The passion, obsession and necessity of building supertall and mega tall structures continues to challenge engineers and architects to reach new heights and ‘go where no man has gone before’.
In this thinking piece, Professor Kourosh Kayvani, one of Australasia's leading structural engineers, discusses the key historic developments, technological advances, current trends and engineering considerations of high-rise buildings.
Vertical habitation isn’t a new trend. It’s one that has been driven by urbanisation and bustling, overpopulated cities for centuries. While the high-rise buildings that we know today became possible with the inventions of elevators, newer building materials and structural engineering systems, multi-storey construction dates back to the Roman Empire and vertical cities have been around for centuries.
A living example is the 16th century Yemeni city of Shibam with mud brick tower houses of five to eight stories high built to protect the occupants from Bedouin attacks.
Social, economic and technological developments in the latter parts of the 19th century created the environment for modern high-rise buildings to emerge in the North American cities of New York and Chicago.
In 1852, Elisha Graves Otis built the first ‘fall safe’ hoisting system (the elevator) allowing vertical transportation of people and goods in multi-storey buildings. This invention made the construction of skyscrapers possible and as a result greatly altering the way modern cities were planned and constructed.
In 1885, the Home Insurance Building in Chicago (originally 10 stories and 42 m high) demonstrated the use of the first steel framed gravity system. It was the first tall building to be supported both internally and externally by a fireproof metal frame, which allowed for large windows at the ground level of high-rise buildings.
This set the trend for use of steel frame gravity systems in tall buildings as the loadbearing masonry system used to that date was very inefficient economically beyond 15 stories. This limitation is evidently demonstrated by Monadnock Building (1893) in Chicago which, at 17 stories high, was the tallest in the world at the time and was the first in use of a portal system for wind bracing. However, owing to its loadbearing masonry system, it had walls up to 1.8 m thick at the ground level that made the ratio of its “net-lettable-area” to its total built area simply too low to be economical.
In 1889, the Eiffel Tower doubled the height of the previously tallest Washington Monument, rising above 300 m with the use of pre-assembled iron components to create what has since become the iconic landmark of Paris. This new architectural concept at the time provided a great boost in general confidence in viability of tall metal structures.
The Ingalls Building (now called the Transit Building) built in 1903 in Cincinnati in the United States of America is considered to be the first “reinforced concrete skyscraper” (Condit, 1968). This 16-storey building was built by monolithically casting the columns, floors and walls in concrete of relatively low strength by modern standards (f’c < 20 MPa).
Cincinnati architectural firm Elzner & Anderson designed what was considered a daring engineering feat at the time (people feared the building would collapse under wind loads or its own weight), but the success of the building led to the team creating the tallest reinforced concrete structure.
From the 1950s through to the 1970s, great technological advancements took place that allowed architects and engineers to aspire to greater heights. Some of these advancements included high-strength bolts replacing hot-driven rivets, the emergence of glass-metal curtain wall facades, the use of electric arc welding in shop fabrication and the compressive strength of concrete catapulting from 40 MPa in the 1960s to 65 MPa in the 1970s (and eventually 100 MPa+ in the 1990s).
These technological advances, combined with a deeper understanding of structural behaviour and analysis under environmental loads (particularly wind loads), led to the emergence of supertall buildings being built in Chicago during the 1960s and 1970s. These supertall buildings were conceived by structural engineers as “tubular” schemes where the entire structure was designed as a cantilevered “tube” (John Hancock Centre, 1969) or a bundle of tubes (Willis Tower, 1974) resisting wind loads.
The increasing rate of urbanisation in recent decades has seen an accelerated trend in the construction of high-rise and tall buildings worldwide, particularly in the emerging economies of the world.
A fundamental economic driver for the growth of tall (particularly residential) buildings is the scarcity of land in the densely urbanised parts of the world. The competition for constructing the tallest building in a city, country, region or the world has acted as another driver for the growth of tall buildings worldwide. In the past two decades or so, the race for constructing the tallest has been extended to include the contest for constructing the most iconic and spectacular high-rise buildings often characterised by complex geometries and leaning/twisting forms.
