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鈥榃e鈥檝e got to get the quality absolutely precise.鈥� It鈥檚 a mantra Nick Moore chants repeatedly on our tour of 70 St Mary Axe, a 21-storey office tower currently under construction in the City of London. Moore is a director of Mace, the contractor responsible for building the curvaceous scheme which, even before completion of its construction, has acquired the moniker of 鈥淭he Can of Ham鈥�. 

Moore鈥檚 focus on construction quality is, one suspects, at least in part due to the unrelenting scrutiny of the design and construction process by the project鈥檚 client, the inimitable Geoff Harris, head of development at TH Real Estate. 鈥淕eoff is quite a character; he鈥檚 very, very particular about how he wants this job to look when it鈥檚 finished, which is great,鈥� Moore says. 鈥淥ver the years I鈥檝e worked with lots of clients, many of whom don鈥檛 take much interest in a project until it鈥檚 almost built and then they turn up on site and say 鈥業 didn鈥檛 think it was going to look like that鈥�; that won鈥檛 happen here because Geoff knows what he wants.鈥� (Harris is, in fact, such a frequent visitor to the construction site that he even has his own PPE locker in Mace鈥檚 site hut.)

The elegantly tapered building Harris has tasked Mace to build will house 28,000m2 of premium office space set over 21 floors. The building鈥檚 unique shape means that each office floor is a slightly different size from the ones above and below it, but all benefit from views out through the building鈥檚 glazed exterior. The building鈥檚 shape and its City location mean that this is proving to be a demanding building to construct. 鈥淭his scheme has challenges you wouldn鈥檛 get on a standard building,鈥� explains Moore. And the story of how those challenges are being met is equally distinctive.

State of play

To Moore鈥檚 credit, construction is actually slightly ahead of programme. Standing in the double-height space that will, within a year, form the building鈥檚 reception area, Moore outlines the current state of play: 鈥淐onstruction of the building鈥檚 concrete core is complete, its steel frame has reached level 22, corrugated steel floor decking is being installed on level 17, the floors are being concreted on level 15 and the cladding has started to be installed on level 7,鈥� he says. 鈥淲e鈥檙e ahead of programme, but not as far ahead as I鈥檇 like us to be,鈥� he adds.

It is, perhaps, no surprise that the project is progressing well because, even at 5pm on a cold, dark winter鈥檚 evening, under the glare of the site鈥檚 artificial lights, the clatter of activity shows no sign of abating. 鈥淣ormally we work till about 9pm, but tonight the steelwork guys will put in a late shift to finish off a level because it is forecast to be windy for the next few days so it鈥檚 unlikely that they鈥檒l be able to work for the rest of the week,鈥� Moore explains.

While standing in the reception area, Moore uses the opportunity to show how the precast concrete fins will be attached to the building鈥檚 core and concrete ribs onto the underside of the soffit to line up precisely with the cladding supports. 鈥淚f these are positioned just half a centimetre out, then you will see it,鈥� explains Moore. 鈥淭hat鈥檚 why the setting-out of the cladding, positioning of core, setting-out of steelwork 鈥� everything, in fact 鈥� has to be spot on.鈥�

One of the more unusual aspects of this project is that, because of the building鈥檚 shape, cladding installation has had to start on the ground floor on both the north and the south elevations. These are the elevations that curve outwards and then in again, to meet in the arch forming the building鈥檚 roof. 鈥淏ecause of the shape of the building we鈥檝e had to clad progressively up from the ground floor on the north and south elevations because if we鈥檇 started the install at level 2, we would not have been able to fit the ground floor cladding,鈥� Moore explains. 鈥淎ll of the vertical ribs have to be aligned exactly, which is why we are so concerned about accuracy.鈥�

The cladding on these elevations is formed from a double skin of cold-curved laminated glass, which is held in place by a precisely curved aluminium frame. The unitised panels are brought to site in 1.5m-wide by 3.8m-high (storey height) modules for installation by Focchi, the cladding package contractor.

鈥淕eoff had a casting vote on the choice of most subcontractors,鈥� says Moore.

Cladding installation is more flexible on the east and west elevations, where flat glazed modules will be fitted. As a consequence, Mace is able to keep the ground floor temporarily free of cladding on these elevations to maintain access to the building for both personnel and materials. The materials and personnel hoists are located on the Goring Street elevation (see plan, right) to provide access to the floor plates.

All the building鈥檚 floorplates are column-free. Cellular beams span the 12m from the concrete core to the faceted steel columns incorporated into the building鈥檚 curved facade. To give the building its curved, organic appearance, these raking columns will be fitted with a curved, anodised aluminium skin to integrate it with the curved facade glazing. 

On the level 3 floorplate, the positioning targets are still visible on the raking steels forming the perimeter columns. 鈥淭he steelwork鈥檚 position is checked against co-ordinates at every level to ensure it鈥檚 within tolerance, because if it were to twist or go out of tolerance then physically you would not be able to fit the cladding to the building,鈥� explains Moore. 鈥淚t鈥檚 all about precision: the steel has to be right because once we鈥檝e concreted it into the floor deck then that鈥檚 it,鈥� he adds, repeating his mantra.

The concrete floor slabs have been spray-painted in a patchwork of colours that define which areas are allocated to storage and access; the painting has ensured a clutter-free corridor around the floor鈥檚 perimeter to allow cladding installation. 鈥淲e have to keep the building as clean and neat as possible 鈥� if not, Mr Harris will be on us like a tonne of bricks,鈥� laughs Moore.

Each floor incorporates what Moore describes as a 鈥済enerously sized鈥� lift lobby formed by the building鈥檚 core: 鈥淭hat鈥檚 what Geoff wants and that鈥檚 what he鈥檚 going to get,鈥� says Moore.

