Two new bridges by Wilkinson Eyre link buildings at Deloitte’s London headquarters

Buildings.

Two new bridges linking buildings at Deloitte’s City of London headquarters “are each a bold expression of their engineering challenges”, says their architect, Wilkinson Eyre. They are “light and logical compositions of folded ‘monocoque’ deck, carrying the glass sides which in turn carry the carbon fibre roof”.

The aerial crossings are part of the ongoing improvement and expansion of Deloitte’s central London ‘campus’, connecting Nos. 1 and 2 New Street Square, which were built 15 years apart, to different designs and with different floor heights. “The brief was to connect two different client-facing areas on the respective first floors of each building, and two staff-only areas, including the main restaurant, on the seventh floor”, explains the architect.  “Additionally, Deloitte was keen for the bridges to signal its presence as a major occupier in this part of the City with a high-quality, attractive landmark”.

The bridges’ design was informed by the level changes, interfaces with existing facades, a planning requirement for visual lightness and the need to provide adequate step-free access. Working with engineer Eckersley O’Callaghan, Wilkinson Eyre devised a switchback ramp on each bridge that dealt with the level change entirely within the bridge environment. In the final design, this was developed into a three-way switchback ramp, combined with two flights of steps, replete with places to pause and even sit to enjoy the view.

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Having established the key design principles, the bridges are articulated as linked monocoque structures with stepped steel bridge floors – gaining far greater strength, through increased rigidity, than a conventional flat section. The glazing of the bridge walls is also stepped to create inherent strength, allowing the glass panels to support the lightweight roof without the need for structural mullions. Negating the need for heavy horizontal beams, the monocoque construction gives the bridges a more refined and lightweight aesthetic.

“A key challenge was addressing the interface between two host buildings with different characteristics and profiles”, says Wilkinson Eyre. “No.1 New Street Square features a curved glass curtain wall while No.2 New Street Square comprises flat glass units set into a pre-cast grid of columns and beams. Each façade has been carefully adapted to receive the bridges in a unique way; they appear to float through No.1 Street Square however they abut No.2. Although the bridges are interstitial, they open out onto the conditioned office floor plate of each host building and as such need to be suitably controlled to maintain a good level of comfort for the users. Localised heating, cooling and ventilation is therefore provided via a series of ‘trench’ units recessed into the floor of each bridge and positioned to help offset cold down-draughts and solar gains”.

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Glass and stainless steel were selected to reduce the need for frequent or expensive maintenance and physical access. The bridge roofs are formed from carbon fibre and incorporate skylights to improve the internal environment, and further reduce the bridges’ apparent mass to the observer. Other materials and finishes, such as polished mirror on the underside of the decks, were selected to give the bridges a shimmering transparency when viewed from the street, and reflect back distorted views of the surrounding townscape.

In contrast, the interior uses the warmth of timber to create a calm space. A light oak floor wraps over the ‘terraform’ surface. The ramps are separated by a balustrade of slats of birch plywood, each individually lit. The ceiling is individual slats of timber hiding ceiling-mounted services. In-built worktops and seating are made of under-lit blocks of Perspex fitted between the plywood slats. “From the exterior”, says Wilkinson Eyre, “the complex geometry of the timber slats creates a contrast with the steel and glass both by day and night”.

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