WATSON STEEL STRUCTURES LTD
HBG CONSTRUCTION LTD
LONDON & REGIONAL PROPERTY LTD
This new structure, presenting a stunning visual statement within the main reception area of the newly refurbished 55 Baker Street, must be one of the most striking examples of how a complex structural solution is needed to produce a seemingly simple architectural concept.
The architectural requirement was for a slim, elegant, angular structure to replace 12 original reinforced concrete ground to first floor columns and present a dramatic backdrop to the reception area.
It was recognised at the outset that the design and engineering of the transfer structure was extremely complicated and a key part of the project. It was essential that a steelwork contractor be brought on board very early in the design process and this was facilitated by a two stage tender process where Watson Steel Structures was appointed, initially based on a set of budget rates and the lump sum fixed price contract was finally agreed once the design was substantially complete.
The client purchased these conventional 1960’s concrete framed office blocks with the intention of creating a single, modern, multiple use building. Rather than demolish the existing buildings and start again it was decided, for programme and economic reasons, to retain the basic concrete frame and replace all the external and internal walls to suit the new uses. Whilst the existing column spacing of 14′ x 14′ (4.27m x 4.27m) was adequate for the client’s use in the general areas, in the main lobby area he wished to have an open, column-free space.
The project involved temporarily propping the concrete frame above, removing 12 of the existing concrete columns, installing the new steel structure and then transferring the load from the floors above to two existing basement columns below.
The transfer structure is in a highly visible area and the architectural requirement was for it to appear as a ‘sculpture’ rather than a conventional transfer frame. The members were therefore required to be as slim as possible with a high quality finish.
There are two separate identical connected structures, mirrored about the centreline of the building, each with four cantilever arms and two stubs that support the floor above and an inclined column down to a single base plate. The legs which each carry an ultimate load of 1200 tonnes are tapered down to just 300mm x 600mm and formed from eight plates which are almost a solid cross section at the base plate level. The arms are tapering box sections fabricated from plates up to 80mm thick whilst the columns are double webbed I sections with additional external structural cladding plates designed to cope with the high stresses involved.
Special attention was paid to the visible connection details between the individual plates, their edge profiles and connecting weld sizes and appearance. A mock up of the column section was produced during the design development stage to allow the architect to define any shadow gaps he wanted to be visible.
Largely hidden within the elegant exterior shape of these transfer structures is an outstanding example of welded fabrication. The technology and craft skills required to weld such thick plate, in such a complex structure, are a great credit to the steelwork contractor. The propping of the existing building, the removal of concrete columns and the transfer of the loads, represents a considerable engineering achievement.
There is no doubt that this challenging fabrication is crucial to the project’s success.