The Future of Tall Buildings
12/2/24
Adapted from his lecture given at Gresham College in London
earlier in 2023, Roger Ridsdill Smith, Head of Structural Engineering at
Foster + Partners, considers the evolution of tall buildings through
history and investigates how their status might be reconsidered in light
of today's climate emergency.
The construction industry currently accounts for around 39 per cent of worldwide carbon dioxide emissions – 28 per cent of which is operational and 11 per cent of which is embodied in construction materials. Further, when we investigate the emissions for tall buildings, we can see that they require relatively larger amounts of carbon in their construction than low-rise structures. How do we reduce these figures? And can we justify the continued construction of tall buildings?
To address these questions, I will look back at the conditions that
led to the explosion of skyscraper construction, then consider modern
tall buildings using projects designed by the integrated architecture
and engineering team at Foster + Partners. Finally, I will look ahead to
the future of cities, and the implications of tall buildings on
humanity's carbon footprint.
A Brief History of Height
The American architecture critic Ada Louise Huxtable wrote in 1982
that 'from the Tower of Babel onward, the fantasies of builders have
been vertical.' For as long as we have told stories, we have imagined
impossibly tall structures that might, somehow, reach the heavens.
However, the desire to build tall has always been limited by the
structural techniques available at the time. The pyramids in Egypt were
the first man-made structures to break through the 100-metre height
mark. The most famous were constructed during the 26th century BC, and
the largest of them, the Great Pyramid of Giza, measured 147 metres
above ground level.
It
is not until the period of European cathedral construction in the
Middle Ages that these heights were surpassed, and even then not by
much. The Lincoln Cathedral spire, built in 1311, was reputedly 160
metres high until it collapsed in 1548. Other spires followed across
Europe, all around 150 metres. In 1880, Cologne Cathedral was the
tallest building in the world with a 157-metre spire.
One
of the primary reasons for these height limits is that medieval
structures were built in the same way as the tall structures that came
before them – with loadbearing masonry. Stone blocks or bricks were
piled on top of one another to create walls. Their height was limited by
the capacity of the ground beneath the wall to support its mass and the
stability of the entire structure against sideways loads acting on the
building, such as wind and earthquakes.
A further factor limiting the height of buildings was the difficulty
of getting to the top of them. In the absence of reliable mechanical
hoists, the building's users were obliged to use stairs, which might be
practical for maintenance and the occasional visit, but unrealistic for
use by large numbers of people.
The
completion of Cologne Cathedral in 1880 – to a centuries-old medieval
design – was one of the last examples of tall structures built with
loadbearing masonry. Meanwhile, across the Atlantic, the first true
'skyscraper' would soon begin its construction. Separated in time by
just a few years, the two buildings would be eons apart in their
structural design.
Entering the High-Rise Era
The Home Insurance Building in Chicago, completed in 1885, was designed by American architect William Le Baron Jenney. It is considered to be the world's first skyscraper due to the incorporation of structural steel in the loadbearing walls. Originally ten stories and 42 metres (138 feet) high, it may be small compared to the towers of today but the Home Insurance Building is recognisable as a modern tall building, something that would not become possible until the innovations of the Industrial Revolution at the end of the 19th Century. This was a period that produced major advances in construction materials and their transportation, as well as developments in structural, electrical and environmental engineering.
The Home Insurance Building, Chicago, following its completion in 1885 and during its demolition in 1931. Designed by American architect William Le Baron Jenney, the building is considered the world's first skyscraper due to the incorporation of structural steel in the walls.
While
Le Baron Jenney's Home Insurance Building is often seen as the first
skyscraper, buildings such as the Reliance Building, 1895, were more
unequivocal in their use of a steel skeleton. Shown here under
construction and completed, the steel-framed Reliance Building was
primarily designed by American architect Charles B. Atwood. The façade
is composed of plate-glass bay windows surrounded by terracotta
cladding. © Tango Images / Alamy Stock Photo
Two
technical advances, in particular, overcame the medieval limitations of
the tall building. In 1854, Elisha Graves Otis demonstrated a new
safety hoist in New York City. Cutting the single rope supporting the
hoist that he was standing on, the platform dropped a few centimetres
and then stopped, a result of the spring attached to the top of the
platform that engaged notches into the guide rails. Popular history
records this incident as the beginning of elevators, but the reality is
more prosaic. By this time, there were already many rival inventors and
companies thinking of ways to raise people through buildings. This
activity was a response to the growing societal need where rising land
values driven by the economic benefits due to urban density justified
higher building costs.
