Ainscough 800 tonner on stilts in London

Heron Tower will be one of the tallest buildings in London, UK. It occupies the entire footprint of a small site bordered by public roads on all four sides.

Three Terex Comedil tower cranes owned by Select have been installed to erect the main building. Their 16 tonne individual lifting capacity was insufficient for the steel columns that had to be craned in to form the basement of the new building.

Individual columns weighed more than 40 tonnes and, combined with the limited space within the footprint of the building, the only answer was to position a large mobile crane immediately adjacent to the site boundary.

Given the restraints of the road system, the only solution was to position the crane in Houndsditch, a major city street next to the site. This presented its own problems as the road width was narrower than the outrigger spread of the crane, a wheeled mobile Terex Demag AC 2000.

The structural steel contractor, Severfield Reeve, called in technical experts from Ainscough Crane Hire to assist with the planning and to liaise with the statutory authorities, which had to approve the use of a large crane in a confined space. Ainscough produced a plan to place outrigger mats for two of the supports spreading over a utility tunnel. The other two outriggers were supported by 4 m high trestles on the base of the excavated site.

With Houndsditch temporarily closed off to vehicle traffic but with a pedestrian route being maintained to provide access to adjacent buildings, the entire area of the road and nearside footpath was covered in a 100 mm thick sand and cement dry mix, over which a double layer of Trax heavy-duty aluminium roadway panels was laid.

Below each roadside outrigger of the 800 tonne capacity crane, five 7.5 x 2.2 m steel mats were positioned, bridged over by an 11 x 3 m steel mat sitting on Ethafoam pads, to give an effective bearing area of 97 square metres. This arrangement brought the bearing pressure at the original road surface down to 1.65 tonnes per square metre, which proved acceptable to the authorities.

The height of this multiple arrangement of load spreading mats meant the wheels of the crane had to be at least 750 mm above road level so that the outrigger legs could swing out into the working position. This was achieved by reversing the crane up a temporary ramp onto a raised pad constructed from timber crane mats.

To support the crane's outriggers where they projected out over the excavated foundations of the new building, construction solutions company Severfield Reeve designed and fabricated 4 m high structural steel trestles, to support Ainscough's load spreading steel mats.

Installing the crane and its supports in 10 truckloads of equipment took 10 people two days. The AC 2000 used 160 tonnes of counterweight and sideways superlift with 55.5 m of main boom. This meant the 40 tonne steel columns could be installed at up to 40 m radius.