“Slender-Scrapers”: Your Guide to the Mega High-rise Buildings of Tomorrow

 

An image showing a city skyline. and "slender-scrapers".

 

You may not be familiar with them, but if you live in a major urban city, you’ve almost certainly seen them. 

The skylines of metropolises such as New York, London, and Hong Kong are changing shape; with tall towers that seem impossibly slender. These “Slender” and even “super-slender” skyscrapers owe their existence to three factors:

  • The lack of available building room in the dense, major cities.
  • The invention of new technology, such as a new combination of steel and cement, that allows architects to build slimmer, sturdier and more rigid structures.
  • A growing demand from wealthy civilians, with a taste for the most scenic views. 

What’s interesting is that most of these slender-skyscrapers are residential buildings. Only a minority of them are mixed-use, meaning offices and hotels also inhabit them (for reasons why we shall see below). 

 

Just what defines a “slender-scraper?”

Slender-skyscrapers have to be considered many times taller than they are wide in order to earn the prefix “slender”. But it is a highly subjective term and whatever answer you get depends on the engineer’s opinion. 

Generally, a height-to-width ratio of 7:1 is enough to consider a skyscraper as a “slender” one. But the Sky High & the Logic of Luxury exhibition at New York City’s Skyscraper Museum has stressed that this ratio must be at least 10:1 or even 12:1. So-called “super-slender” skyscrapers are usually in the range of 1:23 or 1:24 (that is, the building must be twenty-four times taller than it is wide!). 

A building that is exceptionally tall does not make it a slender-skyscraper. For example, New York City’s World Trade Centre may be very tall, but it is quite wide. In fact, the World Trade Centre’s aspect ratio is just 6:1

 

Why are they being built?

It’s an almost universal condition that height > superiority. The Italians have a phrase for it: altezza mezza bellezza, meaning “height is half of beauty”. 

For billionaires in the ‘concrete jungle’ that is New York, London, and the other metropolises, the height advantages of slender-skyscrapers is the equivalent to a bird nest-building on the tallest tree-tops of the Amazon. From such a high position the view is guaranteed to be one ‘overlooking’ all of the other, lower; beneath-them residences. In other words, views have value.

In New York especially, the value is a grand sweeping view of Central Park. In fact, when construction is completed in 2020, the world’s super-slender skyscraper will be 111 West 57th Street — a residential complex only 59 feet wide, but with domineering views over the City’s most famous greenery. 

There is also ‘Billionaire’s Row’, a growing series of super-slender skyscrapers again on 57th Street, New York. The CEO of Dell, Michael Dell, famously paid over $100m for a two-storey penthouse at the top of One 57 in Midtown Manhattan, in a super-slender that is nicknamed the ‘Billionaire Building’ in itself. 

 

How are they built?

Slender-skyscrapers are the impossible dreams of architects only a few decades ago. One critical technological advance is the development of a much stronger form of concrete. 

Not long ago, the typical material strength for concrete was about 12,000 psi (pounds per square inch). But modern concrete today is capable of reaching up to 20,000 psi. This improvement means that the concrete walls no longer have to be so thick, minimising the intrusiveness of the walls into the interior of the building. 

The chemical structure of concrete has also been tweaked to make it more robust, rigid, and able to support heavier loads. Most of these additives to the mix include industrial by-products such as fly ash, pulverised fuel ash, steel slag, and leftover microsilica. 

Like the concrete. architectural steel has also improved in quality. Higher-strength, reinforcing steel with yield strengths of up to 100 ksi (kilopounds per square inch) has become more readily available. 

Engineers have experimented with both of these high-strength steel and concrete types to produce innovative combinations with additional strength and flexibility. 

 

Examples of slender-scrapers in the world today.

As the slender-skyscraper phenomenon is a new one, most of them are currently under construction around the world. Key slender-scraper construction projects include:

Below are images of Victoria One, the Lexicon, and Highcliff, respectively.

 

The Victoria One tower Islington tower The Highcliff tower, Hong Kong

The benefits and peculiarities of slender-scrapers.

Slender skyscrapers are, well… slender. That means there is limited room inside them. In fact, some slender-scale ratios can limit floor plans to as little as 743m2 (which, for $100m, might not seem like a good return on investment). It then becomes logical to ask: “What’s the use of them, then?”

But slender-skyscrapers are still luring in the billionaires and wealthy, even if the apartments contained within them are limited to one or two units per storey. These limitations actually work to generate an ‘exclusive’ feel to them. Despite sharing the slender-scraper with others, it can sometimes feel like the entire building is yours alone. 

That and architects compensate for the relative compactness of the apartments by giving them really high ceilings. Most of them have ceilings of about 4.7 metres high — another reason why the structures are so tall. 

Often the facade treatment is almost always a continuous glass membrane, which looks very attractive from the inside. But this isn’t purely an aesthetic design. Because of slender-scrapers have much less floor area, there is a need to maximise the amount of available interior space. This means that the steel frame grids that characterise other types of high rise buildings are almost absent. Instead of relying on a mesh of interconnected steel, the support for the building comes from the exterior instead. 

 

How expensive are they to build?

