The programme of building initiated by Dubai’s ruling Al-Maktoum family over the last ten years has seen the tiny state’s desert seafront bloom with new towers. On a tour of the emirate, generously paid for by Liebherr, Cranes Today had an opportunity to see some new projects at first hand, and to discover how the German crane manufacturer has brought some unique solutions to the problem of building very tall towers.

Towers around Dubai marina

Towers around Dubai Marina, where Liebherr is working on the Marina Mall project

Al Kazim Towers

The Al Kazim Towers, being built by Al Shafar General Contracting under a design by Dubai’s National Engineering Bureau, will reach 265m, or 53-storeys, high when they are completed in 2008. Liebherr faced two key problems when recommending what cranes to use on this site. Firstly, space around and between the two towers was limited. Secondly, at the end of the job, whatever cranes were used would need to be dismantled, and lowered to the ground.

Liebherr-Werk Biberach project department manager Christoph Schneider explained that the manufacturer had recommended using five luffing jib cranes on the project: “To construct each of the towers, two cranes each are being used inside the buildings because of the heights involved. This means that the cranes are standing and working in close proximity. This is only possible with luffing jib cranes.”

Schneider continued, “The fifth crane however works in the space between the two towers. As the two tower blocks have grown upwards, the lateral movements of this crane have been increasingly restricted. On the other hand, since a good working radius is essential, a luffing jib crane was the only option here, too.

“To construct the two towers, two 112 HC-L cranes each were selected and the fifth crane working between the towers is a 160 HC-L. The cranes were first erected in free-standing position on foundation anchors and prepared for internal climbing inside in the buildings. This is why, instead of the usual base tower section, the tower configuration has a special tower section specifically for internal climbing.”

Liebherr HC-L

A Liebherr HC-L tower crane, mounted on an internal climbing frame

The two 112 HC-Ls selected for each tower have been rigged so that one climbs close to the core of the building, while the second is secured closer to the edge of the tower. As Schneider explained, “The great advantage of ‘climbing inside the building’ lies in the comparative low number of tower sections needed to construct a very tall building. However, this climbing method is only commercially practical at building heights of approx. 200 m and more, firstly because of the amount of work needed for the necessary openings, which have to be closed up again later, and the transmission of stresses out of the crane.”

Liebherr’s project department manager went on, “The second reason is that there are necessarily frequent construction stoppages while cranes are being climbed. Guide frames and supporting girders need to be repositioned again and again, which involves a lot of manpower and time. The third reason is that upon completion the crane needs to be dismantled by some other hoisting equipment such as a derrick, because it cannot climb downwards any more. After the crane has been fully dismantled, the site that the crane occupied and perhaps that of the dismantling derrick will need to be prepared without any hoisting equipment.”

This third point, the problem of dismantling tower cranes at a height, has prompted Liebherr’s designers to come up with a novel solution. As the towers reach completion, they will be topped out with a decorative spire, similar to that on New York’s Empire State building. For this task, only one of the cranes will be needed. At this point, the crane climbing through the core of the building will be dismantled and lowered, using its partner climbing through the building’s edge.

As the spire slopes in, the remaining HC-L 118 will pass through it, and be attached to the exterior of the building, allowing it to climb to a point where its height-under-hook is sufficient to allow the peak to be completed. This second crane will then itself need to be dismantled and lowered. For this task, Liebherr plans to use its new 200 DR 5-10 derrick crane.

Liebherr 200 DR 5-10

Liebherr’s new 200 DR 5-10 dismantling derrick

The 200 DR 5-10 was specifically designed as a dismantling crane for jobs on tall buildings, bridge pylons and communications towers. It is manufactured using parts that are all small enough that it can then itself be dismantled and lowered down a lift shaft, using only a very small hoist. The maximum dimensions for the derricks parts are 2.20m x 1.10m x 1.10m, and no package of parts weighs more than 1,000kg.

The derrick features hoisting equipment for heights of up to 320m, in single or double-reeved configuration, with load moments of up to 200t. A choice of jibs is available. With the first, loads of up to 10t can be lifted, at a radius of up to 20m; the second jib option can lift loads of 7.5t to a distance of 25m. The supports for the derrick are articulated, and can be adapted to the substructure of the building, for the best possible distribution of forces.

When the 200 DR 5-10 has completed dismantling the HC-L 118 being used on the Al Kazim tower it will begin dismantling itself. First, the tower of the derrick will take on the role of the jib bracing frame found in a normal luffing jib crane. Once the jib is dismantled, the tower will be angled and dismantled in the same way that a crane dismantles jib sections. Liebherr predicts that it will be possible to fully dismantle the 200 DR 5-10 in just four days.

Marina Mall

Marina Mall overview

Liebherr HC 280s at work on the Dubai Marina Mall project

The Dubai Marina Mall will comprise two towers, and a number of other buildings, built in part in the water of Dubai’s new marina. The taller tower, at 38 storeys, will contain hotel rooms and apartments. The shorter, eight storey, tower will contain gourmet restaurants and shops. The project has been designed by DP Architects, working with engineers Meinhardt, for Emaar Properties. The lead contractor is Alec.

The construction companies involved in the project had originally planned to use eight trolley jib cranes and one luffing jib. However, Liebherr found that this created a number of problems. The layout the companies had chosen meant that the radii of the cranes overlapped more than was necessary. The design used meant that material flow around the site was poor, and the highest tower could only be serviced by a single crane.

Liebherr’s project design team came up with an alternative solution. Instead of the nine cranes in the original plan, they suggested using just eight 280 EC-H cranes. Six of these will be rigged to lift maximum loads of eight tonnes, and the other two will be configured for the heaviest jobs on the site, at up to 16t.

The cranes were located to balance the need for good material flow around the site with a minimum of overlapping radii. As Liebherr’s Schneider explained, it is essential that when tower cranes are not in use they are able to rotate their jibs freely according to changing wind directions. To manage this on the Marina Mall site without risking collisions between overlapping cranes, it was necessary to set the cranes at three different hook heights.

The two tallest cranes were installed first, near the tower. Once construction reaches a point where it is practical to begin using the lower cranes, the first two cranes will be climbed, and the second group installed, and then the third. Completing the project will require this process to be repeated again. At the end of the job, with just the tower to be completed, the lower cranes will be dismantled, leaving just the two tallest cranes.