For years, engineers and building scientists have relied on tuned mass dampers (TMDs) and tuned sloshing dampers (TSDs) to create some of the most creative and functional high-rise building designs.
But as the demand for more complex design and taller structures increased – combined with a rise in extreme weather conditions in many areas – those involved in tall building design have had to look for alternative solutions to meet their design requirements.
Recent research has shown that performance can be improved when a building is designed with more than one tuned sloshing damper, by tuning the tanks to slightly different frequencies near the structural mode being controlled, instead of having all tanks designed to identical frequencies.
TSDs typically consist of one or more tanks installed near the top of the building that are partially filled with a liquid, which is usually water. As the building begins to move, the liquid in the tank begins to slosh. The dimensions of the tank and the liquid depth are selected (this is the “tuning” part) so the natural sloshing frequency of the TSD will closely match the natural frequency of the structural vibration mode being controlled. The resulting coupled structure-TSD system modifies the mechanical admittance function of the structure, dramatically reducing its resonant response. Loosely translated, that means the vibrational energy from the structure is transferred to the sloshing fluid, where it is dissipated through damping mechanisms within the tank. The more the liquid mass participates in the sloshing, the more the structural motion is reduced.
Traditionally, TSDs have consisted of a single tank tuned to the natural frequency of the structure according to a well-known theory originally developed in the 1980s for tuned mass dampers. When multiple tanks are used in lieu of a single larger tank, they usually are designed to have an identical natural sloshing frequency and damping. This allows a multiple-tank TSD system to be modeled as a single large tank, thus simplifying the model, but introducing a bit more complexity can be well worth the effort.
Theoretical research in the 1990s demonstrated that tuning multiple tanks to different frequencies distributed near the structural frequency of the building’s targeted mode could improve both the performance and robustness of the damping system. Admittedly, modeling each of the tanks separately instead of modeling the entire installation as a single equivalent mechanical damping system made solving the system of equations in the frequency domain more tedious, but no less straightforward.
This is where the passage of time has made the analytical challenge much less daunting for contemporary researchers. Computing capabilities have grown by leaps and bounds since the ‘90s, of course, and by using several daunting equations of motion developed over just the past few years, researchers are gaining confidence in their ability to design these more sophisticated damping systems. They’ve also given them their own new, more descriptive name: structure-MTSD (multiple tuned sloshing damper) systems. And fortunately, in practice, dissimilar tuning for an MTSD system is easily accomplished by having slightly different tank lengths or maintaining slightly different water depths in each tank.
A great example of this is a 56-story building in Toronto, Canada, which was constructed with a three-tank MTSD system. Wind tunnel testing had shown the greatest concerns with wind-induced motion was along one axis of the building. However, structural monitoring for two months in early 2017 — when the building was nearing completion but before the tanks had been filled — confirmed the analytical predictions of the structure’s natural frequencies, raising additional concerns about motion along the building’s other primary axis and torsion as well. Beyond that, the building’s inherent damping was relatively low, so the MTSD performance would be important.
Realizing the opportunity to test the effectiveness of dissimilar tuning, researchers used the equivalent mechanical model previously discussed to investigate how the MTSD design with dissimilar tuning would compare to the traditional (single-tuned) TSD system having the same tank plan dimensions.
To make a long story short, the theoretical acceleration reduction produced by the MTSD was greater than that of the traditional TSD over the entire range considered. Construction continued, with the developer and design teams confident that the implementation of the MTSD system would provide a superior experience for building occupants when compared to the traditional, single frequency tuned TSD.