SKY HIGH IN BIRMINGHAM

The Second City skyline will soon have another skyscraper. Will Mann meets the Wates project team building the 102m tall residential tower at The Bank on Broad Street.

Wates is currently constructing Birmingham’s newest skyscraper – but it nearly wasn’t a skyscraper.

The 33-storey tower is the second the contractor is building at The Bank on Broad Street – though it was originally planned to be two levels lower.

“During our early discussions with Regal, we mentioned that if another couple of floors were added, the building would pass the 100m mark and be classed as a skyscraper,” explains Wates project director David Invernizzi.

• Client Regal
• Main contractor Wates
• Architect Glancy Nicholls
• Structural engineer D2E
• Contract JCT 2011
• Value (Tower 1) £21m
• Value (Tower 2) £36.5m
• Programme (Tower 2) 128 weeks (started July 2017)
• Subcontractors:
• Piling Keller
• Concrete frame MPB
• Cladding Longworth
• Curtain walling Apic
• SFS and partitions Isec
• Electrical Barrie Beard
• Mechanical JS Wright

“The client thought this over, and obviously worked out that the business case would stack up, so went back to the planning authorities and applied for another two storeys. In the meantime, we were instructed to allow for 33 storeys in our foundation design. It was a risk, because if the planners had said ‘no’, the foundations would have been overdesigned. But the planners gave the green light.”

At 102m high, the 217-apartment tower will be the latest in a spate of tall buildings planned for the Second City, which is feeling the bounce from HS2 and major financial institutions like HSBC moving offices up from London. It is also the second project for Wates on the same site.

In July, the contractor completed a 21-floor neighbouring tower, valued at £21m and housing 189 apartments. It was awarded the £36.5m second tower, a negotiated deal, in June 2017. Invernizzi has been project director for both.

As land values rise in central Birmingham, sites are becoming tighter, and this one is no exception. A traffic island 550 sq m in area, there are public interfaces on every side and a listed building on the southern corner serving as Wates’ site office. The two towers are just 7m apart, and with the first handed over and now occupied, there is only 6m to spare around the second tower.

For Invernizzi, this has meant a complex logistics operation, with deliveries planned weeks in advance and specialist contractors working on completed floor plates in the absence of much space at ground level.

Additionally, many trades are overlapping so Wates can deliver a “keen programme”, as the project director puts it, one of many cost savings identified in pre-contract discussions with Regal. The contractor was also able to take its experience – and 90% of the supply chain – from the first tower across to the second, which uses largely similar construction methods.

Wates started work on the first tower in July 2016. “We were in conversation with the client for over a year beforehand to bring the cost within the client’s affordability envelope,” explains Invernizzi. “Other contractors pulled out, but we were asked to do a value engineering exercise, working with the architect Glancy Nicholls and structural engineer D2E, who were subsequently novated to us.”

The site is a tight traffic island

The site was formerly occupied by a bank – hence the scheme’s name – and the vaults are still in place, currently serving as the site canteen. Surveys showed basements from previous buildings, a 30m-deep well and a Virgin Media distribution box which supplies broadband to central Birmingham.

“We had to cap and pressure grout the well and move a couple of central piles to bridge the well and the corner piles where the Virgin box was located,” explains Invernizzi.

The second tower’s foundations involve a 1.1m-thick raft slab and 88 CFA piles, 750mm diameter – slightly wider than the 600m diameter piles used on the shorter Tower 1 – and driven to an average depth of 18m, and to 20m below the core. Some 893 cu m of concrete was used in the foundations.

The value engineering savings, for both towers, chiefly involved the frame and facade. The slabs were reduced to just 205mm thick with post-tensioned cables to stop deflection. “It would be difficult to get them much thinner on a high-rise,” says Invernizzi. “However, the reduction in concrete and steel rebar meant significant value engineering savings.” On Tower 1 alone, this totalled £900,000.

Facade glazing is installed from within the building

Both towers have reinforced concrete frames. Tower 2 has a slightly larger core, using just over 2,000 cu m of concrete and 211 tonnes of rebar. The frame uses shear walls up to level 10 to strengthen the central core, with levels above having a more standardised layout with columns spanning a maximum of 8.9m.

