My Science School

How can a giant crane built without the help of another equally large crane? The answer is that most tower cranes can build themselves once they reach a certain height, and some can haul themselves up completely unassisted.

Freestanding tower cranes are usually bolted to large steel angle brackets that are set deep into concrete, or weighed down under huge concrete blocks. The tower is made up of steel sections about 20ft (6m) tall that are pinned and bolted together.

A smaller mobile crane is needed to place the first few sections on one another before the crane can start building itself.

When the mobile crane can no longer lift the sections high enough the tower crane takes over, using a specialized tower section called a climbing frame. The climbing frame – positioned near the top of the tower – is slightly larger than a standard section, and it has one open side to allow a new section to fit inside it.

The stages of the process are carried out in the following way. Once the crane’s jib and hook have been attached to the top of the tower, the crane lifts a new section up, so that it can be fitted into climbing frame.

A hydraulic ram at the base of the climbing frame then pushes up the new section until it locks into position above.

Another new section is slotted into the now empty climbing frame and fixed in position.

The climbing frame then moves up around the section it has just raised, and again pushes it up, together with the rest of the top of the crane. It is once again empty to receive a new section, and the whole process is repeated. It can sometimes take a whole day to add three new sections to a crane.

When the crane has reached its final height the climbing frame is usually removed from the tower. But sometimes the crane will operate with the frame still in lace. And if the crane’s height need to be altered regularly, the frame may remain on the tower the whole time.

To dismantle a crane the process is reversed – the climbing frame is used to lower the sections rather than raise them.

 

Picture Credit : Google

Many of the world’ most elegant modern buildings have been constructed with a material bearing the unromantic name of prestressed concrete. Skyscrapers, graceful bridges, slender dams, even the Sydney Opera Hose, all rely on its enormous strength.

Prestressed concrete has steel cables running through it that have been stretched by hydraulic jacks. As the cables try to their original length, they pull inwards – compressing the concrete.

The idea of prestressed concrete was first used effectively in 1928 by the French civil engineer Eugene Freyssinet. He developed two sorts – pre-tensioned concrete in which the wires are stressed before the concrete has set, and post-tensioned concrete, stressed after it has set.

In bridge construction today, steel cables are threaded through ready-made holes in precast concrete blocks. Then the cables are stretched and anchored with conical plugs in the blocks at each end of the bridge to maintain their tension. the principle is the same as picking up a row of books by squeezing the ends together. In effect the books create a beam, and the harder you squeeze the stronger the beam becomes. And within limits, the more a bridge’s concrete blocks are compressed, the stronger the bridge will be.

Pre-tensioned concrete is made in factories. Wet concrete is poured over stretched steel wires while they are held under tension. Once the concrete has set, the wires can be cut off at each end of the concrete block, pulling it together. this method is used to produce railway sleepers and strong concrete beams anything up to 150ft (46m) long.

Demolishing a structure built with prestressed concrete can be dangerous and quit unpredictable. When the building does to start to collapse, the locked-up forces and tension in the cables often sends rubble in all directions.

 

Picture Credit : Google

The fastest method of erecting tall concrete buildings is called slip forming. It is widely used for grain silos, chimneys and offshore oil platforms. But the most spectacular example of slip forming is the CN Tower in Toronto, Canada. It is the world’s tallest self-supporting tower at 1815ft (553m).

The usual way of building a high concrete wall is to pour wet concrete into a mould, called ‘shuttering’ or ‘formwork’, and compact it. Once the concrete has set, the shuttering is dismantled and re-erected on top of the new piece of wall, and the next layer of concrete is poured into it.

In slip forming, the shutters are not dismantled but slide continuously up the wall, hanging on vertical steel rods. Quick-drying cement is used which is soon strong enough to support the shutters as they move up.

Hydraulic jacks are fixed to the steel rods, which lift the shuttering up in frequent small steps, 1in (25mm) at a time. The CN Tower was built at an average speed of 20ft (6m) a day.

Slip forming can be used only on buildings that are suitably shaped, circular walls are the simplest. The concrete has to be exactly the right quality, placed evenly and compacted immediately. It is difficult to get the slip going, so once it has started the work usually continues day and night, until the building is finished.

Concrete takes days to harden completely. Although it can support the weight of the shuttering and the fresh concrete above, it is still relatively soft. This resulted in the CN Tower being twisted by the force of the Earth in rotation, and engineers had to use steel ropes to pull the tower through six degrees back to its correct shape.

Another method of continuous construction using jacks is the jackblock system. The top floor is built first, then it is jacked up, allowing the next floor to be built under it. The jacks then lift the two completed floors, and the next storey is built underneath them, and so on.

A Dutch insurance company’s head office in the Hague was built using the jackblock system, and the final weight lifted by the jack was 32,000 tons. This method of building allows work to be done inside the building as it rises, so the top floor is finished first.

 

Picture Credit : Google