Tuned Mass Dampers (TMD)

In tall structures wind and earthquake induced oscillations are usually common. This may not be powerful enough to bring the whole structure to the ground but it will definitely make the occupants feel uncomfortable and unsafe. In some cases, the horizontal sway at the top of a skyscraper can even reach a displacement of 1 m. Therefore, to minimize the effect of oscillation induced by winds and earthquakes, most of the skyscrapers have a device called a Tuned Mass Damper (TMD). There are two types of TMDs, a Horizontal TMD and a Vertical TMD. Horizontal TMDs are used in skyscrapers and Vertical TMDs are used in bridges, floors and walkways. In old structures, the horizontal TMDs are usually embedded into the structure but it was kept visible for the first time in the skyscraper ‘Taipei 101’. A TMD is an oscillating device with a damper attached to it. Due to the damping action, the oscillation will be reduced to zero in an insignificantly short amount of time, when compared to the time taken for the oscillation to stop under normal damping forces such as air drag and resisting forces.

            When winds or an earthquake hits a structure, an induced force will act on it resulting in an acceleration, which will oscillate the whole structure. The amplitude of oscillation depends on the energy transferred from the blowing wind or from the earthquake. Structures can exert a resisting force (damping action) against the oscillation due to its massive mass such that the structure comes back to its steady state. The damping is governed by damping ratio, which could take any values from 0 and above. Structure without TMD normally have a damping ratio less than one, where damping ratio of zero represents no damping action. When the damping ratio is larger than unity the system will oscillate with a greater period and when it is very high the system comes to rest without having any oscillation. This is known as Over-damping and this is also undesirable in structures. Structures without a TMD will take a considerable time to come to rest, depending on several factors. Therefore, a skyscraper without TMD would oscillate longer than one with a TMD. (Hoang, et al., 2016)

There are three major components in TMDs, they are

1) Spring

2) Oscillating mass

3) Viscous damper.

The energy which is transferred to the structure by a wind or an earthquake would eventually make the TMD oscillate as well, but the oscillation of the TMD would create and opposite force to the force which is applied by the wind or the earthquake. This in turn creates a damping action and brings the structure back to its normal state in a short amount of time. Even though the oscillation exists, since the duration is short, the occupants have reassurance about their safety. (Izat, 2012)

The energy from the building is transmitted to the TMD as Kinetic energy by Resonance phenomena. For the better functionality, the TMD must be tuned to the natural frequency of the structure. The natural frequency of the structure is determined by testing scaled models in laboratories. The dampers are set to the TMDs in calculated strength. Due to the resistive action of the dampers, energy will dissipate faster than normal therefore, the oscillation duration decreases. (Engineer, 2016)

In bridges and floors vertical TMDs are commonly used, because a significant amount of force will be applied on the base due to the static and dynamic forces produced by the users. If we consider the London Millennium Footbridge, it showed unexpected sway on the opening day itself. (Anon., 2008)

During the study, it was identified that the nature of the walking pace of humans cause a lateral force which wasn’t accounted for during the design of these bridges. We humans are capable of balancing ourselves to a certain extent. For example, when walking against blowing wind, the walking pattern changes in order to resist the force exerted on us and to move us forward. During the opening of the Millennium bridge, there was a large number of people walking across it, but, the sway was not a result of number of people because the bridge was designed to hold 5000 people at once and there were only about 2000 people onboard. Later the reason for the sway was identified as the change which occurred in the walking pattern of the people. The people who were on the bridge experienced a small lateral 

sway at the beginning and to overcome that motion, the walking pattern changed inadvertently. Since everyone experienced the same motion, the walking pace of every single person on that bridge resembled each other’s. The footsteps synchronized with the motion of the bridge. This created a resultant lateral force which hasn’t been encountered before. Then bridge was closed for repairs and was later reopened with new dampers installed.  (Cathy, 2015)

The lateral force normally doesn’t exist during a random walk, because when a force is exerted by a person, a counter force would be produced by another person. This would end up in negligible lateral force. This initiated new insights for designing bridges to avoid vibration problems and also introduced new regulations for pedestrians using the bridges.

For example, troops must break step when crossing a bridge.

There will always be unexpected issues to face at work. As engineers we should find feasible and economical ways to fix those problems.

           

R.Subakaran

Department of Civil Engineering, UOM.

References

Anon., 2008. PHYS ORG. [Online]
Available at: https://phys.org/news/2008-12-london-millennium-bridge.html
[Accessed 17 04 2020].

Cathy, 2015. June 10 - The Millenium Bridge Opens. [Online]
Available at: http://every-day-is-special.blogspot.com/2015/06/june-10-millennium-bridge-opens.html
[Accessed 17 04 2020].

Engineer, P., 2016. instruutables workshop. [Online]
Available at: https://www.instructables.com/id/Tuned-Mass-Damper-Demonstration/
[Accessed 17 04 2020].

Hoang, T., Ducharme, K. T., Kim, Y. & Okumus, P., 2016. Structural impact mitigation of bridge piers using tuned mass damper. Engineering Structures, , 112(), pp. 287-294.

Izat, M., 2012. The Constructor. [Online]
Available at: https://theconstructor.org/structural-engg/tuned-mass-damper/1198/
[Accessed 17 04 2020].