The
second major innovation was the incorporation of structural steel into
the load-bearing structure of buildings. In Britain, inventor Henry
Bessemer developed a manufacturing process in 1855 which dropped the
price of mild steel by a factor of six, enabling it to be produced more
cheaply than wrought iron.
A
Bessemer converter at the American manufacturer Lackawanna Steel in New
York, 1903. The primary process involved blowing air through molten pig
iron, which results in oxidation, removing impurities. Bessemer's
process took between 10 and 20 minutes to convert three to five tons of
iron into steel, replacing what would previously have been a full day of
heating, stirring and reheating.
This
was the beginning of the modern era of steel production; instead of
buildings using loadbearing masonry walls, they started to be
constructed as a steel skeleton surrounded by lighter-weight walls.
Buildings were no longer limited by the ability of large numbers of
people to walk up them. And their lighter construction reduced the loads
in the walls and in the foundations beneath them.
Of course, the Home Insurance Building in Chicago, a product of the capitalist society of the US in the late 1800s, needed an economic reason to drive its construction in addition to the technological ability to build it. High-rise construction was developing most rapidly in Chicago, and then in New York, where population levels were growing extraordinarily fast and land values were increasing. New York property values doubled between 1840 and 1870, meaning that owners had to maximise their space within more constrained plots.
Tall Buildings at Foster + Partners
Foster + Partners have a long history of tall building construction, starting with the Hongkong and Shanghai Bank Headquarters in Hong Kong in 1986. Other buildings by the studio followed: including Century Tower in Tokyo, Commerzbank Tower in Frankfurt and 30 St Mary Axe in London. Today, Foster + Partners continue to design innovative, high-rise structures working as integrated teams of architects and engineers to provide solutions that are logical, efficient, and elegant.
The north facade of the Hongkong and Shanghai Bank Headquarters in Hong Kong (1986). The Hongkong Bank was the first tower to challenge the central core concept that had persisted since the birth of the high-rise office building in the 1880s. By creating a suspension structure outside of the floor plates and redistributing the elevators, staircases, and bathrooms to the two narrow edges of the plan, it was possible to create uninterrupted, loft-like spaces throughout the building.
A tower is a cantilever, fixed at the base, and subjected to both vertical and lateral loads over its height. The applied 'live' loads are divided into the vertical loads due primarily to the building users, and the lateral loads that are due mainly to wind and earthquakes. The building also must support the 'dead' loads, that is the self-weight of the structure, the cladding, the finishes, and the services. These loads are carried by the structure down to the foundations. The following case studies of high-rise projects designed by the integrated team at Foster + Partners show the structural principles employed.
Lusail Towers: The most common way to resist the lateral loads in modern buildings is by using the building's core, the part of the building that contains the vertical circulation – the stairs and lifts required to provide access, as well as possibly the spaces needed to house the services. Above a certain height, anywhere from 150 to 250 metres depending on the building and the applied loads, the core gradually ceases to be the most efficient method of providing lateral stability on its own.
This
was the case for the Lusail Towers in Doha, designed by the practice
as part of the FIFA World Cup 2022 celebrations in Qatar. The project
consists of four towers – a pair that are 214 metres tall and a pair
that are 300 metres. For the taller pair of towers, the engineering team
mobilised the perimeter structure to provide greater strength and
stiffness. This is achieved with outrigger trusses that link the core to
the perimeter columns at two points over the building height.
425 Park Avenue:
In some instances, it is beneficial to the building plan to move the
core to the edge of the building. An example of this is the recently
completed tower at 425 Park Avenue, the result of a competition that
Foster + Partners won in 2012.
