BAM brings new heart to Sheffield

Faced with a challenging programme, BAM and the design team on Sheffield’s £70m Heart of the City II scheme have modularised key structural and services components to hit the project’s deadline. Will Mann reports.

Heart of the City II is one of Sheffield’s key regeneration projects, and phase one – an office and retail scheme – is arguably the most crucial. The anchor tenant is HSBC, which will move out of its nearby office when the lease expires next year. With the risk of a major local employer leaving the area, Sheffield City Council (SCC) saw an opportunity to rehouse the bank in a new office in its planned retail quarter.

But that meant a drop-dead delivery date for the construction programme, to allow HSBC to move into its new premises in mid-2019 on a 15-year lease.

“Programme has been absolutely key here,” says Nick Howdle, construction manager for BAM, main contractor on the £70.5m project. “SCC saw the project as high risk, so opted for a two-stage approach. We were initially appointed to a preconstruction services agreement in August 2016, working with the council and their design team, before starting on site in April 2017.”

The programme pressure – which increased with discovery of unexpected ground conditions early in the project – was one reason BAM chose to modularise the key building services components and precast the three structural cores.

The contractor devised these solutions with Leonard Design Architects – which worked for SCC in the preconstruction phase, then BAM through a design-and-build contract – plus key suppliers including M&E specialist NG Bailey and Creagh Concrete.

Phase one of Heart of the City II sits on an irregular footprint in the middle of Sheffield, and will comprise one main building, roughly triangular on plan and rising to five above-ground levels, and a smaller, single-storey F&B unit to the west, with the two connected via a capacious basement. New public realm space will sit between them.

Visualisation looking east towards the HSBC building with the single-storey F&B building to the left with green roof

There is an 8m fall across the site, and the split-level basement plus ground floor will accommodate retail and leisure units. Because of the slope, the first uniform level is the first of the five office floors. The gross internal floor area is 30,338 sq m.

The site previously included a hotel, dual carriageway and roundabout, which were demolished ahead of BAM’s start on site. The demolition and site clearance contractors were managed by SCC, with input from BAM. However, during BAM’s site investigation, unexpected problems under the ground emerged.

“Our foundation design was originally ground-bearing,” explains Howdle, “but then we discovered several bell pits beneath the ground, and we had to change our plans – just before the construction programme started.”

Bell pits are a legacy of Sheffield’s distant industrial past – a primitive early form of mining, which left bell-shaped voids a few metres beneath the surface. Their presence meant BAM had to introduce 140 rotary bored piles into the foundation design, mostly 1m diameter, driven to a maximum depth of 24.5m.

Modelling construction of the precast cores in BIM

“To give an idea how large these pits were, we had to pump in an extra 17 cu m of concrete into just one of the piles,” says Howdle. He adds that the demolition contractor also dug out and installed the piling mat in two weeks, so the overall programme was not set back.

The basement is a vast box constructed from in situ reinforced concrete floors and walls. In situ concrete was also planned for the three cores of the superstructure, but BAM was a little nervous about the programme.

“The cores had to be finished absolutely on schedule, so we could move straight on to the steelwork,” says Howdle. “So we decided to switch to precast – just 16 weeks before the units would have to be delivered.”

The contractor visited Creagh Concrete in Northern Ireland to explain the challenge. “We wanted to look at the factory and check the materials were right as this was such a programme-critical item,” says Howdle. “It was tight, but credit to Creagh, they delivered.”

Creagh also digitally modelled the precast units, including cast-in plates for the steelwork connections. “Severfield, our steel fabricator, had to be sure their connections would fit so were also part of these discussions,” says Howdle.

Mast climbing work platforms were used for the facade installation

The 34.5m-tall cores each comprise 17 precast sections, the largest 8m by 4.6m across and 2m high, and weighing up to 26 tonnes. The first was installed in just eight days. “It was a big help on such a tight programme,” says Howdle.

