Over the years, protection and control systems have been developed to solve problems caused by dynamic action. These systems improve the dynamic behaviour of structures by modifying its dynamic characteristics or influencing how the dynamic actions are transmitted to them. The incorporation of damping by including tuned liquid dampers (TLD) in existing buildings is very appealing, since it enables simple installation and reduced extra cost. In new buildings the inclusion of such devices will have the same benefits as those observed for existing ones, adding the possibility that designers can use any system of water storage (which are often thought to taller buildings) and secondary systems. Additionally, systems including liquids may also be used temporarily for controlling vibrations observed during the construction phases.
In this context, a study of the behaviour of TLD and the structures in which they are placed for vibration mitigation was developed at LNEC. The results were part of a PhD thesis presented at the Technical University of Lisbon (“Instituto Superior Técnico”) in december of 2010.
Structure and content
The thesis begins with a description of the evolution of seismic protection systems and a state-of-the-art on passive seismic protective systems, with particular emphasis on TLD. A description of some basic concepts associated with the operation, advantages and limitations of TLD, as well as some examples of TLD practical application to various types of structures, including buildings, is also presented.
An experimental parametric study, in LNEC uniaxial and triaxial seismic platforms, was carried out to evaluate the behaviour of TLD when isolated or embedded in structures and validate their main properties. The structural simplified systems chosen to include the TLD were representative of a single degree of freedom (SDOF) and two degree of freedom (2DOF) systems with adjustable dynamic characteristics. A set of natural frequencies, representative of natural frequencies of current Portuguese buildings, was simulated. It was possible through physical simulations (Figure 1 and Figure 2) to determine the efficiency of TLD in reducing displacement and acceleration in buildings.
The work also lays out some numerical simulations based on mathematical models, performed on variable geometry TLD when isolated, included in SDOF and 2DOF structural systems or included in existing reinforced concrete buildings. The simulated structures and selected TLD correspond to a realistic implementation based on a set of guidelines considered for both the main system (building) and to the secondary system (TLD). The selection of case study buildings for numerical simulations was made on the basis of their geometric characteristics and in-situ dynamic characterization tests previously developed, which enabled the determination of their fundamental vibration frequencies and corresponding mode shapes.
The main guidelines for the conception, design and implementation of TLD on new or pre-existing real structures were also set in the thesis. In addition to guidelines previously established by other authors, guidelines for proper design of these devices were proposed based on efficiency rates obtained experimentally or through numerical simulations. These rates were set based on the reduction of: i) peak displacements or peak accelerations, ii) values of root mean square (RMS) displacement (Figure 3) or RMS acceleration or iii) the frequency response function – FRF (Figure 4).
Some conclusions and considerations related to placement of TLD for seismic vibration mitigation in existing buildings are presented in the final part of the thesis.
The results provided a good understanding of the dynamic behaviour of this type of devices for use in seismic protection of building. Additionally it was proven the adequate approximation of the numerical results with the experimental results in terms of vibration frequencies, the water surface elevation and, consequently, the resulting forces in the walls of the TLD devices when isolated. It was also possible to obtain a very suitable approximation of the dynamic behaviour of TLD when subjected to dynamic actions as well as illustrate the behaviour of the TLD devices included in SDOF or 2DOF structures observed during dynamic tests made in LNEC uniaxial and triaxial seismic platforms.
Considering the hypothetical implementation of TLD on real structures, the main conclusions of the study were: i) reduction of the response (displacements, velocities and accelerations) at the main structural system with the introduction of TLD and ii) response reduction proportional to the increase of the mass ratio between TLD and the main structure. The work revealed that, for a level of vibration mitigation considered as very acceptable the implementation of TLD is presented as financially more appealing and more viable than other solutions frequently used and disseminated through the world.
Falcão Silva, M.J. – Sistemas Passivos de Protecção Sísmica: Uma abordagem baseada no desempenho de amortecedores de líquido sintonizado [Passive Seismic Protection Systems: An approach based on the performance of tuned liquid dampers]. Lisboa: Instituto Superior Técnico, 2010. (PhD thesis).