Going large: flat top tower cranes
By Heinz-Gert Kessel08 July 2019
Over the years, as buildings around the world have become taller and their design more complex and challenging, so too have the machines and technology required to build them. Constantly evolving to meet the construction industry’s latest demands, tower cranes have been continuously redesigned and re-engineered to help construct our modern world and are often equally as impressive as the buildings themselves.
Early giant tower cranes
Heads turned in the industry in 1977 when Linden Alimak won an order to supply custom-built 1,400 tonne-metre class heavy lift versions of its modular 8000 series flat top tower cranes. Destined for a power station project in Poland, their unique design features made people sit up and take notice. They could reach a free-standing height of 139 metres and lift 16.7 tonnes at 46 metres and 50 tonnes at 17.6 m.
Never before had a modular design of saddle jib crane without a classic tower head been manufactured in this size. It was decades until the capacity of these cranes was surpassed by an updated flat top design.
Designer Franc Jost showed the way for a breakthrough in flat top construction tower cranes back in 1998. At that time his unique design for the 400 tonne-metre Potain MDT412 flat top had a 3.1 m high jib profile which looked as mighty as the now-common 800 tonne-metre flat tops of today. It shows the recent improvement in crane design.
The benefits of modern tower cranes
Positive customer experience in the small and medium capacity class has meant flat tops are no longer a niche product. Benefits can be found in several aspects, making such cranes a better choice for standard construction work.
The jib is self-supporting, which means that tension always occurs in the upper member, compression always in the lower members, no matter where the load is located along the jib. Stress in the members will always be in the same direction which helps reduce fatigue and allows economical jib design.
If a jobsite is planned with two or more cranes over slewing each other, the flat top design allows a reduction in the overall tower height for each crane. Foundation costs are then also reduced. Overall height restrictions are more easily complied with and special short jib combinations can more easily be accommodated. Fewer components, for example, no pendants or a tower head, means faster erection time. Reducing the size and weight of components also means a smaller crane for erection and dismantling.
Modern high capacity flat top crane designs
An old argument against large flat tops was that the jib components were too big and heavy. Steel structures have since been optimised so it is no longer valid. Liebherr demonstrates in a comparison between its 280 EC-H with a cat head and its all-new
340 EC-B flat top. Using the same mobile crane the flat top version can be installed 13 m higher than the classic 280 EC-H thanks to not having a tower head up on top. The slewing unit of the flat top can be reduced from 11.8 to 7.1 tonnes while the 13 tonne 280 EC-H slewing unit can be further dismantled to reduce it to 10.2 tonnes.
At the first glance there are obvious disadvantages to high capacity flat top cranes but they can be overcome by proper design solutions. At the high capacity end, flat tops used to be ruled out by the boom profile being outside standard transport dimensions. The base jib section of the original 50 tonne capacity Linden 8000 had the maximum acceptable transport dimensions at 3.7 m high and 1.64 m wide.
Linden produced design studies for a 3,600 tonne-metre class version of the 8000. It showed 100 m outreach and 70 tonnes capacity at 50 m radius. A high profile flat top jib was combined with a conventional tower head and two pendants running to the bottom chords of the triangular jib.
In 2011 Comansa developed patented 6.2 m high jib sections for the LC3000 series where the upper part can be folded to allow the jib sections to be transported, standing, in open-top containers. Feasibility studies on the LC3000 series have been done up to 90 m jib and a capacity of 125 tonnes.
In China where most high capacity flat top tower cranes are manufactured, another solution was chosen by Zhongsheng Construction machinery (ZS), Yongmao, Finehope, Shen Yang Bao Quan and SYM. This patented flat top design has a jib where the triangle is inverted, flat at the top. It allows a reduction in the overall size and weight of the monorail trolley.
In this design the lower chord remains the same but there is a mixed boom tube truss structure with a different profile height which reduces the dead weight of the jib. As the upper part of the jib is wide, the two upper chords can share the high tensile force generated on high capacity cranes. For easy boom section installation in the air two hinge pins can be inserted into latches or double latches on the base boom sections using large safe platforms on the wide top of the triangular boom.
By transporting them lying on their sides acceptable transport dimensions can be achieved. The largest jib section of the 2,200 tonne-metre class Yongamo STT2200 forms a 12.6 m long, 3.70 m wide and 2.40 m high unit weighing 24.2 tonnes. A disadvantage, however, is the tailing operation to get it into an upright position. The inverted triangle-shaped jib section has to be secured by erection outriggers at ground level before being lifted.
