Which is the best tower crane?
By Heinz-Gert Kessel19 April 2022
In the first of a two-part feature, Heinz-Gert Kessel explores whether low top or flat top tower crane designs are leading the way in the high-capacity tower crane classes
At first glance it seems all modern saddle jib tower crane designs are flat tops.
It hasn’t always been the case as many in the industry may consider a flat top tower crane a relatively new kid on the block. Even the Linden 8000 series from the 1970s was not the world’s first construction tower crane without a tower head. In 1930, for example, crane designer Arthur Loeb designed a real topless top slewing internal climbing crane with a 4-chord shaped saddle jib and counter jib. It was on the custom-built cranes used to raise the 65 metre high hotel de ville at Villeurbanne, France.
At the end of the 1940s, under the name of C.A.C.L. J. Weitz, it was again Arthur Loeb who designed the typical low top crane later known as Weitz Richier construction cranes. Milestones were set by the G280B in 1961 with a capacity of 20 tonnes and, in 1968, by the high-capacity low top Richier-Weitz GT1450 versions with up to 62.5 tonnes capacity.
Many crane manufacturers in the 1970s, including Peiner, Potain, Liebherr and Krøll, sought new tower crane capacity records by still relying on classic tower head design with pendants holding the jib. Not Swedish company Linden, however, which set new standards in the modular design of flat top cranes with its famous 8000 series.
In 1977 Linden-Alimak turned heads when supplying 139 metre (free standing), 1,400 tonne-metre class flat top cranes with 50 tonnes capacity at a 17.6 metre radius (and still 16.7 tonnes at 46 metres) for a Polish power plant project. When looking through Linden-Alimak project studies of those days one can also find low top versions pushing the Linden 8000 high-capacity series into the 3,600 tonne-metre class. These Linden HD versions with, for example, 15 tonnes capacity at 100 metres radius and 45 tonnes at an 80 metre radius, never left the drawing board even though they prove the versatility of a combination of modular flat top crane jib design and classic tower head.
In 1978 Peiner carried out tower crane studies for a nuclear power plant project in the USA where 385 tonnes had to be lifted at a 50 metre radius using a hammer head crane. The so called SK20000 with conventional tower head was designed with a 6.40 metre, high profile and heavy-duty jib. Each jib section had to be split by a bolt connection device for transport in two u-shaped frameworks. These were very similar to the Zoomlion W12000-450 flat top crane jib design launched in 2021 in the 12,000 tonne-metre capacity class. This 450 tonner was for a suspension bridge tower project in China.
Also for a bridge project, fellow Chinese manufacturer XCMG raised maximum capacity to 600 tonnes at a 25 metre radius. It had been decades before flat top and low top crane designs entered such mega capacity classes.
Flat top crane design
After the Linden 8000 series it was tower crane designer Franc Jost who focused again on flat top crane design, albeit in the sub-400 tonne-metre class back in 1998. At that time rental fleets were growing with demand for low and medium sized city cranes.
An economical concept for tower crane transport and rigging was needed to reduce the number of support trucks and the rigging time. A brilliant way to achieve this was to eliminate the need to handle pendants during jib installation. It meant the under hook height of the erection or dismantling crane could be reduced by about 6 metres.
On construction sites with several cranes the overall height of the umbrella crane could also be reduced, lowering the necessary tower height. Competing crane manufacturers which had not converted the classic tower head type design into a flat top design tried to address the market requirements in the city crane class. They offered a low top version alongside a classic tower head type in the same capacity class. An example is the Wolff 5520FL-6K developed in 1997.
Customers could change the standard 6.30 metre tower head for a 2.27 metre short version. It was a forerunner of the flat top “Wolff clear” series in the city crane market.
The biggest low top crane in the market
The low top design was reborn by Spanish manufacturer Jaso in 2005, not just as city crane concept but also to reduce erection and transport expense in the high-capacity class. Its J600.20 design provided 20 tonnes capacity and a jib up to 80 metres. Following it was the J700 which itself was updated as a heavy lift version of the J800 and following the same design principles. It offers capacity up to 48 tonnes.
There is no difference in the clearance heights of similarly capable flat top cranes but all the components fit into standard sized containers for easy transport. While jib length can be chosen in 2.5 metre increments and even an extremely short, 23 metre, jib version is available, the counter jib can be adapted in four modular counter jib configurations ranging from 18.6 to 28 metres.
With a 64 tonne capacity the Jaso J1400 is the biggest low top crane in the market. It has a unique rectangular modular counter jib design with four counter jib options from 29 down to 18.1 metres without needing to use different lengths of pendant. This crane giant can be transported entirely in standard containers. For this purpose, the high frame counter jib modules are completely foldable and stackable.
Like the J800, the jib can be erected section by section. If the assist crane capacity is limited, the first two sections, held by the short pendants, can be lifted in to place separately thanks to a special erection lock of the pendant lines. Base boom installation weight can be reduced from 28 to 12.14 tonnes on the J1400 and from 20 to 11.5 tonnes for the J800. Jaso says it sees no disadvantage to its low tower head design against flat top cranes during the installation process. It also highlights that there is less jib deformation with heavy loads and less dead weight of the complete jib structure, when using short pendants for the first two jib sections.
Jib section design
For additional stiffness Jaso opts for four chords in the first four or five jib sections. The same idea lies behind the design features of the all-new Wolff 8076 Compact. In contrast to the four pendants of the Jaso J800 holding the counter jib, in this case only two pendants are used, reducing the installation procedure.
In contrast to the J800 only three and not four counter jib lengths can be selected. The shortest is 22.3 metres while Jaso’s J800 is just 18.6 metres. The Wolff 8076 Compact’s 80 metre jib can be reduced to 30 metres in 5 metre increments while Jaso’s J800 jib can be reduced in 5.5 or 2.5 metre steps to just 23 metres.
