Tuned Mass Dampers

A tuned mass damper, also known as an active mass damper (AMD) or harmonic absorber, is a device mounted in structures to prevent discomfort, damage, or outright structural failure caused by vibration. They are frequently used in power transmission, automobiles, and buildings.

How Tuned Mass Dampers Work

A tuned mass damper (TMD) consists of a mass (m), a spring (k), and a damping device (c), which dissipates the energy created by the motion of the mass (usually in a form of heat). In this figure, M is the structure to which the damper would be attached.

From the laws of physics, we know that F = ma and a = F/m. This means that when an external force is applied to a system, such as wind pushing on a skyscraper, there has to be acceleration. Consequently, the people in the skyscraper would feel this acceleration. In order to make the occupants of the building feel more comfortable, tuned mass dampers are placed in structures where the horizontal deflections from the wind’s force are felt the greatest, effectively making the building stand relatively still.

When the building begins to oscillate or sway, it sets the TMD into motion by means of the spring and, when the building is forced right, the TMD simultaneously forces it to the left.

Ideally, the frequencies and amplitudes of the TMD and the structure should nearly match so that EVERY time the wind pushes the building, the TMD creates an equal and opposite push on the building, keeping its horizontal displacement at or near zero. If their frequencies were significantly different, the TMD would create pushes that were out of sync with the pushes from the wind, and the building’s motion would still be uncomfortable for the occupants. If their amplitudes were significantly different, the TMD would, for example, create pushes that were in sync with the pushes from the wind but not quite the same size and the building would still experience too much motion.

The effectiveness of a TMD is dependent on the mass ratio (of the TMD to the structure itself), the ratio of the frequency of the TMD to the frequency of the structure (which is ideally equal to one), and the damping ratio of the TMD (how well the damping device dissipates energy).

Wide span structures (bridges, spectator stands, large stairs, stadium roofs) as well as slender tall structures (chimneys, high rises) tend to be easily excited to high vibration amplitudes in one of their basic mode shapes, for example by wind or marching and jumping people. Low natural frequencies are typical for this type of structures, due to their dimensions, as is their low damping. With GERB Tuned Mass Dampers (TMD), these vibrations can be reduced very effectively.

The TMD may consist of:

• Spring
• Oscillating Mass
• Viscodamper

Fig. Tuned Mass Damper

as main components, or may be designed as a pendulum, also in combination with a Viscodamper.
Each TMD is tuned exactly to the structure and a certain natural frequency of it. Such TMD have been designed and built with an oscillating mass of 40 to 10.000 kg (90 to 22.000 lbs) and natural frequencies from 0.3 to 30 Hz. Vertical TMD are typically a combination of coil springs and Viscodampers, while in case of horizontal and torsional excitation in the corresponding horizontal TMD the coil springs are replaced by leaf springs or pendulum suspensions.

Fig. Amplitude – Frequency response of a low damped system without (blue) and with (yellow) tuned mass damper

Tuned mass dampers are mainly used in the following applications:

• Tall and slender free-standing structures (bridges, pylons of bridges, chimneys, TV towers) which tend to be excited dangerously in one of their mode shapes by wind,

• Stairs, spectator stands, pedestrian bridges excited by marching or jumping people. These vibrations are usually not dangerous for the structure itself, but may become very unpleasant for the people,

• Steel structures like factory floors excited in one of their natural frequencies by machines , such as screens, centrifuges, fans etc.,

• Ships exited in one of their natural frequencies by the main engines or even by ship motion.

TMD may be already part of the structure’s original design or may be designed and installed later.

Taipei structure has TMD of weight 730 tonnes which is show in below figure: