Raising landmarks in London
By Heinz-Gert Kessel29 April 2013
Looking across the London skyline you will see a healthy number of tower cranes on construction projects in full swing. These mostly luffing jib towers suggest an office space construction boom in the capital city, despite a double-dip recession. In 2012, 34 new buildings raised the total volume of office space under construction to 9.2 million square feet (855,000 square metres).
Construction volumes are three times greater than they were at the bottom of the market in 2008. At the time of the deregulation of the financial market in 1986 many companies signed 25-year leases, which have now come to an end, and many tenants are looking for new offices.
Schemes under development in the inner city district, which are being raised by Terex luffing jib tower cranes, include the 20 Fenchurch Street project, nicknamed the Walkie Talkie due to its appearance. Another well known project is the Leadenhall Building or, more commonly referred to as, The Cheesegrater. These projects make up three quarters of the office space under construction. Typically congested inner city sites and complex building shapes call for special tower crane solutions adapted to the unique site conditions.
The Leadenhall Building
The 224 metre tapering shape of the Leadenhall building will see it become one of the most recognisable in the square mile. It occupies a prominent site between the distinctive Lloyd’s headquarters and the stalled Pinnacle construction project, which was covered in the September 2010 Issue of International Cranes and Specialized Transport magazine.
The slanting wedge-shaped appears to lean away, making it appear to have less impact on the protected sightline of St Paul’s Cathedral when viewed from Fleet Street and the west, than conventional high rise building design. The unusual shape of the 52 storey building will deliver floor plates ranging from 21,000 square feet (1,951 square metres) on the basement to 6,000 square feet (557 square metres) at the top of the tower. The rectangular floor plates decrease in depth by 750 mm per floor towards the apex.
A distinctive steel mega-framework built around a T-shaped strong box in the centre of the building will provide the required stiffness. A traditional central concrete core providing structural stability was ruled out. Unlike most buildings, the service core, which is made up of a yellow coloured steel frame work, is in a detached north–facing tower at the rear of the building. The service core contains passenger and goods lifts, service risers, on-floor plant, washrooms and, in the top cooling tower, generators, switch rooms, etc.
The steel mega frame, designed by Arup, provides the stability of the office tower in front of the north core service tower. It is said to be the tallest in the world. The base features a 30 m (seven-storey) high atrium, which will be open to the public. The atrium will cover nearly half an acre and will extend to the adjacent piazza of St Helen’s square.
In addition to the tower design and shape, the location of the building is on a remarkably tight footprint, allowing no lay down areas. The middle of this financial district of London is surrounded by narrow and densely populated streets and these factors represented considerable obstacles, which could only be solved by a sophisticated construction procedure and a smart crane concept.
In July 2011 British Land and Oxford Properties announced Laing O’Rourke as the main contractor for the works of the new Leadenhall Building. British Land and Oxford Properties noted the benefits of the contractor’s concept to do things differently; in particular to finish at least 83% of the construction work offsite and to base the construction activity on a full Building Information Model (BIM). The model included structure, cladding and all building services, enabling early design co-ordination. The 3-D model also incorporated the location and selection of cranes and their climbing steps. It even evaluated the impact of the cranes on structures and temporary works.
Benefits of prefabrication can be best demonstrated by the construction of the service core. This vertical wall, which runs the full height of the office tower, is built up using Lego-like bricks with a stack 15 m long and 3.5 m wide. The stacks are one story high and are prefabricated as table modules. In these steel frames, preassembled service modules are attached together with precast concrete planks. It takes one truck to deliver the modules.
Arriving on site, the complete table module weighs 35 tonnes, exceeding the capacity of existing Terex luffing jib tower cranes. The weight of the table modules leads to two specially commissioned upgraded 45 tonne luffers, delivered by UK tower crane provider Select Plant.
Handling prefabricated modules for an iconic glass and steel structure relies on very precise steering of all crane functions and wearing white gloves to handle material on site is a major culture change for both the fitter and the tower crane driver.
The 28 m high mega frame columns demand substantial free standing capacity of the chosen tower cranes. The internal climbing crane uses 48 m tower systems connected by two climbing frames within 14 m of the already installed steel work of the building.
During the initial construction phase, because the site was closely surrounded by high buildings, as many hooks as possible had to be installed. To meet the time frame four climbing luffing jib tower cranes were essential. The triangular shape of the building means that at least two of the cranes had to pass through the complex mega frame system of the building. A result was the need for a compact tower system for the climbing cranes, fulfilled by the 2.32 m wide HD23 Terex tower crane mast.
Outfitting work on the massive atrium calls for the early removal of the tower cranes. To cater for this was an internal climbing crane concept, which also had to be very flexible. For example, the cranes serving the north service core tower could only be attached to the central office building framework when using the gap between both structures, not interfering with the cladding process of the yellow coloured wall of table-sized modules. In this case, climbing frames were connected to the mega framework of the building by specially designed tie-in supports. In this way the two tower cranes became “external hanging internal climbing”.
