432 Park Avenue, Manhattan

Tall storeys: building super-slender skyscraper homes

Image credit: Alamy, Getty Images, Newtecnic

Residential towers with extreme width-to-height ratios could define the dimensions of high-rise homes as architects and engineers learn how to build on constrained urban plots.

‘Slender’ and ‘super-slender’ skyscrapers that have arisen across New York’s most exclusive thoroughfares provide luxurious pads for those of far from slender means: Dell Technologies CEO Michael Dell reportedly paid $100.5m for the two-storey penthouse of the 75-storey, 306m-high One 57 (aka the ‘Billionaire Building’) at 157 West 57th Street in Midtown Manhattan.

These new towers, some standing on footprints as narrow as 14m, offer more than just ritzy residences with upmarket zip codes. Their slenderness guarantees spatial exclusivity – rarely more than two apartments per level – and dazzling vistas of the Big Apple. “More than anything, their location is predicated on views of Central Park,” says Carol Willis, founder/director/curator at the Skyscraper Museum. “In New York views have value, and the gold standard is Central Park.”

The prospect of living in such edifices might seem so much pie-in-the-sky musing for the unmoneyed masses. But cities the world over face increasing demand for residential buildings that can use parcels of space too small for traditional apartment blocks. Building tall and thin has previously presented too many challenges to make them economically viable, but now these vertical enclaves – funded from the expectation of assured purchases by rich buyers – could become the shape of high-rise living to come.

“As land becomes scarcer in our cities, developers search for ways to innovatively accommodate growing communities within reach of existing infrastructure,” says Jeff Brown, principal at architects Rothelowman. “Expensive to create and commercialise, slender towers must achieve tight feasibility width-to-height ratios in excess of 1:12, despite highly-complex wind loadings, planning, engineering and site-specific matters – [a combination of factors] often enough to put off [all but] those with the deepest pockets.” Deep pockets are necessary because slender skyscrapers incur high development and build expenses: One 57, for instance, cost $1.5bn.

“Super-slender towers are expensive to build. It took a New York price platform of $3,000 per-square-foot to start to make their basic economics work,” says Willis. “Top prices for the first-completed 57th Street slender towers have already achieved $9,000 to $11,000 per square foot.”

When prices reached this level, slender skyscrapers started to present attractive return-on-investment possibilities for property developers in New York. The last five years has seen a spate of ever-taller slender-build projects including Madison Park Square Tower (237m), 50 West Street (237m), 111 Murray Street (241m), 56 Leonard Street (250m), 53W53 (290m), 220 Central Park South (290m), 9 DeKalb Avenue (310m), 432 Park Avenue (426m), and 111 West 57th Street (435m) – all heights as currently planned.

Rising a stack of apartments up to 90 storeys to heights of up to 300 metres on ground that measures one-twelfth of the total height has set challenges for architects, engineers and construction services experts. Slender high-rises have pioneered design and construction techniques that architects and structural engineers expect to apply to more modest slender residential developments.

The build challenges start with width-to-height ratios. A building’s ‘slenderness’ is an engineering definition. It refers not only to the total height of a skyscraper, but to its (base) width-to-height ratio – the quotient between the width of a building (its widest point at ground level) and its top roof height.

Although used as a defining metric, slenderness ratios can prove imprecise calculations, says Skyscraper Museum’s Willis, because the bases and shafts can be of variable widths as the building rises – they might go thinner at the uppermost levels for architectural, structural or economic reasons. Skyscrapers with a minimum 1:10-1:12 ratio are called ‘slender’. Those with the highest width-to-height ratios – such as 111 West 57th Street, due to complete in 2021, which, at 435m on a base of 18m, will have one of about 1:23-1:24 – are ‘super-slender’.

Slender-scale ratios also define limits on the sizes of apartments they can contain, Willis adds: “Small floor plates – as small as 223 square metres and generally no larger than 743m2 – create the ideal conditions to limit apartments to one or two units per storey.” This lends exclusivity and minimises the number of communal lifts that take up internal space.

