Building on solid ground in central Birmingham

Wedged between the Grade I-listed Birmingham Town Hall and a tram link, Sir Robert McAlpine was happy to discover solid ground on which to build a 10-storey, 42 metre-tall office block

I have been in bedsits smaller than the hoist carrying me up the side of Birmingham’s newest office block. On a grey autumn afternoon, Sir Robert McAlpine (SRM) project manager Chris Hucknall is riding with me to the top of 3 Chamberlain Square, part of Birmingham’s Paradise Street redevelopment. We are joined by chief engineer Andy Towers, four labourers and a neatly stacked pile of ductwork. And there would still be ample room for a game of table tennis.

With a 4,000kg capacity and dimensions of 5×3 metres, the Alimak Mammoth TM 55/50 hoist is one of the biggest in the UK, and the envy of the rest of the firm, Hucknall says. “It has been a revelation for the project, allowing us to move large amounts of materials.” The hoist has carried the bulk of the distinctive terracotta glazing frames that are being fixed to the building’s frame to create fluted vertical ribs stretching to the top of the building.

The 10-storey, 42 metre-tall office block, on the site of the former home of the Birmingham Conservatoire, is setting a new standard as the first office building in the West Midlands designed to achieve a BREEAM Outstanding rating. Upon opening next year, it is set to provide 17,500 square metres of Grade A office space in the Paradise regeneration zone, which is playing host to a rising number of blue-chip financial and law firms.

The building is on solid ground. So solid, in fact, that no piling was required. The sandstone topped with mudstone on the site “supports the vertical very well”, says Hucknall. “If I could take it around the country with me then I would.” In late 2022, SRM, still working on a preconstruction services agreement signed earlier in the year, dug down four metres, removed 10,000 cubic metres of muck and installed a king post wall system using concrete panels cast in situ, to hold back the earth around the footprint of the building.

This method was partially dictated by the tightness of the quarter-hectare site. Hemmed in by buildings on three side – including the Grade I-listed Birmingham Town Hall to the east and a tramline on the south, only a narrow strip was available for works vehicles. The access route was set tight against the foundations and could only allow one vehicle to pass via a one-way system.

Negotiations on the main contract took place during the first half of 2022, at a time when materials prices were rising by up to 20 per cent a month. By the time the contract was signed in May 2023, careful negotiations with the three main subcontractors (frame, MEP and facade) and their suppliers allowed prices to be fixed. After a first terracotta supplier refused a fixed-price arrangement, a second was found, with the price tied to the exchange rate on an agreed date. “We picked a day on the downward trajectory,” Hucknall says.

For speed, the reinforced concreted (RC) core was slipformed, a decision made easier due to the fact that the only element of the core that will remain exposed sits within the stairwells. As we walk down the steps, Hucknall points to the rough walls, whose imperfections are amplified by the temporary lighting. “It does have character, doesn’t it? If you’re in here, it sort of feels a bit edgy,” he says.

That aesthetic approach will extend to the nine office floors, typically 52×47 metres, where concrete ceiling slabs will also remain exposed.

A grid pattern, caused by the joins between the 2.4×2 metre formwork boards, adds a further distinctive feel. To prevent things looking too messy, though, SRM agreed that the 18mm formwork ply wouldn’t be used more than twice. Fit-out work is minimal, restricted to lift lobbies, toilets, stair cores and reception.

One of the main structural innovations in 3 Chamberlain Square is the use of post-tensioned concrete in the slabs. The method allows for a leaner design, with 215mm-thick slabs stretched across a 7.5 metre column grid (565 columns in total across the building). Post-tensioning also minimized the steel content in the slabs by approximately 45-50 per cent, a critical factor in reducing the carbon impact, while maintaining structural integrity. Post-tensioning was used for all slabs, with the exception of the ground floor and basement, where traditional RC was used.

In a late change of plan, the technique was also used within a critical element of the frame. The original plan was for the frame to be made entirely of RC. However, after work had started on site, the team realised that a 3 metre cantilever supporting the levels above the first floor was going to pose a problem. Each corner was supported by three beams in an arrow shape, supported on a column. The congestion of reinforcement at the point where the beams met made  it impossible to build with the original design. “We had to switch to a post-tensioned solution to make the structure buildable,” Hucknall explains.

In total, 8,945m³ of concrete was poured across the substructure and frame, supported with 801 tonnes of rebar. The amount of concrete was minimised by honing the design to reduce the level of support needed to hold up the structure. “Through refinement of the foundations and due to the quality of the strata we are bearing on, we were able to make a judgement of 650 kilonewtons per square metre, compared to the 500 which would have been normal.”

Another challenge for the SRM team related to fire safety, with developers wanting ducts placed as high as possible towards the ceiling and to minimise floor-to ceiling heights. It would have been impossible to certify a solution involving traditional ducts in a wall gap with a deflection head at the top to allow for a 10mm slab movement. “We had to go to a damper that had a frame that is built to the soffit which allows the damper to float and ride between the two,” Hucknall explains.

On the exterior, the building’s distinctive vertical terracotta fins were created via a jigsaw of bespoke panels. The terracotta was fixed to unitised aluminium window frames and hoisted into place by an SRM spider crane on lower floors and, higher up, via the hoist. In between the gaps in the fins, windows are separated vertically by deep red spandrels made from 80 per cent recycled aluminium. At design stage, these were chosen to as a better-value alternative to terracotta, with lower embedded carbon.

The only hiccough came on the seventh floor, where the facade moves from vertical to sloped. Getting the corner units to align proved a “bugbear”, Hucknall says. “The terracotta has been designed in 3D modelling technology, and then it needs to be changed to 2D for cutting lists and so forth. There have been instances where [facade specialist Inasus] have tried to set it up in Spain, and they realised themselves prior to it coming out here. So we’ve never had an instance where it’s come to site and they couldn’t install it, but we have had instances where it’s impacted the programme slightly.”

Internally, the addition of the spandrels creates a noticeably different atmosphere to that experienced within the floor-to-ceiling glazed offices of recent years. “The glass-to-panel ratio is a bit different from most other offices,” says Hucknall. “Having less glazing, you have less solar gain, and so the cooling loads on the plant are much lower.” In addition, one-in-four windows will be openable, providing an alternative cooling method to air conditioning.

At ground floor level, polyester-powder-coated aluminium will be employed to create distinctive “ogees”, which rise from the ground to the cantilevered overhang. They look vaguely like tridents, with a column rising from the ground before splitting into three prongs as it rises to meet the overhanging second floor.

The building has split opinion among the city’s public. “No-one’s got a neutral view about it,” Alex Housden, development manager at client MEPC, says. “There’s a balance of positive reactions, but there are people who really love it… and those who don’t. That’s just the beauty of bold architecture.”

Wherever you stand on the design, the latest addition to Birmingham’s skyline will be hard to ignore. And it will stand testament to a mammoth effort by the SRM team.