Liquid Dampers for Mitigating Wind and Earthquake Induced Vibrations in Buildings- Juniper Publishers

 Civil Engineering Research Journal- Juniper Publishers

 

Abstract

It is important in structural design to efficiently maintain safety, sustainability and stability of structures and therefore, using liquid dampers (LD) to control structural vibrations became popular in engineering design. Due to the nonlinear mechanism of the liquid, the behavior of the LD is very complicated when undergoing strong ground motion. This review focuses on what published research studies, within the field of structural vibration control systems, having accomplished and more specifically, the behavior of the sloshing water and the overall efficiency of the LD as a vibration control system.

Keywords: Liquid damper; Water tank; Vibration control; Wind response; Strong ground motion; Earthquakes

Introduction

Tuned Liquid Damper (TLD) is an auxiliary passive control system that can be used in different structural design applications, such as high -rise buildings, bridges and offshore oil platforms to reduce structural vibrations that can be caused by wind, strong ground motions and other external repeated loadings. The idea of such device relates back to the tuned mass damper (TMD), another form of auxiliary passive control system that uses a huge mass, which is implemented on one of the upper levels of a structure through a spring, to reduce structural vibration. The differences between the two systems are the type of mass and its behavior when it interacts with the structure under different external loadings. For the TMD, the linear behavior of the mass and the spring it is attached to is rather simple to model and analyze and therefore it is easy to adjust, depending on the intensity of the external excitations. However, the TMD was found to be limited in reducing structural vibrations that are caused by seismic excitation, which brings more attention of finding a more efficient solution, the TLD. In this paper a review has been carried out to summarize the efficiency, applicability and shortcomings of LDs.

Discussion

Liquid dampers

The fluid-filled frequency-tunable mass damper (FTMD) is a fluid-based mitigation system where the working mass is all or a portion of the fluid mass that is contained within the geometric configuration of either a channel, pipe, tube duct and/or similar type structure. A compressible mechanism attached at one end of the geometric configuration structure enables minor adjustments that can produce effects on the frequency and/or response attributes of the mitigation system.

Tuned liquid dampers (TLD) rely on the geometry of a container to establish mitigation frequency and internal fluid loss mechanism to set the fundamental mitigation attributes [1]. Since the compressible mechanism is added at the end of the container, it allows for simple alterations to the existing fluid containers to make frequency adjustments. For example, in a multistory building, water from a rooftop tank or a swimming pool could be used to mitigate seismic or wind-induced vibration by simply adding a device that controls the motion of the water in relation to the building. The first category of TLDs is typically a mass composed of waving water, generally located at the top of a building. The container’s shape and dimensions, and the water level define the “sloshing” frequencies. There are different types of devices that belong to this category, and among all, the most common are [2]:

a. Flat bottom (box type) TLD,

b. Sloped bottom TLD,

c. Multiple TLD,

d. TLD with variable density of liquid, Slat screen TLD,

e. Hybrid mass TLD, and

f. Baffle wall TLD.

The second category of the TLDs is the liquid contained within a U-shape tube and is referred as the tuned liquid column damper (TLCD). Usually the orifices are installed inside the container to produce loss of hydraulic pressure and result in the damping effects.

Sloshing phenomenon

Housner showed that the dynamic pressure on accelerated fluid containers varies depending on the geometry of the container [3] while Siekmann and Chang extended the study to the analysis of the dynamic behavior of partially fluid-filled containers with an arbitrary geometry [4]. A significant amount of research on TLDs has focused on the development of analytical models of the sloshing fluid to simulate its nonlinear dynamic behavior, and the response of structures equipped with LDs. The equivalent mechanical models have also been developed to represent the TLD systems allowing the analysis of their efficiency and robustness to be evaluated [5]. Additionally, the comparison of the performance of the TMDs, TLDs and TLCDs were conducted under wind and seismic excitation. The results suggest that the TMD performance is always better than the TLD and TLCD due to unpredictable sloshing effects.

The main problem of TLDs is that the performance depends on the contained liquid interacting with the applied loads and having control over the motion of the disrupted liquid during strong ground motions was not found so easy to deal with. A contained amount of liquid does not have a fixed center of mass, which makes its motion very difficult to evaluate. Unlike TLDs, a TMD system’s center of mass is fixed and so, it is an accessible process to calculate its motion using simple spring-mass models and the differential equations of motion. The main concern about not having enough or very minimal control over the mass center of liquid is that such vibration control systems need to have the ability to be tuned or adjusted, depending on the intensity or the direction of ground motion. Some existing TLD systems have some ability to a certain extent, to adjust their orientation to interact with the seismic motion. One way of doing that is by designing the system on a rotary base and to be connected to computerized sensing devices that can detect upcoming seismic events some enough time in advance and so to adjust the orientation of the damper to maximize its efficiency [6-9].

The directions of wind induced vibration of buildings usually are not in both directions, namely along-wind and across-wind directions, simultaneously. However, changes in wind directions and torsional effects may require dampers to be installed in two orthogonal directions. The bi-directional liquid damper (BLD) is introduced to control two different directions of vibration at the same time [10]. The BLD is like a conventional TLCD, in which in one direction it utilizes the tube-shape of water as a mass damper system while in the other direction the rectangular containers are used as sloshing dampers. Even though new types of LDs are introduced, it is recommended that the variability needs to be carefully evaluated.

Soil-structure interaction

In structural analysis it is generally assumed that the structure is supported by a rigid foundation and the behavior of the surrounding soil is neglected. Over the year’s researchers found that the dynamic response of structures that are located on a soft or flexible soil is completely different from ones that are located on a stiff soil. Considering the effects of soil-structure interaction (SSI) the type and dimensions of the foundation system also play an important role in dynamic analysis of structures. Therefore, it is important to understand and assess the effects of SSI on buildings with damping devices [11,12].

Many literatures in building vibration control with liquid dampers concentrate on the damping devices. Despite the advantages of the TLDs, a drawback remains regarding the fact that the success of TLDs strongly depends on the tuning of the oscillating liquid with that of the primary structure to which it is attached. The SSI cannot be ignored otherwise the induced structural response and the effectiveness of the TLD will be falsely estimated. Consequently, TLDs may be more effective to certain types of soil condition.

Full-scale experiments

Full-scale experiments have always been considered important aspects for validating the designs of damping devices. The behavior of buildings with LDs is a collection of phenomena in the response of buildings caused by the flexibility of the surrounding soils, in the response of soils caused by the response of buildings, as well as in the response of buildings when they interact with the LDs. Experimental studies of those behaviors are best conducted in full-scale in actual buildings [13], during microtremors, forced vibrations, and earthquake excitations. Many published papers present experimental programs on “full-sale” measurements that were conducted in order to prove the efficiency of the TLDs [14- 16]. The difficulties of conducting experiments in the laboratory are not only due to the similarity laws that must be satisfied but are mainly due to modeling the semi-infinite boundary condition of soils and simulating the strong ground motion.

Conclusion

The One Wall Centre, for example, is a 48-storey residential tower in Vancouver, Canada, completed in 2001. One Wall has a tuned liquid column damper at the top level. This damping system is designed to counteract the lateral wind response of the tower. Since the motions of the tower were primarily caused by wind in one direction only, the observed performance of the TLCD may not be applicable to the motions induced by strong ground motions. At the present time many researchers have been focusing on the study of structural vibration control systems such as the sloshing phenomena of water in the LDs and their overall efficiency as a vibration control device. The tuned liquid dampers have indeed a great potential to enhance the wind induced dynamic performance of tall, slender structures, enabling creative architecture, optimizing usable floor space and material use, and resolve the problem of occupant comfort.

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