A challenging angle for tower cranes
By Heinz-Gert Kessel10 November 2014
City centre job sites with complicated high rise requirements and limited space are calling more and more for luffing jib tower cranes instead of saddle jib types. Heinz-Gert Kessel reports
Constraints on crane choice imposed by job site conditions mean that luffing jib tower cranes are an increasingly popular solution on cramped city sites. In addition to restricted space, oversailing of neighbouring properties can also be prohibited and often there is a requirement for flexible multiple crane utilisation.
To save space jibs of luffing cranes can be raised and, at first glance, any difficult construction site layout can be easily solved. Today’s top slewing self-climbing luffers can reach any requested minimum working radius. In so doing it becomes more important how to safely store their jibs when the cranes are out of service. In the distant past the manufacturer’s instructions were to park the jib at maximum radius or, at least, at 45 degrees, so that the crane would be easily rotated by the wind when out of service. Cranes are now often located at the edge of construction sites and not allowed to slew over the edge of the site because of neighbouring high buildings or strict regulations.
In other cases the project-specific placement of certain tower cranes meant a very small distance between the other cranes. Under such circumstances customers will find the minimum radius of the selected model at the final crane lay out planning stage. It might come as a pleasant surprise or, in the worst case, they will find no direct indication of the minimum radius offered by the preferred luffing jib crane. In other cases only one radius is indicated in the technical general information sheets for all jib lengths.
Even somebody with a general understanding will assume that there should be different safe parking radii according to the chosen boom length of the particular crane. The question of out of service position for a luffing jib crane under actual construction site conditions soon then becomes a question for the manufacturer’s technical support department. It might lead to suggestions that influence the construction project. This could be, for example, when a stronger tower system and therewith other crane dimensions and corner forces generated by the crane have to be considered.
When investigating the crane planning it becomes quickly obvious that the question of out of service position of the boom is not only related to the boom length but, in addition, is influenced by the principal crane design. Whether it is a conventional rope-operated luffer, with or without moving counterweight, or an hydraulic luffing jib crane or a folding jib type, the requested minimum out of service radius may be different in the same capacity class. That generally leads to a reduction from the conventional rope luffing jib crane with fixed ballast that has the largest out of service radius, to the folding jib crane type, which can be parked in the most compact way.
Climatic changes mean that higher wind speed has become normal and must also be addressed. A safe parking position will ask for more restrictions, which are at odds with the more and more cramped sites faced by customers applying for luffing jib crane technology as the only real choice to adequately serve the site.
Regardless of chosen design, actual site conditions must be responded to very carefully. In city centres above a height of 100 metres the gust wind speed can be twice as strong as at street level. Nearby buildings may have another significant influence on the same crane when he start its journey to the sky at the building’s foundation. If the crane with its raised jib is at the same height as nearby buildings these structures may provide shelter or create a tunnel effect that can even increase the wind load on the crane. Where surrounding buildings are significantly taller than the crane at the starting point of the construction, they will often generate increased wind loading on nearby lower cranes. If the wind catches the light boom tip of a luffing jib crane parked in a steep position it can throw the jib backwards over the A-frame before the crane itself gets a real chance to turn freely into the wind.
Design for Europe
It is a general key stability factor in European tower crane design to benefit from the rear moment generated by the counter jib and counterweight acting against the larger moment created by the jib surface to let the crane weather vane as soon as the slewing brakes are open and the friction moment of the slewing ring is surpassed. In that way the crane is directed by the wind in a position where it is coming from the rear. Ideally this prevents any side load effect and, essential for luffing jib cranes, the effect of any dangerous wind from the front which could move the jib into an unsafe steep position.
The required effect is easily achieved by a saddle jib crane with a constant long jib. For a luffer, however, it may be critical to keep up the requested front moment of a luffing jib crane when the jib is raised or when a short boom combination is selected. Typical modern rope operated luffing jib cranes have a short machinery deck with fixed ballast plates at the rear. Different rigging conditions have significant influence on the safe out of service position.
Rigged with a 55 to 60 m boom, depending on the manufacturer, the permitted out of service position may be the longest radius. For this a permissible radius for weathervaning is 16 to 20 m. In this situation there is enough front moment created by the jib itself to let the crane slew free in the wind. The risk, however, of a jib overturning due to sudden gusts from the front is essential to consider. At the middle jib length configuration, for example, 40 to 50 m, a 14 to 18 m weathervaning radius can be accepted. In short jib configuration, of 30 to 35 m, the out of service radius increases again, to somewhere between 16 and 20 m.
