Skyscrapers
The tuned mass damper is the most commonly used mass damping system. The main concept in the tuned mass damper is that the damper is tuned to the natural frequency of the building so that it provides counteracting inertia force in the out-of-phase motion and with greater amplitude than the building which leads to dissipation of the building energy. The tuned mass damper is a device consisting of a mass attached to the main structure by a suspension mechanism which includes a restoring mechanism and a damping device. Different suspension mechanisms such as air, viscoelastic and pendulum are used in the tuned mass dampers, but the parallel combination of coil springs and viscous dampers are the most commonly used suspensionmechanism. 16 The most prominent example of this device is the Taipei 101 tuned mass damper. The world’s largest and heaviest tuned mass damper, which is 6 metres in diameter and weighs 660 metric tonnes, is suspended between the 92 nd and 87 th floors on 16 huge steel cables. Rather than being hidden away, which is the traditional approach, the designers decided to use the tuned mass damper as a feature and open it for public viewing. Engineers tuned the frequency of the damper by adjusting vertical locations of the blocks that restrain the suspension cables. To stop the steel ball fromeither rocking excessively or being out of sync with the building, the steel ball is attached to series of hydraulic dashpots, or giant shock absorbers, to resist its movement. Normally, slow sway caused by vortex shedding creates small resistance forces as the steel ball pushes and pulls against the dashpots to convert kinetic energy into heat, while allowing the steel ball to swing like a giant pendulumwhich reduces the amplitudes and accelerations each time it pushes against the dashpots. However, if the tuned mass damper experiences a sudden jolt such as an earthquake, the hydraulic resistance in the dashpots, which are full with oil, will rise drastically and create a ‘lockdown’ e ffect which limits the motion of the mass. 17 The Taipei 101 tuned mass damper is a passive damping system which means that the system does not have a power-drive mechanism or an outside energy source and will absorb energy when it acted upon by wind-loads. On the other hand, the active tuned mass dampers provide more damping using the same mass and can be tuned to different frequencies in different directions by the active control system. This means that they can be used in skyscrapers that cannot support the weight of amassive passive tunedmass damper. However, they are more complex to build and install. 18 The tuned liquid dampers, where the solid mass is replaced by a liquid, are further divided to tuned sloshing dampers and tuned liquid column dampers. Both systems work in a very similar way as they use liquid waves or liquid column motion to absorb energy from vibrating buildings in partially filled tanks of liquid (usually water). The oscillating frequency of the liquid in the tuned liquid damper is determined by the density of the liquid and the tank dimensions which are designed so that the surface wave or liquid column motion has a natural frequency close to the fundamental frequency of the building. Energy is dissipated from the liquid due to turbulence caused by flow-damping devices such as screens, louvers, or posts placed in the path of the motion of the water. 19 The example of the tuned liquid column dampers can be seen in RandomHouse tower inNewYork. It uses themotion of a column of liquid, which rises and falls in opposite upstanding legs, in a U-shaped tank to counteract the forces acting on the building. Damping is introduced when an oscillating column of the liquid passes through an orifice which contains adjustable louver blades to resist the liquid motion and convert the energy of moving liquid into heat. The amount of damping can be changed by varying the louver blade positions
16 DEICON 2016. 17 Poon et al. 2004: 275-276. 18 DEICON 2016. 19 Robinson, Gamble and Myslimaj 2007: 14-15.
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