The internal climbing cranes must have a reasonable free standing capacity to reach the 30 m height, while sitting over the steelwork at level 8 after the first climbing step.
A welcome feature of the Terex tower system is the universal application of the same size for external and internal climbing. For internal climbing, however, the three base sections are reinforced. In contrast to many other crane manufacturers, no climbing leaders are used in combination with an hydraulic jacking system in the base section of the crane tower. In addition, simple hydraulic rams at the lower climbing collar lift the crane by a yoke that grips into the same lugs of the tower used for external climbing. Only the compact hydraulic jacking system has to be reinstalled at a higher climbing frame to jump the crane again. A special base tower section is unnecessary.
The distinctive shape of the building leads to an extension of possible tie-in support points as high as the two cranes in front of the triangular structure are climbing. Like the internal climbing cranes, the wide steel frame structure of the mega frames carrying the whole building load must also be considered. Under these specific circumstances, the movable counterweight, which is typical for all Terex luffing jib cranes above the 300 tonne-metre class, creates a well balanced crane at all working configurations. Structural loadings are reduced, minimising the impact on the building. In addition, the way in which the counterweight is connected to the crane boom also prevents overturning of the jib, which is always a risk when luffing jib cranes have to work on tight city sites.
While two CTL630-32 cranes served the site at the south side of the building with their 40 and 30 m booms, two new CTL650-F45 luffers were installed to work at the north side. They had extra capacity and 35 and 40 m booms.
All the Leadenhall Building towers were rigged in short boom configuration with booms of between 30 and 40 m. While the internal climbing versions have a 48 m freestanding capacity, TC2 will reach a tower height of 150 m in external climbing mode. In the initial construction phase, TC2 is rigged on a fixed undercarriage at the level-3 basement by a truck crane. All other cranes were installed by using existing tower cranes working on the site. They reach their maximum free standing capacity by climbing cage in external climbing mode before three change over to internal climbing.
TC1 and TC2 at Leadenhall Street, both CTL630-32s, are inside the building to keep the length of their upper mast ties within practical limits. Their exact placing is governed by the diamond created from the mega frame through which they must pass between levels 12 and 19. The locations of TC3 and TC4, the two CTL650-F45s, are governed by the request to maintain the access road to the west and the proximity of the nearby Aviva building to the east. Also to be considered is the required 35 tonne lifting capacity to handle the table-shaped modules of the service core, along with the need to allow no interruption of the cladding of the lift shafts. TC4 has to be operated by an electronic restriction program to maintain safe clearance from the Aviva building.
Leadenhall Street is the only suitable access road for dismantling. The building’s shape leans away from this street so a roof mounted derrick cannot be used. The removal sequence is as follows: TC1 was removed on 17 December 2012 to make way for the cladding rising up the slope of the building. It was dismantled by TC2 and lowered to Leadenhall Street. TC3 and TC4 needed to remain in place until the generators were inserted into the service core tower at levels 47 and 48. TC3 will then be dismantled by TC4. TC4 will complete the attic construction, including the plant and building maintenance unit before being dismantled by TC2. Finally, TC2 will climb down, removing its own ties. The lower sections will be removed by truck crane from Leadenhall Street. Curtain wall panels left open after the tower crane is removed will be closed by the building maintenance unit.
In addition to more capacity and a maximum available hook load of 45 tonnes, the new Terex CTL650-F45 includes further advanced design features. The maximum jib length has been increased to 65 m where 7 tonnes can still be handled. While the 45 tonne lifting capacity is restricted to 15.5 m radius, it has 3 tonnes more capacity than the CTL630-32 could offer at 20 m radius by similar crane dimensions. The same tower system may be even more evident for contractors. An essential improvement of working speeds can be expected by the reduction in reeving for the luffing and hoisting operation.
In contrast the CTL630-32 luffing time has being reduced from 2.8 to 2.5 minutes. While the CTL630-32 had to be operated in two- or four-fall mode, the CTL650-F45 can lift up to 16 tonnes in a single fall and 32 tonnes in a double fall condition. Due to the powerful 132 kW winch unit, a maximum lifting speed of 227 m/minute can be achieved in the single fall operation mode. While the CTL630-32 required a fixed ballast block, besides the movable ballast when being operated in 30 – 40 m short jib configuration, the new CTL630-32 needs no fixed ballast. In addition, the stainless steel covered electrical cabinet is on the extended cabin platform, leaving more access space on the machinery deck for the winches. The same platform can be used as access for the machinery deck installation.
Further features include an auxiliary assist winch, which supports the luffing rope reeving, the installation of the retaining slings during the jib installation and the connection of the luffing rope reeving to the jib tie bar. Foldable A-frame and machinery platforms can be loaded on one truck to reduce the transport requirements of the CTL650-F45. The complete upper crane can be transported in 10 HC40 containers for easy shipment all over the world. To extend free standing capacity, besides the proven HD23 tower system, a wider HD33 monoblock tower system and the all new HD45 can be used. While the CTL650-F45 has its debut at the Leadenhall Building in London, requests have already being addressed by Terex from different regions, including Asia, America and Middle East.