Safe application of tower cranes on often cramped inner city sites presents particular challenges to those responsible for the installation. Heinz-Gert Kessel reports
High rise construction work is concentrated in the inner city of metropolitan areas all over the world. Nearby buildings, busy streets, extremely cramped construction sites, existing essential subsurface installations, short construction times and, above all, safety aspects are major challenges for any crane installation work.
The general public is exposed to these inner city construction sites. Unauthorised people sometimes climb tower cranes as thrill-seekers. To combat this a good practice found on many construction sites in London, UK, is to use anti-climbing frames on exposed, free-standing tower cranes. To be functional, however, some design criteria should be respected. To prevent climbing, at least 3 metres of smooth external net type walling or panelling should be fitted around the crane’s tower section. It can be done at the yard and can be transported in place on the mast. No installation work at height is then needed on site, for example, to install a fence like structure resembling an upside down umbrella. Inside the tower a horizontal barrier with trapdoor must be installed in a way that prevents access even to very slim people. On large base tower systems with fat diagonals it is vital to have no space between outer vertical walls surrounding the tower system and the horizontal platform inside.
Airspace over large cities is extremely busy with helicopter flights bringing business people, police helicopters and flights to and from nearby hospitals. They are a permanent risk for tall tower cranes, particularly luffing jib types, where the boom can easily rise an additional 40 to 70 metres above the machinery deck at the top of the crane tower. In January 2013 there was a helicopter crash involving a luffing jib climbing crane enveloped in extreme fog at St Georges Wharf Tower in London. Since then the question of safety lighting on cranes has been discussed. Investigations showed that solar-powered steady red warning lights on top of the crane structure were not switched on because the official requirement is to light them at night, meaning the period between half an hour after sunset and half an hour before sunrise. Such Obstruction lighting is required according to the British rules at least for cranes 150 m high or more. The question arises, however, whether it would improve safety if it should become good practice to switch on the aviation lightning in the day time as well at night, to cope with all weather conditions and no matter how high the tower crane is standing on inner city sites.
When raising the boom even moderately tall luffers quickly become an obstacle far higher than the actual building line of a high rise construction site. Adding aviation lighting to any tower crane, as is standard in Japan, could enhance safety. There are many different rules about how the aviation lighting must be arranged. In Japan, luffing jib cranes are mainly used, generally above 60 m tall. Two intense red sparkling lights are used at the jib head and on the A-frame head. Also requested are seven-section alternating red and white painted booms to provide good visibility day and night.
As it becomes more common to operate the same tower crane with different jib length it could be sufficient to let the red and white colour scheme follow the assembly of the jib sections making up the boom in order to avoid expense of repainting of the boom each time when another boom length is requested.
The number of tall free standing tower cranes on inner city projects with 70 to 100 m towers is increasing. It further raises the question about good practice and using additional steady red lights at the middle of the tower system, especially when heliports are nearby. It would help make the large construction cranes more visible during the early stages of the building project. No single standard defines a minimum mandatory requirement governing design and installation of aviation lighting. With respect to the tower crane rental business and to streamline safety standards there should be a unique European solution.
On many projects in European cities the free standing capacity of modern tower cranes mean they can be set up without needing to climb. It means that the cranes can be set up in just one rigging period. If external climbing cranes are used, they follow the building as it rises during construction. These cranes have to climb several times in the life of the project. In this case the protection of people from falling objects must be not only assured during assembly, rigging and dismantling but also each time that there is a climbing operation. Usually for this period appropriate exclusion zones, at least 20 m from the base of the tower crane, are defined.
It can be difficult the higher the crane is climbing and the closer it is to the edge of a construction site, especially next to a busy street. Here a protection fan surrounding the tower under the climbing frame could prevent tower connection material and climbing tools from falling to the ground. This arrangement, looking like an upside down umbrella, can have a folding mechanism so that it is more compact during the working period and interferes with the operator’s view.
In this way the structure does not need to be dismantled and refitted to a higher tower section in a risky and time-consuming operation when the crane is jacked. Horizontal tower connection devices can be dropped during handling. It is less likely with vertical multiple bolt connections where the heavy bolts can be already inserted in the lugs before the more nuts are added. If these bolted connections are carried out completely inside the mast section, as is common on Japanese tower cranes, there is less risk of falling crane components dropped by riggers leaning out of the tower to reach.
