A dramatic reduction in rigging time characterises tower crane development since their introduction in the second decade of the last century. In the past a 20m high Kaiser crane, with a maximum capacity of 800kg at 12m radius, was set up using an assistant derrick in about 24 hours. The same capacity can be easily achieved today with self-erecting tower cranes that unfold in only a few minutes.
For about 30 years bottom slewing self-erecting erecting and top slewing saddle jib climbing cranes have followed common design principles for the main structural steel components. Gone are the days of technical experiment and alternative design solutions for ways to rig or climb a crane. Today there are two distinct types of tower crane, distinguished by their completely different technical evolution – self-erecting and top slewing – with the following design features:
Self-erecting crane characteristics
• Automatic outrigger setting on screw or hydraulic jacks integrated with the crane base.
• Fast set up by unfolding or telescoping the tower and jib without touching the ground.
• Increasing use of hydraulic erection devices instead of rope mechanisms.
• Self-sufficient ballasting system.
Top slewing crane characteristics
• Modular design of the crane components, mostly with dimensions for containerisation.
• Three main components forming the whole crane:
a) base, alternatively undercarriage or cross frame
b) tower, long sections for non-climbing applications or in shorter sections suitable for climbing
c) upper crane, made up of between two and four modules including at least a machinery platform with incorporated counter jib and a jib made up of modular sections. Traditionally most larger models are split into slewing platform, tower head, counterjib and jib.
The virtual collapse of the tower crane market in the 1980s led European crane manufacturers to concentrate on the only niche in the construction sector that was showing growth – refurbishment of existing buildings in inner cities. As a result, the so-called city crane was born, a milestone in erection and transportation simplification of top slewing tower cranes. While common top slewing tower cranes of the time needed one or two days to rig, the new city cranes could be rigged in between a half and one day. Reasons were narrow (1.2m x 1.2m), 12m-long mast sections, a small fixing base generally with a pin-connected cross section, and easy to assemble module connections.
Peiner was the first German manufacturer to develop this concept with the launch of the SK 70. Liebherr followed and further speeded up erection with the introduction of the EC series one-piece machinery deck, complete with fold away tower head and all ropes installed. During the 1990s the city crane concept was adopted by most European crane manufacturers in the under 100tm class. The rigging advantages mean that city crane uppers also began to be used on standard towers to serve construction sites where more than 40m hook height is needed.
Construction industry requirements changed again with the use of prefabricated elements and larger scale projects which call for cranes with higher load moments. Shortened construction periods lead to multiple cranes on site which in turn lead to rising tower heights.
Above 50m the benefit of the typical one-piece machinery deck quickly turns into a disadvantage as the required lifting capacity increases of the mobile crane used for erection. While a city crane machinery deck for a 100tm crane can easily weigh 10t, a comparable common saddle jib crane can be split down into 5t modules.
Higher efficiency in the construction industry has meant a dramatic reduction in the length of time a tower crane is now kept on site. Instead of six months self erectors are generally now serving a construction site for only eight weeks. Alternative lifting devices such as universal telehandlers are replacing small self-erecting cranes at house building sites. A certain substitution may also be found in new market niches for highly mobile self erectors, suitable for short jobs, that operate like a long reach taxi crane.
Fast rigging devices and the range of transport options are most in demand on modern self-erecting cranes. Even the average top slewing tower crane’s application period has come down from 12 months in the 1980s, and eight months in the 1990s, to five months today.
At the same time, especially in Germany since the reunification boom, demand dried up almost overnight, leading to a sharp decline in rental rates and overwhelming pressure to find effective rigging and transport solutions. Consequently speed of erection and dismantling has gained importance. Most European crane manufacturers have therefore turned their attention to providing designs for rapid and simple assembly of undercarriage, mast and upper crane.
As popularity moves away from the under 100tm city crane class towards larger cranes, proven city crane features are increasingly being incorporated in these larger models. To get an overall view the basic crane components should be examined separately, bearing in mind that only a totally versatile crane concept can cope with market requirements.
