Skyscrapers Pushing Above the Clouds 1

Tuesday, March 17th, 2015
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Failure is the mother of innovation, but it is also the mother of engineering expertise.

Failure is central to the concept of engineering because design engineering is itself the elimination of failure in a structure. When a catastrophic failure occurs, it is indeed tragic, but such failure in itself yields useful lessons to design engineers to make further changes or to pull back in one aspect and move forward in another.

As new materials are developed, design engineers utilise such materials to make structures with greater performance characteristics and potentially higher failure risks. Even when all possible precautions have been taken, a failure can occur in other ways when the structure is subject to environmental or human conditions outside the laboratory or testing facility.

In a world that is pushing for bigger, better, cheaper and faster ways of achieving outcomes, engineers are at the forefront of that cutting edge and are instructed and motivated to push existing boundaries beyond previous limits.

In tackling unprecedented challenges, engineers may have to reduce or refine the safety margin with mechanisms that may work in theory, whereas in practice there are more variables to be accounted for, potentially leading to failure.

Rapid engineering design innovation is likely to yield many missteps, but it is also likely to yield more remarkable successes than a more conservative, less intensive design process.

Engineering firms such as Thornton Tomasetti provide engineering design, investigation and analysis such that they have been involved in engineering six of the 10 tallest buildings in the world.  One such skyscraper currently under construction is the 1,000-metre tall Kingdom Tower in Dubai, scheduled for opening in 2017 at a cost of $1.2 billion.

Robert Sinn of Thornton Tomasetti, said the footprint of the building helps determine how far the structure can extend vertically.

“The so-called aspect ratio, slenderness ratio, or height-to-base dimension ratio is a simple way to gauge potential for resistance to overturning forces from lateral wind and seismic loads,” he said.

In general, the wider the base, the more resilient the building will be. However, thanks to engineering innovation, remarkable thinness can be safely achieved beyond previous limits. The Burj Khalifa’s aspect ratio is close to 9:1 but Kingdom Tower’s aspect ration will exceed 10:1.

Most skyscrapers today use steel-concrete composites to achieve strength while meeting spacing requirements. The engineering challenge is to get those heavy materials into the sky while the building grows.  The innovative solution employed in Kingdom Tower is to have a second set of concrete pumps at about 600 metres above ground. It is one thing to engineer a structure, but hand in hand with that goes the engineering safe building methodology to achieve the completed building.

In constructing the Kingdom Tower skyscraper, engineers also had to help out with building economics, facilitating an expedited construction period. Skyscrapers usually start off by the build with excavating a huge hole in the ground, sinking caissons [piles] and building up columns, so that two years later, one can just see the building appear from beneath the surface of the ground.

Engineers recognised and designed a way around the problem of being unable to start the construction of the tower without waiting until the site was finished by starting to build the tower vertically, while constructing the basement by excavating. This shaves several months off the construction time and results in a significant cost saving. Many builders now go with what is known as top-down construction.

In engineering a one-kilometre high tower, engineers engaged in extensive wind tunnel testing of scaled-tower models and analysis of high-altitude wind-speed data from balloons. Engineers found that a continually tapering vertical profile is very beneficial in terms of the performance under wind.

In addition, the tapering employed in the Kingdom Tower prevents wind vortices from forming and causing discerning noticeable sway.

The soil conditions underlying Kingdom Tower consist of layers made of weak, porous limestone and sandstone. This resulted in a decision to pull back the overall height from one mile to one kilometre.  However, it was only after extensive testing and modeling of the piles and raft system to ensure that settling occurs evenly that the outcome was achievable.

Kingdom Tower required s particularly deep foundation with piles driven down to a depth of 105 metres. A huge slab of concrete called a raft foundation will further undergird the skyscraper. At an estimated weight of approximately 900,000 tonnes, the tower will settle somewhat, but extensive engineering has already allowed for that.

The Kingdom Tower complex – consisting of residential, office and retail – has a remarkable elevator system consisting of 59 elevators, including 54 single-deck and five double-deck elevators, along with 12 escalators. Elevators serving the observatory will travel at a rate of 10 metres per second in both directions. Significant engineering expertise has been deployed in yielding a shorter run time for residents from ground to residence.

Cost-efficient and highly constructible skyscraper design grounded in tradition and aggressively forward-looking engineering, taking advantage of new and innovative thinking about technology, building materials and energy conservation is the result of intensive engineering – something about which all engineers can be proud.

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  1. Gale D.

    While an extraordinary engineering feat, I still can't help but feel that erecting a kilometre high skyscraper in the desert is a somewhat silly and vainglorious undertaking.