Designing safety into luffing jib tower cranes
By Alex Dahm06 April 2009
The rising popularity of saddle jib cranes at the expense of luffers in Europe started in the early 1970s and lasted until the middle of the 1980s. The luffing jib tower crane became a niche market in Europe whereas, especially in Japan, Australia and the USA, luffing jib crane development continued to dominate the high rise climbing crane sector.
Leading European crane manufacturers have rediscovered the luffing jib principle due, largely, to an internalisation of their sales activities. It was further promoted by changing legislation surrounding air space rights, for example, in the United Kingdom. Another major contributor was the growing number of congested inner city sites as construction density increased.
A shift towards luffing jib cranes is now continuing on typical city or high rise building sites. Unfortunately, however, as their popularity increases, so has the number of serious incidents involving luffing jib tower cranes. In several cases it appears that design principles of the crane models have furthered the risk of crane failure, especially in windy conditions. As a result, in addition to careful training of crane drivers operating a luffing jib crane, it may be worth also considering technical features of the crane to minimise the risk of a fatal collapse.
Hazards of operating luffing jib cranes concern particular features of this type of crane and the special site conditions that led to its selection.
A luffing jib type is usually chosen to cope with restricted site conditions caused by surrounding nearby tall buildings or in cases were over sailing of neighbouring sites or buildings is prohibited. In addition, luffing jib climbing cranes are preferred for ultra high building sites to raise the number of hooks serving the building site and keep the required construction speed as the building height increases.
In both cases space for the upper crane is restricted but its working reach should be kept flexible. Principally by changing the jib angle, the load can be placed at various radii. Manufacturers typically allow a jib angle of between 15 and 85 degrees during operation. As jib length increases, the angle gets steeper to reach the area around the crane tower. Especially when the jib foot is fixed at the front edge of the machinery deck, the jib is raised in even steeper operation condition, for example, 87 degrees, to keep the minimum operating radius to about 3 or 5 m.
When fixed ballast is used on tall freestanding cranes, torsion in the tower further increases the resulting angle of the jib in step position because of the backwards bending of the upper crane. If, however, the jib pivot is only 2 m behind the turntable centreline for the same jib length and minimum radius, only an 83 degree jib angle is needed for the minimum operating radius. This reduces the over booming hazard.
As demonstrated by the Korean KNF336i, many long jib luffing jib tower cranes have a larger minimum radius than saddle jib cranes. In this example the rigged 55 m jib gives a 6.5 m minimum operating radius. Especially as an internal climbing crane, this is a disadvantage and may entice the crane driver to hoist up using, where available, the special erection mode instead of the working mode to reach in close to the tower.
In contrast to conventional luffing jib tower cranes with straight boom, articulated jib special cranes, like the BKT BM45, the Tørnborgs Magni jack knife tower crane range or the Swiss-built Cobra crane, follow the double-lever principle often seen in harbour cranes. In this case the jib reaches only about half as high in comparison to conventional single jib cranes, making them less sensitive to wind forces.
For common single jib tower cranes where the boom is almost vertical, the risk that the jib tips over backwards when the crane is facing the wind increases dramatically. Putting the luffing crane out of service means the jib has to be stored not at the minimum radius but at an angle described in the manufacturer's operating manual. This is to ensure that there is sufficient wind area enabling the crane to weathervane.
Only Japanese luffing jib climbing cranes are manufactured according to national regulations so rigid that, even in case of a typhoon, the slewing function of the crane can be blocked. For European style luffers generally 65 to 15 degrees are accepted by the crane manufacturers for out of service position. Again, long jib versions lead to a lower angle meaning that more space is needed for rotation. The Peiner SN166, for example, rigged with 30 m boom can be positioned out of operation at 63 degrees, meaning 13.3 m radius. For the 50 m boom at least 58 degrees is needed, meaning 26.8 m radius.
Again, cranes with fixed ballast have an inherent disadvantage. For the shortest boom combination the out of operation angle of the jib is low to balance the upper crane. That means the required radius is similar to the longest jib rigging condition. Under cramped construction site conditions the required low jib angle in out of service position can lead to challenging crane installation tasks.
