Structural Damping Applications in Seismic Response Modification
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Structural Damping Applications in Seismic Response Modification

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The Authors of  Structural Damping eBook

There are three editors of Structural Damping Applications in Seismic Response Modification:

Zach Liang,

George C. Lee,

Gary F. Dargush

and Jianwei Song.

Preface to Structural Damping Applications in Seismic Response Modification eBook

Today, earthquake engineering research is devoting a major effort toward establishing seismic performance requirements associated with large inelastic deformations of the structure. At the same time, structural response modification systems (certainly, passive energy dissipation and seismic isolation systems) have been widely used.

It is reasonable to expect that the next research emphasis in performance-based engineering is to integrate both frontiers for more optimal seismic performance of structures.

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As far as safety, performance, and cost for both components of inelastic deformation and structural response control are concerned, structural damping is a core knowledge area.

In this Structural Damping Applications in Seismic Response Modification ebook, recent advances in structural damping are presented and their applications to the design of passive structural response modification devices are given to complement the current supplemental damping design practice for high-level damping.

Integration with seismic performance requirement is not addressed in this Structural Damping Applications in Seismic Response Modification ebook. Aseismic design using supplemental dampers has steadily gained popularity in the earthquake engineering profession over the past several decades.

Many practical applications of various dampers can be found worldwide and, in the United States, damper design has been included in building codes.

To date, damping design is primarily based on the concepts of the energy equation, effective proportional damping, and simplified linear single-degree-of-freedom (SDOF) response spectrum.

These concepts, along with their associated underlying assumptions, support the idea that installing supplemental dampers in structures will dissipate energy.

Nonlinear damping is presented by an effective damping ratio through linearization schemes, and the damping coefficients of structures are assumed to be classic damping matrices in order to establish a procedure for damping design.

In addition, it is assumed that statistical procedures that use earthquake records can be carried out through proportional scaling of their amplitudes.

These assumptions have enabled us to develop a design procedure for supplemental dampers. However, it is not well understood that some of these assumptions work well only when the amount of damping in structures is within a specific low level.

The first main objective of this Structural Damping Applications in Seismic Response Modification ebook is to provide a theoretical foundation on the role of damping in the dynamic response of structures, especially when the level of damping is high or when nonlinearities become important design issues.

The second main objective is to provide response spectra–based design principles and guidelines for practical applications of damping devices to reduce earthquake-induced structural vibration.

Generally speaking, structural responses under seismic excitations are dynamic processes. There are three resisting force components to counter the earthquake load, one of which is the damping force.

While damping technology has been developed and advanced in a range of mechanical and aerospace engineering applications over many years, it has become a popular approach in structural engineering only in the later part of the twentieth century.

While the development and application of energy dissipation devices in structural engineering continue to expand, there are a number of fundamental issues related to the dynamic behavior of structures with supplemental damping as a system that require further study.

Limitations and impacts of using energy dissipation devices need to be clarified and established. This Structural Damping Applications in Seismic Response Modification ebook intends to fill the knowledge gap by helping earthquake engineers to better understand the dynamic behavior of structures and to more effectively use the design codes for dampers.

The key elements in this Structural Damping Applications in Seismic Response Modification ebook are summarized as follows. A straightforward concept often advocated in damping design is that “more energy dissipated by the added dampers will result in less vibration energy remaining in the structure, and thus the structural response is reduced.

” This is not always true. A higher level of damping may not effectively reduce the responses of a given structure. In some cases, high-level damping may even magnify the responses, because the level of structural response depends not only on local energy dissipation, but also on energy input and its redistribution.

Thus, minimizing the conservative energy of the vibrating system in structures with supplemental damping is a more appropriate general guiding principle.

Several other basic issues in structural damping are carefully reviewed. These include the maximum energy dissipation principle under preset damping force and allowed structural displacement; the damping adaptability of devices that can operate in a large dynamic range of earthquake loads;

the viscoelastic behavior of any damping elements that take the supporting stiffness as well as installation practice into consideration; nonproportional damping that needs to be minimized as much as possible in design practice;

the limitation of using damping force that provides a practical engineering limit beyond which adding more damping provides diminishing gains; and the problem of damping and stiffness nonlinearity that cannot be accurately approximated by today’s design approaches.

In addition, a design principle based on energy distribution is discussed, which may be useful for generally damped multi-degree-of-freedom (MDOF) systems. The characteristics of nonlinear damping and nonlinear structures are complex issues to address in damping design.

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In this Structural Damping Applications in Seismic Response Modification ebook, nonlinearity is considered in three cases. The first case involves a linear structure with small nonlinear damping, the second case is for a linear structure with larger nonlinear damping, and the third case applies when both the structure and the damping are nonlinear.

In the first case, because the damping force is rather small, almost any type of linearization can be used without causing any significant design discrepancies. In the second case, care must be taken to choose proper linearization; employing nonlinear design spectra can often be a reasonable approach.

In the third case, linearization methods, though adopted by many building codes, can provide highly inaccurate results. Methods such as the equal displacement approach (using R-factors), equal energy approach, and pushover approach all have their limitations.

Since the nonlinear design spectra approach requires too many response spectra, not only for specific damping and stiffness, but also for specific levels of ground excitations, it is not useful in practice. Thus, nonlinear time history analysis must be used.

This latter method, though always workable for the first two cases, can yield an unacceptable computation burden, making it unattractive for use in day-to-day practical damping design. For this reason, time history analysis is not emphasized in all discussions.

Two types of design approaches are provided in this Structural Damping Applications in Seismic Response Modification ebook. The first is the design response spectra approach. Specifically, the design is modified by a simplified factor, the damping ratios of the first several vibration modes of the structure.

For readers familiar with the design response spectra method for an SDOF linear system, this approach provides a good design when damping is small and for structural responses in elastic ranges.

The modified approach addresses cases where damping is large and nonlinear. Examples of how to modify the response spectra design are also provided. The second type of damper design is based on time history analysis, for which several issues are important. 

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