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Advances in almost any field rarely happen until somebody breaks the rules. It takes some daring, creative individual to say, “We don’t actually have to do it that way,” and then try a different approach.
But breaking the rules can be risky. You may have heard the well-known quote by Thomas Edison that goes something like, “I haven’t failed. I’ve just found 10,000 ways that won’t work.” When developing an innovative new idea, you have to be prepared for outright failure, of a sort – but failure is not really an option for structural engineers.
Thankfully, structural engineering, and specifically controlling building motion, is one of the fields that has benefitted from the continued development of computer modeling and analysis capabilities and from the insightful experimentation of building scientists, engineers, architects, and others. This means the field can reduce risk while still allowing for the rule-breaking necessary for innovative development.
Wind-induced motion has been a problem since the inception of skyscrapers. But as high-rise buildings grow even taller and ever more slender, wind effects become more pronounced, thus creating new sets of challenges. Most structures are engineered to move, but building occupants quickly get uncomfortable when a building starts to sway back and forth or otherwise move around too much. And yet, the demand is for buildings to get lighter – and go higher.
Our capacity to predict building movement under different environmental conditions is continually improving, thanks largely to ongoing improvements in a variety of technologies. This is emboldening creative engineers to consider new solutions to old problems.
The challenge
Traditionally, engineers have tried to control excessive structural movement either by adding mass to the building, by making it stiffer, or both. However, the idea of countering a building’s motion with a moveable mass that acts in opposition to the force of the wind has grown in popularity as more installations are proving to be effective.
The idea is to select the appropriate mass and place it in a way that will work against the wind-induced flexure of the structure. When introduced in the early 1980s, these early devices were christened ‘tuned mass dampers’ (TMDs) and consisted of large, custom-sized weights installed in the upper levels of high-rise buildings. They were designed essentially to absorb and dissipate the energy accumulated in the structure’s springiness.
The concept was soon modified to use sloshing tanks of water – called ‘tuned sloshing dampers’ (TSDs)—which were arguably just as effective and could be less costly.
At the risk of oversimplifying this rather amazing technology of tuned dampers, suffice it to say that until lately these were the only two choices from which building scientists and engineers could select. However, recent research has shown that when a building is designed with more than one tuned sloshing damper, performance can be improved by tuning the tanks to slightly different frequencies near the structural mode being controlled, instead of having all tanks designed to identical frequencies.
With this new multi-tuned damper approach, engineers, designers, or building scientists should theoretically be able to tune structural damping devices even more precisely (and even modify that tuning over time as the structure loosens up). But does the concept hold up in the real world? Is it worth all the fuss?
Theory meets real world and breaks with tradition
Yes, and yes. We know this based on real-world data collected from the first-ever installation of ‘multiple tuned sloshing dampers’ (MTSDs) that were designed to be dissimilarly tuned.
Research that compared data from before and after MTSD systems were commissioned, show clearly that the building accelerations are markedly decreased by the presence of the MTSD. Even more encouraging is the fact that at higher wind speeds—above 5 to 10 meters per second—the 50% acceleration reduction predicted theoretically appears consistent with the building’s observed accelerations. This long-term structural monitoring data suggests that the MTSD system is effective in reducing motion and maintaining the serviceability performance of the building in line with the design criteria.
And to think, all of this became a reality because someone, perhaps inspired by Mr. Edison or other innovators, was willing to question tradition and try a different approach.
A much more detailed description of both the virtual and real-world processes and results was presented at the Building Tomorrow’s Society conference in June 2018 in Fredericton, Canada. Read the full presentation, “Full-Scale Monitoring of a Tall Building Equipped with an Efficient Tuned Sloshing Damper System” online here.