Architects compensate for the relative compactness by designing apartments with lofty ceilings of around 4.7 metres – another reason why slender high-rises have to reach for the sky. The prevailing design aesthetic of slenders is not predominantly stylistic, Willis says: “The facade treatment can be a continuous glass membrane or a masonry curtain wall with punch-out windows. The structural system can range from internal shear walls and mega-columns to an exterior bearing wall, to structural expressionism.”

‘The technology to deliver spectacular constructions already exists. In the hands of ambitious and collaborative clients, architects and engineers, complexity can be simplified and delivered at standard prices by working through the design and engineering from first principles.’

Andrew Watts, Newtecnic

Slender skyscrapers are highly-customised buildings. Unlike conventional high-rises for residential or commercial use, there is often a limited amount of big block know-how that can be redeployed for these new towers.

Aerodynamics is a more important factor with tall, thin structures than it is with more conventionally dimensioned towers: strong winds cause them to sway. Slender skyscraper exteriors are hard-tested in wind tunnels to see how well the facades and other architectural features react to issues like ‘vortex shedding’ – the oscillating flow that takes place when air or water flows past a bluff body (a feature that, due to its shape, has separated flow over a large part of its surface) at certain velocities.

Changes to the aerodynamics of features such as ledges and corners can reduce the potential impact wind contact has on a tower’s superstructure. Air current issues are important as the taller and thinner a tower, the more it will sway. Swaying effects grow towards the top and residents who have paid a premium to live up high will feel the most movement.

Holes, notches and slots are variously designed into corners and facade turns. A design feature of the plainly-rectangular 432 Park Avenue, intended to reduce the effects of swaying due to wind vortex loading, is that the window grid and interior space of two floors between every 12 occupied storeys were left open to allow the wind to pass through. These floors also contain modularised mechanical services for the six storeys above and below, to reduce ductwork runs. The five open levels add to the skyscraper’s illumination scheme after dark.

“Digital simulation using computational fluid dynamics (CFD) can simulate the effect of worst-case and general wind load on built structures,” explains Andrew Watts, CEO at building engineering specialist Newtecnic. “We use it to avoid turbulence around the structure, and to break up eddies – both of which produce noise and stress to building components.”

Wind tunnel tests on architectural prototypes usually take place in the kind of facilities used to test aircraft aerodynamics, but technology is catching up. Rustem Baishev, architect at RB Systems, anticipates interactive wind tunnel testing techniques, which, by using actuators inside of a flexible envelope, allow adjustments to be made to the shape of a tower while it is inside the tunnel. “That way you get to see immediate results, as opposed to [going through an] iterative process,” Baishev explains.

Digital simulation, however, is still not yet advanced enough to avoid physical wind tunnel tests, says Watts: “The next generation of software may eliminate physical testing, but currently they are necessary to validate CFD results.”

Dampers are another technology deployed to counteract sway and impose additional stability. These are massive units that work like counterweights installed near the top of some slender skyscrapers. Tuned mass dampers (TMDs) are engineered to adjust their positions slightly when sensors tell them the building is moving. Another kind, tuned sloshing dampers (TSDs), are vast containers of water that help absorb vibrations.

Tuned mass damping is passive technology that is tuned to swing at approximately the same rate as the building’s natural frequency. The TMD physically ‘pulls’ on the building and dissipates energy associated with sway vibrations. At 432 Park Avenue, damper specialist RWDI installed an opposed-pendulum design that reduced the vertical space required by slowing the swing rate of the TMD while maintaining the same internal ‘pull’ on the structure.

To fit the TMD into the constrained available space atop the 85-storey tower, RWDI divided the required 1,200-ton mass into two 600-ton TMDs, one located on either side of the building’s core. The TMDs were configured so 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 required building stability.

Skyscrapers of NYC

Image credit: E&T

As premium-value habitats, slender skyscrapers must maximise the available interior space. But because they have much less floor area to be structurally supported, there is less need for the steel frame grids that characterised conventional high-rise models for decades. Structural support more likely comes from the building’s exterior rather than an internal matrix of interconnected steel. As far as possible, weighty building management mechanical apparatus (lift engines, plumbing, electricals) are based on upper storeys for extra wind-counteracting stability.

