Sulla progettazione in zona sismica di strutture dotate di dissipatori fluido-viscosi: proposta di una metodologia
Creators: name: Dellavalle, Alberto <1977>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: ICAR/09 Tecnica delle costruzioni

description: CHAPTER 1:FLUID-VISCOUS DAMPERS
In this chapter the fluid-viscous dampers are introduced.
The first section is focused on the technical characteristics of these devices, their mechanical
behavior and the latest evolution of the technology whose they are equipped.
In the second section we report the definitions and the guide lines about the design of these
devices included in some international codes.
In the third section the results of some experimental tests carried out by some authors on the
response of these devices to external forces are discussed. On this purpose we report some technical
schedules that are usually enclosed to the devices now available on the international market. In the third
section we show also some analytic models proposed by various authors, which are able to describe
efficiently the physical behavior of the fluid-viscous dampers.
In the last section we propose some cases of application of these devices on existing structures
and on new-construction structures. We show also some cases in which these devices have been revealed
good for aims that lies outside the reduction of seismic actions on the structures.
CHAPTER 2:DESIGN METHODS PROPOSED IN LITERATURE
In this chapter the more widespread design methods proposed in literature for structures equipped
by fluid-viscous dampers are introduced. In the first part the response of sdf systems in the case of
harmonic external force is studied, in the last part the response in the case of random external force is
discussed.
In the first section the equations of motion in the case of an elastic-linear sdf system equipped
with a non-linear fluid-viscous damper undergoing a harmonic force are introduced. This differential
problem is analytically quite complex and it’s not possible to be solved in a closed form. Therefore some
authors have proposed approximate solution methods. The more widespread methods are based on
equivalence principles between a non-linear device and an equivalent linear one. Operating in this way it
is possible to define an equivalent damping ratio and the problem becomes linear; the solution of the
equivalent problem is well-known.
In the following section two techniques of linearization, proposed by some authors in literature,
are described: the first technique is based on the equivalence of the energy dissipated by the two devices
and the second one is based on the equivalence of power consumption. After that we compare these two
techniques by studying the response of a sdf system undergoing a harmonic force.
By introducing the equivalent damping ratio we can write the equation of motion of the non-linear
differential problem in an implicit form, by dividing, as usual, for the mass of the system. In this way, we
get a reduction of the number of variables, by introducing the natural frequency of the system. The
equation of motion written in this form has two important properties: the response is linear dependent on
the amplitude of the external force and the response is dependent on the ratio of the frequency of the
external harmonic force and the natural frequency of the system only, and not on their single values.
All these considerations, in the last section, are extended to the case of a random external force.
CHAPTER 3: DESIGN METHOD PROPOSED
In this chapter the theoretical basis of the design method proposed are introduced.
The need to propose a new design method for structures equipped with fluid-viscous dampers
arises from the observation that the methods reported in literature are always iterative, because the
response affects some parameters included in the equation of motion (such as the equivalent damping
ratio).
In the first section the dimensionless parameterε is introduced. This parameter has been obtained
from the definition of equivalent damping ratio. The implicit form of the equation of motion is written by
introducing the parameter ε, instead of the equivalent damping ratio. This new implicit equation of
motions has not any terms affected by the response, so that once ε is known the response can be
evaluated directly.
In the second section it is discussed how the parameter ε affects some characteristics of the
response: drift, velocity and base shear.
All the results described till this point have been obtained by keeping the non-linearity of the
behavior of the dampers. In order to get a linear formulation of the problem, that is possible to solve by
using the well-known methods of the dynamics of structures, as we did before for the iterative methods
by introducing the equivalent damping ratio, it is shown how the equivalent damping ratio can be
evaluated from knowing the value of ε. Operating in this way, once the parameter ε is known, it is quite
easy to estimate the equivalent damping ratio and to proceed with a classic linear analysis.
In the last section it is shown how the parameter ε could be taken as reference for the evaluation of
the convenience of using non-linear dampers instead of linear ones on the basis of the type of external
force and the characteristics of the system.
CHAPTER 4: MULTI-DEGREE OF FREEDOM SYSTEMS
In this chapter the design methods of a elastic-linear mdf system equipped with non-linear fluidviscous
dampers are introduced.
It has already been shown that, in the sdf systems, the response of the structure can be evaluated
through the estimation of the equivalent damping ratio (ξsd) assuming the behavior of the structure
elastic-linear. We would to mention that some adjusting coefficients, to be applied to the equivalent
damping ratio in order to consider the actual behavior of the structure (that is non-linear), have already
been proposed in literature; such coefficients are usually expressed in terms of ductility, but their
treatment is over the aims of this thesis and we does not go into further.
The method usually proposed in literature is based on energy equivalence: even though this
procedure has solid theoretical basis, it must necessary include some iterative process, because the
expression of the equivalent damping ratio contains a term of the response. This procedure has been
introduced primarily by Ramirez, Constantinou et al. in 2000. This procedure is reported in the first
section and it is defined “Iterative Method”.
Following the guide lines about sdf systems reported in the previous chapters, it is introduced a
procedure for the assessment of the parameter ε in the case of mdf systems. Operating in this way the
evaluation of the equivalent damping ratio (ξsd) can be done directly without implementing iterative
processes. This procedure is defined “Direct Method” and it is reported in the second section.
In the third section the two methods are analyzed by studying 4 cases of two moment-resisting
steel frames undergoing real accelerogramms: the response of the system calculated by using the two
methods is compared with the numerical response obtained from the software called SAP2000-NL, CSI
product.
In the last section a procedure to create spectra of the equivalent damping ratio, affected by the
parameter ε and the natural period of the system for a fixed value of exponent α, starting from the elasticresponse
spectra provided by any international code, is introduced.