The Burj Dubai—meaning “The Tower of Dubai” in Arabic—is now the tallest building in the world and it's still rising. At the time of publication, the building's height has reached 1921 feet (totaling 156 floors), making it 251-feet taller than the Taipei 101 building in Taipei, Taiwan, the previous tallest building. The frame is structurally reinforced concrete with a structural steel spire at the top. The structural concrete portion now is complete and the structural steel part is underway. With the completion of the structural steel work, the public finally will discover the total height.
There's an increasing trend toward constructing structural concrete super-tall buildings (more than 80 stories) for the following reasons:
- The mass and rigidity of concrete provides twice the dampening effect compared to steel, reducing forces on super-tall buildings due to wind and the cost of construction. Building occupants prefer buildings with little sway as well.
- Improvements in concrete mixes, including strength and modulus of elasticity (E) have made high-rise construction more attractive. Self-consolidating concrete (SCC) is increasing in use too.
- Concrete buildings are quiet, especially important for residential occupation.
- Structural concrete is naturally fire resistant.
- By using “flat plate” floor construction methods the distance between floors is minimized, saving money.
- Modern forming systems for both vertical and floor construction greatly increase productivity.
- Advancements in concrete pumping technology, including the introduction of placing booms, make easy, fast delivery of concrete possible, freeing tower cranes for other work.
- Many general contractors perform their own concrete work (they tend not to do that with structural steel) and are more able to suggest changes and ideas.
Design and engineering develop together
Close teamwork between structural engineering and architects occurs at the start of a project like the Burj Dubai. The forces generated by wind and wind behavior are a significant engineering challenge, partially mitigated by surface shapes. Bill Baker, a partner at Skidmore, Owings & Merrill (SOM), Chicago, says they first developed design models and tested them in a wind tunnel to minimize wind shear forces (the force that wind exerts on a structure). “Vortex shedding” also is an important consideration. When wind moves around a structure, it causes spirals of wind that move a building from side to side generating harmonic wind forces that can have a great effect. These forces are minimized by altering the width and shape of floors along the height of a structure. During the wind tunnel testing process, Baker and the project's chief architect made structural and design changes to the model after each test until wind forces were minimized. They also used the wind tunnel to perform flexible aero elastic model tests.
SOM also managed wind forces on the Burj Dubai by adding a “buttressed core system”—three wings in a “Y” pattern that brace the core structure. Like the horizontal root system of a tree, the buttresses support the structure and reduce torsional forces on the core regardless of the direction of the wind.
Larry Novak, an associate director at SOM says that the structure of the Trump Tower in Chicago, (which SOM also designed) is different than the Burj Dubai because its column lines shift as the building steps in, requiring transfer beams to carry the load from one column line to the next (see the June 2007 Concrete Construction cover article “Reaching New Heights in Chicago”). For the Burj Dubai, the columns are in line to the top of the structure. As the building floor plan diminishes in size, column lines terminate. There are 27 such reductions in floor size to control wind shear and vortex shedding.
Workers placed concrete outrigger walls every 30 floors to provide additional strength and stiffness to the frame.