Unveiled to a crowd of thousands amid an impressive firework and laser display, the Burj Khalifa – renamed from Burj Dubai after the president of the UAE Sheikh Khalifa Bin Zayed Al Nahyan – soars 828 metres into the sky, breaking many records; most significantly having surpassed the height of Taipei 101 by 320 metres to steal the ‘world’s tallest building’ crown. But also hugely significant are the design, engineering and construction firsts. The 160-storey Burj Khalifa developed by Emaar Properties employs a record breaking 330,000 cubic metres of concrete, 39,000 metric tons of steel rebar and 142,000m² of glass, And it took 22 million hours’ labour to construct. But where did it all begin?
Seven years ago, Emaar Properties approached Chicago-based architecture firm Skidmore, Owings & Merrill (SOM) with the intention of building the world’s tallest building.
Robert Booth, Emaar International MD for Canada and the USA and then Emaar Design Studio MD Mark Amirault (now a senior director at JZMK Partners) met with SOM’s Adrian Smith (now of Adrian Smith + Gordon Gill Architecture) and two colleagues in New York on the basis of seeing the Jin Mao Tower that Smith had designed in 1993.
“At the end of the discussion they asked me how I thought they should go about selecting an architect for this project because they had interviewed several and felt each had the ability to perform the task,” recalls Smith. He suggested Emaar host a two-week ‘ideas’ competition, which it did, only to opt for Smith’s entry.
Smith says the initial inspiration was a combination of factors: “The tripod like shape of the tower was developed on the basis of previous structures used for residential towers, such as the Tower Palace III I designed for Samsung in Seoul Korea. That was also used in Chicago’s Lake Point Tower Palace and architect Mies Van De Rohe’s Friedrichstrassa prototype tower he planned for Berlin in the 1920s. “I wanted the tower to feel as though it was growing out of the ground and organic, with a spiral quality; this was achieved by stepping the length of the legs back as it climbed upwards.”
Smith was inspired by a regional desert flower, the hymenocallis. Like the petals from its stem, the tower’s wings extend from the central core. In three months, the basic concept of the tower was established, but the design work lasted for one and a half years and the construction documents were completed within three years.
The major challenge was to design the building so it would not move with a high level of acceleration during peak winds, according to Smith. This was helped by the size and frequency of the steps as was limiting the stack effect (or the movement of air inside the tower); an issue that governs the design and construction of all tall buildings. Turner Associates was appointed construction manager on the project and the general contractor was a joint-venture between Korea-based Samsung Corporation, Belgium-based Besix and Arabtec, headquartered in Dubai.
Ahmad Abdelrazaq, vice president and executive director of the High-rise Building and Structural Engineering Division, Samsung Corporation, was involved in the structural planning and structural design of the Burj Khalifa. He reveals that it was also a challenge to complete the building within the allocated time, taking into account the harsh environment. From the onset of the project, the plan was to mechanise and streamline the entire construction process for it to become repetitive and speedy. “In addition to the tight construction schedule, we needed to place a huge volume of concrete; pumping it up to 600m above ground level and transporting materials and resources to the highest level,” says Abdelrazaq.
This had never been done before, adds Hyder Consulting mechanical engineer Alastair Mitchell, who took on full responsibility for the planning, execution and ultimately the closure of the mechanical engineering activity for Burj Khalifa. The tower also had to be built to within acceptable tolerances (vertical and lateral) that would not compromise the technical execution of the project and its long-term performance, including elevator systems, mechanical systems, cladding and so on, Abdelrazaq explains. To overcome this, he performed an extensive construction sequence analysis to predict the actual movement of the building as it was built.
“This was probably one of the most comprehensive construction sequences that had ever been done before for a tower of this height, since it included the prediction of the short- and long displacements (vertical and horizontal) of the tower due to elastic, creep and shrinkage as a function of every single construction activity, says Abdelrazaq.
“We extensively monitored the movements of the tower from the foundation to the tip of the pinnacle to correlate the predicted movement with the actual behaviour. The agreement between our prediction and what we actually measured was excellent despite it being a concrete building.” The tower required a significant amount of early planning for all concrete works and full-scale testing was performed to simulate the actual construction to avoid any technical issues that could have arisen due to any unusual and unpredictable conditions.
Abdelrazaq says the original construction detailing at the outrigger levels was difficult and could have resulted in potential delay on completing the mechanical floors on time. Therefore on these levels, the contractors recommended changing the outrigger wall panels into composite elements – in agreement with design consultants and the owner – to speed up the construction.
Constructing the spire was an accomplishment in itself. Erecting the steel structure at heights above 600m was never going to be easy, considering the wind effects and the time involved transporting the steel components to such heights. This was made even more difficult by the thin design of the spire and the fact that cranes could only be climbed up to certain heights. “Just like the empire state building, the pinnacle was built from within the tower at level 156 and lifted to the highest structure within a very short period of time”, recalls Abdelrazaq.
“The pinnacle and its cladding attachment were lifted in three steps and erected with a high degree of tolerance in spite of wind conditions and the spire geometry. The lifting was a novel one and it took a lot of planning before the operation started,” he says.
Furthermore, contractors had build the tower within a strict tolerance while it was constantly moving in the wind. To overcome this obstacle, a creative and innovative survey procedure was put in place from the onset and GPS survey technology was utilised to build the tower when at great heights.
Wind engineering management started from the early design stage, with wind forces on the tower reduced by disorganising the vortex shedding along the height by constantly changing the shape of the tower; creating proper orientation of the tower to the wind and, most importantly, reducing the wind vibration of the tower. “Almost every wind engineering trick was used from the foundation to the very pinnacle. The tower shaping added spoilers and the helical shaping strategies (setbacks) had significant effects in reducing the cross-wind response and the creation of different resonance conditions along the height of the tower.
