Sky high solutions: developers look to go higher and higher

As the Burj Dubai rises from the desert, plans are already afoot for its ‘world’s tallest building’ tag to be a short-lived moment on route to far greater heights. With architects and developers now anticipating breaking the mile high barrier, Steve Skinner looks at the techniques behind going tall and the developments needed to keep pace.

To see the Burj Dubai quite literally dwarf a host of high rise towers huddled beneath its shadow, or to glimpse Taipei 101 (at 508 m) looming over everything else in Taiwan bar Jade mountain, one could be forgiven for thinking that we’ve perhaps reached the physical boundaries of building tall.

However, only seven years after the World Trade Center tragedy - a time when many predicted the end of high rise structures - it is safe to say that quite to the contrary of doubters, high rise towers are on the up and the world’s first 1 km tall building is now a project much nearer to reality than it is to fiction.

Particularly in the BRIC (Brasil, Russia, India and China) economies, population traditionally migrates to the major cities in the pursuit of wealth and employment opportunities. This continual migration, by its very nature, sees a huge demand placed upon a relatively restricted area for both housing and commercial construction.

Of this geo-economic trend, Hyder Consulting’s global director Stewart Tyler recently suggested that, “As the percentage of the world’s population living in cities passes 50% and continues to grow from 3.5 billion to an estimated 5.0 billion by 2030, the creation of new cities or of major satellites to existing cities becomes inevitable. In other words, the number of ‘mega-cities’ will double every 10 years.”

Currently the Middle East simply dominates in the high rise business, with the Burj Dubai scheduled for completion by this time next year. Before this feat of engineering and construction excellence is even completed however, further towers of even greater magnitude have been proposed. The Mubarak Tower at 1001 m is designed to break the 1 km barrier for Kuwait, while proposals for the first mile high (1610 m) tower were recently put forward for Jeddah by Prince Al-Walid Bin Talal of the Saudi Royal Family.

Nakheel, the developer already behind Dubai’s World island will be launching a tower project in Dubai in the coming months too, but at present the company will say little more. Rumours persist that this will be the Al Burj tower, predicted to stand at somewhere between 1200 m and 1400 m.

Still climbing

November 8 2007 saw the Burj Dubai officially hit 601 m to become the tallest man made structure ever, and by June of 2008 this had increased by a further 35 m to 636 m. This construction feat has perhaps already given an insight into the future techniques that will dominate high rise construction as the honeycombed structural design; with multiple airframe-like stiffening reinforcements and 12000 tonnes of Eversendai’s steelwork necessitated the forming of some 430000 m² of wall - double the area of that formed for the floors.

Never before has climbing formwork been deployed at such a great height and under such extreme conditions. Furthermore, the architectural design of the Burj Dubai made for exceptionally tough demands of the formwork system because as the Y-shaped, wing-walled structural core tapered upwards; it underwent some 32 calculated changes in layout to counter wind induced structural sway.

“There was one direction that the wind was more active than others so we rotated the building 120⁰,” said partner in charge of structural engineering at Skidmore, Owings and Merrill, Bill Baker. “Vortices created by wind wrapping around behind the structure create pulses, and if these pulses occur at the natural frequency of the building you can get very large forces. We specifically designed the 32 different steps to break the frequency on the Burj Dubai,” confirmed Mr. Baker.

In planning the forming system, Doka had to make allowance for each of the 32 adaptations. Separation points in the platform system enabled the SKE100 formwork system to be efficiently modified every time the layout changed, causing only a minimal impact upon the regular three day cycle time. Samsung’s project director, Kyung-Jun Kim said of the system, “As the in-situ concrete core was being constructed ahead of the floor slabs, construction progress was entirely dependent on the self-climbing formwork. The system functioned with machine-like precision, allowing us to complete the core within the original timetable.”

While completing the formwork on the world’s tallest building presented Doka with a range of unique demands, raising the concrete to 606 m proved to be an equally significant challenge for Putzmeister concrete pumps.

Utilising the 14000 SHP D super high pressure trailer pump, concrete delivery above 600 m took 40 minutes from the filling of the hopper at ground level to final delivery line discharge, with the pump operating at a pressure of more than 200 bar. The weight of concrete on the pump during the final phases of construction peaked at a staggering 26 tonnes.

Challenging shapes

Aside from pure height, perhaps one of the defining attributes of current high rise projects is the incorporation of bold designs and inspiring external profiles. Whereas, in past decades, high rise structures were predominantly about height with limited scope for artistic flair, the current crop is visually and structurally fascinating.

