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
