Damping Technologies for Tall Buildings: New Trends in Comfort and Safety

Alberto Lago, CTBUH/IUAV University of Venice - Antony Wood CTBUH - Dario Trabucco CTBUH/Iuav University of Venice

Damping technologiesTall buildings have become a prominent solution for increasing density in major cities around the world. The trend of the last years is to build taller, slimmer, and lighter structures, as the recent tall buildings completed or under construction in Manhattan demonstrate. Indeed, the latest advancements in high-strength materials, with the same modulus of elasticity (i.e. less stiff structures) and construction methods have lead to more efficient solutions (Ali and Moon, 2007). However, these lighter systems could lead to structures more prone to vibrations, which can cause discomfort, damages, and eventually, structural failure.

Taller and slimmer buildings need to withstand a variety of external forces that are different from those of low-rise constructions, and, as a consequence, different structural solutions need to be used. Moreover, many major cities are threatened by a variety of extreme events such as earthquakes and strong winds. However, in recent years, global warming and climate change appear to have caused these extreme wind events to occur more frequently and in less predictable areas.

Motion control of tall buildings, therefore, should take into consideration both static and dynamic loads. This can be accomplished by increasing the structural stiffness and damping while keeping the material amount at a minimum. A stiffer building can be achieved with a proper selection of the structural configuration. Tubes, diagrids, and core-supported outrigger structures could be considered more optimal solutions than others. Increasing damping, instead, can mainly be achieved by installing auxiliary damping devices, since the damping characteristics of the main structural system (i.e. inherent damping) is quite uncertain until the building is complete (Smith et al., 2010). On the contrary, damping provided by external devices can be estimated quite accurately. This is thanks to the extensive research conducted in the last decades, making them a reliable solution for reducing the vibration problems in tall buildings.

Damping systems were first introduced to the mechanical industry and experimentation with damping in building industries started during the 1950s. The first application in tall buildings dates back to 1969 with the World Trade Center. Damping was used extensively in the 1980s as part of retrofits of existing buildings (e.g. John Hancock Tower, Boston) or in new constructions (e.g. Citicorp, New York; One Canada Square, London; and Yokohama Landmark Tower, Yokohama). Nowadays, there is a wide range of damping systems available, and their selection depends on the primary structural system of the building, the main external forces that they need to resist, and the performance criteria that the building needs to meet. The current available damping systems can be subdivided into two main categories: passive and active systems (Kareem et al., 1999). Passive systems have fixed properties while active systems change their properties based on the load demand and require an external energy source to be activated. Therefore, while more efficient, active systems are less common due to economic and reliability constraints.

Passive systems can be further divided in two subcategories: (i) material based dissipation systems (e.g. viscous and visco-elastic dampers), and (ii) additional mass generating counteracting inertia forces (e.g. tuned mass dampers and tuned liquid dampers).

The first category of passive dampers is typically an integral part of the primary structural systems, and they are positioned in optimal locations (e.g. in bracing systems) to reduce the building’s dynamic motion. The force generated by these devices is a function of the time rate of change of deformations, and the relative damping comes from the phase shift between force and displacement. There are different types of devices that belong to this category, and among all, the most important are: viscous (Lee and Taylor, 2001; Chen et al., 2008; and McNamara and Taylor, 2003), viscoelastic (Samali and Kwok, 1995), hysteretic, friction (Kaura et al., 2012), and electro-magnetic dampers (Datta , 2003).

The second category of passive systems is based on the counteracting inertial force created by an additional mass generally allocated at the top of a building. There are two main categories of devices belonging to this family: tuned mass dampers (TMD) and tuned liquid dampers (TLD). In the TMD, the mass is supported by an appropriate mechanical system that allows it to move out-of-phase with the fundamental period of the building (Tanaka and Mak, 1983; Xu et al., 1992 and Wong and Chee, 2004). In TLD (Wakahara et al., 1992) the mass is composed of waving water, and for this reason, the existing building’s water source can be utilized (e.g. water tanks located near the top of the building). In the case of a rectangular tank, its dimensions and the water level define the “sloshing” frequencies; this device is called a tuned sloshing damper (TSD). When the tank is a U-shaped vessel, it is called a tuned liquid column damper (TLCD).