This modular design of extremely short counter jib and jib is essential for the PPVC construction market increasingly prevalent in Singapore in recent years. The first two jib sections of the Wolff 8076 Compact, including the pendants, forms one erection unit of 23 tonnes. The lifting capacity of the Wolff low top is well above the J800 Jaso. For example, 24 tonnes can be lifted at a 40 metre radius, and 7.6 tonnes at 80 metres. It appears from the design that with a double trolley arrangement even more than the actual maximum of 40 tonnes could be handled.
Wolff engineers say the low top 8076 Compact is designed to optimise the size and weight of all crane components. They estimate that in comparison to a standard 800 tonne-metre class crane with tower head the overall jib weight is 20-30 per cent higher. At the same time the weight reduction in contrast to a flat top version is about 18 to 25 %.
High-capacity class cranes
In contrast to conventional triangular jibs on flat top cranes the short pendants, together with the strong rectangular base boom section, add to the stiffness of the jib when the crane is slewing.
In 2021 Krøll introduced the short jib and boom version of its K860F into the PPVC construction market. It can be operated with just a 16 metre tail radius. Capacity is 48 tonnes, to a 19.6 metre outreach. To meet rising demand for high-capacity class cranes in general construction Krøll is developing a K1030F with a tip load of 9 tonnes at 80 metres and a 48 tonne maximum. At 44 metres 24 tonnes can be lifted and, at 22.5 metres, capacity is 48 tonnes.
In this crane class transport dimensions must cope with street regulations. The maximum base jib section height is restricted to 3.20 metres. In 2019 a custom-built K1230F was delivered for a Canadian oilfield project. In this case, on a 30.5 metre radius, 48 tonnes can be lifted. It has a 3.6 metre boom system over its full length of 46.3 metres. If the jib is extended to 80 metres the outer sections are 2.56 metres tall to fit inside a high cube container. All jib sections are low weight designs, each between 1.8 and 9.3 tonnes.
Much larger flat top cranes could be realised by Krøll at short notice as a result of the company’s broad experience in the heavy lift crane sector.
After launching the 66 tonne capacity 21LC1400 with large PPVC construction work in mind, the 21CM1100 was launched from the Comansa China factory, built to European EN standards. Most boom sections can be interchanged with 21CM750 standard sections. A short counter jib version between 19.9 and 27.9 metres allows adaptation to cramped sites. Even at an 80 metre outreach 10.2 tonnes can be lifted.
As with the earlier Linden Cranes the Comansa models all have triangular jibs. It means 6.2 metre high base sections on the LC3000 series cranes, with capacity of up to 90 tonnes. Even these fit into a standard open-top container because the upper part of the jib sections can be folded down for transport.
Design studies for the LC3000 series include a jib length of 90 metres and a capacity of 125 tonnes. A special heavy duty version with 40 metre jib and 125 tonne capacity at 13.2 metre radius have been on the drawing board for a wind turbine installation project.
The Potain MDT 809-M40 flat top crane
In contrast to Wolffkran, a conventional flat top crane version is offered by Potain in the 800 tonne-metre class. At the end of its 80 metre jib 8.3 tonnes can be handled by the MDT 809-M40. It is close to the 8.4 tonnes of the Wolff 8076 Compact. In the short jib range up to 40 metres, some 17 to 20 % more capacity is provided by the Wolff 8076, showing the benefit of the low top design.
On the other hand, the Potain MDT 809 has already demonstrated its fast-rigging capability on several construction projects. It turned out that the MDT 809 can be assembled twice as fast as conventional tower cranes in the 40 tonne capacity range with a tower head, underlining the benefit of a flat top crane design for easy rigging, especially in the higher capacity class.
Another benefit is the moderate erection weight. When installing the jib in three sections the maximum load for the assist crane is less than 13 tonnes. Following the original Jost flat top design principle, as many European crane manufacturers have, China’s Zoomlion applies the jib foot section as a central unit mounted above the slewing unit, where jib and counter jib are connected.
In contrast to many other Chinese crane manufacturers a rectangular framework is used as a reinforced part of the base boom section, added to with conventional triangular jib sections for the outer boom. The rectangular lattice boom design is also used for the counter jib, reducing the weight of the segments. A simple modular design for pendant-free design solutions shows maximum flexibility for a short counter jib version.
On top of the rectangular counter jib the hoisting winch can be installed to allow the hoist rope to move freely above the central part of the tower crane before it is led over a pulley on the second jib section. In contrast to a monorail concept, both chords of the jib are used as the runway for the trolley. All crane manufacturers preferring this design claim it leads to a better load distribution and a higher torsion moment resistance of the trolley when the crane is slewing.
A further aspect is the overall length of the trolley in heavy lift operation. When using a monorail system, it will lead to a longer unit with multiple rollers which in turn will reduce the maximum available hook radius at a comparable jib length. Inverted triangular jibs with H-beams as a trolley runway may lead to safety issues during pre-installation at ground level due to their small base.
Rigging support jacks have been developed to secure the jib during erection at ground level but this can be avoided with conventional rectangular and triangular jib designs like Zoomlion. Its T1200-64Q lifts a 64 tonne maximum and 15 tonnes at an 80 metre radius. There are three counterweight radius options from 25.1 to 17.1 metres. It has transport-optimised component heights of a maximum 2.88 metres for the jib. Its 22 tonne turntable and slewing ring unit is the heaviest single load. This crane finds a ready market in the growing prefabricated building market.
END OF PART ONE
See next month’s issue of ICST to continue reading about high capacity, primarily flat top and low top tower cranes, with a focus on developments among the Chinese manufacturers