To simplify weather vaning some manufacturers, for example, Potain, offer wind sail plates to be installed on the jib. Their number varies depending on jib length and more can be added if the jib has to be stored at a steep angle to cope with site conditions.
On occasion spectacular measures have to be considered to keep a luffing jib crane weather vaning. For the Peiner SN 630 and SN 1000 used to build the Hong Kong Shanghai Bank headquarters, "typhoon sails" were constructed which were hooked up by the cranes to enlarge the wind sail area of the jib when it was parked at 75 degrees instead of 50 degrees.
Concerning design principles there seems to be a correlation between the use of fixed or moving ballast and incidents under high wind conditions. For example, the Jaso 138PA that collapsed in Liverpool in 2007 was rigged with fixed ballast. As initial cause for the accident the UK Health and Safety Executive (HSE) identified a short gust of wind hitting the crane with raised jib from the front.
Until the end of the 1980s Favco cranes had a travelling counterweight system. The counterweight was pinned to a pair of four-wheel trolleys which travel up and down the inclined surface of the machinery deck guyed by two ropes connected with the jib foot. When the jib is lowered the counterweight moves away from the mast and closer to the mast when the jib is raised. At minimum radius the return force that the ballast exerts on the jib is at its maximum. This means that the counterweights tend to retard further upward movement of the jib, preventing the jib from being pushed backward by wind forces.
Designs with a movable counterweight arrangement, however, also have disadvantages. Located under the machinery deck, installation is difficult - it can be hazardous and time consuming for the erection team.
Moving parts require maintenance and inspection. Saddle bag platforms should be installed aside the machinery deck to check that counterweights are moving correctly for their entire range of travel.
To balance the crane during climbing the counterweight has to be kept in position at the top of the counterweight rail, while the jib has to be lowered. The counterweight is kept in position by means of a latch that locks onto a lug on the bottom of the rail. The potential risk arises that a loose counterweight carrier will run down the slope and put the crane out of balance. To help prevent inadvertent release of the latch, a secondary means of securing the latch in place is needed.
Moving to fixed
In 1987 Favco changed to a fixed ballast system in its new crane range. Many old machines have been converted to fixed ballast versions.
Incidents caused by wind force on cranes in out of service condition have risen. In several cases the luffing rope has had to be replaced because it jumped out of the sheaves on top of the A-frame when the jib was pressed into the buffers. In 2005 a Favelle Favco M440D lost its jib in Melbourne, Australia where it had been blown over the machinery deck, destroying the A-frame.
Since the 1980s Peiner equipped its SN Series cranes with rope guyed moving ballast travelling on a slope under the machinery deck. In this case the counterweight blocks are attached to the counterweight carriage beam beside the machinery deck. This facilitates erection in comparison to the counterweight blocks, which have to be placed under the machinery deck, but it remains a risky and delicate procedure for the rigging team.
The counterweight blocks have to be carefully lifted into the insertion position outside the machinery platform. During crane erection the ropes for moving the counterweight carriage must be adjusted with turnbuckles to be sure that the counterweight carriage runs parallel to its rails. Rope failure is always a risk for rope operated ballast systems. A rope breakage safeguard is installed at the fixing point of the ropes on the machinery deck. The integrated counterweight monitoring device must be kept maintained properly for safety reasons.
The Swiss Cobra crane design optimises the return force on the jib by moving the counterweight on a guide track with a variable incline. The level of the centre of gravity of the counterweight is close to the level of the running track to avoid rocking during movement of the crane.
For the MR 300 Potain developed a system of two pushers approached by the raised jib, which press on the rods, forming a broken line, lifting up the machinery deck foot. Due to the rope guyed ballast under the machinery platform foot, the system generates enough energy to return the jib against wind force. This system, however, is an expensive design with many moving parts.
At the beginning of the 1990s the Potain MR 150 used a safety device for returning the jib out of the minimum radius while using fixed ballast for speedy erection of the crane. This system was later used by many Chinese luffer manufacturers who followed the Potain design principle in their mid-class capacity ranges.