In this respect, the China Central Television CNY 5 billion (US$ 730 million) CCTV Towers in Beijing, commissioned specifically to host the international broadcasting centre for the Olympic Games, clearly achieves the aims set out at concept. The Office for Metropolitan Architecture (OMA) specified that the building “should be an iconic constellation of two high rise structures that actively engaged the city space and consolidated the entire TV program in a single building.”

“From the outset, the vision was to achieve something more than a two dimensional tower soaring skyward,” said an OMA spokesperson, and the resultant three dimensional structure with its inspiring canopy certainly breaks the mould in the most spectacular of fashions.

Similarly striking are the Alberghi del Nuovo polo Fieristico di Milano towers which dominate the trade fair centre in Milan, Italy. Designed by French architect, Dominique Perrault, each of the twin towers employs a 5⁰ inclination - a greater tilt than that found in the leaning tower of Pisa.

With the top floors offset by some 6 m from the vertical axes, Peri’s engineers had to design a specially customised climbing formwork system for the project.

Commenting on the system, a Peri spokesperson said, “In association with the Vario GT24 wall formwork, the SKS climbing system employed for the inclined exteriors meant that the additional concrete loads and lifting forces from the forward and reverse inclines were safely transferred into the building without the need for any additional measures. Even confronted with these complexities, we were able to complete a floor each week.”

While the leaning towers in Milan near completion, the main contractor for Abu Dhabi’s striking Tameer Towers has been appointed. The project, which fronts the Shams development on Al Reem Island will be headlined by the commercial tower at 300 m.

Explaining the structural intricacies of the striking commercial tower, lead architect Duncan Swinhoe said, “The diagrid shell that we’re employing is primarily the lateral stability system of the building - It’s not the entire gravity bearing structure.

“If you think of a conventional tower, the central core provides most of the proportional and lateral stability. In this situation, the centre of the building vanishes from the lower 20 stories, so the idea was to place the lateral stability system as the perimeter, to trap the load to the perimeter and thus reduce strain on the core. I think this is the first building of this concept to be built from concrete,” continued Mr Swinhoe.

The commercial tower represents just one of a number of high rise projects that are currently underway in Abu Dhabi as the capital of the UAE breaks cover from the shadows of Dubai. The Landmark tower, on the Corniche, is set to rise to 324 m to become the second tallest building in the city.

Although still at the groundwork stage, five Liebherr tower cranes, including three luffing jib models are already carrying out site duties. At the top of the 72 storey tower, a 52 m high concave steel structure will be installed in-situ with the external climbing Liebherr 160HC-L 8/16 litronic luffing jib crane, tied-in nine times at 34 m intervals, lifting the final two 10 tonne elements.

Liebherr is also at the heart of Ground Zero in New York, where work is underway to construct the 541 m Freedom Tower. The five machines on site include three 90 tonne capacity HS855HD duty cycle crawler cranes and two 200 tonne capacity LR1200 lift crawler cranes.

Since October 2006, the Liebherr crawler cranes were used for deep foundation works and slurry walls, and following the delivery and placement of some 300 m³ of concrete, one of the company’s LR1200 lift cranes conducted the symbolic task of installing the tower’s first 10 m long, 25 tonne steel element.

In Chile, the Costanera center in Santiago will be flanked by four high rise towers, the tallest of which will be the 300 m Gran Torre Costanera. Due for completion in 2010, the tower will be the tallest building in South America.

The 270 m tall in-situ concrete core of the Gran Torre Costanera is characterised by honeycombed twin shaft geometry and a continuous upward taper. Forming for the tower is being conducted by Doka using its SKE100 self-climbing formwork, the same system as adopted for the Burj Dubai.

Doka is also supplying its profile guided windshield GCS to offer workers maximum levels of protection. Encircling the façade around the top three floors, the 28 sections of windshield GCS can be lifted in a single crane cycle and the system provides high levels of fall and environmental protection.

Sky’s the limit?

One clear benefit of the continuing high rise boom is the engineering advancements that these buildings spur. In structural terms these breakthroughs are clearly leading to ever more interesting and exciting buildings.

But as the parameters are moved ever upwards, one has to wonder just how high can we go? Perhaps the answer lies with Duncan Swinhoe, lead architect on the Tameer Towers project, “As an architect, the only limits really are client aspirations, budget and what’s considered appropriate.

“From a practical point of view we’re still limited by physical transportation. Also, these tall buildings with huge floor areas have a significant impact on the infrastructure at ground level that’s needed to support them. I think the blight on the groundscape, caused by parking lots and all of the services that are needed to support these super-tall buildings, is severe, and the danger is that huge ‘no go’ zones will effectively surround these structures and sterilise the environment in which they sit.”