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A pencil tower (also known as a skinny skyscraper, pencil-thin tower, super-slender tower, or super-slim tower) is a
high-rise building
A tower block, high-rise, apartment tower, residential tower, apartment block, block of flats, or office tower is a tall building, as opposed to a low-rise building and is defined differently in terms of height depending on the jurisdictio ...
or
skyscraper
A skyscraper is a tall continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least or in height, though there is no universally accepted definition. Skyscrapers are very tall high-ris ...
with a very high
slenderness ratio
In architecture, the slenderness ratio, or simply slenderness, is an aspect ratio, the quotient between the height and the width of a building.
In structural engineering, slenderness is used to calculate the propensity of a column to buckle. It ...
that is very tall and thin.
There is no universal definition of how slender these buildings are to be categorised, but some definitions of 10:1 or 12:1 ratios and higher have been used.
[
]Hong Kong
Hong Kong ( (US) or (UK); , ), officially the Hong Kong Special Administrative Region of the People's Republic of China ( abbr. Hong Kong SAR or HKSAR), is a city and special administrative region of China on the eastern Pearl River Delt ...
started developing pencil towers in the 1970s. Residential buildings of twenty or more storeys with one unit per floor were built over small lots.[ It has become one of the most common types of buildings in the city, making Hong Kong the world's highest concentration of pencil towers. Hong Kong's most notable towers are the 72-storey ]Highcliff
Highcliff is a luxury apartment on a south slope of Happy Valley on the Hong Kong Island in Hong Kong. The 75- storey building's construction (70 floors of which are liveable space) began in 2000 and was completed in 2003 under a design by ...
tower, which has a slenderness ratio of 20:1, and its neighbour, The Summit, a 65-storey residential building.[
In the 2010s, pencil towers became a new phenomenon of building design in ]New York City
New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...
. The newer pencil towers on Manhattan's " Billionaires' Row" (a thin strip of Midtown near Central Park
Central Park is an urban park in New York City located between the Upper West Side, Upper West and Upper East Sides of Manhattan. It is the List of New York City parks, fifth-largest park in the city, covering . It is the most visited urban par ...
) are mostly supertall
A supertall building is an occupied "supertall" structure higher than and beneath . A form of skyscraper, it falls midway between a common minimum definition of "skyscraper" (a building taller ) and a " megatall" building (taller than ).
Diff ...
s. The first of this new crop of super-slim towers was the 306 m One57
One57, formerly known as Carnegie 57, is a 75-story, supertall skyscraper at 157 West 57th Street between Sixth and Seventh Avenues in the Midtown neighborhood of Manhattan in New York City. The building has 92 condominium units on top of ...
tower. Two pencil towers on a section of 57th Street made the street the most expensive address in the global real estate market, with 41 transactions above US$25 million from 2015 to 2019.
Outside of Hong Kong and New York City, Melbourne
Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a met ...
has become the centre of pencil towers.
History
Early slender skyscrapers were first developed in Manhattan
Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...
in the late 19th and early 20th century (before zoning laws
Zoning is a method of urban planning in which a municipality or other tier of government divides land into areas called zones, each of which has a set of regulations for new development that differs from other zones. Zones may be defined for a si ...
). The new designs were fostered by an increase in the price of land and enabled by the use of elevators and steel frame
Steel frame is a building technique with a "skeleton frame" of vertical steel columns and horizontal I-beams, constructed in a rectangular grid to support the floors, roof and walls of a building which are all attached to the frame. The developm ...
construction which allowed buildings to be built taller. The Metropolitan Life Insurance Company Tower
The Metropolitan Life Insurance Company Tower (colloquially known as the Met Life Tower and also as the South Building) is a skyscraper occupying a full block in the Flatiron District of Manhattan in New York City. The building is composed of ...
is an example of the slim buildings of that era. Finished 1911, it is tall with floor dimensions of .[
When the Equitable Building's shadow influenced the passage of a 1916 Zoning Resolution, street canyon shapes became regulated but 25% of the property was exempt. This caused building designs of the era to have a wide base and thinner tower covering a quarter of the lot area. ]The Pierre
The Pierre is a luxury hotel located at 2 East 61st Street, at the intersection of that street with Fifth Avenue, in Manhattan, New York City, facing Central Park. Designed by Schultze & Weaver, the hotel opened in 1930 with 100+ employees, n ...
and The Sherry-Netherland
The Sherry-Netherland is a 38-story apartment hotel located at 781 Fifth Avenue on the corner of East 59th Street in the Upper East Side neighborhood of Manhattan, New York City. It was designed and built by Schultze & Weaver with Buchman & Ka ...
are examples of slender towers of the time which were allowed to be built close to Central Park
Central Park is an urban park in New York City located between the Upper West Side, Upper West and Upper East Sides of Manhattan. It is the List of New York City parks, fifth-largest park in the city, covering . It is the most visited urban par ...