A rod is pin-connected to the front of the machinery deck and two cylinders with springs connect the upper jib foot with the root. When raising the jib, the rod comes to rest against the machinery deck front. By further raising the jib, the springs are compressed, supplying enough energy to return the jib against wind force from the front. No rope is used.
On the 500HC-L Liebherr described a way to move ballast without ropes by carrying the ballast at one end of a rocking arm as a pendular counterweight. The position of the arm is related to the angle of the jib connected with the rocking arm. The counterweight is mounted close to the rotation axis of the mast when the jib is raised and moves away from it in a pendular movement when the jib is lowered. The rocking arm gives the ballast an active force on the jib foot top when the jib is raised in step position, acting against wind force from the front.
A complicated rigging procedure was necessary because the ballast moved completely below the machinery deck in every jib position. To fit the central ballast blocks diagonal members inside the machinery deck had to be removed. Care had to be taken considering the swinging of the ballast during operation. Dampers were installed to absorb shock when booming up and down.
Tower crane designer Franc Jost improved the swinging ballast principle on the BKT series, still seen in the large capacity Potain MR range. In assembly position the ballast lever is behind the machinery platform, facilitating counterweight installation, although access through the erection basket is limited, which still required rigging personnel to step out on the ballast beam. The counterweight blocks rest side by side on the ballast beam, secured by connecting each at the top because shaking of the whole crane caused by, for example, an earthquake could lead to individual oscillation of the counterweight blocks.
On the Comedil CTL400 and the Jost JL series Franc Jost minimised that risk by using a ballast basket where the centre of gravity of the filled-in ballast blocks lies under the mechanical link to the ballast lever. This means that no shock absorbers are required. In addition, the patented Jamoba (Jost Moving Ballast) system shifts the mechanical link between the counterweight basket and the jib under the machinery platform, giving more space above for the winch units.
An issue with this, however, is that all movable connection parts cannot be easily reached from top of the machinery platform for easy erection and maintenance. Terex Comedil improved safe access to the ballast basket by adding platforms with handrail on the CTL630 and CTL 340.
In the JTL series Jost set a milestone in luffing jib crane technology. The jib of the flat top luffer is moved by an hydraulic ram. The counterweight basket at the short counter jib moves down when the jib moves up. That mechanism means the crane is in balance at every angle of the jib. No ropes are used during the jib movement and the number of moving parts is reduced so safety is improved. In out of service condition Jost claims the ability to store the jib at 75 or even 85 degrees, making it especially suitable for cramped city sites.
In areas with high earthquake risk, for example, Japan it may be preferred to use fix ballast systems to avoid shock created by oscillating ballast. In general, buffers ensure that the jib is not luffed beyond the design limitations on luffing jib cranes with fixed counterweight. To reduce the risk of damage from wind force from the front, additional design features are seen on Japanese tower cranes:
* Instead of jib suspension bars - preferred in Europe for speedy erection - only ropes are used. This means that even if wind force raises the jib, dangerous side loads created by oscillating bars are eliminated.
* The risk of the boom rebounding is reduced by locating the pivot point three to six metres behind the turntable centre line. At minimum working radius the jib is not in its steepest position. The maximum available angle will only be used when the crane is being raised by inserting new tower sections through the slewing ring.
* In cases where pulley blocks integrate hoisting rope sheaves for horizontal load movement heavy designs are used, adding to the tension of the luffing rope to prevent slack rope conditions.
* At every sheave individual rope guards are fitted to prevent the rope from jumping or riding off the sheave.
* The buffers are integrated in the very top of the A-frame, acting well above the jib foot on the jib in its fully raised position.
Most new European design luffers have fixed counterweight to reduce fabrication costs, rigging expenses and maintenance. Some, for example, Potain and Terex Comedil prefer moving ballast systems for large capacity class machines above 300 tonne-metres. A few manufacturers, for example, Jost and BKT Engineering GmbH, have movable ballast systems for their entire luffing jib ranges.
Although fixed ballast systems are gaining popularity in luffing jib crane design it must be considered that safe application requires the careful choice of safety devices against unintentional raising of the boom.