Some 2,500 tonnes of steelwork are being used. The biggest beam, weighing 18 tonnes, spans between the two buildings in the basement, carrying the public realm space above. “The rest are fairly standard castellated beams,” says Howdle. “Their design was coordinated with the M&E in the BIM model so the service runs can run through the castellated beams.”

The steel frame grid is largely 7.5m by 7.5m, allowing for the building’s triangular shape and the large, 350 sq m atrium, which runs the full building height. The facade is curtain-walled, with a rainscreen using four different terracotta finishes plus glazing to create a mosaic effect.

Design and installation was coordinated closely with Severfield using Navisworks for clash detection, explains Leonard associate James Dealtry-Smith: “A typical clash would be an end plate where two steel beams connect, and M20 bolts would be projecting out, where the mullion runs up the building. So, using the BIM model, Severfield would reengineer the connection so that the curtain walling could fly through.”

With clash detection, adds Howdle, it can be tricky deciding which company changes its design first. “In this scenario, it’s logical for the steelwork contractor to go first as it is further on in the programme than the curtain walling,” he says.

The facade units were installed using mast-climbing work platforms (MCWPs). “The tie positions had to be modelled in BIM, so the MCWP provider supplied 2D drawings to Henshaw who added them to the 3D model,” says Dealtry-Smith.

The glass installation has been driven by safety. Some 8,149 sq m of glazing has been used, with 600 terracotta panels, and the largest units 3m by 2.2m.

“As an industry, we don’t have a great safety record on handling glass, so we have tried something different here,” explains Howdle. “Using the tower crane, we lift a two-tonne stillage of glass on to the MCWP platform, where we have a runway beam at the top and a 500kg Minifor winch with Oktopus vacuum attachment. For the operatives, the only manual handling is attaching the Oktopus attachment to the glass, lifting it off the stillage and fitting it onto the facade. It also saves on tower crane hook time.”

The £15m M&E package includes the project’s standout offsite innovation. “We decided to get as many of the service runs, as well as the plant rooms, manufactured offsite as possible,” says Dealtry-Smith. A visit to NG Bailey’s Bradford factory showed what was feasible.

Floor plate modules, service risers, riser caps and plant rooms have all been manufactured offsite. The largest plant room measures 7.6m by 5.8m by 3.7m, weighs 21 tonnes, and was split into two units for transport. There are five complete riser caps, made up of 11 sections, the largest 9.5m by 3.5m x 3.5m.

Though generally happy with the offsite approach, Howdle says: “There are some tolerance issues and we will carry out an independent air test on zones where two offsite-manufactured components interface, for example, where a riser butts up against a precast unit. That wouldn’t have been an issue building in situ, but the tolerances are harder to achieve with different components manufactured in different geographic locations coming together on site.”

BAM is keeping a close eye on quality concerns through two M&E managers – and keeping a close eye on them are two clerks of works employed by project manager Turner & Townsend. Building control is handled by the local authority.

The city-centre location has meant operational constraints as well as public safety considerations. “Just 1.3m separates the face of the building from the face of the kerb on Furnival Gate [the south end of the site],” explains Howdle. “Here, we erected a temporary scaffold structure to protect the road and also used this as support for three of the MCWPs. Additionally we used nets for further protection during the steelwork erection. Happily the works have passed without incident.”

Future operational requirements have also been considered. “The client was originally planning to scaffold the whole atrium for maintenance, which has wear-and-tear implications, so we suggested using MEWPs instead, which they agreed with,” says Howdle. “So we have sized the doorways and lifts to take a spider MEWP.”

Provision was made for MEWP access for external window cleaning, instead of a window cleaner system, says Dealtry-Smith: “We tracked MEWPs around the building using the BIM model – the MEWPs are available in the NBS BIM library – to prove that access was always possible and altered the external landscaping in cases where there were bollards close to the building, for example.”

The Cat B fit out for HSBC will be handled by Styles & Wood, with BAM to hand over the building on 10 January.

The project, where the peak workforce was 304, is on schedule, says Howdle. “Working with an educated client and design team has been a big help,” he adds.