Different from passive systems that are tuned to work on a narrow range of loading conditions, active systems perform more efficiently over a wider range (Connor, 2003). There are many different types of active devices, but the most prominent ones are active mass dampers (AMD; Xu, 1996 and Mackriell et al., 1997) and active variable stiffness devices (AVSD; Lin et al., 2015). Both devices rely on the same principles of mass and material based dissipation, respectively, similarly to passive dampers. However, their properties are adjusted from a computer control system. While very promising as a better solution for auxiliary damping in tall buildings, their applications have been limited due to relative high-costs and reliability issues. It is believed that further research on these devices might lead to increases in their utilization.

Several of these solutions were originally developed for low-rise buildings and have been subsequently adopted for tall buildings. There are also several examples of innovative solutions adopted only for tall buildings, and among all, the most relevant are:

  • Inclusion of dampers in outrigger systems. (Ahn et al. 2004; Smith and Willford, 2007; Joung and Kim, 2011; and Asai et al., 2013).
  • Dampers in shear walls. The paper by Madsen et al. (2003) introduces dampers in shear walls, even though this solution was already proposed for low-rise building. In recent years, several papers have discussed their application for tall buildings (e.g. Pant et al., 2015).
  • Adjacent buildings equipped with dampers (Bharti et al., 2010). These are utilized to reduce and avoid the possible pounding between abjection buildings.
  • Double skin façade as mass damper (Lago et al., 2010; Moon, 2011; and Fu, 2013). Double skin façades can be utilized as a structural motion control device in tall buildings. Two different strategies have been developed (Moon, 2011): one with low connection stiffness between inner and outer skins together with a damping mechanism, and one with inserting additional masses in the cavities of the double skin façade that could act as distributed tuned mass dampers.
  • Self Mass Damper (SMD; Kidokoro, 2008). Based on a project completed in Tokyo in 2007, and inspired by the pendulum movement of an antique clock, this system utilizes the existing mass of the building to act as a mass damper without adding any additional mass. The author explains how the system is created by disconnecting the upper floors via a system of sliders and high-damping rubber bearings.

Another important feature, peculiar to tall buildings, is higher modes of vibration contribution. These frequency characteristics of tall buildings can be very relevant for the design of passive damping systems (Lago, 2010; and Au et al., 2012). This is particularly true for mass damper systems since they are tuned to the building’s first mode of vibration.

In addition to these innovative damping solutions, auxiliary damping has also been used quite extensively in the retrofit of existing tall buildings. As an example, the 54-story Shinjuku Center Building in Tokyo has been retrofitted with deformation dependent oil dampers to overcome problems in the existing building’s structural capacity under long-period ground motions (Aono et al., 2011).

When dealing with auxiliary damping, some essential aspects to take into consideration are device maintenance and reliability. This is the reason why health monitoring has become a relevant topic for the life-cycle behavior of structures with additional damping devices. Several papers have been written on this topic (e.g. Tamura et al., 1995; and Bashor et al, 2012). It is important, therefore, to consider the life-cost analysis of structures with auxiliary damping in order to evaluate their cost-effectiveness. Several authors have discussed this relevant topic (e.g. King et al., 2001; Chen et al., 2008; and Hahm et al., 2013). Especially in the paper by King et al., 2001; a procedure is outlined for the estimation of the life-cycle costs and benefits of new and existing structures under earthquake demands. This method has been showcased against a high-rise steel moment frame building for different times periods and levels of added damping. The results show that the utilization of added damping devices is beneficial depending on specific thresholds of the additional costs coming from the installation of these devices (e.g. added damping would be beneficial for a 30-year time period and 10% added damping if the added cost is less than $4.9 million).

Reference

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State of the art in the Damping Technologies for Tall Buildings

There have been a large number of studies conducted on tall buildings with damping technologies, and as a result, it would be almost impossible to review all the available publications on the subject. Therefore, the following review attempts to give relevance to some of the major aspects of research related to tall buildings and damping technologies, without trying to be an exhaustive and complete review on the subject.

Some of the first practical considerations for the vibration control of tall buildings with mass dampers are given in “Performance of tuned mass dampers under wind loads” (Kwok and Samali, 1995). The selection of the optimal vibration control system is a function of several factors including: efficiency, compactness and weight, capital cost, operating cost, maintenance requirements, and safety. Mass dampers are usually the most utilized system in one of the following forms: passive, active, and hybrid. The paper reviews these systems with particular reference to full-scale verifications. For passive tuned mass dampers several design considerations need to be taken into account to achieve an economical solution. First, the additional mass on the top of the building does not have to exceed one to two percent of the building’s modal mass. Another practical aspect involves moving the additional mass even during small excitations. The easiest way to obtain this is with a pendulum. However, this requires a lot of headroom; that is why alternative solutions have been proposed (like inverted pendulums or multistage pendulums).

