Favelle Favco cranes take on a London landmark. Heinz-Gert Kessel reports
By Euan Youdale15 September 2010
A new generation of Favelle Favco cranes set to work in London's financial district. Heinz-Gert Kessel reports.
With its height of 288 m, The Pinnacle, also known as The Bishopsgate Tower, will be the centrepiece of London's financial district. It will be taller than the nearby Heron Tower which stands at 230 m in the UK's capital city.
The 63 storey office tower is going to set a landmark by the twisting design of the roof and curling patterns in the façade. Like the Gherkin, also in the neighbourhood, the new building will have a double-layered skin to reduce energy consumption. Containing more solar panelling than any other building in the UK, the 2,000 square metres of photovoltaic solar cells integrated in the façade are designed to generate 200 kilowatts of electricity.
The Pinnacle is surrounded by the busy Bishopsgate, the Deutsche Bank building and the Hiscox Syndicates building. Demolition of existing buildings on the site was completed in 2008. Sixty of the largest ever piles in the UK were then sunk into the ground to 65.5 m below street level before excavation work started.
In October 2009 the first of four Favelle Favco tower cranes, specially ordered by main contractor Brookfield Construction (UK) Ltd, was installed on the site to deliver building material for the basement construction while excavation work was ongoing. During the pre-construction service period, Brookfield brought in experienced experts from Dubai and Australia to optimise the construction plans and logistics of the site. From the structural construction point of view Brookfield changed the original design in opting for a concrete core, giving a much stiffer back bone to the building than the former planned pure steel construction.
Towers for all
When the project is in full swing, the core will be about 20 storeys above the steel work. In addition, the cranes serving the core will have to fit into the small lift shafts of the building and maintain clearance with the jump form structure. Due to the shape of the construction site, the centre distance between the two cranes in the building core is just 17.93 m, while at the same time 42 m working radius is necessary to cover the site and to reach the only two access areas where construction material can be delivered. In addition to general lifting work, the 25 tonne steel columns have to be handled by the tower cranes.
For full site coverage two additional external climbing tower cranes have to be positioned at the edge of the building site. To further complicate the crane planning, it must be considered that one of the internal climbing cranes, TC2, has to be climbed externally after reaching level 34 as the core drops at this location. TC1 remains an internal climbing crane topping out the building after 15 stages with the slewing ring 269 m above foundation level.
In addition to special steel grids in the foundation area to support the cranes, purpose made steel supports have to be designed where the shaft shape becomes eccentric in correspondence to the mast centre of the cranes. Consideration to nearby high buildings and other sites must be considered for the crane installations.
Confronted with the many restrictions, one leading European crane manufacturer asked Brookfield to change its construction scheme to enable four tower cranes into the project, recalls Dean Mitchell, Brookfield project manager, with a smile. For Brookfield it was clear that the project required special custom designed cranes which could also be a benefit for future cramped city site projects.
Luffing jib cranes are the only answer for such restricted site conditions. As Craig Taylor, Brookfield fleet manager, points out, two additional aspects were critical for the tower crane concept for the Pinnacle site. First, the requirement was for a very small mast size on the internal climbing cranes, especially in relation very high lifting capacity requirement. Capacity needed to be 25 tonnes at 17 m radius and even to 22 m on TC1.
Second, out of service space required by the luffing jib cranes should be, as a minimum, achievable. From the Jost topless luffers, the benefit of placing the jib in a steep position in parked conditions is well known in London, but for high capacity cranes, this device was missing until now.
Brookfield found an open ear for its wishes concerning the crane design by Favelle Favco and went through a two year intensive co-operation with the its project group and the crane manufacturer's engineers before the first crane was delivered. The four Favelle Favco cranes meanwhile on site are the first Favco cranes in Great Britain since the development of Canary Wharf in 1989, which were US-built Link-Belt designs.
In addition, the newcomers are also the very first electrically driven Favelle Favco cranes in Europe. They represent a global combination of crane technology in accordance with all relevant national and the European standards. To ensure legal operation of the cranes HSE (Health and Safety Executive) was invited by Brookfield to attend the crane project in the early design steps.
The drive units and slewing systems, including the DC hoist winches, Flender gear units, Siemens slew motors and Siemens frequency inverters for stepless control were supplied by Krøll in Denmark. Like Favelle Favco, Krøll is part of the Muhibbah Engineering Group. The steel structures were manufactured in Malaysia by Favelle Favco. It is the first time that the two members of the same group have co-operated like this for the European luffing jib crane market.
In out of service position, the crane boom can be stored at 12 m radius using additional hydraulic buffer rams. They are beside the conventional spring buffers and act against the boom, preventing it from rebounding if there is wind pressure from the front. As a second unique feature the cranes have an hydraulically operated slew park brake that mechanically blocks the internal slew ring teeth.