Over the years, the Council of Tall Buildings and Urban Habitat (CTBUH) has been recording data of tall building structures, showcasing how they continue to rise in height.
Below is a graph of the timeline and height of tall buildings, culminating at the 828 m-tall Burj Khalifa in Dubai in 2008. (Click image to enlarge).
According to the CTBUH’s annual review of tall buildings, 97 buildings taller than 200 m were completed in 2014, which is the most ever in a year, with 60% of completions being in China. 11 supertall buildings (more than 300 m high) were reached in 2010, 2011 and 2012. South America completed its first supertall building, the 300 m-tall Torre Costanera, and the tallest building completed in 2014 was the One World Trade Centre in New York at 541 m.
The definition of ‘tall’, however, has changed over time. According to the definition given by CTBUH, a 200 m+ building is ‘tall’, 300 m+ is ‘supertall’ and 600 m+ is ‘mega tall’.
However, architectural and structural factors such the context of the building, location, and the slenderness of the building (i.e., its height-over-base ratio) would demand more flexible definitions. Skinny skyscrapers, for instance, aren’t possible everywhere and this has more to do with the target market and location than engineering and design capabilities. An example of this is the super skinny tall buildings in New York City, where each apartment is a penthouse that occupies an entire floor of the building. Engineers consider tall buildings with a height-to-base ratio in excess of 1:10 or 1:12 to be slender or skinny.
While tall, skinny buildings present a number of design challenges, the developers of the property need to be able to fill it with tenants who are willing to pay for the special views that a penthouse-style, super tall and skinny building offers. Location and environmental factors also influence how slender the building can go. Wind engineering is, for example, a fundamental aspect in creating tall, skinny buildings.
As design philosophy evolved, architects started to use the structure to inspire the forms of the buildings they designed. Examples of this include Bank of China Tower in Hong Kong (which at the time of its construction set the record of the tallest building outside North America) and Gherkin in London, where architects and engineers took adaptable approaches to the diagonalised grid structures on the building facade to create structures of efficiency and elegance.
The design and form of original supertall buildings used to be structurally-driven, such as the John Hancock Centre of 1969 that had exposed structural steel as part of the design.
Today, architects and designers have more form freedom than ever before. Advancement in design and construction techniques allows engineers to assist their architect colleagues to create buildings that would have been unimaginable a few decades ago.
While complex forms would often result in an increase in construction cost, a careful and sophisticated engineering approach is required to achieve the architectural vision without unnecessary cost overruns. Whether it is in the choice of the lateral load-resisting structure and/or floor systems, or in the approach for integrating the structure in the overall geometry and architecture of the building, the decisions made by the structural engineer have a profound impact on the cost, amenity, constructibility, and sustainability of tall buildings.
The locations of the tallest buildings in the world, as well as the function of the buildings and the materials used to construct these buildings, is rapidly changing. Only 20 years ago, 75% of the 100 tallest buildings in the world were located in North America and as of 2014, this figure is less than 25%, with the shift occurring predominantly to Asia and the Middle East.
The function of tall buildings has also changed in a significant way over the past five years. In the past, the function of the 100 tallest buildings in the world moved away from the predominantly office buildings that have dominated the tallest lists for many decades to more residential and mixed-use functions. Growing populations and rapid urbanisation in developing countries explain why so many tall buildings are being developed for residential and mixed-use purposes instead of for commercial office use.
The structural materials used in high-rise buildings are typically one or a combination of (reinforced or pre-stressed) concrete, structural steel and composite systems.
Structural material systems for high-rise buildings should be chosen by carefully considering architectural, economical and site factors. The economic drivers vary by geography as the relative costs of material, labour, time and space vary from one location to another. Other factors to consider in choosing the structural material include:
Preferences and the economic viability of the different structural materials that are used in tall buildings’ construction are also changing. In 1970, 90% of the 100 world tallest buildings were all-steel buildings. Today, all-steel buildings account for less than 15% in favour of concrete or composite structures. The cost of the material, technological expertise and the way that tall buildings are being built all influence this change in material selection of tall buildings.