Another important aspect is that these systems can be precisely tuned after the building characteristics are known (i.e. when the building is complete). In the case of large movements of mass, safety devices need to be installed to limit the motion. In most cases, these passive systems can all also be designed to operate as active systems with servo-hydraulic actuators and/or servomotors as driven mechanisms. An important aspect to taken into consideration with these devices is the additional requirements for safety measures in order to control the forces generated.

The advantage of an active system is the smaller mass and the higher additional damping provided, that can be 10 percent or more, compared to three to four percent foe a passive system. However, this increases the costs. A passive system can reach up to one percent of the building cost, while an active system costs up to two percent (with maintenance costs being higher too). Looking at the properties of both systems, the ideal case would be to have a hybrid that takes the advantages of both passive and active systems. This system would work as passive or active depending on the loading conditions. When larger damping and motion control are required, active systems are usually used, and for low damping and motion control, passive systems are utilized. These systems, while being more costly than the passive ones due to the presence of an active damper, achieve considerable savings in operating and maintenance costs. In order to take into account all these factors, parametric studies are the best means for selecting the most appropriate vibration control system. These, together with experimental tests, will provide the most appropriate tools.

The paper “Structural systems to improve wind induced dynamic performance of High Rise Buildings” (Banavalkar, 1990) deals with the modification of vibrational mode shape without significantly affecting the overall stiffness of the building. Therefore, the focus is on modifying the mass and the damping of the building. The author tries to optimize the building mode shape in order to reduce the floor acceleration for two different case studies, and it illustrates how adding damping can be an effective means to reach this scope.

Modal properties are very important parameters to be estimated in the design of tall buildings. Researchers have conducted studies to determine a better estimate of these properties through building monitoring, as shown in “Field observations on modal properties of two tall buildings under strong wind” (Au et al., 2012). The authors describe how two tall buildings in Hong Kong are analyzed during typhoon and monsoon events. The buildings’ recorded accelerations are utilized to identify the modal properties of the building. Window frames of 30 minutes and a fast Bayesian frequency domain method are utilized to determine the natural frequencies, damping ratio, and mode shapes. The comparison between different events shows that the procedure is an effective way to identify the natural frequencies, while damping ratio estimations show more scatters.

Full-scale measurements have always been considered important aspects for validating the designs of TMD devices. One of the first papers on this subject was written by Tamura et al. in 1995  (“Effectiveness of tuned liquid dampers under wind excitation”). The paper describes an experimental program on full-scale measurements that was conducted in order to prove the efficiency of TLDs. Building performance was measured under wind vibration with and without TLDs being installed. Four structures were analyzed (three towers and one hotel). From the analyses of the data, the authors found that the efficiency of the TLDs depends on its mass (compared to the building mass) and on the damping of the sloshing motion. In general, a larger mass provides more damping in the building, with the damping of the sloshing motion considered optimal. This damping can be easily controlled with floating particles, baffles, nets, and other means.

The reliability of the estimation of building acceleration and dynamic properties is an important topic, since in most of cases it is a design factor for tall buildings. For these reasons, in “Full-scale performance evaluation of tall buildings under wind” by Bashor et al. (2012), the importance of full-scale monitoring is emphasized especially in regard to the Chicago Full-Scale Monitoring Program (CFSMP). This program monitors three Chicago buildings under a wide range of wind environment conditions. The results display significant scatters that are due to different factors, and, among all, the most important are: temperature effects, features of wind induced response, and errors in the analysis technique utilized. In particular, the damping estimation is less effective than the natural frequency calculation. As a consequence, the authors suggest that these results need to be analyzed in a meaningful sense and that the variability needs to be carefully evaluated.

In “Mass Dampers and their Optimal Designs for Building Vibration Control”, Chang (1999) describes the control performance of three different damper devices: tuned mass dampers (TMD), tuned liquid column dampers (TLCD) and liquid column vibration absorbers (LCVA). The comparison of the three systems has been carried from a SDOF point of view in a theoretical manner. Two sets of different formulas are proposed and tested under wind and seismic white-noise excitation. The results show that the performance of the mass damper is a function of the efficiency index (i.e. ratio between mass damper and the building), and that the TMD performance control is always better than the LCVA and TLCD. Moreover, LCVA performance is better than the TLCD when the ratio between the vertical and horizontal area of the mass damper is greater than one. When the ratio is less than one, the TLCD performance is better than that of the LCVA.