In this case the restrictions are caused by the missing front moment of a short raised jib. Wind sails can help the crane to slew into the weathervaning position. A 10 square metre sail at the jib end would allow the same crane to raise its 30 m jib from a 16 m out of service position to just 10 m. For a conventional luffing jib crane, with a maximum 60 m jib, a positive effect of additional wind sails can be expected at jibs shorter than 45 m.
Note that actual conditions must be confirmed by the manufacturer and that the figures quoted in this article just demonstrate a trend in the general information of, for example, a 30 m out of service position for all jib lengths in the data sheet. Also highlighted by this is the limited use of a single out of service angle for parking the jib, no matter which jib length is rigged. At a boom angle of more than 70 degrees the risk of a lack of tension on the flying sheave block increases. A short gust of wind could move the jib backwards. To prevent this it can help to put extra weight on the hook when the crane is in the out of service condition.
Some manufacturers, especially in Asia, advise customers to fix the hook block at the machinery deck when the crane is parked at a steep out of service position. Alternatively, the crane boom can be fixed to the building under construction. In this case, however, it cannot weathervane and massive side loadings must be accepted by the crane and building structure.
For the prestigious Tokyo Skytree tower project Japanese manufacturer IHI developed a patented jib restriction device consisting of a winch holding back the jib and acting against the main luffing winch when the jib is parked. In every jib position the jib is fixed between the tensioned luffing ropes and the ropes of the assistance luffing winch connected by sheaves to the jib base acting in the opposite direction. The four cranes working closely together at the TV tower project could weathervane individually. IHI synchronised their movement with a Slewing Assist Function. In the out of service condition the computers of all three cranes were linked. Following the signal of the lead crane all other three were orientated in the same direction when there was a change in wind direction.
In the UK
For the UK market Spanish manufacturer Jaso, with its representative Falcon Crane Hire, developed special equipment consisting of an additional boom stop. It is lowered from the A-frame into a rest position to fix the raised jib when the crane is moved into a steep out of service position. In this way the weathervaning radius of the J180PA with a 55 m jib can be reduced from 20 m to 11 m.
Making use of large wind sails can impact the crane’s precise steering under working conditions, especially when booming out to maximum radius. To keep the crane in position even with wind from the side, the slewing unit capacity must be considered. Wind sails may lead to a significant reduction in acceptable wind speed during crane operation. Consideration will have to be given to higher side load, generated by the covered jib surface, plus the strength of the jib structure and the jib connection devices.
To extend the wind sail area in the 75 degree out of service position to its maximum, the Peiner SN 630 and SN 1000 cranes raising the Hong Kong Shanghai Bank headquarters in Hong Kong lifted up so-called Typhoon sails. These box-shaped large steel structures were lifted using the tower cranes’ hooks into the maximum lifting position. They added dramatically to the jib end surface area.
A modern moving ballast system allows a steep out of service position to be increased for two main reasons. In the parking position the ballast pushed the jib forward, making it difficult for wind from the front to bend the jib backwards over the A-frame. In addition, when booming up, the counterweight is moving in towards the tower, meaning that the back moment forces are reduced so the jib can lead the crane to turn into the wind more easily than cranes with fixed ballast.
Everdigm Corporation in Korea offers an example of this latest breed in the form of the ED580L. Anthony Chang, Everdigm spokesperson, points out how the moving ballast design allows the crane to weathervane at 15 m/s in parking position. To bend the jib backwards would require a 50 m/s wind speed from the front, Chang said.
When tower crane designer Franc Jost pioneered the original moving ballast system in 1994 he gave a maximum safe out of service position of 86 degrees with 30 m jib on the BN 355. It should be noted that such extreme applications demand a rigid tower and generate extreme corner forces.
At first glance the recently popular hydraulic luffing jib city cranes offer the best design features for a steep out of service position and, therefore, are very popular in the UK with its strict air space rights. Originally designed by Jost in 2005 with its JTL series, Wolff followed with its 166B and this year two more crane manufactures, Jaso and MTI-Lux, entered the scene. While the MTL220-10 of MTI-Lux in many ways appears to be an upgrade of the Jost design, Jaso has taken a different approach by preferring a boom held by pendants instead of the common flat top design.
In all the designs when the boom is moved up using an hydraulic ram, the counter jib moves down and the counterweight approaches the tower of the crane. The back and forward movement of the crane means that the backward moment changes visually when the boom is raised. Less forward moment is needed for weathervaning the crane in comparison to a standard luffing jib crane.
Without a luffing rope holding the boom it is the bolt connections on the boom sections and the strength of the hydraulic ram that dictate how well the crane can withstand gusty winds. Incidents with Jost JTL158 hydraulic luffing jib cranes during storms with 160 km/h winds hitting London in the last couple of years raised safety concerns about the best crane type for steep out of service positions. In all cases Jost said it demonstrated, as a result of investigations, that the crane jibs were damaged by high side wind loadings because the cranes had not weathervaned.
