Tower cranes flock to money. During the economic boom years, high-rise construction cranes migrated from Beijing to Shanghai to Dubai, where it was estimated in 2006 that there was one tower crane for every 44 residents of the desert boom-opolis.
Tower cranes are feats of structural engineering that often outshine their creations. They are designed to stand 80 metres tall and reach 80 metres out supported only by a narrow steel-frame mast, a concrete foundation and several counterweights.
The engineering principle that keeps the twiggy tower crane from tipping over is something called a ‘moment’. If you hang a weight from the crane’s jib arm, it exerts a rotational force or torque where the arm connects to the top of the mast. The magnitude and direction of this force (clockwise or anti-clockwise) is called the moment. If the weight is hung close to the mast, the magnitude of the moment is lower than if the weight is hung far out on the jib. To keep the crane upright, counterweights are used to create a moment of equal magnitude in the opposite direction, balancing out the rotational forces.
Once a tower crane meets its maximum unsupported height, it can be tethered to the building itself and continue to grow with the rising skyscraper. The tower cranes that rose with the construction of the record-breaking Burj Khalifa skyscraper in Dubai reached a truly dizzying height of 750 metres.
How does crane assemble itself?
One of the most remarkable engineering feats of tower cranes is that they can literally build themselves. With help from a large mobile crane, construction workers secure the base sections of the tower and assemble the top unit of the crane – the slewing unit, jib and machinery arm.
But before the top section of the crane is attached, workers slide a hydraulic climbing unit around the base of the tower. Once everything is in place, the hydraulic climbing unit lifts the entire top section of the crane (including the horizontal jib and operator’s cab) just enough to slide in a new section of tower beneath. Once the new section is secured, the hydraulic unit continues to climb up, section by section, as the crane slowly builds itself higher.
Jib arm – The horizontal arm of a tower crane can extend outward 85m. The arm has three sides forming an isosceles triangle with a trolley track running along the bottom section.
Trolley – The trolley and hook are connected by cables to a trolley motor mounted on the upper side of the jib arm. The operator can roll the trolley back and forth with hand controls.
Cat head tower – On hammerhead tower cranes, the cat head tower reinforces the jib arm and counterweight jib using thick steel cables called pendants.
Operator’s cab – It’s a long climb to the cab, where the crane operator has a bird’s-eye view of the construction site through floor-to-ceiling windows.
Machinery arm – The power to raise and lower the load line is supplied by a huge winch located along the counterweight jib or machinery arm.
Counterweights – Multiple concrete slabs – each weighing several tons – are hung or piled on the very back end of the counterweight jib to overcompensate for the crane’s lifting capacity.
Hydraulic climbing section – The hydraulic unit attaches to the outside of the tower. A powerful hydraulic arm lifts the entire top section of the crane just enough for the crane to insert a new section beneath.
Slewing unit – This motorized pivot allows the jib arm to rotate nearly 360 degrees to lift and drop materials all across the construction site.
Concrete foundation – Large tower cranes get their core stability by burying the bottom of the tower in several metres of concrete weighing 185 tons.