A unique shape and an unusual mechanism for controlling wind-induced motion in the world’s second-tallest building
When completed in 2016, the 128-storey Shanghai Tower became the second-tallest skyscraper on earth, after the Burj Khalifa. In addition to its height and LEED Platinum certification, the tower is known for its unusual double-façade design: it’s composed of nine cylindrical segments stacked on top of each other, all encased in a single, twisting glass sheath.
The building’s exceptional height would pose wind engineering challenges regardless of where it was situated. But in the case of Shanghai Tower, engineers also had to consider its location in a seismic zone and the fact that typhoons are regular occurrences in the region. Extensive wind tunnel testing and the optimization of the building’s aerodynamic form by RWDI in collaboration with the design team helped to mitigate most of the wind-induced motion, but Shanghai Tower’s designers approached us in search of additional help. They wanted a tuned mass damper (TMD) for the building that would not only provide an extra guarantee of occupant comfort on the upper floors, but also serve as a tourist attraction.
Our damping systems team has designed dozens of TMDs for buildings around the world. One core property of these passive devices is always the same: they include a mass that’s designed to move back and forth, opposing movement in the larger structure (like a building or a bridge) they’re intended to steady. The Shanghai Tower TMD was no different – but it had a couple of special features: one aesthetic, and the other technical.
On the aesthetic side, parts of the TMD – notably the cables that suspend the thousand-tonne steel mass – are visible from an observation area. The device extends through five floors at the top of the building, and designers chose to expose some of its mechanics to highlight the tower’s remarkable engineering.
Another notable feature of the TMD is how its movement is controlled. In tall buildings, TMDs are essentially pendulums: they’re designed to swing in a way that counteracts the buildings’ own wind-induced swaying movement. But the mass’s swinging motion must be properly controlled to ensure that the building’s sway is minimized. Typically, we use hydraulic systems, which are similar to shock absorbers, to constrain the movement of the TMDs and draw energy out of the system.
The Shanghai Tower TMD uses some hydraulics to control extreme amplitude motions, but it primarily uses another unique mechanism: eddy current. RWDI worked with the Shanghai Research Institute of Materials, the TMD supplier and eddy current developer, to optimize what is the largest application of eddy current damping ever used on a TMD. The bottom of the steel pendulum is fitted with 1800 brick-sized neodymium iron boron magnets. A large sheet of copper sits on the floor over which the pendulum is suspended. The copper neither attracts nor repels the magnets and doesn’t make physical contact with them, but electromagnetic effects create a drag force as the pendulum moves laterally over the copper. Just as a hydraulic piston can be adjusted to create more or less resistance, the amount of frictionless drag this set-up creates can be adjusted by varying the distance between the magnets and the copper sheet.
This innovative TMD delivers a 45% reduction in building acceleration under windy conditions.
Shanghai Tower was completed and opened to the public in 2015, and now functions as an office tower and tourist destination. Its TMD is performing well, providing a damping effect that means most people will not be able to detect movement at the top of the tower. Their only clue that the tower is moving will be watching the gentle sway of the TMD if they visit the deck from which it can be observed in action.