425
Park Avenue is the first full-block office building on New York's Park
Avenue in over 50 years. The 47-story tower includes a triple-height
lobby, world-class office accommodation, external green spaces, an
expansive social amenity level and a 38-foot-tall penthouse floor. Built
to LEED Gold standard, the building has earned Well Core certification
at Gold level, in recognition of its features which enhance the health
and wellbeing of occupants. © Nigel Young / Foster + Partners
This
project had two main structural drivers. Firstly, tall buildings in
Manhattan are legally required to fit within a tapering volume, which
becomes thinner over the building height to allow more light to
penetrate down to street level. Secondly, the client wished to move the
vertical circulation to the rear of the building, away from the Park
Avenue entrance, to create whole floorplates with views of the
streetscape and Central Park.
The resulting structure was conceived to be as minimal a response as
possible to resist the vertical and lateral loads acting on the tower
and where architecture and structure are completely aligned. The facade
could be removed entirely and the skeleton revealed would be completely
consistent with the way the building appears from the outside.
The
card model of 425 Park Avenue demonstrates the tower's structural
principles. A single line of columns starts at the top of the building
and bifurcates at two levels as it descends. Each bifurcation creates
diagonals that also connect to the core so that the columns and the core
act together to provide stiffness and strength in both orientations. ©
Foster + Partners
China Merchants Bank:
At 388 metres tall, the China Merchants Bank tower will be the tallest
building to date in Shenzhen. The stability for the building is provided
by a combination of a reinforced concrete core coupled with the
external columns around the perimeter of the tower. The area is subject
to seismic activity and the energy transferred into the building by the
ground movements is dissipated by 'buckling restrained braces' – a new
technology that forms part of the outrigger trusses linking the cores to
the perimeter frame.
270 Park Avenue:
In 2017, Foster + Partners entered a competition for the new
headquarters of JP Morgan at 270 Park Avenue. The brief called for a
building around 420 metres tall, on a narrow site just 43 metres wide
and, crucially, over underground railway lines that lead into Grand
Central Station.
During
the competition, the teams were informed that they would all be obliged
to adopt a specified structural arrangement with multiple columns
leading down into the basement to avoid the trains underneath. But the
Foster team created another approach, gathering the columns into fan
structures and bringing the loads down from each facade at just three
points. Inside the building, the loads come down on double columns at a
further three points.
The
structural solution is driven by the constraints of the site. The
external bracing on the east and west facade provides stiffness and
strength to the building over and above that provided by the core and
outriggers. The building doubles the density of the previous building on
the site and 97 per cent of the building materials from the original
building were recycled, reused or upcycled.
Density Debate: Carbon analysis of tall buildings
Despite the advances in efficient high-rise design and construction, tall buildings are still synonymous with high consumption. The question remains: are tall buildings a feature of a sustainable, carbon- and material-aware future? Or should they be consigned to the past?
Research into this area has been developing recently, and urban economists in particular have led the way. Harvard professor Edward L. Glaeser has published extensively on the subject and his work directs us to an initial answer. He writes: 'Low-density suburbs will leave a significantly deeper carbon footprint, it turns out, than Americans who live cheek by jowl in urban towers … When environmentalists resist new construction in their dense but environmentally friendly cities, they inadvertently ensure that it will take place somewhere else – somewhere with higher carbon emissions.'
Compared with suburban and rural areas, residents in urban environments have lower annual carbon emissions – a key metric for measuring environmental impact. What's more, the tall buildings can more densely accommodate a growing global population, two thirds of which will be living in urban zones by 2050.
Two carbon footprint metrics are useful here: the whole-life carbon
of a building, and the household carbon footprint of the building users.
The whole-life carbon footprint accounts for the carbon produced during
construction (embodied carbon) and the carbon produced during its use
(operational carbon). Improvements in the insulation and power
requirements of modern buildings have resulted in a substantial drop in
their operational carbon footprint and, as a result, the embodied carbon
of a construction has become relatively more important. The second
metric is the household carbon footprint, which measures the greenhouse
gas emissions of the individuals within a household, including their
requirements for energy, transportation, food, goods and services.
Tall buildings can more densely accommodate a growing global population, two thirds of which will be living in cities by 2050.
