Innovative TMD design addresses wind-induced motion without sacrificing premium space.
The 1396-foot (426m) 432 Park Avenue in the center of Manhattan became the tallest residential tower in the western hemisphere and the third tallest building in the U.S. when it was completed in December 2015. More than twice the height of even its tallest neighbors, the structure expresses architect Rafael Viñoly’s vision of pure pragmatic form – a square shape, extruded straight into the sky, the exposed concrete simply and elegantly expressing pure structure.
Various codes of practice have guidelines that identify any structure with a height-to-width aspect ratio over 4:1 or 5:1 as being sensitive to dynamic wind effects due to vortex shedding. The 432 Park Avenue aspect ratio is 15:1, so even for regularly occurring winds sway control measures were clearly indicated. Our experts on the project team knew that owners and occupants making a premium investment in this “estate living in the sky” would appreciate their efforts to control wind effects and they were fully up to the task.
Through one of our shaping workshops, we explored the concept of opening up some floors to allow wind to pass through the building and break up the coherence of the vortices while maintaining the building’s pure prismatic form. Extensive wind tunnel testing showed that opening up two adjacent floors at five locations evenly spaced throughout the building’s height would be the optimal arrangement to significantly reduce cross wind oscillations. Even with these design solutions, however, there was still a need to further reduce lateral accelerations to achieve the project’s performance goals in terms of motion.
Our wind tunnel testing provided information about how the building would perform in a variety of extreme windstorm scenarios. Using that information, we determined that a 1200-ton tuned mass damper (TMD) system would keep the motion of the building within acceptable levels. Such a system would also be the largest TMD we had implemented to date.
Tuned mass damping is a passive technology that is tuned to swing at approximately the same rate as the building’s natural frequency. A TMD reduces the movement of the building and improves comfort of the occupants. Generally placed near the top of the building, for maximum effectiveness, the TMD physically “pulls” on the building and dissipates energy associated with vibrations of the building.
A pendulum is the simplest implementation of a TMD. However, to directly match the frequency of 432 Park Avenue would have required a pendulum about 120 feet long, roughly eight stories, taking up space that would be unusable for other functions. Instead, we chose an opposed-pendulum design that reduces the needed vertical space by slowing the swing rate of the TMD while maintaining the same pull on the building. Even that required creative implementation because the only locations available for the TMD were two long and narrow spaces on each side of the tower core.
To fit the TMD into the available space, we modified an innovative opposed-pendulum design concept we previously had implemented on other projects. This approach uses a combination of traditional and inverted pendulums, but for 432 Park Avenue we reconceptualized the design in two ways. First, we divided the required 1200-ton mass into two identical 600-ton TMDs, locating one on either side of the core. Second, we configured the twin TMDs such that each has two conventional pendulums on either side of an inverted pendulum, allowing the TMD to swing at a slower rate but with sufficient force for good building performance. Linking these three pendulums in this way resulted in a stable system that matched the tower’s frequency and worked well with the available long and narrow floor space.
This design innovation achieved performance objectives for occupant comfort. As customized for 432 Park Avenue, the TMDs we designed fit within three stories, instead of the eight stories that would be required for a conventional design, while still allowing travel up to 136 inches peak-to-peak in any direction to effectively control building sway. This innovative TMD design enabled the designers of the building to address wind-induced motion issues without adding expensive structural materials or sacrificing valuable penthouse-level interior space.
Furthermore, with its multipart arrangement, this innovative and proven TMD design concept can be modified for other projects to accommodate various sizes and configurations of available floor space.