Final Height of 'On-Deck' Tallest Tower Shrouded in Secrecy

But residential concrete frame, with short spans, makes it easier to exceed 700

11/06/2006 By Peter Reina in Dubai with Nadine Post

	Concrete pumps must reach 550 m. Photo: MichaelGoodman/ENR

Speculation among supertall-building pundits once had the $1.1-billion Burj Dubai rising 800 meters or more. Currently, the developer admits to only 700-plus. The tower’s architect-engineer is sworn to say only 600-m-plus. And the main contractor claims not to know the final height.

The secret gets harder to keep as the tower, now at about 80 stories, rises toward its pinnacle, at an average of one floor every three days. Whatever its final stopping point, Burj Dubai is on course to supersede the world record holder, the 509-m-tall Taipei 101, by summer, says Greg Sang, assistant director for projects at local owner Emaar Properties PJSC.

The largely residential and hotel burj—Arabic for tower—already surpasses Europe’s tallest building, Moscow’s 264-m Triumph-Palace. Located just south of the United Arab Emirate city, the iconic centerpiece of Emaar’s $20-billion Burj Dubai Down- town mixed development, is taking the form of a tapering, three-winged tower, mainly of over 280,000 cu m of concrete rising 162 floors. From level 156, the tower will be framed with some 0.5 million tonnes of structural steel in equipment floors and the spire.

Top steel structure (model, below) may be about 200 m tall. Concrete setbacks help fool wind. Photo: MichaelGoodman/ENR

The steel frame alone will be “almost as tall” as the U.K.’s tallest building at London’s Canary Wharf, which is 225 m, says Rob Pickering, project director of Hyder Consulting Middle East Ltd., the tower’s construction supervisor and designer of record.

The reinforced concrete structure serves the quest for height of Emaar, which is a third owned by Dubai’s ruling family. But it meant taking wind engineering to new heights. “We virtually designed [the tower] in a wind tunnel,” says Bill Baker, structural partner at architect-engineer Skidmore, Owings & Merrill LLP, Chicago. “In the old days, wind tunnels were for confirmation of forces, not to tell you what the building would look like.”

Adds Anton Davies, a director of Burj Dubai’s wind consultant, Rowan Williams Davies & Irwin Inc. (RWDI), Guelph, Ontario: “We probably did a million dollars worth of work on this.”

The largely residential tower’s smaller room sizes allowed greater use of structural walls than would have been possible for an office building, notes Eric Tomich, SOM’s associate partner on site. Having designed a broadly similar structure for Korea’s 264-m-tall Tower Palace III a few years ago, Baker says “we knew we could get this structural system pretty damn high. Essentially, every piece of vertical concrete is part of the lateral system.”

Stiff Core

The stiff, six-sided core will reduce twisting to below residents’ discomfort threshold. Lateral stability comes from pairs of roughly 60-centimeter-thick, reinforced concrete shear walls forming central corridors of the three symmetrically distributed wings.

Lower wings, which project over 60 m from the core, shorten in turn by 9-m bays creating 27 setbacks spiralling up the tower. Perimeter columns, generally 1.9 m in diameter, support outermost bays.

Baker.

The constantly changing profile may complicate construction but it prevents the building from resonating to wind-induced vibrations. Vortex shedding “is going to happen, but you don’t want it organized,” says Baker.

Shear walls at the tower’s two-floor- deep mechanical levels act as outriggers. The four levels of complete outriggers, and one partial, mobilize all vertical elements to resist lateral loads.

Irwin (left) used 1:50 scale for wind tunnel model of top of tower. Photo: NRC Canada	Tower was virtually designed in wind tunnel, says engineer. Photo: SOM

Wind consultant RWDI worked with SOM during design to refine the shape and orientation. Using local and regional wind data, RWDI proved the original 45-m-per-sec design wind speed would be excessive. Based on the data, authorities allowed a design wind speed over 15% lower than the value set by their interpretation of the local code, says Peter Irwin, RWDI’s president. Otherwise, it “would certainly have been a much more expensive structure,” adds Baker.

