Isolation

Isolation pads are used to protect the structure against the effects of Vibration – how well the isolator works depends on its deflection. The greater the deflection, the less the vibration transmission.

There are three classes of vibration isolators which include:

Isolation Pads which are made from either Rubber or Neoprene which are fitted between the source of the vibration and its support. They offer the least in terms of deflection but stacking a number of pads will improve the situation, so they are used mostly to interrupt hard surface to surface contact to reduce noise and other high frequency vibration.

Isolation Mounts are one step ahead of the isolation pad in that they will offer greater deflection.

Spring Mounts offer the greatest deflection for maximum vibration reduction.

The source and location of the equipment in the building is the main deciding factor in selecting either Neoprene of Spring Mounts. If the source is located on a stiff support structure such as a ground supported concrete slab, the Neoprene mounts should be satisfactory – however, if the source is mounted on the upper levels of a structure which are more susceptible to vibration transmission, the Spring Mounts would be advised.

The key to proper isolator selection where the source is mounted on the upper levels of the building is to make sure the deflection is significantly greater than the floor deflection. The importance of location is the same for suspended equipment.

For more information on vibration and its effects go to Vibration

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Damping

Mechanical damping is today often used in the construction of high-rise structures to counterbalance the effects of lateral loads by means of either pendulums or viscous (liquid mass) damping. Pendulums are usually suspended by cables at or near the top of the structure, and when the building sways the pendulum’s inertia acts as a counterweight pulling it in the opposite direction. Viscous dampers work in a similar manner whereby the natural flow of the liquid in a tank, acts as a counterbalance to the movement of the structure.

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‘Tuning’ the mass damper refers to the additional element to limit the pendulum or viscous dampers own motion by means of passive mechanical damping. Tuning is necessary because the frequencies and amplitudes of the damper must match that of the structure, so that when the wind pushes the structure the damper should generate an equal and opposite reaction keeping the horizontal displacement at or near zero. If their frequencies were significantly different the damper could act in synch with the movement of the building, making the situation worse.

The effectiveness of a Tuned Mass Damper 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).

‘Tuning’ a viscous (liquid mass) damper is usually done by means of placing baffles in the tank to control and limit the velocity and mass of the liquid as it moves. Another method is to build the entire tank on wheels while controlling the movement by means of hydraulic damping. These two systems may work together or independently.

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‘Tuning’ a pendulum is usually done by installing hydraulic dampers around the counterweight which must be set and adjusted manually to accommodate the vibration frequencies of the structure.

Another method employed at the Shanghai Tower, called the ‘eddy current damper’ is to place a large copper plate covered with strong magnets under the suspended pendulum, which when it sways induces an electrical current which in turn, creates an opposing magnetic field counteracting the pendulum’s motion.

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