Material Marvels: Climbing high
The Burj Khalifa, United Arab Emirates, stands as an impressive feat of architecture and engineering, comprising meticulous structural design, as Ellis Davies reports.
Dubai, it seems, is somewhat of a playground – a canvas on which to let the ideas and imaginings of people run wild. With areas, such as the infamous Palm Islands, made entirely by humans, the city is a physical representation of the species imposing its will on the world. A fitting place, then, for the world’s tallest building, Burj Khalifa, which is seen to have redefined the possibilities for the design and construction of super-tall buildings.
Located in Dubai’s downtown, the building stands at 828m with 200 storeys – 160 inhabitable. It contains residences, hotels, a restaurant and bar, an observatory, and corporate suites, as well as the tallest service elevator and longest elevator in the world to navigate it all.
Excavation work began in January 2004, and it took 12,000 workers a collective 22 million man-hours to complete its construction by September 2009, for an official opening in January 2010. The resulting work is the holder of a number of records, including those already mentioned. It also holds the world records for tallest freestanding structure, highest number of stories, and highest occupied floor and observation deck.
The design and structure
The architects in charge of building the world’s tallest tower was the Chicago office of Skidmore, Owings & Merrill LLP, USA, with Adrian Smith as a consulting design partner. The exterior of the building features a triple-lobed footprint, and is designed to replicate the hymenocallis, or spider lily, a sprawling white flower native to southeastern USA, Mexico, Central America, the Caribbean, and northern South America. It has three main elements surrounding a hexagonal central hub, with a modular, Y-shaped structure. The setbacks – recesses in a wall that reassemble steps that feature along each of the wings extending out from the hub, help to provide stability.
The core of the building provides torsional resistance to the structure, and it is from here that corridor walls extend to the near end of each wing, where they fix to thickened hammer head walls. These are described as behaving in a similar way to the webs and flanges of a beam to resist the wind shears and moments. The system is capped off with perimeter columns and a flat plate floor construction.
Gravity is dealt with by using outrigger walls to link the perimeter columns to the interior wall system on mechanical floors, which allows the vertical concrete structures to support both gravity and lateral loads. This makes the tower very efficient as the gravity loading system is used to benefit the resistance to lateral loads.
As expected in the design of a super tall building, wind had to be taken into account. The Y-shaped plan is spiralling, providing aesthetic and functional advantages, while also helping to reduce the wind forces upon the tower. It confuses wind. As the tower extends upwards, the profile of the building regularly changes making the boundary layer of wind turbulent. This reduces wind force, deflecting it around the structure and preventing the formation of organised vortices that would rock the tower from side to side and damage the building. However, even with these precautions, the building can move by up to two metres at the top – although, this is below the three metre mark at which people start to experience discomfort.
Over 40 wind tunnel tests were carried out during the design of the building to examine the possible affects. Large structural analysis models were used, along with façade pressure tests and microclimate analysis of the effects at terraces and at the base. A special consideration was also paid to the stack effect – a phenomenon that is caused by the changes in pressure and temperature with height – that is closely linked to the design of super tall buildings.
According to consulting engineering firm Rowan Williams Davies & Irwin Inc., which worked on the design and testing of the Burj Khalifa in relation to wind resistance, the original design was based on a building that was not as tall – approximately 300m shorter. The increase in height meant that the group was charged with finding ways to reduce wind load effects on the building foundation to accommodate the change. One method used was to change the number of setbacks – making it possible to substantially reduce forces by encouraging vortex shedding – an oscillating flow of air past a body, creating low-pressure areas to which the body will tend to move towards – that is not synchronised over the height of the tower.
The foundation of the building is a large reinforced concrete mat supported by bored reinforced concrete piles. This was chosen after geotechnical and seismic studies – Dubai is not particularly susceptible to earthquakes, but, given the Burj Khalifa’s size, it’s best to be safe. The mat is 3.7m thick, and is the result of four pours of concrete measuring 12,500 cubic metres. The piles are the largest and longest conventionally available in the region at 1.5 x 43m. Corrosive chemicals present in groundwater are also guarded against using high density, low permeability concrete in the foundations and a cathodic protection system beneath the mat.
In all, 45,000m3 of concrete were used in the construction of the foundations, clocking in at 110,000 tonnes and stretching to 50m deep. Alongside this, 39,000 tonnes of steel rebar was used throughout the structure, and a total of 330,000m3 of concrete. If the rebar were laid end-to-end, it would extend around one quarter of the world’s circumference.
A high façade
The façade of the Burj Khalifa accomplished a world record in being the highest installation of an aluminium and glass façade – 512m up. The aluminium used is the equivalent weight to five A380 aircraft, and forms the frames for the 28,000 prefabricated panels of double layer glass that make up the curtain wall. The wall also features stainless steel bull nose fins, which in total length is 293 times the height of the Eiffel Tower, France.
The façade is coated, and transmits 20% of visible light and 15% solar energy. If the wall was flat, this would have caused the building to become blinding. It is, however, a rounded design, but as curved glass was outside the budget, this is achieved using flat panels of glass with angled joints concealed behind the steel fins.
The final design is one of three full-scale mock-ups presented by Skidmore, Owings & Merrill. The façade was one of the first elements of the building to come together, but would later cause delays due to a contract handover to the Far East Group.
In 2015, the façade had a new addition in the form of the world’s largest LED screen. Covering a total area of 33,000m2, the screen was first used to bring in the New Year in 2015, and was the canvas for a display of dynamic art installations in 2016.
The inside of the Burj Khalifa, however, was also meticulously designed to fit the city and clientele it now caters for.
The Chicago office of Skidmore, Owings & Merrill, led by designer Nada Andric, took on interior design. The tower features an array of glass, stainless steel, and polished dark stone, as well as silver travertine flooring, Venetian stucco walls, and handmade rugs and stone flooring. Inspired by local culture, the interior also features 1,000 pieces of art from Middle Eastern and international artists, many of which were commissioned for purpose.
The first eight levels of the building are home to the Armani Hotel Dubai, as are levels 38 and 39. Residences are found on levels nine-to-16, as well as 45–108.
Capping of the building is its telescopic spire. Covering the final 200m, the spire was constructed from the inside of the building before being hauled into place with the use of a hydraulic pump. Comprising 4,000 tonnes of structural steel, the spire is where the building’s communications equipment is housed. The top of the spire is not accessible, with the 163rd floor being the final observation deck.
Burj Khalifa has held its place atop the hierarchy of super tall buildings since the completion of the spire in January 2009. Having sat pretty for nearly a decade, its crown may soon be toppled by the planned, and under construction, Jeddah Tower, Saudi Arabia, which is set to stand at 1,000m tall when it opens in 2020. But, for now, the Burj Khalifa remains on top.