SKIDMORE OWINGS & MERRILL
SKIDMORE OWINGS & MERRILL
WILLIAM HARE LTD
BOVIS LEND LEASE LTD
BRITISH LAND PLC
The development consists of a 12-storey office building known as 201 Bishopsgate and the 35-storey Broadgate Tower, which reaches a height of 165m above ground level. Between the two buildings is a Galleria area complete with a glazed roof which is supported as a cantilevered structure from the Broadgate Tower structure above level 5.
The development, which is located directly above the railway route at Liverpool Street station, was only structurally possible after significant enabling works were completed to create a raft structure over the live railway. Due to design changes the original 6,000 tonne raft structure needed extensive modification from the design constructed in the mid 1990’s, and resulted in an additional 1,000 tonnes of steelwork being incorporated.
In order to limit deflections and settlement it was necessary to distribute the resulting vertical forces from the two buildings evenly across the raft support structure, and thus the option of a heavy centralised RC core was not a viable structural solution for either building. Hence, the vertical load-path for the tower includes a series of raking A-frame legs that effectively prop the structure at level 5 and transfer the loads to strategically located transfer beams and columns within the raft structure.
The specification of 5-storey high raking struts provided a challenge during the construction of the frame, and an extensive arrangement of temporary props and trestles was required in order to build the structure up to level 5 before installation of the A-frame legs could begin. This included a sophisticated load transfer operation using a jacking system to transfer the loads from the temporary supports onto the permanent main raking columns. These raking columns and their associated bracing members were delivered as individual components of up to 26m in length.
The structural form of the Broadgate Tower is an elongated rhombus shape and the lateral stability is provided by a perimeter diagrid bracing system to all four sides of the structure located just inside the external cladding. The diagrid nodes occur at 6-storey increments, with plan bracing and RC diaphragm floor plates distributing the horizontal loadings into the external bracing lines.
Intermediate floor stability is provided by a secondary internal bracing system that spans between each 6-storey module.
The main service cores of the structure are constructed in steel and house the lift voids, staircases and service risers. The floor plates are RC on steel decking supported by long span cellular beams in the E-W direction.
In order to simplify the transport, handling and construction of the main perimeter members, the structure was constructed in 3- storey increments with the central welded splice being carried out on site to form the continuous 6-storey elements.
Wherever possible the main diagrid column and bracing connections were designed as a bolted joint but in certain locations, due to the resulting forces, it was necessary to site weld the joints. This detail required a robust temporary connection to be installed during erection in order that construction could continue above the splice location, and this methodology assisted in allowing control of the tower verticality during construction.
The building façade is designed to mimic the structural form so that the diagonal shape of the perimeter bracing system is visible from the outside. This gives the structure a unique feature that differentiates it from other buildings in and around the development.
These landmark offices are prominent and prestigious. The steel-framed Tower is sited above the Liverpool Street railway, above a bridging raft which had been constructed for an earlier, smaller scheme. The East façade has therefore been carried on large raking steel shores across to the raft abutment.
An heroic solution creating interesting spaces as a bonus.