Feet firmly on the ground

At just 16 storeys high, 6 Bevis Marks is relatively diminutive compared with the many new towers now dominating the City of London skyline.

The Fletcher Priest-designed office development is dwarfed by the adjacent Swiss Re ‘Gherkin’ and nearby Leadenhall ‘Cheesegrater’, and at 52.4m is nowhere near tall enough to attract a nickname of its own yet. But there are ways other than height to create distinctiveness on the skyline – in this case a steel and ETFE rooftop canopy that wraps over the 21,370m2 building to capture an all-weather roof terrace, before continuing in a lattice down the side.

The site was occupied by a 1980s eight-storey structure, and client AXA Real Estate & MGPA naturally wanted to maximise its potential with a much larger development. After experimenting with several massing options, Fletcher Priest designed a 15 storey tower that steps down to 11. The footprint is slightly reduced from 27.5m to 24.5m to maximise public space on Bevis Marks, with new pedestrian links created from the rear to Bury Court and the Swiss Re building.

But while the top of the £52m building grabs attention, it’s what’s going on at the bottom that provides the key to the whole scheme.

With limited scope to install new foundations, options quickly moved from demolition to the more cost effective re-use of what was already there, according to Julian Traxler, director of structural engineer Waterman Structures. This meant retaining the original 67 piles, basement slab and retaining walls and rebuilding the cores in the same place. These original piles support 56% of the new building with the rest provided by 37 new piles and 66 mini piles.

To make this solution viable, the new, taller building needed to be as lightweight as possible, using a superstructure far lighter than the one it replaced. 
‘This wouldn’t have been possible if it had been a concrete building,’ says Traxler.

The structural design uses a 150mm composite steel and concrete deck slab supported by 600mm deep fabricated composite steel beams with circular and rectangular web penetration for service distribution. The total floor zone is 1100mm deep. These beams were fabricated by steelwork contractor William Hare from three plates welded together to the engineer’s specification to ensure the lightest possible outcome for the job. 

According to Traxler, the new steelwork for the main structure was relatively straightforward. The structural grid is formed using 13.4 m long secondary beams and 9m primary beams with perimeter columns varying from 300mm circular hollow sections (CHS) to 550mm by 350mm rectangular hollow sections. This gives clear spans across the plan depth with the exception of three CHS columns on every floor excluding reception level. 

Floor plates total approximately 1255m2 on levels 1-10 and 650m2 on the smaller levels 11-13. All have 2.75m floor to ceiling heights. 

The only major complication was the inclusion of a number of large transfer beams including three at first floor level. The biggest was needed above the loading bay. Weighing 38t, this 15m long beam measures 1500mm deep with 1000mm by 100mm flanges and 50mm webs. Because of its size and weight it had to be brought to site in two pieces and welded; then it was installed with the help of kentledge blocks on the ends of the beams to shift the centre of gravity and avoid the core. All this had to be achieved within a very tight programme window as a result of Olympic Games-related road closures.

On level 11, a 10.5m long plated section – again delivered in two pieces – supports the plant unit enclosure.

Two further 9m transfer beams, each weighting 25t, were incorporated over the reception to avoid columns within the entrance space. All these transfer beams were essential for the success of the development.

‘If you don’t have a decent reception or operational loading bay, you can’t let the building,’ explains Traxler.

Aesthetically, a key reference point for the architects was the nearby, HP Berlage-designed Holland House, in particular its vertical emphasis and the way its ribbed green faience appears ‘closed’ when viewed obliquely. Fletcher Priest aimed to reference this through its choice of textured green glass column cladding panels brought forward from the glass facade line. This cladding is interspersed after every two panels of windows to form a strong vertical rhythm down the building. 

‘Like Holland House this looks like a closed façade and you get a sense of solidity of the material which goes as you walk past,’ says project architect Mareike Langkitsch.
From a distance however, the roof canopy is the most striking feature, providing all-weather protection for the 204m2 sky court – the largest of three roof gardens in the development. The architect toyed with the idea of a running track or tennis court, but wisely settled on a garden with obvious potential for corporate events as well as an ideal lunch spot for tenants. It is protected by a steel and fritted ETFE canopy, which frames views in two directions over London (see box) though it screens out much of the Swiss Re building. The canopy – fabricated and installed by Tubecon, the exposed steelwork division of Billington Structures – wraps over the garden and down the south facade to level 11 in a diagrid to assist solar shading. 

‘The roof is a very special top to the building, picking up on the same criss-cross diamond grid as that of the Gherkin,’ says Langkitsch.

Careful re-use of existing foundations is an increasingly popular option for developers on crowded City of London sites, aided by the better quality engineering records available for 1980s buildings – the era increasingly coming up for demolition. Load capacity testing of existing piles will be required to enable engineers to prove the foundations but, if they can be incorporated into the new building, massive programme and cost-savings can be achieved.

At 6 Bevis Marks, the re-use of 52% of the original structural mass shortened the construction programme and contributed to its BREEAM Excellent rating.

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Roof canopy

The light steel roof structure covers a bird’s eye footprint of around 33m by 25m and oversails approximately 28m down the south facade, leaving the terrace open at two ends. The canopy is supported on eight branched 355mm diameter circular hollow section columns. These tree-like columns support 45, CHS branches of 193mm each – up to seven per column – which form the cranked and curved diagrid canopy arranged in 6m bays. 

Canopy aprons are fixed back to the building’s structural frame through the cladding, at level 11 on the south side opposite Swiss Re and on level 15 on the other. 

Tubecon fabricated and welded over 1000 bespoke t-saddle brackets to the structure to accommodate the 4m by 4m ETFE cushions that clad the canopy.

Due to site constraints, the only feasible option for installation was to fabricate the structure in small pieces that could be bolted and assembled on site. To conceal the bolted splice connections, Tubecon used its ‘invisible’ connection technique with the help of CNC laser-cut ‘cod mouth’ cuts on the ends of each CHS member, according to parent Billington Structures’ process planner, Simon Ward. The canopy was test erected at Tubecon’s Bristol facility using bespoke steel jigs.

Customised structural nodes are covered in a 3D-printed architectural nylon shroud, designed to fit tightly to the steelwork to achieve a smooth transition from the structural column to the branches.