Controlling the load moment of long jibs
On large cranes with long jibs it is important to minimise the decrease in load moment. A way to reduce dead weight is to use a monorail trolley. The other is to apply a double trolley system where the inner heavy lift trolley is parked at the jib base section and only the outer light weight trolley is used to apply maximum load moment at maximum radius. In best practise a changeover from double to single trolley operation can be controlled by the operator from the cabin as realised by Comansa and Krøll.
Saving weight is essential. On the Krøll K630F with a single trolley as a 12/24 tones version, the trolley and hook weighs 1.6 tonnes. On the 12/36 tonne version, the front trolley of the double trolley weighs 950 kg including the hook block.
On high capacity tower cranes with long jib options it is common to offer two counterweight length options. If rigging component weights are reduced the minimum lifting weight becomes important, not only the overall length of the counter jib.
In the 400 to 600 tonne-metre class there is a clear trend for flat tops to replace conventional saddle jib cranes with tower heads. It helps minimise transport and riggings costs. A new market has emerged in the heavy lift sector above 800 tonne-metres in niche markets formerly served mainly by large mobile cranes or luffing jib cranes.
For heavy steel construction flat top giants rated to lift 180 tonnes have been built in China. They are used for boiler house construction as an alternative to heavy luffing jib cranes. A requirement for low headroom above the maximum hook height allows them to work together for tandem lifting. The horizontal load path and no luffing winch is an economic advantage over luffers. One example is the ZSC3600 with 180 tonnes capacity at 20 m radius.
The Zoomlion T3000-160V incorporates Jost design features, for example, connecting the first reinforced jib section directly on top of the slewing unit. Capacity is 160 tonnes at 19.5 m radius.
The future of giant tower cranes
Even larger flat tops are under development for bridge pylon construction. The ZS product line is led by the ZSC5200 with 260 tonnes capacity at 20 m radius and still 87 tonnes at 50 m. It has a patented inverted triangle-shaped boom and box-shaped modular counter jib where the winch unit is then added on top.
Of similar design is the Yongmao STT3930-200t developed for the Nanjing Puyi Highway Bridge project. At a 21 m radius it will lift 200 tonnes to a height of 250 m with two 162 kW hoisting winches connected with 8 falls in double trolley mode. At the maximum 50 m radius with a single trolley 79.1 tonnes can be handled. With a transport-friendly 3.6 x 3.6 m monoblock tower system maximum free standing height is 67.3 m.
All the mast and boom connection pins can be easily inserted using hydraulic pullers. Under test is a Yongmao double hoisting winch flat top version. It is a simple design with the two winch platforms located at different heights on top of the rectangular counter jib end section. The hoist ropes are guided over the top tower section to the tension pulleys away in the second jib section.
The long distance between the first sheaves and winch units means a better fleet angle for rope spooling. It also adds to the maximum under hook height of the crane by storing hook rope outside the two winches on top of the counter jib and jib. In addition, the loaded hook rope compensates deflection of the boom.
The all new Finehope FHTT2000 also shows the typically Chinese inverted triangle boom configuration. This 100 tonne capacity flat top is serving a power station project in Dubai. It has an unusual additional moving ballast carrier added to the fixed ballast. The additional travelling counterweight carrier under the counter jib works as a patented automatic load balancing mechanism. This reduces the bending moment on the tower.
The above feature is especially suitable for flat top designs in contrast to jibs supported by pendants which tend to show more deflection under full load. The maximum load moment is claimed to be substantially being increased over common flat tops. It should be considered, however, that moving ballast systems have more moving parts and need more maintenance and monitoring.
XCMG exhibits more European roots in its XT1200 design. The first jib section is rectangular and directly connected to the slewing unit. Remaining jib sections are a conventional triangular type allowing up to 80 m outreach with 10.5 tonnes tip load. On a 50 m jib capacity is 63 tonnes in double trolley operation mode. As seen on the Potain MD series the ballast frame at the back of the winch platform is inclined to allow the ballast blocks to self lock by gravity. The rectangular modular counter jib is 18.2 m long up to a 50 m jib and then 28.2 m for longer jibs.
Zoomlion’s all new T1200-64Q offers a capacity of 64 tonnes and then 15 tonnes at 80 m radius. No crane part is wider or higher than 3.2 m for easy transport.
Lifting prefabricated components
The government in Singapore is encouraging construction companies to develop pre-fabricated pre-finished volumetric construction (PPVC) methods to improve productivity by up to 50 % in terms of labour and time savings. PPVC involves stacking, in a Lego-like manner, factory-built building modules complete with internal finishes, fixtures and fittings, to form residential and hotel buildings. In addition to quickly providing much-needed accommodation, it also helps get around the lack of suitably skilled labour.