.[
New York's subsequent 1961 Zoning Resolution set tip height limits for the first time and replaced the shape limits with ]floor area ratio
Floor area ratio (FAR) is the ratio of a building's total floor area (gross floor area) to the size of the piece of land upon which it is built. It is often used as one of the regulations in city planning along with the building-to-land ratio. The ...
ones. It also allowed transferable development rights which gave birth to the concept of buying air rights
Air rights are the property interest in the "space" above the earth's surface. Generally speaking, owning, or renting, land or a building includes the right to use and build in the space above the land without interference by others.
This legal ...
from nearby lots.[
In the 1970s Hong Kong was in a similar position of high land values and lax zoning laws, and started building pencil towers.][ Multiple factors contributed: in 1964 the Hong Kong Housing Authority finished the ]Choi Hung Estate
Choi Hung Estate () is a public housing estate in Ngau Chi Wan, Kowloon, Hong Kong. It was built by the former Hong Kong Housing Authority () and is now managed by the current Hong Kong Housing Authority (). It received a Silver Medal at the 1965 ...
for 43,000 residents who suffered from a massive fire in 1953. Those public housing units were to which set a new expectation for the sizes of residential units in private developments. The cost of land leasing for residential developments was also a big factor in the building designs at that time. Hong Kong's government owned all the land and leased some out for private development through auctions for various terms, such as 50, 75, 99 and 999 years. Developers needed to pay tens of thousands of HK$ for leasing each square metre (11 sq ft) of land. Meanwhile, some innovative building designs emerged. The use of scissor staircases, in which two fire escape staircases intertwined, shrank the width of the fire escape void, allowing buildings to be narrower. These factors incentivized developers to build slender residential towers on small lots with one unit per floor. Developers only needed to acquire a lot with a frontage area of two small retail shops (two tong lau
Tong lau or ke lau are tenement buildings built in late 19th century to the 1960s in Hong Kong, Taiwan, Southern China, and Southeast Asia. Designed for both residential and commercial uses, they are similar in style and function to the shophouse ...
buildings) in order to build a tower of 20 stories or more. This was when the term "pencil towers" was used to describe these micro-developments. Currently, a unit on a pencil tower has an average price of HK$4.3 million. (US$0.55 million)
Pencil towers became one of the most typical building types in Hong Kong, beside tong lau and cruciform
Cruciform is a term for physical manifestations resembling a common cross or Christian cross. The label can be extended to architectural shapes, biology, art, and design.
Cruciform architectural plan
Christian churches are commonly described ...
apartments. Overall, buildings with a slenderness ratio of up to 18:1 have dominated the Mid-Levels residential district, making Hong Kong the "world capital" of pencil-thin towers. In the 21st century Hong Kong pencil tower developments started to get much larger in scale than previous decades. In 2001, The Summit, a 65-story upscale residential building was completed. It was followed in 2003 by the world record (until 2019, when 111 West 57th Street topped out
In building construction, topping out (sometimes referred to as topping off) is a builders' rite traditionally held when the last beam (or its equivalent) is placed atop a structure during its construction. Nowadays, the ceremony is often parlaye ...
) 20:1 ratio, 72-story building, the Highcliff
Highcliff is a luxury apartment on a south slope of Happy Valley on the Hong Kong Island in Hong Kong. The 75- storey building's construction (70 floors of which are liveable space) began in 2000 and was completed in 2003 under a design by ...
. These developments highlighted the potentials of tall luxury pencil towers.
In the early 21st century, the pencil tower concept for residential towers also expanded outside of Hong Kong and returned to New York City. Developers utilized the transferable development rights and innovative structural engineering to elevate the buildings as high as possible for spectacular views while requiring the smallest footprints possible. The term "pencil towers" has been used to describe a new type of tall and slender buildings in New York City as well.[ Early examples of the new trend were Sky House Condominium, a building on a lot, and ]One Madison
One Madison is a luxury residential condominium tower located on 23rd Street between Broadway and Park Avenue South, at the foot of Madison Avenue, across from Madison Square Park in the Flatiron District of Manhattan, New York City. The build ...