For detailed analysis, numerous tunnel tests were run, first on simple force balance models. Tests on designs using full aeroelastic models gave the “ultimate in precision,” says Irwin. Special tests on a 1:50 model of the tower top proved previous work to be free of scale effects, he adds.

Because of the height, RWDI used weather forecasting software to assess structural effects of the regional Shamal wind that “can cause unusual profiles of...speed with height,” says Irwin.

For the burj’s higher levels, RWDI had only one source of balloon data—from Abu Dhabi. “We were missing information and used MM5 [weather software] to help fill in gaps and to extrapolate to Dubai,” adds Irwin.

RWDI revealed the occurrence of high-speed Shamal winds during night time temperature inversions between 300 m and 500 m above ground. But their structural effect was secondary to ordinary wind loads higher up, says Irwin. Even at 700 m, the tower needs no mechanical damping, though there will be space left for some.

The burj, which started off to be the world’s tallest, grew as tests revealed the structure’s inherent ability to satisfy Emaar’s desire for ever greater height as a selling point. The strategy worked—the tower is “pretty much sold out,” says Sang.

For statutory requirements no one code applies. SOM used the (U.S.) International Building Code overlain with the (U.S.) National Fire Protection Association’s code for fire and emergency escape. And it adopted British standards for fire detection and alarm systems. Among safety “enhancements” the building includes air-conditioned, pressurized refuge areas at four levels, connected by corridors to stairs of other wings.

Environmentally, “we haven’t exploited a lot of experimental technology, partly because of the scale,” says SOM’s Tomich. Rather than harness Dubai’s abundant solar energy, the design aims to cut power demand. With the area’s extreme heat, the biggest problem for energy consumption is cooling. Heat gain will be cut by different coatings on both layers of the twin-leaf curtain walls. One will reflect daytime solar heat and the other will block long-wave radiation during hot nights.

In 2003, within weeks of winning a limited design competition, SOM started work. Also in 2003, for project management, Emaar hired New York City-based Turner International, a subsidiary of Turner Construction Corp., Dallas. This year, Emaar and Turner Corp. announced a new entity, Turner International Middle East Ltd., to jointly pursue projects in the Gulf.

To work out the burj’s constructibility, SOM and Hyder involved Turner very early in the design. The design is based on high-strength concrete, with most of of the tower’s vertical concrete rated at 80 mpa and the rest at 50 mpa. In Dubai, they had produced 80 mpa only in the last four to five years, says David Bradford, Turner’s construction manager.

	Work continues around the clock to keep schedule and take advantage of cooler evenings in extremely hot climate. Photo: MichaelGoodman/ENR

Because of the structural sophistication, SOM produced full working drawings. Coordinating positions of 100,000 openings and holes in walls was a big challenge. “With...high-strength concrete, you can’t drill it or just cut it,” says Tomich.

Hyder, in the Gulf for over 20 years, was hired because SOM has no local presence. Hyder has 60 of its 350 people in the area monitoring the burj. With 200 design submittals a week, 40 of its staff concentrate on detail design issues only. Hyder also handled foundation design.

Construction began late in 2003, with a 15-m-deep basement excavation through sand and brackish water. In January 2004, following pile tests at up to 6,000 tonnes, workers began installing more than 190 skin-friction piles, 1.5 m in diameter, down to a 50 m depth.

Germany’s Bauer Spezialtiefbau with Middle East Foundations (UAE) handled piling under a $15-million contract.

A switch to polymer, not bentonite, to support the bores improved skin friction, reducing the required pile length from 55 m to 49.5 m. That saved 15% on costs, says Turner’s Bradford. Polymer also avoided increasing environmental concerns over bentonite disposal.

Starting in April 2004, Australian-owned Nasa Multiplex followed piling by casting the 18,000 cu m of concrete for the 3.7-m-deep mat. Most of the underground structures are in groundwater, which is unusually rich in chlorides and sulphates, threatening to attack the steel and concrete respectively. With a 100-year design life, the piles and mat foundation concrete has epoxy-coated rebar and cathodic protection.

