Super high rise towers with a concrete core, relatively small floor area and on a small job site very close to neighbouring buildings are all challenges for tower crane designers and equipment providers. Heinz Kessel reports
More than half of the global population now live in cities. By the end of the century another 3 billion will have followed. High rise construction is an essential way to achieve sufficient population density in urban development and to avoid urban sprawl into the countryside.
Whereas the earliest skyscrapers were purely office towers, many of today’s high profile towers combine residential, commercial, leisure and even green space into a kind of vertical village. Another trend is for slender high rise luxury residential projects. These have an astonishing height-to-width ratio far beyond 9:1. An example is the breath taking 22.5:1 of the 438 metre Steinway Tower in New York, USA.
Manhattan is at the forefront of the development of this kind of record-breaking condo tower. The apartments will each occupy either a full or a half floor. Far fewer elevators are needed than in an office building. A large glass area allows a spectacular view but requires design solutions that minimise the impact of structural elements on the already limited usable space.
Construction trends like the above directly influence crane application. In Melbourne, Australia, at the time of writing 75 per cent of the city’s tower crane fleet was engaged on residential tower sites. Choosing adequate lifting equipment is mainly a matter of space on site and the type and size of building materials to be handled. Climbing tower cranes working on inner city residential super tall projects have to cope with limited set up space as a result of surrounding infrastructure, neighbouring buildings and the narrow core and lift shaft size of the building under construction.
In addition, the wide application of concrete requires fast load cycles. Strong competition among tower crane manufacturers drives provision of special slim tower systems under 2 x 2 metres for internal climbing. In 2009 Favelle Favco delivered its compact 223.19 type 1.9 x 1.9 m tower for the London Pinnacle project. For the Melbourne Empire apartment tower project a step down to the even slimmer 1.6 x 1.6 m type 223.16 tower system was made.
A transition tower section provides adaptation to the standard 2.017 m slewing ring support of the upper crane. On the Favelle Favco M220DX luffing jib tower crane with 30 m boom a free standing height of still 40.6 m can be achieved with the 223.16 tower system. Access in very narrow lift shafts is afforded by a vertical connection device. It consists of six bolts per angle in the pockets of the reinforced main chords of the mast. Nothing extra has to be considered for removing horizontal connection devices when de-rigging the internal climbing tower crane after topping out the core.
Adding to the compactness is the three beam internal climbing system. Space constraints can become a major issue. When conventional climbing collars or systems using leaders, plus the form work, force the use of a smaller crane than is possible using the three beam lift shaft climbing device. During the first climbing step of a three beam system the middle beam extends at both ends into core wall pockets while telescopic sections of the two other beams are retracted.
After jacking the crane by the amount corresponding to the length of the central hydraulic stroke, the two lateral beams are extracted from the core wall pockets leaving the whole crane supported on the next working level. Then the hydraulic cylinder with the central retracted beam follows up into the working position of the crane. Three-beam climbing systems were originally widely used on Favelle Favco cranes. They are self-contained and minimise the preparation work needed before jumping a crane. They require, however, protrusions in the core at many short intervals as defined by the cylinder stroke of the climbing unit.
The system is especially popular in Australia. On Jaso J208PA and J280PA luffing jib cranes the Spanish manufacturer developed a similar climbing device based on 1.7 x 1.7 m towers to fit 2.09 x 2.09 m lift shafts for its local representative Titan. Adjacent high rise buildings and other neighbouring obstacles demand a short out of service position in the initial phase of the construction project. To accommodate this Jaso developed a folding structure acting as an additional boom stop which can be manually or automatically activated when the crane is parked in a steep out of service position. It prevents the jib from being pushed backwards by wind from the front.
The J280PA can be parked with a 30 m jib at only 5.75m radius. To help the crane to weathervane a large windsail banner was added at the jib end. For the Crocus City Manhattan project in Moscow, Russia, two J280PAs were delivered climbing inside the building’s core. Both had the special Jaso jib park system so that the 40 m booms could weathervane at 7.5 m radius instead of the standard 16 m. On such cramped sites Favelle Favco fits a storm parking system consisting of separate hydraulic buffers added to the A-frame against which the boom is parked in a raised position.