, a 50-story building with a 12:1 ratio. These were followed by more than a dozen pencil towers built in the 2010s.
Challenges
Engineering
Building materials have improved in recent decades. The strengths of concrete have been changed from 10,000 to 12,000 psi to 18,000–20,000 psi (69–83 to 120–140 MPa). Reinforcing steel with higher strengths of 100 ksi
Olajide Olayinka Williams "JJ" Olatunji (born 19 June 1993), known professionally as KSI, is an English YouTuber and rapper. He is a co-founder and member of the British YouTube group known as the Sidemen. He is the CEO of Misfits Boxing and ...
(100,000 psi, 690 MPa) became more available. Higher-strength materials can reduce thickness of concrete walls, reducing the amount of required interior structures. This is a critical factor that enables super-slender building designs.
A main challenge of super-slender buildings is the management of lateral movements due to wind load
Wind engineering is a subset of mechanical engineering, structural engineering, meteorology, and applied physics that analyzes the effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits w ...
s, which can cause discomfort for occupants. Various engineering techniques are used to address this issue. One approach to manage the wind loads is to use an open mechanical floor
A mechanical floor, mechanical penthouse, mechanical layer or mechanical level is a story of a high-rise building that is dedicated to mechanical and electronics equipment. "Mechanical" is the most commonly used term, but words such as ''utilit ...
in the middle of a building to allow wind to flow through. This can reduce the wind loads and the resulting movements by 10%-15%. An example of this technique can be seen in 432 Park Avenue
432 Park Avenue is a residential skyscraper at 57th Street and Park Avenue in Midtown Manhattan in New York City, overlooking Central Park. The tower was developed by CIM Group and Harry B. Macklowe and designed by Rafael Viñoly. A part of ...
, a 15:1 ratio building that has many open-air mechanical floors. Another approach is to stiffen the structure to resist the loads. This can be done by using outrigger walls to connect the building core to the perimeter columns on two sides to distribute the loads. The structure of 111 West 57th Street uses shear wall
In structural engineering, a shear wall is a vertical element of a system that is designed to resist in- plane lateral forces, typically wind and seismic loads. In many jurisdictions, the International Building Code and International Residential Co ...
s on the east and west facades to stiffen the structure while leaving the north and south views unobstructed.[
To manage building movements, a ]tuned mass damper
A tuned mass damper (TMD), also known as a harmonic absorber or seismic damper, is a device mounted in structures to reduce mechanical vibrations, consisting of a mass mounted on one or more damped springs. Its oscillation frequency is tuned ...
can be used. This is a passive device with a large mass mounted on shock absorbers to slow the movements of the building. Typically they are made of solid mass, but recently liquid damping systems have been used. A liquid damping system or sloshing damper is a tank of water with screens to control the movements of the water, specifically tuned such that when building is moved in one direction, the liquid force goes in the opposite direction to offset the movements. Highcliff
Highcliff is a luxury apartment on a south slope of Happy Valley on the Hong Kong Island in Hong Kong. The 75- storey building's construction (70 floors of which are liveable space) began in 2000 and was completed in 2003 under a design by ...
is one of the early adopters of such system.[
Building footing is another area of concerns. As the slender buildings have smaller footprints, the foundations need to go down deeper.][
]
Firefighting
Very tall and very narrow pencil towers present new challenges to fire fighting. With narrower buildings, the scissor staircases are narrower and they may need to have five or more turns to rise for each floor. This presents impacts to the performance of the fire hose
A fire hose (or firehose) is a high-pressure hose that carries water or other fire retardant (such as foam) to a fire to extinguish it. Outdoors, it attaches either to a fire engine, fire hydrant, or a portable fire pump. Indoors, it can perma ...
s that are twisted over a long length, and small areas that can be used for fire fighting operations and medical staging. Another area of concern is their open-air mechanical floors, especially those of New York City pencil towers. If the doors are left open, wind could enter the staircase fueling the fire.
