Land parcels in the world’s most popular cities are getting scarcer, yet the appetite and market remains for more skyscrapers.
New technology combined with engineering innovation is helping developers meet the demand for more skyscrapers on even the tightest and most compact of sites.
Invariably, this trend for taller, more slender buildings, combined with the implicit challenges of making these buildings stable and serviceable, has necessitated the development of structural solutions that meet all of the governing relevant building codes with even more onerous wind, earthquake and robustness requirements. These more complicated sites are requiring very sophisticated and innovative structural solutions.
Melbourne has seen a surge of skyscraper projects. Five tall buildings were recently approved on what has come to be known as "Super Tuesday" adding to the 31 tall buildings already currently under construction or topped out in the Australian city, with a further 43 skyscraper projects proposed or recently approved.
Prima Pearl and 568 Collins Street are two projects currently under construction which fall within the super skinny category.
Optimising the building stability solution to meet strength and serviceability requirements is a challenge owing to the slenderness involved.
On Prima Pearl, engineers were able to incorporate a number of very stiff shear walls up through the majority of the building height to complement the core stiffness. On 568 Collins Street, also engineered by Meinhardt, this was not possible as the number of shear walls required to be effective could not reach a sufficient height of the building.
Instead, the solution adopted involved a core linked at two levels to four large outrigger columns - two on both the east and west sides of the building. These columns are then further coupled to the boundary walls on the east and west sides within the podium (the bottom 11 levels) to provide more stiffness in the weak axis direction.
Optimising the building stability solution to meet occupant comfort requirements in terms of perceived movement (horizontal accelerations from high wind activity) is also another major consideration for super skinny skyscrapers. At 568 Collins Street, the provision for a tuned liquid damper tank at the top of the building will reduce acceleration under wind load should testing of the near-completed building show that additional damping is required.
Overall, the engineers have sought to optimise the structural solution of both towers by precluding or minimising the need for transfer beam structures which are very expensive and slow to build and which take up valuable height in the building.
Of course, when it comes to tall buildings, New York is the mecca, as well as a city where the super skinny trend is even more prevalent.
When completed, One 57 will be one of the tallest residential buildings in New York. The 1,005-foot concrete-structured tower will include 95 luxurious condominiums and a Park Hyatt hotel.
Engineers WSP has achieved the tower’s slenderness and unique shape by using the latest design technology and the most advanced high-performance materials. Concrete with a strength in excess of 12,000 psi (83MPa) was specified for the shear walls and columns.
It will be eclipsed however by 432 Park Avenue, which upon completion is expected to be the second-tallest building in New York City and the tallest residential building in the Western Hemisphere.
Also engineered by WSP, the structure consists of an architecturally-exposed concrete tube system, coupled to a central core with concrete strength of more than 14,000 psi. A flat slab connects the core to the perimeter with no interior columns, giving interior space planning the maximum possible amount of flexibility.
Given the tower's height and aspect ratio of more than 15:1, the efficiency of the structure is of the utmost importance in order to create a structure that works harmoniously with the architecture. To that end, every inch of concrete is utilized to provide lateral stability and maximize efficiency.
To control the perception of lateral motion under high wind conditions, a series of openings throughout the structure have been used to improve its aerodynamics, the design of which was achieved by means of wind-tunnel testing. In addition, two tuned-mass dampers were designed to further reduce lateral acceleration to acceptable amount for the comfort of occupants.
The crown for skinniness, however, goes to 111 W. 57th St., designed by SHoP Architects, with structural engineering again by WSP Group. With a base width to height ratio of 1:23, the 1,350-plus-foot tower will be the most slender building in the world.
To stiffen the structure, the shear walls that run the full length of the east and west exterior walls have been thickened to as much as three feet and then linked to the core with deep beams. These stiffening beams are concealed above the ceilings, something that has been easy to achieve due to the generous 15 ½-foot floor-to-floor heights.
As developers continue to look to squeeze as much value out of sites as they can, engineers are continuing to rise to the challenge.