civil Engineering

Seismic isolation (also often referred to as base isolation) is a technique for mitigating the effects of earthquakes on structures through the introduction of flexibility and energy absorption capability. Various practical means for introducing this desired flexibility and energy absorption capability are described.

Sliding seismic isolation systems may consist of a variety of components, of which the most important one is the sliding bearing. Typically, sliding bearings consist of interfaces made of PTFE or PTFE-based composites and highly polished stainless steel. The properties of these interfaces are described.

The design of sliding isolation systems and relevant applications are presented. Sliding isolation systems which found application are:

Under a joint research program between Kyoto University and the Hanshin Expressway Public Corporation, five isolation bearings of different damping mechanisms were tested under identical test conditions in order to evaluate their suitability and applicability to different types of bridges. The isolation bearings, including the lead-rubber bearing, two types of high-damping rubber bearings, sliding rubber bearing, and a conventional rubber bearing were tested under cyclic loading and hybrid earthquake (pseudo-dynamic) test methods. In addition to the fundamental cyclic load-deformation behavior, earthquake and resonant response behavior of the five isolation bearings were obtained. From the test results, equivalent damping ratios are evaluated based on the usual cyclic loading tests as well as from earthquake response of equivalent linear elastic models. In addition, the effectiveness of isolation is evaluated based on acceleration and displacement amplifications using earthquake response results obtained from the hybrid earthquake loading tests. In order to evaluate the effect of initial first-cycle stiffness of rubber bearings on the earthquake response, a second identical specimen of each type of isolation bearings was tested for its virgin earthquake response behavior.

New earthquake energy spectra are proposed for practical design procedures using energy concepts. The spectra show integrated earthquake input energy of a unit mass for a given natural period and damping ratio. The total input energy and its partitioning in complex structures can be evaluated with the conventional procedure of modal analysis and the proposed spectra. Numerical simulations are carried out for inelastic structures to show the earthquake energy partitioning in time and in space. Inelastic earthquake response and earthquake energy partitioning of bridge structures with and without seismic isolators subjected to earthquake ground motions is evaluated to verify their engineering significance.

This section describes the development and application of seismic (or base) isolation in the United States. Significant strides have been made in the implementation of the technology. This is attributed to recent advances in seismic isolation hardware and the removal of a number of impediments, which delayed widespread implementation in the near past.

In recent years, base isolation techniques are actively adopted in construction of bridges, buildings and other structures in Japan. This paper summarizes the code provisions for base isolated bridges and buildings in Japan, as well as their implementation. Earthquake records obtained from base isolated buildings are also reported to verify reduction of earthquake response in base-isolated structures.

This section presents a brief account of the development and application of seismic (or base) isolation in Europe. It appears that the first documented attempts to mitigate seismic hazard by seismic isolation were made in Europe.

This section describes passive energy dissipating devices which may be used within a structural system to absorb seismic energy. These devices are capable of producing significant reductions of interstory drifts in moment-resisting frames. Furthermore, these devices may, under elastic conditions, reduce the design forces.

In this chapter, the principles of TMD (Tuned Mass Damper) are explained. Selection of structural parameters of TMD is discussed based on Den Hartog’s optimum theory for a simplified 2-DOF (degree-of-freedom) system. Design procedure of TMD for implementation to structures is also introduced. Discussions are extended to TLD (Tuned Liquid Damper). Shaking-table tests of TLD for flexible structures at Kyoto University are also introduced.

Passive energy dissipation systems were developed in the United Stated either specifically for civil engineering applications or they evolved from devices and materials used in industrial, automotive, military and aerospace applications. Yielding steel and frictional devices were specifically developed for structural applications, whereas viscoelastic dampers and fluid viscous devices were developed for other applications and adapted for structural applications.

