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Excessive wind-induced motion is an increasing concern for tall buildings. This is because of several factors, such as their lightweight construction, increasing height, slenderness, and low inherent damping. Excessive motion can cause significant discomfort for tenants, particularly on the top floors of a building. Other effects of excessive wind-induced motion include issues with elevator function as well as creaking or groaning noises.
This is precisely why tall buildings are using tuned mass dampers (TMDs) at a growing rate to control wind-induced motion.
What Are Tuned Mass Dampers?
Tuned mass dampers are mechanical devices that control the motion of tall buildings under dynamic loads, such as wind. Typically installed at the top of a building, the tuned mass damper will move out-of-phase with the building’s motion, provided force that directly opposes it. The structural vibrational energy will transition from the building to the damper, where it will dissipate and increase the building’s damping.
How Friction Impacts the Performance of Tuned Mass Dampers
In the theoretical world, we do not need to worry about friction and can model a damper quite easily. We can simulate the ideal conditions and ideal damper performance as the variables for doing so are easily controlled.
However, tuned mass dampers experience friction in the real world. This is because dampers are composed of a variety of mechanical components, such as cables, springs, rollers, and viscous damping devices. All the mechanical components experience some level of friction in the real world, and this effect is passed on to the entire damper. Thus, dampers must overcome this friction before they can move relative to the building, which is what they must do to perform properly.
Accounting for Friction in Tuned Mass Damper Design
Accounting for friction is a critical component in the effective design of tuned mass dampers, as friction has a significant impact on a damper’s response to motion.
Friction adds additional damping to the system. This must be accommodated, as too much friction damping can move the TMD away from the optimal damping desired. During light winds, friction can be the dominant form of energy dissipation in the damper, which generally results in too much energy dissipation.
In other cases, friction can lock out the motion of the damper, specifically if there is a low level of excitation. Low levels of excitation occur under light wind events where the building may not move much, causing the damper to be unable to overcome the forces of friction. When this happens, the damper will not move relative to the building, and thus, will not add any supplementary damping.
How Much Friction Is Okay?
Even a small amount of friction can have a significant effect on the performance of a damper. The amount of friction considered okay can vary depending on the amount of motion or acceleration that must be controlled. The easiest method of quantifying the acceptable level of friction is to identify the acceleration at which the damper would break free and begin to move.
Many designers may believe the damper will function as intended as soon as it overcomes the forces of friction and as a result, only needs to break free at accelerations that are less than those considered problematic. One example of this assumption is if tenants of a building feel accelerations of 5 milli-g, the damper will still perform well if it breaks free at accelerations of less than that. Unfortunately, this is not enough. Friction will remain a dominant force over damping and stiffness and continue to compromise the damper’s performance significantly.
The friction in the damper should be overcome at one-tenth of the acceleration you are attempting to control. So, if you are trying to control a building’s acceleration of 10 milli-g, which is 10/1000 of gravitational acceleration, you want the damper to overcome friction at a force no greater than 1 milli-g
Clearly, unintended friction has a dramatic impact on the performance of tuned mass dampers in the real world and therefore, must be accommodated. For more information, check out the previous chapter in our Damping Explained series here.