An obvious measure to enable the crane to weathervane is to keep an eye on the slewing brake that must be released. In the author’s opinion it would be a good practice standard to use electrical release systems for the slewing brake.
Safety design standards have also been improved to allow a crane to start weathervaning. In the former FEM1.004 code the safety factor is the moment of the upper crane out of wind force at 72 km/h wind speed, divided by the friction moment of the slewing ring, which must be more than two. This safety factor has to be more than three according to the EN14439-C25 norm.
For the above reason Jost is considering provision of wind sails in the base jib section for all jib lengths. This is to ensure sufficient wind area to enable the crane to weather vane more easily. Wind sails only make sense when the jib connection devices are strengthened to cope with side wind loadings. In addition, the advertised out of service position of hydraulic luffers has to be re-examined. An example is that older Jost data sheets claim 85 degree out of service position for the 50 m boom. That may be possible with special means but years of experience lets 75 degrees be a safer standard position meeting the new EN norm.
Wolffkran opted for a heavy rigid boom design to cope with possible side loadings and indicates a conservative 12 m minimum out of service position for all boom lengths up to 50 m jib. The out of service position of the 50 m jib is indicated by a 76 degree angle of the boom.
While at first glance the MTI-Lux MTL220-10 looks very similar to a Jost crane, Reinhold Bräuner, head of the company, claims essential safety and rigging improvements. To increase the rigidity of the boom four bolts and one pin connect each section. At the tower head where the boom foot is connected it is reinforced with additional welded plates. Bräuner claims that the crane stands wind speeds of 160 km/h with the boom parked at a 9.6 m out of service radius.
To speed up erection the complete reeving of the hoisting rope can be arranged at ground level and the boom connection part, including tower head with preinstalled hydraulic ram, form one transport and rigging unit. Electrical installation is arranged in a weatherproofed E-cabinet accessible by a separate door behind the driver’s cabin. For safe counterweight block installation on the swinging beam mechanism, access platforms are integrated into the counter jib. The entire 220 tonne-metre class crane can be installed within six hours and the complete upper crane can be delivered in three truck loads, the company says.
In the first quarter of 2014 Jaso entered the UK hydraulic luffing jib crane market with the J168HPA. Instead of following competitor crane designs, the luffing ram is positioned in the back of the tower head, protecting it against possible collusion of loads handled at minimum radius. In out of service position the ram is retracted and also in a protected position, instead of being extended as is a characteristic of all other hydraulic luffing jib cranes.
Holding the jib by pendants means that it can be designed very light weight, reducing luffing power consumption and enabling transportation of the slim jib sections inside the tower segments of the crane. The minimum out of service position with a 50 m boom is claimed as 8 m and then 12 m for the 30 m short boom configuration. To reduce power consumption the luffing speed of hydraulic luffing jib cranes is low. The Jost and Jaso types need two minutes to boom up, while Wolff and MTI-Lux claim 1.5 minutes. For working speed and for safety reasons a short luffing time should be recommended.
Articulated jib (jack knife) tower cranes offer an unsurpassed minimum out of service position. An example is the Raptor 84 developed by Artic Crane AB. As a special small city crane marketed by City Lifting for the demanding UK market, its 32 m jib can be stored at just 4 m radius, in contrast to the required 12 to 15 m typically required. With the double lever principle the jib reaches only about half the height of conventional single jib cranes. The geometric lay out of the folding mechanism means that it cannot be blown over the A-frame, even from an almost vertical parking position. This crane design is restricted to small capacities for economic reasons as the sophisticated upper is an expensive design. The city crane can handle 4 tonnes to 21 m radius and the tip load at 32 m is 2 tonnes.
Clearly the raised jib is the weakest point of a luffing jib crane in high winds. Some design weaknesses might only become evident after extensive field testing. An example came to light after a test erection and long term measurements at the Health and Safety Laboratory (HSL) in Buxton, UK. The spring buffers of a common rope luffing jib crane were extended to avoid uncontrolled movement of the jib at minimum radius. Manufacturers entering the market with a new crane design should consider the experience of competitors and crane users who may have already collected valuable data concerning the effect of wind on luffing jib cranes.
In addition to careful calculation, in the author’s opinion, it should become good practice to make use of wind tunnel tests or extensive field monitoring on the manufacturer’s test ground for every jib rigging condition of the prototype crane, before publishing out of service radius and permitted wind speeds. One should always be aware that if the wind speed doubles, wind pressure increases by a factor of four times. This means a small increase of wind speed can have significant effect on the safety of the tower crane.