In addition, there is no need to install work platforms outside the tower system when the tower is erected using an assist crane. On inner city sites most external climbing cranes are close to public areas. Falling objects during rigging, maintenance and climbing will not only be a hazard to workers on site but also to pedestrians. Risk zones on tower cranes are platforms where small hand tools and crane parts are temporarily stored.
Increasingly around the world one can witness the good practice of requesting the installation of safety nets around the handrails. If a tower crane works for a longer time, however, nets break up. Instead durable perforated steel panels installed by the crane supplier are a much better solution than a classic multi-rail system which is enclosed temporarily by nets.
On the outside
An external climbing crane will generally reach heights where it has to be tied in to the building under construction. Generally for each tie-in support a massive collar surrounding the crane tower must be installed and then three anchorage struts must be connected to this collar. All the work is normally a risky operation in which riggers lean out of a man basket or balance over the steel collar to insert large pins.
In the UK, HTC Wolffkran (formerly HTC Plant) designed an installation platform surrounding the tower under the steel collar to give a safe place of work and to reduce the risk of falling items. In Japan Yoshinaga Manufacturing developed collars of two u-shape units where an erection platform is already mounted. In two lifts it can be rigged at the tower and only two vertical pin connections have to be installed to form up the complete collar. Each tie-in support is done the same, with its own work platform, so access does not have to be reinstalled during the climbing procedure. In addition, these are safe points from which to inspect the anchorage struts.
The standardised tower dimensions of all the major Japanese climbing cranes means that this equipment can be used with different brands. Restricted site conditions and safety aspects may also lead to more floor climbing application. The main advantages are as follows:
- The crane can raise a high building without a massive tower or foundation and it generates much lower forces.
- It can be climbed up to the standard in-service wind speed.
- The crane may be climbed safely at night as all work is carried out from the illuminated inside of the building.
- The crane driver does not have to climb very high towers.
- A working permit alongside railways is easier to get because of the reduced collapse radius.
Jaso has developed an internal three-beam climbing system where jacking and support beams are inserted into six wall pockets of the concrete lift shafts carrying the crane. The three support beams are integrated with the hydraulic climbing system in the special base section of the tower crane. In working condition the crane is resting on all three parallel beams in the wall pockets.
When climbing the shoes of the outer beams are inserted hydraulically while the crane is still resting on the central beam. Then the complete crane is lifted by the central hydraulic ram of the base tower section so that, at the next level of wall pockets, the two outer beams at the base tower section can be secured to base the crane. Then the central beam follows onto this level by retracting the central hydraulic ram. The maximum stroke of the Jaso climbing ram is 4 m so there must be a wall pocket every 4m in the concrete shaft cast ahead of the rest of the building.
Upper and lower tower guide systems can be added to adapt the system to different lift shaft dimensions. Jaso has developed the lift shaft system for two tower systems requiring openings of just 2.5 x 2.5 m, suitable for J380PA and J450PA models and 2.09 x 2.09 m for the J180PA / J208PA crane. The crane tower is guided in the lift shaft by rollers fixed to the tower. In addition to its small size there are safety and time benefits. No work is needed outside the platforms integrated in the tower system. No crane components have to be added or removed by hand during the climbing process.
City construction sites are often already surrounded by tall buildings so Jaso has developed special equipment for safe out of service positioning of rope-luffing jib cranes. In this case, aside from the original boom stops, the boom can be raised against a steel frame lowered from the A-frame. With that device the standard out of service radius of the popular Jaso J180PA with 55 m boom could, for example, be reduced from 20 to 11 m.
Taking a view
For the drivers of core-climbing cranes it will be hard to see the loading zones on the ground. In Japan tower cranes generally have a jib-top camera to give a “bird’s eye view” of the load on a large monitor in the operator cabin. In other parts of the world cameras to assist the crane driver are still not standard, even though it could improve the safety of load handling procedures because the crane driver is no longer relying solely on radio communication. It lets the operator see hand signals and he can check if the load is safely slung before its leaves the ground.