The crane base
Due to site requirements in Europe many tower cranes are not installed on expensive concrete foundations, and rail mounted versions are also expensive. Undercarriage bogies can be replaced with concrete blocks but the time consuming rigging of the undercarriage itself remains. A Wolff 8m x 8m UW8 undercarriage, for example, still requires five hours to rig it, while when using a cross frame the same capacity class base can be installed in only two hours.
Free standing tower heights are increasing, even for medium capacity cranes, and the associated mast system calls for heavy duty undercarriages or cross frames. Wolff and Comansa use large cross frames, with the benefit of fast set up time, up to the 500tm class. The same cross base can be used for different mast sizes thanks to interchangeable bolt-holes on the base itself. Other manufacturers including Liebherr and Potain offer compact cross frames for models up to 300tm, particularly suitable for cramped city sites, but they argue that common undercarriages still provide more flexibility. In their opinion the missing base tower section when using cross frames leads to a significant reduction in the maximum available free standing under-hook height. This disadvantage can be compensated for by using stronger tower sections at the tower foot.
Retaining the benefit of the classic undercarriage while facilitating its erection was a major aim of Jost’s design for models up to at least 800tm. A central undercarriage tower section can be easily levelled by jacks on the construction site, whereupon four triangular struts resting on concrete blocks are connected to the central part. If central ballast is required, the struts are connected by sleepers.
Quickly gaining height
Tower cranes up to 60m high in Europe are normally rigged by truck cranes and fast installation depends on their ability to handle long tower sections, up to the maximum transport length of about 12m. When climbing the crane to get different heights smaller tower sections must be available. Fast, easy and safe tower section connection is fundamental for reduced rigging costs. One-piece, or monoblock, tower sections, therefore, up to the maximum allowable transport dimensions are used wherever possible. Assembling each tower section from four panel pieces, as was common up to the 1970s, is no longer acceptable, even for cranes in the 500tm to 800tm class, except where extreme, 100m, free standing heights must be reached.
In 1979 the Linden 8952 flat top crane set a world record with a maximum capacity of 50t and a free standing tower height of 135m. The 8952 is available as a modular tower crane system from Swedish rental company Lambertssons. Still this crane has an unsurpassed erection system, in addition to the economic Linden modular design for fast rigging in this class. Despite the fact that, for example, the tower sections are 5.5m x 5.5m, an external telescopic climbing cage is used, where complete mast sections can be inserted. On today’s cramped construction sites it is a real benefit of the Linden series 8000 mast system that the tower sections can be inserted or extracted from a 90° angle. Even for this tonne-metre class there is a convenient mast coupling device consisting of two half shells at each corner which are connected by four screws.
The number of connection devices, generally located at the mast corner, should be reduced as far as possible but the opinion of whether high-strength bolts or slug-bolts are the best choice varies from one manufacturer to the next. Assist devices such as the pneumatic pin handling system developed by Potain for the new Potain K-200 mast system improve tower erection.
For medium capacity tower cranes that need high free standing heights a versatile modular mast system allowing different sizes to be combined can cut costs dramatically. In 1992 Liebherr launched a three stage telescopic tower which could be hydraulically extended from 15.4m to 37.9m, suitable for the 70tm to 140tm crane range. By adding further standard sections the cranes could be rigged even higher. Up to now only minor market success has been achieved, perhaps due to the heavy weight of the system and the complicated integrated hydraulic climbing system. Under new market conditions it is possible that the idea will reappear in a modified, more simple and therefore more cost effective form.
Tower and jib are normally the largest crane volumes to be transported and, especially on long distance transport of medium sized and large tower cranes, it is a real benefit to be able to transport jib sections inside the containerised tower sections. It must be considered, however, that every inserted crane component causes delay and additional lifts to be made at the construction site.
Upper crane components
How many components should form the crane upper? The complete one-piece machinery deck design, well established in the city crane category by, for example, Liebherr and Raimondi loses its advantage on cranes above 100tm due to its weight and dimensions.
If the required number of lifts is reduced from the traditional four (slewing frame, tower head, counterjib and jib) to only three, besides the ballast blocks, options are limited.
Flat top crane design can be followed, where there is no tower head. This crane type has the most sophisticated and economic design for erection as jib pendants are also not normally necessary, thereby speeding up erection. Furthermore, the jib can be split into weights that reduce the cost of the assist crane. The price for speedy erection, however, is in a higher jib profile and an upper crane that is normally between 5% and 15% heavier.