In “Suppression of vortex-excited vibration of buildings using tuned liquid dampers” (Chang and Gu, 1999), the authors try to use previous studies on TLD to effectively reduce tall building vibrations coming from vortex excitation. The design optimization studies have been validated through several wind tunnel tests. The authors state that if the TLDs are optimally tuned to their damping ratio the vibration suppression will be at a maximum even if, due to scaling factors, this is hard to achieve in wind tunnel tests. Indeed, one of the major concerns with wind tunnel tests and TLDs is that the size of the facilities has to be quite large or the mass of the testing liquid damper needs to be really small (e.g. even smaller than 5gr).

A new type of liquid damper is introduced in the paper “Reducing acceleration response of a SDOF structure with Bi-directional Liquid Damper” (Lee and Min, 2011). Buildings vibrate under along-wind and across-wind directions and, even if they usually do not vibrate simultaneously in both directions, changes in wind direction and torsional vibration would require dampers installed in two orthogonal directions. The authors introduce a single vibration absorber that can control two different vibration modes at the same time. The so-called bi-directional liquid column damper (BLD) is similar to a convectional TLCD, in which in one direction utilizes the column of water as a mass damper system and in the other direction the vertical columns are used as sloshing dampers.  While proposing a methodology, the authors also validate the proposed scheme with shaking table tests of a single-story shear model. They state the importance of fine-tuning between the two directions to avoid the adverse effect of the interaction between the two systems.

Ikago et al. (2012) in their paper “Seismic Control Design of Tall Buildings using Tuned Viscous Mass Dampers” introduce a new tuned mass damper system that could be effective for both wind and earthquake induced vibrations. The system utilizes a rotating viscous mass damper with a soft spring connection to the main building. This system, called a tuned viscous mass damper (TVMD), allows for the creation of a large apparent mass that would be more efficient for earthquake vibration control. The authors propose also a simple response estimation method that is useful for the design of structures with TVMDs.

An innovative structural design solution strategy for tall buildings and damping devices was proposed by Moon (2001) in his paper “Structural Design of Double Skin Facades as Damping Devices for Tall Buildings”. The author outlines a new damping strategy configuration that takes advantage of double skin façade systems that nowadays are very common in tall buildings. Two different solutions are studied. The first one takes into consideration the modification of connectors between the main structure and the façade. They have very low stiffness and additional damping. While a very promising structural solution, this system has serious problems with façade vibration. The second one is investigated to overcome these issues, and it utilizes small masses in the façade cavities. Therefore this technique is similar to a TMD but with the advantaged of utilizing unused space in the building. Moreover, this second system would allow a greater reliability than standard TMD since there are more masses, and it can also be used to get a better control on higher modes of vibration.

One of the first studies on soil structure interaction (SSI) on tall building with damping devices was proposed by Liu et al. (2008) in their paper “Wind-induced vibration of high-rise building with tuned mass damper including soil–structure interaction.” The authors state the importance of understanding the implications of having more flexible and less damping structures constructed on flexible soils. For these reasons, they developed a time domain mathematical model in order to take into account the SSI interaction of tall buildings under wind loads. Several numerical examples were conducted to show the validity of the model. The results demonstrate that SSI cannot be ignored for low soil stiffness, otherwise the induced response will be overestimated and the effectiveness of the TMD underestimated. As a consequence, TMDs are more effective to suppress vibration with higher soil stiffness.

“Seismic retrofit of high-rise building with deformation-dependent oil-dampers” (Aono et al., 2011) treats the problem of long-period ground motion on existing high-rise buildings in Japan. To overcome this problem the authors suggest that the most advantageous solution is to utilize a deformation-dependent oil damper that eliminates the requirements of additional reinforcement in those areas where these devices are installed (which is typical when other devices are utilized). Indeed, this damper limits its force when the frame deformation comes close to its limit. The proposed solution has been utilized in a 54-story office building in Japan (Shinjuku Center Building). Dynamic analyses under long-period earthquakes were conducted and compared with the observed response during the 2011 earthquake off the Pacific coast of Tohoku. The results show the good agreement between the model and the real response.

In “The Damped outrigger concept for tall buildings” (Smith and Willford, 2007), the authors describe a new philosophy for the design of tall buildings with added damping systems. This consists of inserting dampers in the outrigger of a building. One of the main advantages of this system is to increase the damping (around five to 10 percent), and to reduce the variability of the inherent damping.  Viscous dampers are utilized as damping devices, and two different configurations are proposed: dampers at the connection of the external column with the outrigger, and dampers in the coupling beams. Together with these innovative solutions, the authors provide a design procedure for tall buildings that adopt this technology. The paper concludes with an example of a building that is under construction (at the time of the paper) with this technique.