Up until recently, building tall and thin has presented many challenges to even make them economically viable. And even with the technology, construction is very expensive. One 57, for example, cost $1.5bn to build. 

In New York especially, building a slender-scraper was out of the question unless returns could be guaranteed on the estimated cost of $3,000 per-square-foot of the construction. But some of the behemoths on Billionaire’s Row have already achieved $11,000 per square foot. 

 

A construction worker looking at a skyscraper with blueprints.

The difficulty of building super-skyscrapers.

The construction of slender-skyscrapers has been a process of discovery in the construction world, pioneering new construction and design techniques on the way.

Imprecise ratio measurements

The obvious challenges come from the width-to-height ratios. An over-reliance on these ratios can make for imprecise calculations because the bases and shafts can vary in size for structural or economic reasons. (For example, it is common for the width of a skyscraper to contract nearer to the top.)

Aerodynamic problems

Slender-skyscrapers are also highly customised buildings. Because of their enormous height, sometimes eccentric architectural shapes are needed to keep them sturdy in the face of strong winds.

It is impossible for a skyscraper to be “blown over” but at high altitudes, they can sway, which can feel very unsafe (even though it isn’t). Furthermore, this swaying is more apparent at the top, which is where the highest premium residents are likely to live.

Because of this, slender-skyscrapers are hard-tested in wind tunnels to see how the facades deal with and distribute gusts of wind. If a building is large enough — as most high rise buildings are — they can disrupt the flow of wind and split it into two paths. This “vortex shredding” is what causes swaying. 

Changes to the aerodynamics of features such as ledges and corners can reduce vortex shedding and the impact of the wind in general. The slender-scraper 432 Park Avenue in New York has unique holes, notches, and slots designed into its corners and facade turns. But perhaps the most impressive way to deal with wind shear is the wind tunnels that the building itself incorporates. Between every 12 storeys, two floors are left empty, so the wind can pass through unimpeded, reducing its impact on the structure at large.

An expensive reliance on wind tunnels

There is a type of computer software known as Computational fluid dynamics (or CFD) that may one day replace wind tunnels altogether — though the technology is not there yet. 

The purpose of the software is to simulate high altitude and powerful winds on the design of a building to see how it deals with issues such as vortex shedding and sway. At the moment, the technology is not at the point where it could replace wind tunnels alone, but it is promising. 

With CFD, engineers can figure out how to tweak the design, and where to prevent turbulence and break up eddies — both of which generate noise and can stress the architecture of the building. 

A lack of weight to combat sway at the top of the towers.

As the effects of the wind disproportionately impact on the top part of the tower, engineers have had to tweak another method of combating sway: that is, to use what is known as a “damper”. Dampers are essentially counterweights. There are two types of dampers:

  • Tuned mass dampers.
  • Tuned sloshing dampers.

The mass dampers physically pull on the slender-scrapers as the towers themselves are blown in the opposite direction. the mass dampers then dissipate the energy associated with the sway vibrations. Sometimes these mass dampers can take the form of pendulums. 

Tuned sloshing dampers, on the other hand, work on a simpler basis. They are essentially large containers of water that work to absorb the vibrations. The Highcliff tower, in Hong Kong, has a sloshing damper.

There is a third way of adding weight to the top of a slender-scraper, that does not include dampers. That is, as far as is possible, all of the weighty mechanical apparatus for building management is kept near to the upper stories. Meaning the lift engines, plumbing, and electricals are all stored high up as a further way to counteract the wind. 

 

Benefits and controversies

We are living through a rare moment in history where a new type of architecture is being constructed. The Romans discovered concrete and built huge, concrete domes. The Victorians developed steel and built great railways and bridges. And in the last century, the Americans invented elevator lifts, which led to the creation of the world’s first skyscrapers (in Chicago). 

The growth of this new type of architectural creation is spurring on new innovations in steel and cement production, and may also have other great benefits by proxy. For example, the building of Turkey’s Istanbul K࣫uçuk Çamlica TV Tower was key to helping in the development of understanding the impact of facade loads on the structural behaviour of the new concrete composition. 

The technology that currently exists is also not adequate for developing slender-scrapers. Cranes and construction rigs may have to be re-designed — especially as there are fewer shafts and thinner cores in slender-scrapers where they might traditionally have operated from. 

But the development of super-scrapers is not without its drawbacks. Critics have likened their appearance on major urban skylines as a sort of fungus that only sprouts from extreme inequality. There are also concerns of the colossal shadows these skyscrapers produce, and if the snuffing out of the sunlight they could have on nearby, lower down apartments. There are also fears that such buildings could be just “trophy-properties” in name only, and perhaps hardly even inhabited. The City of San Francisco has already banned the construction of slender-scrapers, for fears of sunlight-blocking alone.

 

The future of slender-skyscrapers

With wealth concentrating in the mega-metropolises of the world, and with construction planning-space increasingly limited, it seems a certainty that the slender-skyscrapers phenomenon will continue for the time being. Most of them aren’t even built yet, and are expected to be completed in 2020 and beyond. 

With their development, they are sure to encourage more innovation in the construction world. Whether this progress is enough to justify their creation or to placate the remaining 99 per cent of the population, well only hindsight will be able to answer that — once the skylines have already been transformed. 

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