Joint WSP/RWDI presentation on the design of super-slender towers
This joint WES-IstructE event featured a summary of the wind engineering and structural principals used by RWDI and WSP in their collaborative super-tall building projects across the world.
As new buildings are becoming taller and skinnier, their natural periods of vibration are getting longer, with low inherent damping characteristics. Traditional structural and wind engineering principals alone are not always sufficient to achieve the desired wind performance targets on such buildings, particularly in terms of motion comfort (acceleration) and drift. Many such slender towers feature aerodynamic or innovative damping solutions (fun fact: only 4 buildings in NY featured dampers prior to 2006, with 27 buildings incorporating dampers since then).
A particular example of an innovative design was 432 Park Avenue in New York, which features a slenderness ratio of approximately 15. The uniform shape of the tower and its exposure give rise to substantial vortex-induced vibration problems, which are dealt with using two innovative 660 ton dampers, as well as aerodynamic modifications to reduce the coherence of vortices.
An interesting feature of slender buildings like the 432 Park Ave is that the critical wind effects occur at very low return periods, in the region of 1 month average occurrence rates, due to the dynamic and aerodynamic properties of such towers. Most acceleration criteria are geared towards limiting 1yr, 5yr or 10yr return acceleration values, and extrapolating these to 1yr return may not be appropriate. Therefore a careful review of the acceleration criteria is required for such towers.
Higher mode participation in the wind effects, storey drifts (and associated noise and cladding design problems), fatigue concerns and even visual perception considerations are other interesting factors that can become critical for very slender towers.
Some of these towers, including the 432 Park Avenue, are being monitored by RWDI to assess the performance of the dampers, and to develop more robust solutions for other projects in the future.