Nonlinear tuned mass damper devices are compared with linear ones in the paper “The behavior of simply non-linear tuned mass dampers” by Vickery et al. (2001). Two simple, commonly utilized non-linear forms are discussed: dry-friction and “velocity squared.” To compare linear and non-linear TMD the latter ones need to be linearized. The linearization assumption is that the individual cycles are sinusoidal with slowly varying amplitude that follow the Rayleigh form associated with a narrow-band Gaussian process. These assumptions were verified with a true non-linear time domain simulation for an example application. The results shows that velocity squared TMD significantly reduce motions with little sacrifice in performance compared to the optimum linear TMD. Instead, friction TMD performs well only around the design point.

Seismic isolation in tall buildings have been not extensively studied but the paper by Tatemichi et al. (“A Study on Pendulum Seismic Isolators for High-Rise Buildings”, 2004) deals with one type of isolator device: the so-called non-parallel swing system. This system is considered suitable for tall buildings because it provides longer periods and it is not influenced by the weight of the building, contrary to standard isolation device solution (e.g. rubber bearing). Indeed, the period of the system depends only on the length of the hanging system. The authors suggest that a possible application of this technology is for the isolation of individual floors. Furthermore, a conceptual scheme for high-rise building is proposed that is tested on a small scale. The results show the potential of the system, although further studies needs to be carried out before a full-scale application can be done.

Kwok, K. C. S., & Samali, B. (1995). Performance of Tuned Mass Dampers under Wind Loads. Engineering Structures, 17(9), 655–667.
doi:10.1016/0141-0296(95)00035-6

Banavalkar, P. V. (1990). Structural systems to improve wind induced dynamic performance of high rise buildings. Journal of Wind Engineering and Industrial Aerodynamics, 36, 213–224.
doi:10.1016/0167-6105(90)90306-W

Au, S.-K., Zhang, F.-L., & To, P. (2012). Field observations on modal properties of two tall buildings under strong wind. Journal of Wind Engineering and Industrial Aerodynamics, 101, 12–23.
doi:10.1016/j.jweia.2011.12.002

Tamura, Y., Fujii, K., Ohtsuki, T., Wakahara, T., & Kohsaka, R. (1995). Effectiveness of Tuned Liquid Dampers under Wind Excitation. Engineering Structures, 17(9), 609–621.
doi:10.1016/0141-0296(95)00031-2

Bashor, R., Bobby, S., Kijewski-Correa, T., & Kareem, A. (2012). Full-scale Performance Evaluation of Tall Buildings under Wind. Journal of Wind Engineering and Industrial Aerodynamics, 104-106, 88–97.
doi:10.1016/j.jweia.2012.04.007

Chang, C. C. (1999). Mass dampers and their optimal designs for building vibration control. Engineering Structures, 21(5), 454–463.
doi:10.1016/S0141-0296(97)00213-7

Chang, C. C., & Gu, M. (1999). Suppression of vortex-excited vibration of tall buildings using tuned liquid dampers. Journal of Wind Engineering and Industrial Aerodynamics, 83(1-3), 225–237.
doi:10.1016/S0167-6105(99)00074-4

Lee, H. R., & Min, K. W. (2011). Reducing Acceleration Response of a SDOF Structure with a Bi-Directional Liquid Damper. Procedia Engineering, 14, 1237–1244.
doi:10.1016/j.proeng.2011.07.155

Liu, M.-Y., Chiang, W.-L., Hwang, J.-H., & Chu, C.-R. (2008). Wind-induced vibration of high-rise building with tuned mass damper including soil–structure interaction. Journal of Wind Engineering and Industrial Aerodynamics, 96(6-7), 1092–1102.
doi:10.1016/j.jweia.2007.06.034

Moon, K. S. (2011). Structural Design of Double Skin Facades as Damping Devices for Tall Buildings. Procedia Engineering, 14, 1351–1358.
doi:10.1016/j.proeng.2011.07.170

Kohji Ikago, Yoshifumi Sugimura, K. S. and N. I. (2012). Seismic Control design of tall buildings using tuned viscous mass dampers. CTBUH 9th World Congress Shanghai 2012 Proceedings, 854–860.

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Smith, R. J., & Willford, M. R. (2007). The Damped Outrigger Concept for Tall Buildings. The Structural Design of Tall and Special Buildings, 16(4), 501–517.

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