Introduction.
Vibration (vibration) control (damping) technology is used to reduce vibration (sway) in buildings and other structures in addition to vibration control technology used in earthquakes. This section focuses on vibration control and vibration control devices for environmental vibrations other than earthquakes. We are surrounded by a wide variety of vibrations (shaking), but what are the types of vibrations for which vibration control devices are effective? The key points will be explained in an easy-to-understand manner.
Cases where vibration control devices are effective
Introduction to Vibration Control (1)Seismic dampers, one of the vibration control devices introduced in Section 2.1, are only effective in large vibrations such as those caused by earthquakes. Therefore, the vibration control devices here are mainly mass dampers.
Mass dampers are effective against the natural frequencies of structures and the swaying around them, or resonance. Understanding this property is the key to effective use of vibration control technology.
Natural frequency and resonance
The natural frequency is the frequency of vibration at which a structure is most likely to sway, expressed in units of [Hz] (hertz). At the natural frequency and its surrounding frequencies, the structure tends to sway easily, and even weak forces can cause it to sway greatly. This is called resonance.Resonance, the phenomenon of increased shaking - a comprehensive guide.(Please refer to the following table).
Example: Vibration of a pedestrian bridge
In the case of a pedestrian bridge, the typical natural frequency of vibration is that the bridge portion vibrates in the vertical direction, as shown in Figure 1. Suppose the natural frequency of this pedestrian bridge is 3 Hz. The 3 Hz component of the force generated by vehicles passing on the road or people walking on the pedestrian bridge will cause the pedestrian bridge to resonate, resulting in significant swaying. When crossing a pedestrian bridge, the uncomfortable fluffy swaying is often due to resonant natural frequency swaying.
In the case of vibration problems caused by such resonance, the installation of mass dampers will likely reduce vibration and solve the problem.
Cases where mass damper is not effective
In general, most cases where vibration is a problem are caused by resonance, but some are not. In such cases, mass dampers are not expected to have much of a reduction effect.
Example: building vibration and electric drill vibration
Let us consider a model in which a mass damper is installed on the roof to reduce the lateral sway of the entire building caused by road traffic vibration (let us assume a natural frequency of 1 Hz).
When traffic vibration, including building natural frequency vibration, is input from the ground, it resonates with the building's natural frequency and becomes a large vibration. In this case, mass dampers are effective.
On the other hand, the vibration component of the electric drill contains almost no vibration of the natural frequency of the building, so resonance does not occur. The higher frequency component of the electric drill's vibration is simply transmitted. For these non-resonant vibrations, vibration control devices are ineffective.
Frequency response characteristics of vibration
The graph in Figure 3 is called the frequency response characteristics of vibration, and it shows the building's tendency to sway as described in the previous section, for each frequency. The frequency on the horizontal axis represents the period of shaking, with slow shaking on the left and faster shaking as one moves to the right. The vertical axis is a value representing the ease of shaking at each frequency, with larger values indicating greater ease of shaking.
The blue line shows the case of resonance without a mass damper, and the red line shows the case where a mass damper is installed. It can be seen that the installation of the damping device makes the area around 1 Hz, which was originally prone to swaying, a little less swaying. On the other hand, away from this peak, the blue and red lines overlap, showing no effect from vibration control.
Mass damper application examples
Mass dampers, TMDs and AMDs, can be applied to a wide range of vibrations, from minute tremors such as environmental vibrations to large tremors caused by strong winds or earthquakes. They can also be applied to both horizontal (lateral) and vertical (longitudinal) vibration.
TMDs are passive vibration control devices used to reduce structural sway.
AMDs are electrical vibration-damping devices that use active control. It requires a power supply and electrical components and requires maintenance. However, it has the same vibration-damping effect as a TMD with approximately 10 times the weight, and can be installed in locations with severe load and footprint requirements. It can also be effective for multiple natural frequencies.