Driving economic design in the construction of high-rise buildings isn’t the same all over the world. What is cost-effective in one country won’t necessarily be cost-effective half way around the world. Materials, labour cost, the value of time and the value of space all need to be carefully weighed in order to drive economic design.
From a structural engineering point of view, as high-rise buildings get taller and more slender, their design becomes increasingly (and fundamentally) influenced by specific behavioural factors that are much less significant for shorter buildings.
These factors include the dynamic response of tall buildings to wind loads both in the ultimate and serviceability limit states, and the differential axial shortenings of the vertical elements of tall buildings under gravity load effects. As far as these factors are concerned, the absolute height of the building is not necessarily the best measure for “tall behaviour”. In particular, the magnitude of the dynamic wind response is more significantly influenced by the overall slenderness of the building and the natural frequencies of its fundamental modes (i.e., the first two sway modes about the principal axes of the building and its first torsional mode) than its absolute height.
The overall slenderness of a tall building is usually defined by its “height-to-base ratio”, being the height of the building divided by its narrowest plan dimension. Essentially, higher height-to-base ratios and lower natural frequencies increase the dynamic component of the response to wind. A building with a height-to-base ratio of more than around 5 and/or a fundamental natural frequency of less than approximately 0.2 Hz is expected to respond to wind loads in a significantly dynamic way (where building inertial effects are significant) or even in a potentially aero-elastic fashion (where building motion interacts with and influences the wind flow).
Wind loads affecting the design and construction of high-rise buildings are intrinsically dynamic and random in nature (in both time and space).
Wind speed can be described as a mean value upon which random fluctuations or gusts are superimposed. The wind loads arising from the mean and gust wind speeds are called Mean and Background components, respectively.
For slender tall buildings, there is a third component of wind load namely the resonant component that dominates the structural behaviour. The Resonant wind load is the result of the fluctuating frequency of wind effects (for example “vortex shedding” effects) matching the natural frequency of the building structure. This results in amplifying effects much the same as the amplifying effect resulting from a playground swing or a pendulum being pushed at the same time interval (i.e., frequency or period) as its natural swinging frequency or period.
When designing a building, the Mean, Background, and Resonant wind loads need to be considered:
|Building type||Mean wind load||Background wind load||Resonant wind load|
|Low rise||Significant||Very significant||Negligible|
Wind has always been an important consideration when erecting tall buildings and it becomes more important and complex as the height increases.
Wind loading on tall buildings can have a direct impact on the serviceability of the building as well as the comfort of occupants, as they will be able to perceive building motion (acceleration) due to the Resonant component of wind if appropriate design measures aren’t used to mitigate excessive building vibrations.
Controlling wind motion or acceleration to acceptable levels for human comfort is often a critical aspect of tall building design. As is the case with other resonant dynamic effects, increasing damping is often the most effective method in controlling wind motions. However, the issue is that a reliable level of damping in buildings is often quite limited (i.e. equal or less than 1% of critical damping). Therefore, additional dampers would often be required. A common approach is to add tuned mass dampers (TMDs) at or near the top of the tall building. A TMD acts essentially as a shock absorber, pushing the building against the force of the wind.
The prime objective of seismic design is clearly to provide life safety. The common practice to achieve economic and safe design is to dissipate seismic energy in the structure during an earthquake by forming controlled and stable “damages” in the structure (in the so called “plastic hinges”).
To ensure that damage is distributed rather uniformly among floors and that the gravity load path is not compromised, engineers often use what is called a “strong column/weak beam” design philosophy, which stipulates that the columns of a joint needs to be at least 20% stronger than the beams framing the same joint. While this philosophy provides life safety, the implication of widespread plastic hinges is extensive damage throughout the structure to the extent that the building might be damaged beyond repair due to an earthquake.