Examples
Australia
Melbourne
Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a met ...
entered the global pencil tower scene with the construction of the Phoenix Apartments building that rose up on a wide lot in 2013. The plan originally called for a 40-story building. After oppositions from concerned neighbors, the design was revised down to 28 stories at a height of , still with a high slenderness ratio of 13.5:1. That was soon followed by Collins House, a skyscraper that has the width of its base at . It slenderness ratio of 16.5:1 was the fourth most slender tower in the world at the time of completion. Collins House incorporated many of advancements in technology including the first of such towers to use prefabrication
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located. The term is u ...
of 3D structural elements to help navigate narrow project site. A combination of air rights purchase and large cantilever
A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a canti ...
s helped maximizing its floor space. In 2018, a design of a tower sitting on to of a triangular plot in Melbourne was submitted for planning approval. If built, The Magic would become world's slenderest by a ratio of 26.6:1 based on the shorter side of its widths in a triangular footprint. If it would have a rectangular base, a similar size building would be at 18:1 ratio.
Canada
In 2020, Dutch developers proposed to build an 87-story mixed-use pencil tower at 1200 Bay Street in Toronto
Toronto ( ; or ) is the capital city of the Canadian province of Ontario. With a recorded population of 2,794,356 in 2021, it is the most populous city in Canada and the fourth most populous city in North America. The city is the ancho ...
to replace a 12-story office building. It follows the form of pencil towers of New York. However, with its 11:1 ratio, the slenderest in Toronto, it falls short of matching those New York towers.
Hong Kong
In Hong Kong, the Mid-Levels area is home of many pencil towers, such as Tregunter 3 of the Tregunter Towers. The Highcliff
Highcliff is a luxury apartment on a south slope of Happy Valley on the Hong Kong Island in Hong Kong. The 75- storey building's construction (70 floors of which are liveable space) began in 2000 and was completed in 2003 under a design by ...
tower, located in the neighbouring Happy Valley area, was the world's slenderest tower (20:1 ratio) at the time of completion.[
]
United Kingdom
In 2020, Birmingham City Council
Birmingham City Council is the local government body responsible for the governance of the City of Birmingham in England, which has been a metropolitan district since 1974. It is the most populated local council area in the United Kingdom (e ...
approved a plan to construct a wide and tall building. This would be the first super-slender tower in the United Kingdom.
United States
The Skyscraper Museum
The Skyscraper Museum is an architecture museum located in Battery Park City, Manhattan, New York City and founded in 1996. As the name suggests, the museum focuses on high-rise buildings as "products of technology, objects of design, sites of c ...
cited Sky House (2008), a 55-story building made possible by a purchase of air rights from the Little Church Around the Corner
The Church of the Transfiguration, also known as the Little Church Around the Corner, is an Episcopal parish church located at 1 East 29th Street, between Madison and Fifth Avenues in the NoMad neighborhood of Manhattan, New York City. The con ...
, as an early example of New York's super-slender towers. One Madison
One Madison is a luxury residential condominium tower located on 23rd Street between Broadway and Park Avenue South, at the foot of Madison Avenue, across from Madison Square Park in the Flatiron District of Manhattan, New York City. The build ...
, a tower that incorporates seven volumetric pods to extend the tower's floor plate, was completed soon after Sky House.[
There were several more buildings completed after 2014. These include: ]One57
One57, formerly known as Carnegie 57, is a 75-story, supertall skyscraper at 157 West 57th Street between Sixth and Seventh Avenues in the Midtown neighborhood of Manhattan in New York City. The building has 92 condominium units on top of ...
, the first supertall pencil tower in New York;[ ]432 Park Avenue
432 Park Avenue is a residential skyscraper at 57th Street and Park Avenue in Midtown Manhattan in New York City, overlooking Central Park. The tower was developed by CIM Group and Harry B. Macklowe and designed by Rafael Viñoly. A part of ...
, a 15:1 ratio building that has five open-air mechanical floors throughout its height;[ 220 Central Park South, a 18:1 ratio building that uses the first of its height for mechanical equipment to stiffen the building;][ 111 West 57th Street, the world's slenderest tower at a ratio of 24:1;][ and ]Central Park Tower
Central Park Tower is a residential supertall skyscraper at 225 West 57th Street in the Midtown Manhattan neighborhood of New York City, along Billionaires' Row. Designed by Adrian Smith + Gordon Gill Architecture, the building rises with ...
, a tower that sets aside as mechanical space to build taller than would have otherwise been allowed.
References
{{Reflist
Pencil towers
Skyscrapers