The
diagram below shows an estimate of the household carbon footprints of
residents across 31,500 zip codes in the United States by Christopher
Jones and Daniel Kammen of the University of California, Berkeley. There
is a recognisable pattern. The densest areas, the city centres, show
the lowest household carbon footprints, represented in green. The
suburbs show the highest, seen in red. The primary reason for the
difference between suburb and city centre is the reduction in private
transport. Transportation's carbon footprint is about 50 per cent higher
in large suburbs compared to large principal cities, while total
household carbon footprints are about 25 per cent higher.
Other
studies are consistent with these results. Professor Glaeser examined
metropolitan areas across the United States to compare the difference in
the household carbon footprints between the suburb and city centre for
each area. In the graph below, each bar represents the increase in
household carbon footprint between the city centre and its corresponding
suburb. Households in northern cities (New York, Boston) use more
natural gas for heating (shown in light blue), while households in
southern cities (Atlanta, Houston) use more electricity for cooling
(shown in dark blue). And the carbon due to transport (in magenta) is
consistently higher in the suburbs compared with the city centres.
The
same question was addressed by Colin Beattie and Peter Newman in
Sydney, Australia, where the carbon footprint of 4,000 living spaces – a
mix of high-rise, mid-rise, low-rise and individual houses – was
compared. Once again, the study found that the higher emissions due to
private transport (in magenta) in detached houses in low density areas
was the defining factor that led to these areas having the largest
carbon footprint.
But
what happens when we include the embodied carbon of construction into
the calculation? Are the environmental benefits of dense city living
cancelled out by the additional carbon emissions of construction of tall
buildings?
This is an emerging field, but comparisons have been carried out by
the Foster + Partners Sustainability Group and published by the World
Green Building Council. These compared the whole-life carbon of a tall
building (sixty storeys) in a dense city with strong public transport
connections to a low-rise development (three-to-five storeys) in a
suburban location accessed almost exclusively through private vehicle
transport. The results also anticipate the future electrification of
transport, as well as the reduced carbon content of the materials due to
projected reductions in fossil fuel use.
The
results show that even though the embodied carbon of the structure
(shown in yellow) is greater for the high-rise than for the low-rise,
the carbon footprint of private transport (in magenta) for the
less-dense development outweighs the embodied carbon savings from
low-rise construction.
Economically speaking, the density that tall buildings unlock in our
cities enables public transport to become viable, and local travel
distances become shorter, permitting more walking and cycling.
Additionally, there is an operational decrease in carbon emissions from
shared heating and cooling in densely populated structures. As a result,
and as the research set out above has shown, despite the higher
embodied carbon cost, the total carbon footprint of a tower in a dense
city centre is still lower than a low-rise low density suburban
development.
The Future
This brief dive into the history of tall buildings and their structural development – up to some of Foster + Partners' most advanced and contemporary high-rise designs – has shown a consistent march towards structural lightness and efficiency. It was the shedding of weight, from loadbearing wall structures to frame systems, that enabled us to climb past millennia-old height constraints.
We need to continue to design and build lean buildings, not only to reflect the forces acting on them and the constraints of their locations, but also to reflect a design philosophy that considers resource scarcity. Any departure from efficient design needs to be justifiable through the benefits to those who use the building or its vicinity.
Broadening our understanding of historical structures, we also know
that, wherever possible, we should consider reusing existing buildings
to avoid incurring the embodied carbon of new construction. When this is
not practical, we should recycle the materials from the buildings that
we remove.
And we need to make buildings that last. A city street can last for
500 or 1,000 years, while a building might be pulled down after twenty.
Our philosophy needs to change; we need to think of buildings as
vertical streets adding useful density to our cities, whose functions
can change, but whose structure remains.
We need to build efficiently, reuse and recycle wherever possible, and minimise carbon in building construction and use.
Tall
buildings came about through a combination of technological innovation
and societal need. A tipping point occurred around the second half of
the nineteenth century, and a period of rapid growth followed. We are at
a tipping point now with regards to our climate. We need to build
efficiently, reuse and recycle wherever possible, and minimise carbon in
building construction and use. But we cannot consider buildings in
isolation from their urban context. Increased density results in lower
overall carbon emissions and creates places for people to commune and to
thrive.