Retail units will occupy the ground floor of the second tower, where the floor height is 6m to the underside of the first floor slab. Otherwise, the storeys are 2.85m from slab to slab, except for the two extra levels at the top, where the floor height is 3m to allow for “penthouse” style apartments.

The single cores on each tower, which contain two lifts, a staircase and the main M&E risers, have been constructed using full-storey-height slipform rigs. The concrete is poured into the rig, which then moves up a level, and then the slab construction follows behind. The only precast element is the stairs.

“As there is limited space at ground level, the entire frame construction has had to operate off floor plates, moving upwards as the structure advances,” explains Invernizzi. “So the frame contractor occupies the four or five plates below the top of the build, where they store materials and assemble rebar and other prefabricated elements.

“Then these materials and elements are wheeled out onto a CantiDeck loading platform, which projects off the face of the building by 5m, and lifted by the tower crane up to the top of the structure.”

For the envelope construction, the first tower used a punched window detail, but Tower 2 has a curtain walling system. “Punched windows would not have worked because of the wind loading on the higher levels,” says Invernizzi. “There are similar cladding details, but additional fixings to cope with the wind loads. This was modelled using a wind tunnel.

“Also, the original design of the cladding detailing used a secret fixing, but we have reverted to face-fixed detailing, which has saved hundreds of thousands of pounds.”

The facade comprises glazing plus rainscreen cladding – a brass and tortoiseshell effect – with Rockwool Duo Slab insulation, and a Siniat weather defence board, fixed back to a lightweight steel framing system. “We were halfway through constructing Tower 1  when the Grenfell fire happened, and we went through numerous external consultations to validate our facade design,” says Invernizzi.

The facade is constructed in situ, with mast climbing work platforms (MCWPs) used for facade build-up and the glazing then installed from inside the buildings.

The facade comprises glazing plus rainscreen cladding – a brass and tortoiseshell effect – with Rockwool Duo Slab insulation, and a Siniat weather defence board

“We use a Glassboy floor crane, with hydraulic lifting arm and vacuum cups to lift up the glass,” says Invernizzi. “The heaviest glazing units weigh 150kg. The external panels are relatively lightweight – the heaviest is around 25kg – so these are handled manually, but all workers on the MCWPs use tool and material tethering.”

There are eight MCWPs on Tower 2, supported by scaffold structures at ground level which also protect pedestrian walkways. “We are using two on each elevation, because when we have to fit the curtain walling at the corners, it involves two MCWPs, which stop other trades from working.” says Invernizzi.

Invernizzi says Wates would have opted for a unitised facade system for a tower above 33 storeys. “That comes with a higher price though,” he adds, “but it does mitigate the risk of downtime from poor weather that we have with insitu construction.”

The lean programme is targeting 18 weeks per floor plate on Tower 2, where there are seven apartments per floor (there are nine on Tower 1, reflecting its slightly higher spec).

“We release the envelope in bands of five to the facade contractors,” explains Invernizzi. “These are then completed and waterproofed, at which point the facade contractors move up to the next band of five and the fit out starts. And so on up the building.”

“Obviously if we have an issue with the frame or the facade – and we have had delays due to weather, notably during the ‘Beast from the East’ in March – then it can impact all the trades following behind. And the further up the building you get, the higher the risk of bad weather delays as the winds are higher and the temperature colder.”

When the hoist comes out and then the tower crane, two more MCWPs will be installed in their place to accelerate the final stages of the programme. “Obviously we cannot install the curtain walling and fit out the apartments where the hoist stands, so those areas have been temporarily waterproofed to stop rain getting in,” says Invernizzi.

The apartments are serviced by gas and electric – only the retail space has gas – and both towers have individual sub stations. The plant is housed mostly on the roof along with the sprinkler tank. For speed, the M&E first fix is proceeding in front of the internal wall partitions. The high-spec fit out is under way with the show home nearly finished.

Wates is “on schedule” for completion in autumn 2019, says Invernizzi. Logistical challenges have included closing Sheepcote Street on the site’s northern edge for a weekend to allow erection of the tower crane using a 500 tonne mobile crane – and organising vehicle movements around the utility works for the tram extension along Broad Street.

“The tram extension is a bonus for the client – an extra selling point for the apartments,” says Invernizzi.