In fact, “the wind engineering techniques used will be incorporated in future tall-building planning; a testament to successful collaboration between architecture and structural engineering concepts,” asserts Abdelrazaq. Since the building was constantly moving, the geometric centre had to be established before setting the steel and concrete elements into their theoretical design elevations.
Extensive analytical work was also performed to study the interaction of the spire and the crane during construction as the crane imposed significant loads on the spire and potential effects of high winds and major seismic events had to be taken into account,” explains Abdelrazaq.
“All sorts of due diligence works were performed to ensure the safety, stability, and the behavioural characteristics of these elements during construction. In fact, a big seismic event did happen during the spire construction and the building behaved exactly as predicted.”
There were many lessons learned during the design and construction of the Burj Khalifa, all of which were observed with interest by Chicago-based Council on Tall Buildings and Urban Habitat (CTBUH); the organisation that determines the world’s tallest building based on specific criteria.
“It’s not one single revolutionary finding that make taller buildings possible; it’s a whole range of small inventions, efficiencies and improvements of materials, methods and management,” asserts CTBUH research and communications manager Jan Klerks. But technically, Klerks says the Burj Khalifa uses “pretty straightforward techniques to ensure the building stands up”. The tower is built of concrete with embedded steel plates until the 156th floor. From there a lighter steel structure takes over to the top.
Although, he points out that usually a tower has a rectangular plan, but such a plan would not suffice for a building of that height. By designing a Y-shaped structural plan, one wing buttresses the other two wings. In the centre of the building, there is a hexagonal concrete core that encloses all the elevators and acts as a giant axle. This design is referred to as the Buttress Core.
SHAPING THE FUTURE
In fact, Burj Khalifa represents various advancements in construction, materials, engineering and technologies; mainly in the use of concrete.
“In the early 1900s when concrete was proposed for tall-buildings, engineers were concerned about its behaviour as there was very little information available. And even in the 1960s-80s, concrete strength was limited; it was very heavy and wasn’t an economically-viable solution due to the large member sizes and the effect on the floor efficiency and construction period of the tower, which was longer than steel construction; thus a major impact on financial viability. Generally steel was looked at as the solution for super-tall buildings,” says Abdelrazaq. However, now concrete has become a highly-competitive solution with the availability of high performance or high-strength concrete, asserts CTBUH research and communications manager Jan Klerks. But technically, Klerks says the Burj Khalifa uses “pretty straightforward techniques to ensure the building stands up”. The tower is built of concrete with embedded steel recent advancements in concrete materials and technologies, the availability of high capacity pump, high-flow concrete and automatic formwork systems. In addition, concrete is easier to plan, needs less leading time, requires a less killed labour force; particularly significant in regions where skilled labour is scarce, and – because of concrete’s high strength, stiffness, added damping etc.– structural concrete is an economically-viable solution.
“Burj Khalifa is a testament that concrete is a viable option for supertall buildings,” he adds. Abdelrazaq tells The Big Project that many structural options were considered for the development of the Burj Khalifa, including a composite solution.
“But after my extensive review of the construction market in Dubai, I was convinced the concrete option was the best solution for the tower. We came up with a solution that addressed all the fundamental issues typically faced in highrise building construction.
He was easily able to predict the structural behaviour with a high degree of accuracy and without fully relying on powerful computer analytical tools that were only used to verify the solution.
“This structural concept of the tower had approached 95% cantilever efficiency, which is the best we can do in designing super-tall buildings. This simplicity in design led to simplicity in construction,” asserts Abdelrazaq.
Today, some of the highest skyscrapers proposed use similar systems as Burj Khalifa, which proves its efficiency in design and construction.
“The development of the building has proven to be a testament to engineering and construction success stories; built efficiently, designed with due diligence to excellent engineering solutions and on the way we managed the gravity loads, wind loads, seismic loads and elimination of relative shortening, while minimising the amount of materials used for the project.”
No other tall building has been given the “quality of materials, coupled with the slender spire the Burj Khalifa was afforded and these elements will be very difficult to duplicate”, adds Smith. But with various projects proposed that surpass the height of Burj Khalifa, incorporating many of the design and construction concepts discovered during the development of the tower, the question is raised of ‘how long will the Burj Khalifa be able to maintain its world’s tallest building title for?’
Council on Tall Buildings and Urban Habitat’s Klerks says he cannot tell. The 1001m tall Kingdom Tower project in Saudi Arabia seems to be the most serious contender for the title, but planning a next world’s tallest and completing one are two different things and even if someone started constructing it today, it would take five years to build,” he asserts.
However, Klerks can offer a clue as to what future trends will be seen in the development of tall buildings. “We’ve already witnessed a more sustainable awareness when it comes to designing, constructing and managing tall buildings; Steven Holl’s Linked Hybrid in Shanghai is a good example”.
“Another trend is tall buildings with more interconnections between them, instead of a standalone development; this could be done through sky bridges or even platforms of parks between two buildings, such as the Marina Sands development in Singapore,” adds Klerks.
He says the Burj itself has been quite a trendsetter in the way it has been developed in conjunction with the surrounding Burj Khalifa area is “different from most generic tall buildings, which are usually developed as a standalone project”.
Abdelrazaq concludes: “Burj Khalifa was the catalyst for new surge in tall building buildings all around the world and in the Middle East in particular. “Burj Khalifa made Dubai the hub of financing in the Middle East and has brought so much investment to the region regardless of the financial crisis. Development visionaries had the courage to go into this venture and bring fame to Dubai from the desert sand.”