While steel modules generally weigh as much as 20 tonnes, today’s preferred reinforced concrete modules weigh about 35 tonnes. It is good practice to hoist PPVC modules with the aid of a special steel collar frame so that the module will not be subject to inclined forces from slings.
Extra weight from slinging equipment must be added, meaning at least 25 to 40 tonnes of capacity is required at a radius of 35 to 40 m. At least 1,000 tonne-metres – a 1,600 tonne-metre class – self-climbing crane is necessary when the building height exceeds the economic utilisation of mobile cranes.
With PPVC the horizontal and vertical joint tolerance of the modules is crucial, which means smooth and exact placement within plus or minus 2 mm at very slow speed is necessary. In many countries oversailing of neighbouring properties is not allowed, so extremely short counter radius is a benefit in addition to being an economical fast rigging crane design.
All these requirements are best served by the modular design of large flat top cranes with shortened boom and counter jib. Yongmao has found broad market acceptance in Singapore for its STT1330-64t with a capacity of 64 tonnes at 21 m radius and 34.25 tonnes at 35 m.
Installation typically uses a 350 tonne telescopic boom crane at 17.1 m basic height before self climbing on the 3.2 x 3.2 m tower system. Tiong Woon Tower Crane Pte Ltd ordered 20 large Yongmao STT1330 and STT1830 towers. In Singapore around 50 tower cranes with more than a 1,000 tonne-metre load moment are working on PPVC projects.
Liebherr’s 1000 EC-B 125 Litronic with a capacity of 125 tonnes at 9 m radius demonstrates how custom-designed flat top climbing cranes can be an economic alternative for 5 MW wind turbine construction. Optimized for this application, the crane has a fast climbing system, 17.5 m
short counter jib and improved transport features. The boom is only 2.70 m high and can be fitted into a 3.40 x 3.40 m tower section. In contrast to a luffing jib tower crane the driver always sits above the load and has a clear view.
The new SoLite synthetic fibre hoist rope on the Liebherr only weights around a fifth of steel rope so it boosts capacity of the 370 tonne-metre class crane on two falls. It could become even more significant for the development of Liebherr heavy lift tower cranes where four- or even eight-fall mode operation is requested.
A new application for custom-modified heavy duty flat top tower cranes will be the gravity-powered long-duration energy storage proposed by Swiss-Southern Californian start-up company Energy Vault.
A six-armed flat top tower crane stacks 35 tonne concrete blocks up to 120 m high with abundant grid power and drops them down to retrieve electricity from the speedy descent. Operation of the free standing climbing tower crane surrounded by concentric rings of concrete blocks must be fully automated, meaning the control software has to move the blocks precisely in all weathers. The first 35 MWh gravity-based battery tower served by a custom designed multi-flat top jib crane system will soon be installed by India’s Tata Power Company.
Manufacturers in Europe have also added to the top end of the flat top market. While European manufactures Liebherr, Krøll, Potain, Wolff and Wilbert still prefer conventional tower head hammerhead designs for their highest capacity cranes, they also push their flat top models in higher capacity ranges. A recent example is Wilbert with its all new WT 720 e.tronic. It offers 32 tonnes and a 30 to 85 m jib adjustable in 2.5 m increments. Three counter jib lengths are available and it has a 3 m high top tower section. All components fit standard containers. The 110 kW hoist gives 240 m/min lifting speed and the tip load on the 85 m jib is 5.4 tonnes.
The new Potain MTC 565 flat top reaches its maximum 80 m jib in 5 m increments. Three counter jib radii are available from 17 to 24 m and capacity is 20, .25 or 32 tonnes. In Denmark Krøll has continued to raise the size and capacity of its large flat top series, starting with the K630F in 2016. That was followed in 2018 by the K830F and in 2019 with the massive K1230F, in production at the time of writing.
Preliminary studies have already been carried out for an even larger version, the K1830F. Back to the K630F, there is a 12/24 tonne single trolley version and also an all new, automatic switch-over, double trolley 12/36 tonne system where the operator can park the heavy inner trolley at the jib foot section without leaving his seat. The maximum jib length is 80.5 m where capacity is 5.4 tonnes and the base jib sections are less than 2.6 m tall so they fit in high cube containers. Two pendant-supported counter jib lengths are available and the 2.85 tonne top tower has pins for easy disconnection from the 14.5 tonne slewing unit.
The K830F provides the same maximum outreach and trolley alternatives and a slightly better load moment. It also has the same overall jib height but a different counter jib design. Again, two length versions can be rigged but, thanks to the box-shaped lattice modules, pendants are no longer necessary and installation weight is reduced.
