ISAAC’s technology: how do active mass dampers work?
All structures, from the terraced houses of the city suburbs to the taller and taller skyscrapers of the financial districts, from motorway viaducts to the longest suspension bridges in the world, are subject to vibrations of various kinds and magnitude. When we think of swaying phenomena affecting structures, earthquakes are undoubtedly the first things that comes to mind.
The action of an earthquake, however, is not the only phenomenon that can induce significant vibration phenomena. Consider, for example, the wind that has an extremely important effect on tall and slender structures, or the vibrations induced by heavy transport traffic, such as lorries and trains, a problem well known to those who live near an underground line in a big city.
The reasons, therefore, for reducing the swaying that affect a structure can be different and range from comfort inside environments, to the protection of vital systems, to the protection of structural elements and protection from earthquakes in general.
Active Mass Damper (AMD)
An AMD is an active vibration damper, a device that was created precisely from the need to eliminate, or reduce, the oscillations structures are subjected to. It is a swaying mass, free to move in one or more directions, actuated by a mechanical actuator capable of generating high forces, installed on a structure in order to reduce its oscillations in the presence of an oscillation induced, for example, by seismic action.
The force generated by the AMD depends on the real-time reading of the accelerometer sensors installed at crucial points on the structure and follows a control rule defined during the design phase, which allows for achievement of the desired vibration reduction.
To better understand the principle of operation from a comparison of the AMD and its “passive” cousin, the TMD.
The “passive” cousin: the TMD
The TMD is an oscillating mass capable of discharging forces on the structural elements it is anchored to via a spring-damper connection.
The TMD allows for “capturing” the motion energy of the structure around its resonant frequency. To work with good efficiency, the TMD needs to be “calibrated” in terms of mass and stiffness in such a way as to “couple” appropriately with the natural resonant frequency of the structure it is installed on.
The most famous example of a TMD in existence is on the Taipei 101 skyscraper in Taiwan, where a 660-ton pendulum equipped with a total of eight huge hydraulic dampers has been installed.
The advantages of the active mass damper
In this sense, the main advantage obtained from using an active mass damper compared to a tuned mass damper is that the former does not require ad hoc design for each structure it must be applied to. Not to mention that in the event of damage to the structure itself, its natural frequency can change over time, making the action of the TMD less effective, almost totally cancelled out, due to very significant changes in the building’s dynamics.
In addition to the great flexibility with which an active mass damper is able to adapt to dynamic changes in structures, another great advantage obtained by using this technology lies in the forces generated with the same installed mass. The installation of large masses at the top of structures involves considerable problems of a design and structural nature. The forces that can be generated with an active mass damper do not depend on overall mass, but only on the nature of the actuator with which it is moved.
The I-Pro 1, the first active mass damper developed and tested by ISAAC Antisismica, has a total mass of 4 tons (of which 2.2 tons of mobile mass and 1.8 tons of fixed mass) and, thanks to a hydraulic actuator that works with pressures up to 280 bar, is able to generate horizontal thrusts exceeding 20 tons of force.
Figure 2 – I-Pro 1 installed at the Polytechnic University of Milan for functional tests
Electro-Pro20x, another AMD also developed at ISAAC Antisismica’s laboratories, meets the need for extreme simplicity of installation in the field. With a fixed mass anchored to the structure that varies between 100 and 200 kg (depending on the overall length) and a mobile mass that can be adjusted between a minimum of 250 kg and a maximum of 1,000 kg, it can be fully assembled at the installation site. The actuator is a linear electric motor capable of developing horizontal forces greater than 20 kN (2 tons of thrust).
Figure 3 – Electro Pro 20x installed at ISAAC Labs for performance testing
The device’s compactness has allowed for preliminary commercial applications of the technology which, until now, due to high costs and technical difficulties, had only been applicable in theory.
Author: Stefano Cii