To enhance safe working conditions, Brookfield opted for additional band brakes on the winches as a second independent braking mechanism. An hydraulic power pack in the crane's slew well has two stages of operation. The first stage is used for operating the band brake while the cranes are working and for the slew lock and hydraulic buffer for the parking position. The second stage operates the climbing system to raise the height.
Not allowing the cranes to weathervane means additional forces acting on the tower system, exacerbated by the lift shaft dimensions restricting mast size. Favelle Favco chose a very stiff mast system leading to unique limited deflection. Both core climbing cranes, the MK440 (TC1) with 50 tonnes capacity in two fall operation, and the MK380 (TC2) with 32 tonnes capacity on two falls, are mounted on 47 m free standing towers of 1.9 x 1.9 m horizontal dimension.
The stronger 5.8 m long base tower sections are connected by six bolts per corner while four bolts are used on the lighter upper sections. Under the maximum calculated load, the corresponding deflection of the 47 m tower is not more than 0.505 m. Counterjib radius is 7.65 m on the MK440 and 7.48 m on the MK380. At maximum calculated backward deflection there is a minimum distance of 1.82 m between the two core crane machinery deck tails.
These figures indicate, however, that the 45.8 m booms should be stored in opposite directions in parking position when they are luffed into a steep position. The two external climbing cranes on the site have the same boom length and hoisting units but different tower systems and load moments. While the MK380 (TC3) is rigged on a 44 m tall tower made up of 2.4 x 2.4 m square tower sections, the MK440, (TC4) is set up on a 28 m tall 2.7 x 2.7 m square sized tower for the basement construction of the building.
At a later stage it will be relocated outside the footprint of the building next to Bishopsgate. At the time of writing in early August it was waiting for a new crane base to be poured. It was delayed to install the underground supply infrastructure for the building. The crane will then be raised to a 48 m free standing version and externally climb alongside the building to 224 m mast height with six ties.
The cabins with integrated electrical cabinets can be fitted in the field on either side of the machinery deck. For the external climbing cranes the most suitable side was chosen by the site manager to minimise space problems when climbing down the crane alongside the building façade.
On the move
During the project the cranes will be relocated several times. This will include changes of the rigging condition. TC2, for example, started in the project as the first crane with TC6, rigged on the concrete foundation of TC4 inside the building's excavation, to cover the basement construction area with 64.2 m boom. When the other cranes arrived it was relocated into the core of the building and the boom had to be shortened. It was then renamed TC2.
The tower for TC4 was installed on a second set of anchors in the concrete base where formerly TC6, with its slim internal tower, was standing. At the busy Bishopsgate, permissions for any road closures required for tower crane erection are granted under strict regulations and authorisation must be sought at least three months ahead of work when every attempt is made to reduce the need for mobile crane assistance.
Due to the split-deck design of the machinery deck, the heaviest crane component is 13.9 tonnes for the hoist winch with 600 m rope. A 200 tonne mobile crane can be used to install the first crane and dismantle the last, climbed down, one. All other crane rigging, relocation and de-rigging work during the project will be carried out by the tower cranes on site.
Speeding up the crane rigging procedure are fixed counterweight, made up of steel plates, and erection pendants that hold the boom foot during boom assembly. Service, rigging and climbing work is carried out by tower crane specialist Bennetts Cranes Limited.
According to Craig Taylor, working speed was a further issue leading to the decision to order Favelle Favco luffing jib cranes. All tower cranes on the site can be operated in one or two fall hoisting rope conditions. The MK440 is one of the largest capacity construction tower cranes in the UK. The main benefit, however, is the two fall operation, whereas European competitors currently would require four lines.
With the 200 kW winch a maximum speed of 80 m/min in two fall operation mode is possible, which is doubled as soon as the crane changes into single fall mode for loads up to 25 tonnes when half the final building height has been reached. For a comparable European luffing jib crane, the luffing rope reeving must be done with five or more rope sheave blocks in the bridle, while the MK440 will just have two. The 56 kW luffing winch provides a boom movement from maximum to minimum radius in just 90 seconds while European style luffers often need two minutes.
In the operator cabins, touch screen load moment indicators providing the driver with a clear and continuous picture of the operation condition. At the back of the cabin two warning lights are installed indicating to other crane drivers and the site manager if the crane is parked with boom and slew lock.
The four cranes are responsible for all lifting work in the four sectors of the building. If a crane has to enter the working area of another one the site crane co-ordinator has to be informed. Also, due to the limited distance between the cranes, special lightning protection devices for each machine were arranged.
Even at first glance the four Favelle Favco cranes appear to be purpose built for the multi-million pound Pinnacle project. Dean Mitchell at Brookfield recognizes a unique sales proposition in the new Favelle Favco cranes joining the Liebherr 160HC-L luffing jib crane in the company's tower crane fleet. He forecasts that the new Favelle Favco crane generation may become a common sight on other Brookfield city site locations.