Some manufacturers including Potain, Terex-Peiner and Raimondi build top slewing tower cranes with a fitted counterjib that carries an articulated A-frame strut instead of the cat head. If when the strut is in transport condition it forms part of the jib this design significantly speeds up erection.
Other ways to avoid a separate tower head lift are to unfold the tower head which is resting on the inner counterjib section during transport, as on the Terex-Peiner SK 315, or to split the tower head into two A-frames, connected to the counterjib and jib through the ties. This last design feature is typical of Potain MD-Geo models. It must be stated, however, that in both cases erection steps which were made on top of the tower are now done at ground level, increasing the pre-assembly time. The question remains whether the number of lifts to the tower top, or more the length of the whole set up time, is essential for cutting costs.
Speeding up erection can also be achieved with small design features that have major influence on the whole crane system. An example is the Potain MD city crane range up to 170tm. The single jib tie, incorporating the hoist winch, can be fully assembled at ground level pre-fitted with all ropes. The counterjib is a simple lightweight support for the ballast. All components are equipped with lifting slings that stay in place and hold the modules exactly at their centre of gravity when lifted, thereby avoiding time consuming repositioning of lifting slings. Also helping the erection procedure is the application of collapsible joints for the pendant connection, as well as long holes at the tower head where the pendants are fixed with one bolt.
Reeving the hoist and trolley rope can be sped up by reducing the number of sheaves. This aspect should be considered for medium and large capacity tower cranes in particular, where a separation of the winch unit from the jib is needed to get enough flexibility regarding the winch capacity actually required. A side folding cabin as found on Terex-Peiner and Potain MDT models helps to reduce transport width, and speeds up erection as the electrical connections are kept in place at the slewing frame. And thanks to modern frequency regulated drives the cabinet size can be reduced and so compact winch platforms for easy transportation are available. Often they no longer have to be separated from the counterjib for transport.
In the city crane class latest developments are compact crane designs, where the number of trucks needed to shift the crane has been reduced. Jaso’s J47NS, for example, equipped with 48.5m jib and 37.6m under hook height, including the cross shaped travelling base, needs just two truck loads. Compared with similar standard tower cranes of the 1970s three fewer trucks are needed. For the near future rental companies hope that this evolution will lead to 200tm cranes, with jibs up to 60m, where the upper can be transported in just two truck loads.
Although luffing jib cranes are still a niche market fast rigging procedures are essential to cope with today’s market requirements. Moving counterweight designs are more time consuming and complicated to rig than those where the ballast is simply stored on the machinery deck. Jib sections of older models were connected using a number of screws but now pin connection is common.
Wolff luffers such as the 180B prove that this type of crane can be equipped with fast rigging devices. The hoisting winch, for example, is fitted at the jib base section so the hoist rope can be installed at ground level after assembling the jib, and the luffing winch and roller block are integrated into the tower top. As a result there is no time consuming reeving of the luffing rope at height.
A further reduction of main crane components is a feature of the Jost JUL series where up to 8t load and 50m jib designs are under development. In this case the classic A-frame or tower top will be missing. The uncommon design looks like a flat top crane with a luffing jib and it will be interesting to see if this feature will be the future of the next luffing jib crane generation.
Self-erecting flexibility
Increasingly restricted construction sites mean that rental companies are looking for new reliable crane concepts. Liebherr’s TT range with telescoping jibs may set new standards. A semi-automatic ballasting device allows one person to do it without climbing onto the ballast. Although the idea of a double telescoping system of the tower and jib is not new, improvements have increased versatility – the jib that can be telescoped under load.
TT cranes have six or seven jib lengths and the associated load capacity curves, either with the jib horizontal or raised. If site conditions call for limited slewing when out of service, the TT crane simply retracts and the slewing brake is applied. Guying the jib is no longer necessary. With jib retracted the load moment can be increased, for example, the 24 TT has a load capacity of 2.5t at 14m radius which is enough to lift small excavators into a construction pit or to handle precast building elements. Considering the fast set up time, depending on the site conditions, the TT range can be an economic alternative to larger, i.e. more expensive, tower cranes.