There is the principle choice to add a camera at the trolley or luffing boom head of luffers or to use a hook block camera system. Recently three Terex luffing jib tower cranes serving the Berkeley Group’s Corniche project at the River Thames in London had the HookCam wireless camera system fitted. It just requires an antenna at the jib end section and the camera directly fixed to the hook block.
HookCam cites university studies showing that the system not only improves safety but also reduces lift time by an average 26.7 % in open space and 38.9 % in blind space. The quality of hook camera systems is mainly determined on battery capacity. The operator’s view, however, is limited to the scene under the hook. In contrast, with a zoom camera installed at a trolley or, on luffers, at the end of the boom, the crane driver can zoom out to give a vertical view over the whole lifting area.
Manufacturer Orlaco claims that the 0-lux night version of its 216 times zoom camera offers a better view than the naked eye alone. With the brightness function even dark shadowed work areas can be lightened up. On cramped inner city sites with many cranes, often at the beginning a mix of mobiles and towers, working together this visibility to the rear of the machine becomes a safety factor. When lifting heavy loads a quick look at the winch spooling may help to avoid problems when another layer starts. For this purpose this Orlaco camera and the load view monitor can be used simultaneously with the rear view camera.
Following major fatal incidents in the recent years, especially in New York, it has been suggested that older diesel hydraulic cranes less safe than newer cranes. Former New York City Mayor Bloomberg proposed a new law that would ban all tower cranes from operating when they reach 25 years old and require all cranes, no matter how old they are, to have load-cycle counters. In the last year a notable shift has taken place from diesel-hydraulic tower cranes towards electric tower cranes. While in 2011 still 68 % of New York’s tower cranes were diesel-hydraulic machines, partly to cope with the poor energy supply at construction sites and to handle the high lifting loads on ultra-high rise sites in 2014, their chare fell to 42 %.
Arguments against diesel-hydraulics focus on noise pollution and other environmental impact but a diesel-hydraulic drive with up to date technology is by no less safe than an electric crane. The risk of, for example, dropping loads can be reduced by permitting only re-powered cranes with fail-safe mechanisms for the hydraulic motors and disc brakes. In addition, collision avoidance systems are not restricted to electric cranes.
Using the Limatlas system, jointly developed in Australia by Atlas and Favelle Favco, specific work zones are programmable to stop hydraulically controlled diesel cranes operating outside set areas, for example, dangerously close to other tower cranes, railways, pedestrian crossings or public buildings. It is capable of interfacing with the SMIE anti-collision system for all cranes on a site, whether diesel or electric.
To improve safety several accident examinations indicate the importance of maintenance and skilled service above the pure age of a crane. The common multi-layer sub-contracting system for tower crane erection and dismantling work is an important risk because of the more complicated communication between the companies and riggers involved. Where main contractors rely on formal control by paperwork it doesn’t reveal any inadequate training of the sub-contractor riggers.
Failure of tower crane components as a result of inadequate maintenance is one of the major causes of incidents, other than faults in the erection and assembly process. Crane driver elevators and remote controls should also be considered under the safety aspect of who cares for the inspection of important crane parts, like tower section connection devices at the requested intervals.
Tower cranes are of modular design but inner city job sites often demand specially designed components which are used together with standard crane parts. Advanced technology to track the crane components should become standard. In addition, such a system would provide a means to determine the age of each individual load bearing crane component. Stability relies on every crane module and a construction tower crane is made up of a combination of different modules. It should be noted that some of them may have different history.
Another key point for tower crane stability is its foundation. Adapted crane bases and purpose-built grillages are often necessary for narrow city sites. These are usually carefully engineered but standard foundations, for example, anchors or ballasted stationary undercarriages, should be installed with the same respect to all safety factors. The widespread practice designing a foundation prior to the tendering process of the tower crane may quickly raise safety issues and, therefore, have to be avoided. The foundation should also be designed specifically for the crane that is actually used. When rigging tower cranes on concrete foundations the first tower section together with the anchors should be installed before the concrete foundation is poured to prevent dangerous mast lean.