Christchurch’s central business district saw the devastating impact that an earthquake can have on a city after the 6.3 magnitude earthquake of 22 February 2011 led to the death of 183 people in New Zealand’s second-largest city.
The seismic forces generated by very large ground accelerations (peak of 1.8 g horizontal and 2.2 g vertical) caused widespread damage to buildings and infrastructure. While the majority of high-rise buildings performed satisfactorily in so far as providing safety to the public during and immediately after the earthquake, the extent of damage to many high-rise buildings were beyond repair. Originally the total cost to insurers for rebuilding was estimated at NZ billion, but this skyrocketed to NZ billion within two years.
The heavy financial cost of the Christchurch earthquake demonstrated the value that a “low-damage” design philosophy could have offered. At Aurecon, we are a world leader in low damage design solutions for buildings, aimed at reducing primary structural damage in bracing systems. By concentrating and dissipating seismic energy in predefined parts of the building, a low damage design philosophy creates a more resilient system that increases post event operability for owners and tenants. The additional capital cost of this investment can range from negligible to approximately 4% with a base isolation system. Downstream advantages include secure tenants, lower insurance premiums and fundamentally a safer built environment.
While base-isolation systems, such as friction pendulum bearings, have been in use for some time, we have developed innovative “rocking” and “sliding” frame systems in collaboration with major universities in New Zealand. For example, we have incorporated post-tensioned rocking walls in high-rise building designs to increase the natural period of the building (without damaging it) and thereby reduce how much force a building “feels” during an earthquake (see image below). The post-tensioning elements provide restoring forces to bring the building back to its position after the rocking motion caused by earthquake pulses.
The global impact of buildings means that engineers and designers need to start creating more sustainable high-rise buildings. Currently, buildings account for 40% of global energy use, 15% of water use and 30% of the waste that is generated.
Although the drive to deliver good, functional and economical designs for high-rise buildings is not changing fundamentally, the focus on produce energy efficient and sustainable designs is expected to increase at an accelerating pace. Tall buildings are proportionally more material- and energy-hungry than lower rise buildings. In high-rise buildings the structure is a large portion of the overall cost and embodied energy, and hence, the structural engineer can significantly influence the overall sustainable design outcome.
Sustainable structural design goals can be achieved by addressing the following three objectives: reduce, reuse and recycle. Advanced analysis and design methodologies allow us to design increasingly more efficient structures (with just the required amount of material and no more). Also, new material technology is opening the way for the reduction of the embodied energy per unit of material (in terms of transport energy, sustainable supplies, and the like). The use of industrial by-products such as fly-ash, slag and silica fume as a cement substitute can drastically reduce the embodied energy of concrete.
“Reuse” is about adapting the use of a high-rise building while keeping the original structure. There are growing examples of adaptive reuse of high rise buildings globally. To achieve future reusability of high-rise buildings, an important design consideration is the provision of “planning flexibility”. This can be achieved in the design phase by a generous choice of structural grid, live load allowances and the like (i.e. use longer spans and larger live loads that are more adaptable to future reuse). The emergence of building information modelling (BIM) as a repository of information for asset management (as-built drawings, mill certificates and the like) is also expected to facilitate future reuse opportunities. Future high-rise buildings are likely to be designed with more consideration given to the recyclability of structural components (beams, columns, etc.).
While the trend in the development of higher strength steel and concrete is not stopping, use of new material with superior performance (like fibre reinforced concrete) and/or superior sustainability (such as engineered timber) is gaining significant momentum. While timber buildings of taller than ten stories have already been achieved, there are significant research and development projects underway globally aiming to construct buildings as tall as 40 stories in steel-reinforced timber.
The other growing trend is in offsite fabrication of high-rise buildings. As labour costs escalate relative to material costs and as the construction safety and quality gain increasing attention, solutions involving prefabricated or manufactured structural components and building modules are gaining increasing popularity. There is a growing trend in construction of high-rise buildings from fully modular systems.