AWARD – 8 Bishopsgate, London

High sustainability credentials and an evolving design have all played an important role in the delivery of a 50-storey landmark City of London development.

FACT FILE
Architect: WilkinsonEyre
Structural engineer: Arup
Steelwork contractor: William Hare Limited
Main contractor: Lendlease
Client: Stanhope PLC

Rising up on a plot bounded by two of the City of London’s busiest thoroughfares, 8 Bishopsgate features a host of sustainability credentials. It is an EPC A rated building and the UK’s tallest structure to achieve a BREEAM ‘Outstanding’ rating. With 30% less structural embodied carbon than London tall building benchmarks, 8 Bishopsgate is a stand-out development in many ways.

According to the project team, the 50-storey structure challenges the traditional office tower design by breaking the massing into smaller blocks, allowing the tower form to address the site constraints and bring human scale to the scheme.

The blocks are said to be differentiated by scale, materiality, and structural function, while the building’s stepped form is accentuated by terraces and cantilevers that contribute to a cohesive and visually dynamic composition.

A stepped design was also important as the project is located within the City of London cluster, moderated by the constraints of the London View Management Framework; the terraces help satisfy these important planning requirements.

According to WilkinsonEyre Director Oliver Tyler, sustainability has been a key design driver from the project’s inception and one of the main goals was to achieve material efficiency.

“We used bespoke fabricated steel sections throughout, which were optimised for individual unique loads. By rationalising the building’s frame, we needed 25% less steel, which saved approximately 5,000 tonnes of carbon.”

Another initiative, to make sure the steel frame met the project’s high sustainability goals, was to leave the majority of it exposed within the completed structure.

“The steelwork was painted offsite and once it was erected, we only needed to do some minor touch-up work,” explains Lendlease Project Director Paul Gransby.

“The exposed steel frame provides the building with a modern industrial-looking office environment, but importantly we used less materials, which is good for sustainability, to enclose the columns and beams.”

As the steelwork is on show, a lot of care was taken with the choice and design of the connections. Two splice arrangements for the columns and bracings have been used by steelwork contractor William Hare. They are: an internal splice plate connection, with countersunk bolts that minimise protrusion from the outer section diameter; and a recessed end plate connection, which butt the connecting member’s webs and flanges up against each other.

Starting at ground floor level, the steel frame incorporates three distinct blocks that are stacked-up to form the overall building.

Each has a unique identity related to the cladding module, the addition of fins to the mullions or how the structure is seen through it. While predominantly glazed in a double skin curtain walling, the lowest block is clad in stone to respond to the street level context.

A low-rise block extends up to level 11, a mid-rise one, which actually sits behind the lower block, starts at sixth floor and goes up to level 26, and then a slim high-rise block tops out at floor 48. A pavilion is then positioned above this floor, incorporating a public viewing gallery and a plant zone.

The primary internal grid for the low and middle blocks is based around a 12m × 9m column pattern, using 620mm-deep plate girders with circular and rectangular openings to accommodate the building services.

“The column positions were selected in order to mitigate any column transfers, whereby the internal columns in the low-rise block continue upward to become perimeter members on the mid-rise part of the project,” says Arup Project Engineer Jeremy Edwards.

Using the same column grid pattern, the high-rise block has no internal columns as beams span directly between perimeter steel columns and the core, creating 21-storeys of column-free space.

Providing the stability for the 50-storey building are two concrete cores. The north core extends from the basement up to level 51, while the south core terminates at level 24.

Providing some extra stiffness to the structure, the mid-rise block is a braced box, featuring perimeter steel bracing that connects the two cores together and mobilises the perimeter columns to resist horizontal loads. This stiffens up the low and mid-rise blocks and allows for a narrower north core for the uppermost block.

Helping to further maximise available floor space, the building cantilevers out along its western elevation, over-sailing the pavement along Bishopsgate.

This overhanging west face reinforces the architectural concept, while providing 15% of the Net Internal Area, thereby improving the viability of the scheme and enhancing its identity.

“For the transfer of vertical loads, the west face can be viewed as a 20-storey truss, which is supported on its north edge by a series of raking columns that connect to the north core and by a series of further columns on the southern edge,” adds Mr Edwards.

The western elevation overhang commences at level six and initially extends up to floor 26. This is the truss, and it forms a 5.5m-wide cantilever on the north, which increases to 9.5m-wide at the south.

Further cantilevers begin at level 26 and 48, with the former overhanging by up to 3.8m. This feature is formed by three inclined columns, sat atop the truss and extending up to level 30 (four floors), where they are tied into the steel frame and anchored into the diaphragm.

Finally, the uppermost overhang at level 48 is created by two inclined columns, which extend up to floor 50, where they are also tied into the frame.

In summary, the judges say eschewing the creation of a simplistic City icon, this 50-storey building successfully combines a fragmented form, determined by function and site constraints, with a rigorous structural system. The resulting variety of spaces has proved a letting masterstroke. Impeccably constructed through a Construction Management contract, this is a top-class project.