18th Novemebr 2008
First Stage, Bulk, Excavation
The centre of the Heron tower site has now been executed down to approximately 7 metres below street. This has included breaking out the remainder of the reinforced concrete foundations of the former buildings and a substantial proportion of the mass concrete fill that had been placed in the City Ditch under the former Kempson House.
The Museum of London Archaeological Service maintained a watching brief throughout the operation to record the materials found and evidence of the Ditch profile.
No further excavation will be undertaken until the perimeter secant piled wall has been propped by the construction of the permanent ground floor. Once this has been completed in the New Year, excavation will recommence with teams mining beneath the completed ground floor and spoil being raised to street level through a central hole in the middle of the floor plate, a “mole hole”, left clear for the purpose.
First Stage Steel Erection
Timely completion of the first stage of excavation released the site for the start of first stage steel erection. This consists of the columns and beams that form the structural frame from Basement 3 level to 2nd floor. The ground floor frame will support the reinforced concrete ground floor slab and provide the propping action needed to support the perimeter retaining walls. The second floor frame will allow the second floor slab to be cast, which in due course will separate a “mole hole” operation, excavating underneath from the ground floor, from a continued Tower construction from 2nd floor upwards.
Pile caps have been constructed at Basement 3 level on the top of each of the bearing piles. As previously described, these were constructed from the original basement level within temporary shafts.
Severfield Rowen Structures, the steelwork subcontractor deployed an 800 tonne hydraulic mobile crane within the closed section of Houndsditch, to lift and position the very substantial columns that stand on these piles and carry the Tower. A group of central columns and beams is being installed and temporarily braced to act as a temporary stiffening core.
This core is temporary because unlike most tall buildings which have a central core, the stiffness in the Tower’s permanent structure is provided by the structural steel frame at the perimeter. Stiff, “moment”, connections join substantial perimeter columns and beams together to carry all of the horizontal and torsional forces from the Tower – a “perimeter stability frame”.
The perimeter stability frame commences in a substantial deep steel, upper capping beam placed on the reinforced concrete capping beam that has been formed on the top of perimeter secant piled wall. Connections between this deep beam and the perimeter columns have stiff, moment connections. To allow these connections to be made in the factory, the erection component is an inverted T consisting of a column with a section of perimeter beam attached to the bottom of it. Multiple plated and bolted connections are being made between the sections of steel capping beam. At first and second floor the stiff connections between columns and beams are again made by welding beam stubs to the column in the factory. Heavy, floor level, beams are then installed between the beam stubs with plated and bolted joints. Once the perimeter stability frame is completed and the floors are concreted, the components providing the temporary stiffening core will be removed.
The southern part of the structural frame, that facing Camomile Street, consists of a series of cantilever beams which will stabilise the scenic lifts and the cladding to encase them, and two steel staircases.
The weight and size of these substantial steel members require careful planning and timing of deliveries. Abnormal components are being delivered to arrive after the evening rush hour to comply with transport restrictions, and with little or no space on site to store fabricated sections of these sizes, components for the day’s work must arrive the evening before.
The steelwork is now erected to level 2 and will stop at that level whilst the upper capping beams and ground floor slab are constructed. Once these are completed in mid-January 2009, then the steelwork can recommence at the same time as the “top-down” construction of 3 basements. This is an engineering constraint that requires the low level steelwork to be fully constrained before erection continues. The completed ground floor slab is also required to act as the servicing area for the basement excavation.
Houndsditch Bridge
To transport men and material up the tower during the construction process a number of temporary external hoists will be installed. These are to be located above the closed section of Houndsditch and will be mounted on a steel bridge spanning over the road. Foundations for this bridge will be constructed during November and erection of the bridge is during in December/January.