Building engineers experiment with composite structures that combine high-strength steel and concrete in innovative ways to find the right balance of strength and flexibility. Where builders were limited in the past, stronger materials mean they can build taller while maintaining the same-size structural elements. Recent formulations of concrete’s chemical composition – such as addition of industrial by-products like fly ash, leftover microsilica (spheres of silicon dioxide), pulverised fuel ash and steel slag – make it more rigid, robust enough to support heavier loads.

Like other fields of architecture and structural design, slender skyscrapers are customarily designed using computer modelling tools that enable architects to refine their designs in accordance with both logistical considerations as they are discovered, and budgetary limits. Advances in modelling technology itself will play a part in a move toward enabling slender skyscrapers to be built on sites previously deemed unsuitable for high-rise structures, predicts Baishev at RB Systems.

“Some multi-objective optimisation software workflows in programming [in architecture the term describes composition of contents of a building in response to set typology] of a tower and in designing of its structural concepts, as well as of its facades, are in wide use already,” he says, “but [as yet] are still largely set up and controlled by humans. A mutual interest in an initiative could unite experts in creating a knowledge base for a wider involvement of artificial intelligence in the process, but skyscraper projects are budgeted scrupulously and those who commission them are not always willing to wait or experiment, therefore this idea – although great – might take time.”

According to Newtecnic’s Watts, to construct new slender buildings at low cost requires those who commission buildings to “become more demanding of engineers. The technology to deliver spectacular constructions exists. In the hands of ambitious and collaborative clients, architects and engineers, complexity can be simplified and delivered at standard prices by working through the design and engineering from first principles.”

Further challenges come from the limitations of construction technology, says Baishev, where slender towers are being built with cranes and construction rigs designed for larger-scale high-rise constructions: “For instance, typically, the cranes are located inside of elevator shafts; in slender towers there are fewer shafts and thinner cores, so the cranes’ robustness and operations might need to be improved.”

More information about the history of high-rise buildings is available from New York’s skyscraper museum.

Case study

Istanbul’s Küçük Çamlıca TV Tower

Although slender towers to date have been mainly residential projects, the lessons learned are being explored by other tall building use-cases. Istanbul’s highest building – the just-completed 365m-high Küçük Çamlıca TV Radio Tower (KCTV) – is a £36m structure by Melike Altinisik Architects that features facades design-engineered by building engineering specialist Newtecnic.

“The complexity and cost of building towers of this height usually means that they have accommodation only at the top,” says Andrew Watts, CEO at Newtecnic. “The load-bearing structure of KCTV’s tower is a concrete tube, with constant geometry throughout its height. Using specially-developed computer algorithms we devised a design that allows lightweight pre-fabricated glass-reinforced concrete (GRC) panels to be attached all the way up the central column.”

Newtecnic developed a facade concept that allows inhabitable spaces to be attached to the whole of the tower’s core – an unconventional development for buildings of this kind where core internal space is required for technological equipment, leaving no space for other uses, such as paying visitors.

These spaces hang like a curtain and are securely clipped to the main central core to create large interiors, Watts explains: “The envelope system was designed to reduce installation time and uses a newly-developed method that integrates thin GRC rainscreen panels, stiffened by a steel frame. This is fixed directly to a backing wall that incorporates integrated glazed openings.”

The algorithms provided the data that enabled Newtecnic to understand and engineer solutions around the relationships between building components and environmental factors such as gravity, seismic activity, temperature and positive/negative wind pressure.

Costs of materials and processes are also incorporated through the algorithms, which helps reduce project risk and increases cost control.

The KCTV tower, which hosts 125 broadcasting transmitters, was wind-tunnel tested. This allowed Newtecnic to develop accurately sized facade components from the first stage studies. It also provided the data to optimise the envelope build-up and obtain an accurate understanding of the impact of the facade loads on the structural behaviour of the concrete structure.

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