Instead of using a common fixed windsail banner Select delivers its new Terex luffing jib cranes in London, UK, with a curtain-like windsail. When the crane is working this windsail will be folded and parked in a shelter on the tip jib section. For weathervaning the crane driver activates an electric motor by pressing a button in the cabin to unfold the banner which is guided in a curtain track inside the jib.
In contrast to three-beam climbing, a typical climbing system from European manufacturers allows more flexibility as the crane can jump several levels in a single climb without needing core wall pockets arranged close together. In this case the inner climbing crane is resting on a set of two climbing collars surrounding the crane tower which, in turn, are based on custom-designed sub-beams connected to the core wall. By using the hydraulic ram in the base tower section climbing catch hook past, and then rest on, ladder rungs moving up the crane. The whole crane is held by a ladder fixed to the lower climbing collar during this process. After the crane has climbed through the first collar, crane supports extended at the base tower section will rest on this collar while the ladder has to be moved to the second collar above. Before jumping the crane to the next level, a third upper collar must be installed, including sub-beams. It will guide the crane when leaving the lowest collar.
Even when using a slim tower system the dimension of the climbing collars must also fit into the lift shaft. To save space and climbing time for the core construction, crane manufacturer Wolff developed a special version of its new KSH 23 inner climbing device. Fitted with collars the necessary floor opening is 3.54 x 3.19 m for the 2.30 x 2.30 m tower system. Without the collars the modified ladder climbing system, called KSH E 23, can be installed in a 2.70 x 2.70 m opening.
Without collars a new guide system for the tower and a connection point for the ladders must be found. Two telescopic cross beams with extendable shoes replace the upper collar and sets of mast corner guides. To clamp the tower they must be fixed to the core wall. During operation the whole crane is resting on a massive extendable support girder which is integrated in the lowest climbing tower element. When climbing, the support girder is retracted and catch hooks at the piston cross beam fall out and latch into the climbing ladders.
Regardless of whether a Wolff KSH 23 standard or a modified KSH E 23 internal climbing version is used, the 2.30 x 2.30 m tower system with strong corner posts is very resistant to torsional distortion. Its characteristics allow the tower to be up to 36 m above the clamping of the internal climber. It is suitable for internal or external climbing so it brings flexibility to the construction site. An example was on the prestigious Jeddah Tower project where Wolff 355 Bs with 40 m jibs were set up on 48 m towers. These cranes are climbed in 12 to 15.5 m steps at the core before one crane is removed outside the building to carry on as an external climbing crane using the same tower system to achieve an 800 m final hook height.
Terex has an improved HD19 26.6.ladder climbing system. It was used for the first time installed on the two CTL430-24 core climbing cranes raising the 40 storey 100 Bishopsgate project in east London, UK. No climbing collars or sub-beams are used with this internal climbing device. Core wall pockets in the elevator shaft allow positioning of the climbing ladder support beams during climbing and crane supports during crane work.
During a climbing operation specially shaped catch hooks flip by gravity into the lugs when passing the ladder. Two guiding fix frames connected to the base section and third tower section, allow continuous climbing. By using transfer mast sections CTL 180-CTL430 luffing jib tower cranes can be mounted on the slim 1.90 x1.90 m mast system.
Key to making good progress is the speed at which the core can be raised by slipforming, to provide stability as the building takes shape. The tower cranes need to grow at the same speed to ensure that a light weight crane can be installed on top of the slipform with a fixed short tower length. Alternatively the crane following the rising core must be fast climbing to minimise its time out of service for climbing. A solution is to choose a rigid compact tower system allowing high free standing capacity above the last tie-in support to the building.
Three Liebherr 180HC-L luffing jib climbing cranes with 50 m jibs are mounted on the 1.90 x 1.90 m 355IC tower system to raise the 325 m supertall “Worli” residential project in Mumbai, India. The cranes started with 52 m free standing height before climbing with the core raised by a large slipform leading to the extraordinary 41 m free standing height above the climbing collar. The cranes are based in 2.60 x 2.80 m lift shafts.
