Pier Giorgio Graziano describes the role cable cranes can play on dam construction sites
Cable cranes are a familiar sight in the dam construction industry and can still be one of the best ways to transport materials on project sites. From a safety perspective they offer advantages over tower or derrick cranes. Dam construction sites can be at risk from flooding from the nearby river, with the possibility that the building yard will be inundated. Cable cranes can place the most important equipment out of the way of flood water during construction. Furthermore, it has been known that when flooding has occurred minor operational equipment, and even workmen, have been rescued by cable cranes. Bulldozers, weighing tens of tons, are also known to have been recov-ered by two cable cranes coupled with lifting beams.
The main safety developments intro-duced in the passenger transportation industry over the past few years have been successfully transferred to cable cranes. Such developments include electronic overspeed devices, controlling the mechanical transmission of the winch, and maximum torque and torque gradient control.
Cable cranes allow for greater organisation in building yards. The crushing and screening plant; the batching and mixing plant; and the platform of the runway for loading the cranes’ buckets, can be arranged according to organic criteria and in the most appropriate position to rationalise the flow from one plant to another without worrying about the distance to the location where concrete has to be placed. There is also the great advantage of having a system which passes high over the dam area, without interfering with the dam works on site.
There are also economic advantages to using this type of crane. Although the initial investment required can sometimes be noticeably higher than for other methods, economy of operation not only recovers the price difference but considerable savings can be made as well. Such economic benefits have developed through improved logistics and reduced handling. Concrete is transferred from the batching and mixing plant into the buckets of the cable crane which carries it directly to the casting block, eliminating the transfers and trans-shipments necessary with other means. Indeed other machines (several would be required) often have to be moved around the site during construction.
The cable crane is controlled by a single operator from a cabin positioned in a suitable place, allowing the best view of the bucket loading and unloading area. A special indicator shows the position of the bucket allowing ease of operation in poor visibility. Since the anchorage position of the track ropes are placed well outside of the dam area and the ropes are usually positioned high above the dam site, the cable cranes are kept clear of any mine explosions.
Even if the main duty of the crane is concrete transportation, the installation is still suitable for other auxiliary functions such as transporting formwork, reinforcing bars and equipment from one yard to another.
Cable cranes have proved to be a valuable addition to the construction site for high dams and large arc dams. The main examples of dams being built or which have been built utilising cable cranes include:
•Rules dam in Velez de Benaudalla, Spain (built by the Auxini-Agroman consortium).
•Foz Coa in Portugal (built by the Somague-Agroman-Dumez-Bento Pedroso consortium).
•Deriner dam in Turkey (built by the Erg company).
•Karun III in Iran (built by Sabir). A new feature of such cranes has been developed at the Karun III arc dam in Iran, which is being built by Sabir using two 9m3 cable cranes. Parallel cable cranes have two sides of track rope fixed to mobile towers, each moving on a runway.
One technical problem with parallel cranes is that the winches have to be on board one of the mobile cars, and the hoisting rope has to be kept in line with the track ropes. As the hoisting rope is moved by a drum winch there is the problem of keeping the hoisting rope aligned with the track rope as the drum moves. The standard solution is to have the drum axle parallel to the track rope, running the hoisting rope on a pulley, the movement of which is synchronised with the drum rotation. In this arrangement the pulley has to withstand all the force of the hoisting rope. But at Karun III Poma Italia came up with a different solution. The hoisting drum axle is perpendicular to the track rope axle and the hoisting winch is moved synchronously with the drum rotation. In this way the pull of the hoisting rope is not conflicting with the winch movement.
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|The origin of cable cranes|
| On 30 June 1859 Jean François Gravelet Blondin, a French acrobat known as The Great Blondin, crossed over the Niagra Falls walking a rope tensioned across the top of the waterfall. Over one hundred and forty years later his name lives on. In France and Italy in particular blondin is the name given to cable cranes.
The first known cable cranes were built around 1860 and used mainly for wood transportation. The cranes consist of a single or twin track rope pulled between the two sides of a valley where a concrete dam is being built. On the track rope the carriage goes to and fro moved by a ring of rope (the translation rope). The winch moving this rope usually has a driving sheave. A hook transporting the bucket is suspended from the carriage with a hoisting rope, usually moved by a drum winch.