The Burj Dubai

Source: CONCRETE CONSTRUCTION MAGAZINE
Publication date: February 1, 2008

By Joe Nasvik

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.

The Burj Dubai is now the tallest building in the world. It's a structural concrete building with a structural steel spire. Concrete floor work is now complete and the structural steel component is just starting. Photo: SOM

• 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.

Structural elements

The building is supported by 194 caissons, 5 feet in diameter and approximately 150 feet deep. Baker says that soils in the region have high chloride and high sulfate contents and are composed of silt-formed calcium type rock. The caissons depend on "skin friction"—the resistance between the concrete caisson and the surrounding soil—to provide the necessary support. High-performance, dense concrete resists the high sulfate soils.

Resting on the caissons is a 12-foot-thick mat slab cast with SCC placed in three lifts. The core structure, the buttresses, and the columns are all supported by the mat slab. The core walls start at 26 inches thick, diminishing to a 20-inch thickness at the top of the structure. Floor thicknesses for the bottom floors are typically 8 inches thick and mechanical floors are 12 inches thick. There is no post-tension (PT) reinforcement used anywhere in the building.

Concrete requirements

SOM believes that the modulus (E) number of concrete is as important as its compressive strength for super-tall building construction. Normal-weight concrete has an E of 2000 to 6000 ksi, with the requirement for the Burj being 6300 ksi at 90 days. The quality of aggregate material has much to do with E but SOM decided to specify the E they required and let the contractor be responsible for the mix details.

When there are hundreds of concrete placements over the course of construction, shrinkage and creep, occurring at different rates over time, can be critical to a building like the Burj Dubai. For that reason, Novak says they decided to use one mix for all the vertical work on a given floor level, keeping surface-to-volume ratios the same for columns and core walls (column and wall thicknesses are the same). This way shrinkage and creep would be the same and have minimal influence on the structure.

Shown in the plan diagram is the "buttressed core system" that increases wind shear resistance and reduces torsional movement of the core structure. Photo: SOM

James Aldred from the Independent Verification and Testing Agency (IVTA) says that most of the mixes are "triple blends" including portland cement, fly ash, and silica fume. They have a relatively high fine aggregate fraction as well as containing up to 650 pounds/cubic yard of cementitious content. Flowability is increased with polycarboxylate superplasticizers while keeping water-cement (w/c) ratios below 0.32 for higher strength concretes. Although some vibration was used during casting, the concrete could be considered to be SCC, according to Aldred. Three-quarter inch maximum aggregate was used up to the 100th floor and 9/16 inch at higher elevations to reduce pumping pressures. Significant amounts of ice were added to keep concrete temperatures between 75° F to 90° F. Even with placements conducted at night, ambient temperatures could be up to 105° F from daily highs of 120° F in the middle of summer.

Concrete cube strengths specified for building elements includes the following:

• Caissons: 9000 psi minimum strength
• Mat slab: SCC with 6000 psi minimum cube strength
• Core walls and columns < 126th floor and floors 154 and 155: 11,600 psi minimum strength
• Core walls and columns > 126th floor: 9000 psi minimum strength
• Floors: 5000 psi minimum strength

Novak says the quality of the concrete was excellent. Column and core wall mixes specified for 11,600 psi compressive strengths were actually developing an average 56-day strength of 15,000 psi with a modulus of 7000 ksi.

Forming and placing

Baker says that recent advancements in forming technology have helped to make structural concrete construction attractive. The same is true for concrete pumps because they effectively deliver concrete—1900 feet straight up in this case.

Doka, Lawrenceville, Ga., supplied the vertical formwork for the 4.6 million square feet of walls. Core walls were constructed with self-rising or "jump forms" with the concrete placing boom mounted on the top of the forms. The boom advances as the forms move upward.

Winds in the region didn't permit the use of table forms for floor construction so workers used an efficient handset forming system manufactured by Meva Formwork Systems, Springfield, Ohio, for the floor construction.

The concrete pump for the project is the largest one Putzmeister, Aichtel, Germany, manufactures. Bill Carbeau, a sales and product manager for Putzmeister, says it can develop as much as 5500 psi pressure on the material, although 3000 psi is all that's needed for this project; the rest being reserve capacity. At the placing boom, the pressure is approximately 50 psi to ensure safe delivery. The pump weighs 22,000 pounds and is powered by a 630-hp Caterpillar engine. Carbeau says they decided to use a 6-inch-diameter pipeline instead of the customary 5-inch to reduce friction in the line and decrease the chance for plug-ups (there was a reduction back to a 5-inch line at the placing boom). This decision, along with the well-designed concrete mixes, significantly reduced pipeline wear. Putzmeister also developed a special tool that could be wheeled onto any floor in the building to quickly lift vertical lines, assisting in the process of removing blockages should they occur.

Carbeau says you can pump concrete five times further horizontally than vertically so it was important to run tests before the project started using the approved mix design. Horizontal tests were conducted adjacent to the Burj site, pumping different concrete mixes through special test lines to determine friction coefficients, assuring that the pump could deliver concrete to the top of the building.

You might wonder how to clean the world's tallest pipe line. It's done the same way as for any other vertical building pipeline. Rubber balls are placed in the line at the top, the line is capped and air is pumped in behind the ball, pushing approximately 15 yards of concrete down the pipe (assisted by gravity) where it is diverted into ready-mix trucks.

Project challenges

It's hard to imagine how tall the Burj Dubai really is. George Efstathiou, a partner at SOM, likes to tell people that the building is approximately the same height as the John Hancock Center in Chicago, (the world's 16th tallest building) stacked on top of the Sears Tower (the world's fourth tallest structure).

Efstathiou, says the owner Emaar Properties, Dubai, wanted a building that would project a strong image and one that would have world impact. To do this, they invited a select group of design firms to a paid competition. SOM's first challenge was to provide concept drawings to meet those criteria. Then the challenge was to combine the talents of architecture and structural engineering to develop an aesthetic building that could hold up to the riggers of the wind, temperature, and soil conditions of Dubai. At the peak of the design process, there were more than 90 architects and engineers working on the project.

The general contractor decided not to use table forms for the project, preferring a panelized, "drop head" system that can be erected safely, quickly, and efficiently. Photos: Meva

As important as image is, integrating structural walls and columns into pleasing user spaces was also important. And designing typical building systems such as HVAC, lighting, and home systems, and figuring out how to wash windows also presented challenges. For example, examining satellite weather data of the extreme heat and humidity conditions of Dubai helped designers develop HVAC equipment that would remove enough condensation from the air inside the building to fill 14 Olympic-sized swimming pools annually.

Efstathiou says that coordination and communication for a project of this complexity located several time zones away continues to present significant challenges. The contractor uses sophisticated computer technology for planning, graphing, and animations to help subcontractors and workers understand critical parts of the construction process.

The future of super-tall buildings

There are a couple of projects on the horizon that could take super-tall building construction to the next level, says Efstathiou. One currently being discussed is a 3280-foot building in Kuwait, and in Jeddah, Saudi Arabia, there is consideration of a building rising 5249 feet—almost a mile high.

What part will concrete play in super-tall buildings? Baker thinks that residential buildings will continue to use structural concrete. Commercial high-rise buildings mostly will be composite structures with structural concrete cores and long-span structural steel decks. The next tallest building in the world? It could be structural concrete with its many advantages.

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About Burj Dubai

• Number of stories: 160
• Total building height: Currently unknown
• Usable square footage: 5 million square feet
• Cubic yards of concrete: 300,000 cubic yards

Project Participants

• Owner: Emaar Properties, Dubai
• Architect/engineer: Skidmore, Owings & Merrill, Chicago
• Project management: Turner International, Dubai
• Supervision consultant: Hyder Consulting, Dubai
• General contractor: Samsung/BeSix/Arabtec, Seoul, South Korea
• Independent verification and testing agency: GHD Global Pty Ltd, Dubai U.A.E.
• Vertical formwork: Doka, Amstetten, Austria
• Horizontal formwork: Meva Formwork Systems, Springfield, Ohio
• Concrete pumping: Putzmeister Aichtel, Germany
• Concrete producer: Unimix, Dubai