To date, the biggest Krøll flat top is the K1230F. Capacity is 48 tonnes at 30.5 m radius and 30 tonnes at 46.3 m. Despite their 3.8 m height, the rectangular counter jib sections and the triangular jib sections can be transported either lying down at 1.9 m height and 3.8 m wide or upright on flat beds at 3.9 m high and 1.9 m wide. When rigged with its maximum 80 m jib the outer sections are just 2.56 m tall to fit inside a high cube container. Depending on the jib length the standard counter jib can be shortened from 28.5 to 22.5 m. The heaviest part, the 3.30 x 3.30 m wide and 1.8 m high slewing unit, weighs 23.6 tonnes.
With a heritage going back to the original Linden 8000 series of the 1970s, Spanish manufacturer Comansa has the most experience of large flat top cranes. Even on the largest model in the LC3000 series, features include high speed winches, PowerLift moment control and the automatic controlled change over from single to double trolley. The LC3000 series has a unique patented foldable jib and counter jib design. Each section forms a main rigid body giving stability to the jib sections. Over that there are two panels articulated with hinges so they can be folded and connected in the upper part of the jib with bolts to form the 4.7 or 6 m high base sections. Capacity at 80 m radius is 13.5 tonnes while the maximum varies between 32 and 90 tonnes depending on model.
Tower crane range expansion
Comansa’s popular 2100 series recently got a new addition, the 21LC1050, improving standards in the 1,000 tonne-metre class for transportability and fast erection. Only the slewing part has to be delivered in a 20 foot
open top container and five triangular base, counter jib and jib sections, which are 3 m high, need to be shipped in open top containers. The rest of the crane with up to 85 m jib and a capacity of 50 tonnes, fit into standard containers. The slewing unit is the heaviest part at 19.5 tonnes.
The 21LC1050 shares up to five jib sections and four counter jib sections with the smaller 21LC660 and 21LC750 models. This modularity allows important savings for those companies who operate different Comansa models. The design of the counter jib allows six standard lengths from 21.2 m to 31.1 m radius. By using steel ballast the counter jib radius can be further shortened by another metre for all six configurations. Jib length can be set between 30 and 85 m in 5 m steps. The short counter jib version with 35 m jib is particularly well suited to 25 to 35 tonne PPVC module projects.
For its H-series of heavy load saddle jib cranes Jaso favours a low top design principle. A low tower head and short pendants keeps the transport dimensions of the reinforced base jib sections to a minimum. At the same time the application advantage of the conventional flat top design remains, for example, reduced overall height of the tower crane and the possibility of jib installation in sections directly from the delivery trucks.
At first glance it seems that the two base sections held by pendants must be rigged as one heavy unit. A smart design, however, allows the first jib section to be lifted in separately and fixed to the low tower head by a pair of pendants bolted to the upper chords of the rectangular jib base section. Then the second jib section can be added with the remaining pair of pendants already stored on top. The pendants are then connected and the additional attachment of the first pendant set to the first jib section is disconnected.
Up to now the J700 was the largest model, with 5.5 tonnes capacity at 80 m radius and a maximum of 24 tonnes to 30 m outreach. The strong low-profile counter jib beam allows a choice of two lengths. The new recently tested giant J1400 has a different counter jib design allowing four configurations from 18.1 to
29 m. On request the crane can be delivered with steel counterweight shortening the counter jib radius another 1.8 m.
Pendant length from the 5.7 m-high tower head to the first counter jib section was minimised on the J1400. At the counter jib tail section are the hoisting winch platform and easy-to-install counterweight blocks. For easy transport and storage the voluminous counter jib sections are completely foldable. Pendants holding the first two jib sections means no section of the jib is higher than 2.65 m. All components can be transported in high cube 40 foot containers.
Component weights, especially the slewing unit, are generally higher on flat top cranes than with conventional tower head models. Jaso‘s low top design benefits from a relatively light slewing unit. In standard erection condition it is 21.6 tonnes but can be split down to 10 tonne units to minimise the size of the assembly crane. The boom can be extended from 25 to 80 m in 5 m increments with a tip load of 10.5 tonnes. Maximum load is 64 tonnes at 24.3 m radius.
Due to the reduced hosting rope weight and thanks to the maximum 4 fall operation mode up to 70 m under hook height there is no load chart reduction to be considered by the customer. Optional additional safety features are secondary clamp brakes for the hoisting and trolley winch. PPVC is a target application with its short counter jib and impressive capacity of 42 tonnes at the 35 m maximum outreach.