For the tower itself, Emaar selected a single main contractor. Subcontractors are building specialty systems, such as the curtain wall, building infrastructure systems and fit-outs.

The 47-month main contract, initiated in January 2005, went to a joint venture led by South Korea’s Samsung Corp. Samsung had helped build Malaysia’s Petronas Towers, the world’s tallest until 2005.

	Kim

The joint venture bid against six teams before reaching a shortlist of two. It won on technical merit, though its price was higher, says Kyung-Jun Kim, Samsung’s vice president and project director. Prices were very close, adds Bradford.

Since the Petronas towers, “we have a special team for high- rise,” says Kim, who worked on the Malaysian project. Samsung’s partners, Arabtec Construction LLC and the local unit of Belgium’s Besix Group, bring experience of local conditions and major projects to the team, he adds.

With Dubai’s explosive growth in construction, Samsung was soon surprised by problems securing quality labor. “Four to five years ago it was easy,” says Kim. For the tower, “we had to train them [on] new systems. It was very hard.” Engineers and supervisors are no easier to find, he adds.

Photo: MichaelGoodman/ENR

This fall, the joint venture’s labor force reached over 2,500, working three shifts around the clock, seven days a week. The crew is set to approach 6,000 at next year’s peak, says Kim.

To ease construction, Kim early on asked for their the highly congested reinforced concrete outrigger design to be replaced with easier-to-build structural steel. Outriggers now each contain 24 tonnes of 4-cm-thick, double steel plate which is around 1 m deep.

Some 2,500 tonnes of hydraulically powered climbing formwork for core and walls, from Austria’s Doka Schalungstechnik GmbH, is the joint venture’s key construction tool. “We had to have a good system to account for all the changes,” says Kim.

	Self-climbing formwork is used for concrete walls. Photo: MichaelGoodman/ENR

Facing record-breaking pumping heights, the contractor ran tests on pipes zigzagging over the ground. “We are going to pump it [about] 550 m. That requires extremely sophisticated concrete design,” says Turner’s Bradford.

Universal Concrete Products Ltd. Co. (Unimix) LLC is supplying all the concrete and delivering it, mainly with two super high-pressure pumps from Germany’s Putzmeister A.G.

Raising wall-size rebar panels is the main task of the tower’s three large Australian-made Favco hydraulic luffing jib cranes. “We had these machines and we discussed (modifications) with Favco,” says Kim.

For such heights, the drums needed designing for 800 m of wire. Fourteen high-speed hoists from Czech supplier Pega Hoists Ltd. transport people, cladding and other items.

The joint venture’s learning curve started at a concrete casting rate of one floor in seven days, accelerating to the current three days some 50 floors up, says Kim. In summer, pumping is possible only after 7 p.m., when temperatures fall below the 40°C threshold. The tower is due to reach level 100 by this year’s end and top off concrete, at floor 156, by next October.

First, workers cast the six-sided central core. Wing walls and their slabs follow several floors behind, using the self-climbers for the walls and a panel form system for the slabs. “Nose” columns at the wing tips come next, with their slabs, using circular steel forms for columns and a panel form system for slabs.

The tower has 140,000 sq m of cladding. The system consists of reflective glass in aluminum and stainless steel spandrel panels with vertical tubular fins of stainless steel.

Installation of the curtain wall became troublesome when the Swiss-based contractor Schmidlin A.G. went bankrupt early this year. Its local operations were then saved by Arab investors.

Curtain wall installation just began. Schmidlin’s problems caused “a few months’ delay,” says Kim. “They will operate two shifts to recover,” he adds, forecasting no adverse effects on the overall schedule.

The main hold-up now is in fitting out the classy Armani Hotel Dubai, filling the tower’s lower quarter. Giorgio Armani, whose company is responsible for the hotel’s styling, seems less at home with high-rise than with haute couture.

Otherwise, Turner’s schedule for the whole project, planned over three years ago, still holds. “We are actually slightly ahead,” says Bradford.