Service risers run up through the core. Many of the services have already been installed. The services were delivered to site as prefabricated modules and lowered down the riser for connection by services contractor Mace MEP. The modules incorporate a checker-plate platform where they pass through each floorplate to close off the riser.

The cores will also incorporate the office toilets. These will be fitted out using a prefabricated solution, the components of which are currently being fabricated in Scotland for delivery to site as flat-packed units. It is a solution intended to deliver a high-quality finish by harnessing the efficiency and accuracy of off-site manufacture.

While the steel structure races upwards, Moore is hurrying to complete work in the building鈥檚 three basement levels. The basement is home to the plant that was too heavy to be installed in the roof-level plant room, such as the domestic water and sprinkler storage tanks, electrical distribution boards, and huge oil tanks to fuel the standby generators. The oil tanks are so large they had to be craned into position in basement level 2 before the level 3 slab was constructed above it.

The chiller plant is being installed by craning it in through holes strategically left in the floor slabs above. In preparation, the floor was painted and the services fitted to the soffit above the chillers. The basement also includes 35 showers to cater for cyclists using the building鈥檚 360 bicycle parking spaces. 鈥淚 want to get the basement areas completed by June so that I can shut it off from the site,鈥� says Moore.

There is still plenty of work to be done: the landlord鈥檚 specification is for the scheme to be shell, core and floor; Mace has yet to start installation of the raised access floor. There are also three floors, which will be fitted out to Category A, each with a different style, for use by TH Real Estate to market the scheme.  

The real challenge is still to come

TH Real Estate鈥檚 office is just a 10-minute walk from the site. In pride of place in its reception is a detailed architectural model of the scheme Moore is constructing. Geoff Harris uses the model to explain how architect Foggo Associates鈥� distinctive design exploits the constrained site while preserving strategic local views of Tower Bridge. 鈥淚f you鈥檙e clever about your design it gives you the ability to build it quite efficiently, because you鈥檙e getting decent floor areas for the amount of walls you build 鈥� because the walls are quite expensive,鈥� he says. 

The scheme鈥檚 50m by 50m plan means that it takes up an entire City block. Its flanking walls taper outwards to add width to the public footpaths. However, even in its tapering form the office floors are super-efficient, having been designed to allow tenants to subdivide them without wasting space. 鈥淎s a developer, this scheme is about as textbook as a centre-core building gets: the old magical 12m wall-to-core dimension works perfectly for a range of occupiers,鈥�  Harris explains. 

There is also a commercial element to Harris鈥� description of the floor areas because the net floor areas are enshrined in the building contract. 鈥淣ick [Moore] must be within 1.5% target area on each individual floor and by total floor area,鈥� says Harris. 鈥淚f there is a diminution of more than 1.5% from that area then there is a financial penalty because floor areas are what we generate our income from; get it wrong and the consequences are massive,鈥� he explains.

The contract is a two-stage modified JCT with an element of contractor鈥檚 design. 鈥淚t was locked down with Mace at 80% fixity on cost at the second stage with the supply chain procured as part of the process,鈥� says Harris. The project reached 100% surety on cost at Christmas 2017. 鈥淚t is a testament to everybody working on the project that it sits on budget and it has advanced on programme,鈥� says Harris.

With its current rate of progress, the project鈥檚 target completion date of December 2018 is looking achievable. However, Harris knows that the most challenging aspect of the build has yet to come: the works to the rooftop plant room.

The two-storey plant room is located beneath the arched structure at the top of the building, 100m or so above the City鈥檚 bustling streets. It will house cooling towers, air handling units and two generators. The space is open to the elements: structural steels arch over the space but instead of glazing, the fins support perforated mesh to help with cooling, coloured to look like glass. 

鈥淭he building needs to read as one, so instead of glass the infill is a perforated mesh of a colour preselected so that when you look at the building from a distance, the mesh will appear to reflect the light in the same way as the glass does,鈥� says Harris.

Its porous mesh cladding means that the plant room and its plant will be visible. So as not to detract from the form of the building, Harris says that the plant room has to be 鈥渂eautifully planned鈥� with all plant finished in the same colour. The plant will even be a feature at night. 鈥淭he danger is that the building鈥檚 form might make it look like a boiled egg with the lid off at night, so the plant room will be illuminated,鈥� Harris explains.

鈥淭he roof works will be our biggest challenge,鈥� acknowledges Mace鈥檚 Moore. In recognition of this, Mace has produced a video animation to show how it plans to sequence these high-level works. 鈥淲hat we must do is get the concrete floors on levels 21 and 22 cast as quickly as we can, because we need the concrete to have cured so that we can waterproof it before we start to land plant on it,鈥� explains Moore. 鈥淭o save time, we鈥檒l use precast up-stands, pre-coated with waterproofing, around all openings.鈥�

Similarly, much of the plant installation will also be prefabricated. 鈥淭here is a lot of plant to go in so we鈥檒l prefabricate as much as possible in the largest-sized pieces we can possibly lift using the core-mounted tower crane, which will save us time and help minimise the number of people that will have to work at roof level,鈥� says Moore. In addition, work platforms will be attached to the main items of plant to enable the cladding contractor to safely install the mesh panels from inside the building.

The size of the prefabricated plant means that some elements will be too large to fit through the 12m spacing between structural steels. As a consequence, parts of the roof structure will have to be fitted after the plant has been installed. 鈥淭he structure has to be installed once the plant is in place,鈥� he says. He admits that the procedure could be 鈥渜uite risky鈥�, but says Mace has come up with a solution to protect the plant that will ensure the installation is 鈥渁bsolutely precise鈥� 鈥� which should ensure that it, along with the rest of the building, meets with Geoff Harris鈥� exacting requirements.