In comparison with passive energy dissipation, research and development of active structural control technology has a more recent origin. In structural engineering, active structural control is an area of research in which the motion of a structure is controlled or modified by means of the action of a control system through some external energy supply. In comparison with passive systems, a number of advantages associated with active systems can be cited; among them are (a) enhanced effectiveness in motion control. The degree of effectiveness is, by and large, only limited by the capacity of the control system; (b) relative insensitivity to site conditions and ground motion; (c) applicability to multi-hazard mitigation situations. An active system can be used, for example, for motion control against both strong wind and earthquakes; and (d) selectivity of control objectives. One may emphasize, for example, human comfort over other aspects of structural motion.

Thus motivated, considerable attention has been paid to active structural control research in recent years. It is now at the stage where actual systems have been designed, fabricated and installed in full-scale structures. A number of review articles (Miller et al., 1988; Kobori, 1988; Masri, 1988; Soong, 1988; Yang and Soong, 1988; Reinhorn and Manolis, 1989; Soong et al., 1991) and a book (Soong, 1990) have provided the reader with information and assessment on recent advances in this emerging area.

This chapter describes some concepts and methods related to the design of control laws and algorithms for active control of structures. The first point will be to identify second order differential equations as models to describe the essential components of a control system. Then some issues about the state space representation of these models will be reviewed. State space is the mathematical framework most frequently used to formulate active control laws. Optimal control will be presented as a representative methodology for continuous time control. Predictive control will be formulated as representative of discrete time control methods, also pointing out the issue of the time delay and some questions about robustness.

This chapter does not try to be exhaustive. Control (in general) is a very wide area of knowledge and active control of structures is adopting more and more of its concepts, methods and techniques. The purpose of this chapter is to serve as an introduction to the problem of analysis and design of control laws. Many topics are left and can be covered going through the bibliography suggested in the last Section.

As in all other new technological innovations, experimental verification constitutes a crucial element in the maturing process as active structural control progresses from conceptualization to actual implementation. Experimental studies are particularly important in this area since hardware requirements for the fabrication of a feasible active control system for structural applications are in many ways unique. As an example, control of civil engineering structures requires the ability on the part of the control device to generate large control forces with high velocities and fast reaction times. Experimentation on various designs of possible control devices is thus necessary to assess the implementability of theoretical results in the laboratory and in the field.

In order to perform feasibility studies and to carry out control experiments, investigations on active control have focused on several control mechanisms as described below.

As we have seen, full-scale implementation of active control devices in buildings has taken place, but all installations in either full-scale test structures or new buildings can only be found in Japan. However, some of the operational full-scale active systems are the result of US-Japan research collaborations and U.S. researchers were responsible for the design, fabrication and installation of an active bracing system in a full-scale dedicated test structure for structural response control under seismic loads (Soong et al., 1991; Reinhorn et al., 1992; Reinhorn et al., 1993). In addition, at least two designs have been completed for the purpose of retrofitting existing deficient structures in the U.S. The active bracing system and one of the design projects are briefly described below.

In this chapter, recent developments in active and hybrid control techniques is reviewed by introducing experimental projects and some implementations being conducted in Japan. Active and hybrid control techniques are categorized into AMD (active mass damper), ATMD (active tuned Mass Damper), AB (active brace), AVS (active variable stiffness), A VD (active variable damper) and ABI (active base isolation). Shaking-table tests of flexible frame models with TMD, AMD, and ATMD at Kyoto University are also introduced.

As we have seen, remarkable progress has been made in the area of base isolation, passive energy dissipation, and active control. Recent advances in seismic isolation hardware and the benefit offered by this technology have been responsible for a rapid increase in the number of base-isolated buildings in the world, both new construction and retrofit.

The basic role of passive energy dissipation devices when incorporated into a structure is to consume a portion of the input energy, thereby reducing energy dissipation demand on primary structural members and minimizing possible structural damage. Over the last twenty years, serious efforts have been undertaken to develop the concept of supplemental damping into a workable technology and it has now reached the stage where a number of these devices have been installed in structures throughout the world.

In comparison with base isolation and passive energy dissipation, active control is a relatively new area of research and technology development. However, we again see a rapid development in this area, to the point that active systems have been installed in several structures in Japan.