For larger cranes up to the 1,000 tonne-metre class Liebherr developed its new 24HC monoblock tower system with an outer dimension of 2.40 x 2.40 m. It is suitable for internal and external climbing. An impressive 74.80 m free standing capacity can be achieved for the new 710HC-L luffing jib crane rigged with 30 m jib.
Component weight is important as it dictates the size of the derrick crane needed for dismantling. In addition to the crane upper, on an internal climbing crane it is also important to consider the weight of the internal climbing system components. To that end the new 24HC internal climbing section can be easily split in two tower sections by the standard tower connection devices. So, instead of 18.67 tonnes in one piece, only 7.73 tonnes has to be handled, well below the 9 tonnes of a standard tower section.
Around the core
As the core rises the installation of permanent lift shafts is a substantial benefit to accelerate construction time. As each floor is constructed around the core, the lift capacity is already there to serve it. In comparison to external construction hoists, hours can be saved for each worker just travelling up and down by using the building’s super-fast lifts inside the core. In this case, however, the crane should be installed externally.
A solution can be conventional tie-in support frames. For buildings with only one rigid core surrounded by open space floors that means long, custom-designed tie-in supports which soon become expensive and an obstacle to the complex facade installation. In addition, the crane will lose its central position in the building under construction, leading to significant expense for extra mast sections and higher capacity because of the requested extended outreach.
A solution is to climb the crane outside the core. In normal operation two out of three supporting brackets are used to secure the crane at the outer core wall. For jacking operations three brackets alternate in their operation. In China the large number of patents since 2007 indicates that this concept is widely used for ultra-high rise construction.
While the upper frame tightly holds the crane tower with horizontal loading acting on it, the lower frame has also to take the vertical loading. Forces generated can be absorbed either by compression members under the frame or tension members above it, or a combination of both is preferred in order to distribute the loads. For the New Comcast Tower project in Philadelphia, USA, Lawrence Shapiro modified this externally hung crane principle using a cable suspended system as a result of the difficulty in removing large compression struts under the bracket using the climbing crane itself.
Lawrence Shapiro is a principal of Howard I. Shapiro & Associates consulting engineers based in New York and is experienced in special tower crane support designs. In some cases it is still necessary to use compression struts for tension members acting on core formwork. An improvement will be found in the application of chain blocks and steel cables attaching the dissembled lower support framework members to the crane base when it is climbing. Re-installation during the next jacking step will be speeded up and additional storage space on the construction site is not needed.
Made in Japan
The unique Japanese fast internal climbing system was conceived for skeletal steel framed buildings. As a benefit the upper crane can be jacked alongside its complete tower in one operation. That allows it to easily jump 12 floors at a time. At every stage all forces generated by the crane must be carried by one floor level. Reinforced concrete structures or precast concrete construction are becoming popular on new residential towers in Japan. They are a challenge because of the necessary load distribution over several floors. Sumitomo Mitsui Construction’s solution is to share the load between two floors. The crane’s base section rests on a specially designed support framework. A crane situated inside a lift shaft will be rooted on wall anchor screws which prevent bending moment on the shaft wall between two floor levels. The crane uses its standard foldable crane base to climb from one such temporary support table to the next one.
IHI with Shimizu developed another way to distribute the forces generated by the crane on different floor levels with a wider spacing. The upper main support is formed by a special tower section with ball support concentrating the forces generated by the crane in the centre of the crane tower. It is to distribute the vertical forces equally in all four corners of the mast and thereby the support beams and into the building under construction. Dimensions of the upper support can be minimised to a quarter of the original design. The bottom support only takes horizontal forces. After testing in the 350 tonne-metre class IHI is now adapting its new JCC-TS500 500 tonne-metre class luffer for this innovation.
On narrow job sites a short tail radius becomes important also during crane installation. In Japan Kitagawa, for example, claims just 6.80 m in the 700 tonne-metre class and only 5.80 m in the 350 tonne-metre class. Spanish manufacturer Comansa claimed that only its LCL165 with standard counterweight would fit into a building gap no more than one metre apart from the neighbouring building in Singapore. It was based free-standing on a 44.90 m tower. By using steel ballast its tail radius can be reduced even further, to less than 6 m.