Tower cranes are selected for use based on factors such as:
Weight, dimensions, lift height and radius of the heaviest loads
Frequency and duration of usage
Maximum free-standing height of the crane
Maximum braced height of the crane
Machine-climbing arrangement
Weight of crane supported by the structure
Available headroom which can be developed
Area which must be reached
Hoisting speed of the crane
Length of cable that the hoist drum can carry
Other normal procurement factors such as price, reputation and service back-up of manufacturer/ agent
After erecting the whole tower crane, it was ready for "raising" to the required operating height. This was achieved by jacking up hydraulically the telescopic cage section and then bringing extra standard mast ( 3m height )sections into the telescopic cage consecutively until sufficient height was reached.
According to statistics available, about 125,000 tower cranes are operating worldwide. Industry experts cautiously estimate that 15 to 25 per cent of the world's cranes are here.
Patrick Mullaney, Tower Crane Operations Manager for Select Plant Gulf, the heavy machinery arm of Al Naboodah Laing O'Rourke, arrived in Dubai three years ago to work on the new airport site.
On arrival he approached the Guinness Book of World Records to enter Dubai for the high number of cranes on the site at the time. "There were 50 cranes, all over the 12-tonne capacity. It's the most we've had at the site and we are just one contractor. We tried to get it in the Guinness Book of World Records, but they weren't that interested," said Mullaney.
Tower cranes used to build skyscrapers are put up in two days on average. "The tallest free standing crane we used was at Ski Dubai, which measured 92.5 metres," said Mullaney.
The crane at Burj Dubai will eventually be 750 metres above ground level. Laing O'Rourke is currently working on four big projects Dubai airport, Festival city, Burj Dubai and the Old Town and Atlantis hotel on Palm Island.
The construction boom in Dubai is very fast paced due to the overwhelming workforce and building around the clock. "At the most, we as a contractor had about 11,000 men working at the airport. Sites are just flooded with workers," said Mullaney.
"Tower cranes help the whole site. You can't do without them. Mobile cranes are limited because you keep having to move them. Tower cranes can be placed closer together and have a radius of about 140 square metres," he said.
Bridge building can be traced back several -thousand years. In primitive times, man used fallen trees, arches made of a rock fall and suspension. bridges made of vines and creepers to cross streams and ravines. These are the three natural forms of bridging methods, namely, the beam, the arch and the suspension structure. Despite the technological advances of the recent past, these three forms, either singularly or in combination, remain the basis of all bridge construction.
The vast majority of arch bridges were built with stone masonry. A few were also built of iron. Modern arch bridges are built with reinforced concrete.
Suspension bridges are used to form exceptionally large Spans. They are a highly specialized form of construction and far more expensive than beam bridges. Their use is therefore limited to a very small number of applications where their ability to span large distances is paramount.
KUALA LUMPUR: The Smart Tunnel closure on Monday was justified because it was the first time the system was put to the test and proved effective in diverting floodwater, Smart project director John Abraham said.
“An alert from the Drainage and Irrigation Department’s Stormwater Control Centre (SCC) was received when rainwater reached critical levels at the monitoring station at 8pm.
“As a result of the closure, the tunnel managed to divert about 500,000 cubic meters of rainwater into the Taman Desa holding pond, keeping the water at a safe level of 28m at the monitoring stations,” he said during a press briefing at the Smart Motorway Control Centre in Jalan Davis, off Jalan Tun Razak, here yesterday.
The briefing was called in response to public queries on the necessity of the tunnel closure that resulted in hundreds of motorists being caught in a massive traffic jam.
He said although the rainwater diversion involved only the lower deck of the tunnel, the upper deck or the motorway tunnel was also closed for the safety of highway users.
The tunnel was closed at 9pm on Monday and re-opened at 5am the following day.
Abraham said the closure also gave the authorities the chance to put the system to a real test rather than basing its effectiveness on a wet-test. It also allowed the system to be fine-tuned.
“An important implication of the tunnel closure is that the system was able to divert rainwater away from the confluence of Sg Klang and Sg Ampang, to avert flash floods in the city.
“The level of water at the Tun Perak measuring station remained at 28m throughout the downpour, meaning that the tunnel helped to keep the water lower there,” said Abraham, adding that the river would overflow if the water level reached 29.5m.
He also said the system was able to give an accurate prediction of rising water levels at the rivers 30 minutes prior to it happening, while water levels at the 28 catchments areas were updated every five minutes.
The information was then relayed to the SCC before any closure instructions were given.
The first is the Rotary Cutter Head with tungsten pick bits used for excavation of soil and disc cutters used for the excavation of rock.
The second part of the machine is the Bulkhead located immediately behind the Rotary Cutter Head. This is where the pressured bentonite slurry shield is formed to provide stability during the tunnel excavation.
The third part of this machine consists of hydraulic rams, which are used to propel the machine forward and to keep the tunnel in its right alignment.
The fourth and the last main part of the machine is the tunnel lining erector used to install the pre-cast concrete lining of the tunnel.
Other parts complementing the main Special Features are two boogies on rails that house electrical, slurry pumping, ventilation equipment and cable and pipes.
The pre-cast concrete linings for the tunnel are transported continuously to the erector by boogies on rails. These linings are then hoisted and placed in the correct position in the tunnel with the help of vacuum pads attached to hydraulics arms. The arms are ex tended to keep the linings in place while the linings are bolted together.
The void between the excavated ground surface and the linings is immediately filled with cement mortar grout under pressurized conditions to ensure all the voids are fully filled.
For SMART, two Slurry Shield machines are used, both starting from the JKR field area in Jalan Chan Sow Lin. The first machine will head North under Jalan Tun Razak and Jalan Desa Pandan before terminating at the pond at Ampang behind Gleneagles Hospital.
The second machine will head south under the Jalan Chan Sow Lin and the KL-Seremban Highway next to the Sg Besi airfield before terminating at the existing pond in Taman Desa.
The Smart Tunnel operates in 3 modes:
Some of the Progress Photos:
