Harris Partnership and Renaissance deliver a transport hub with two acres of green space on the roof
The Greater Manchester town of Stockport has a new centre of gravity. From above, the £140m Stockport Interchange looks like a void, cut out from the surrounding collar of green space. In reality, it’s more of a vortex. Bus, pedestrian and cycle routes all revolve around this mysterious hole in the ground, drawing together disparate elements of the town, including the station and main shopping centre.
Designed by architect The Harris Partnership and structural engineer Renaissance, the Interchange is a mainly single-storey bus station, with a public entrance and passenger concourse at one end, and a vehicular entrance and back-of-house spaces at the other. Between the two is the large oval-shaped opening, known as the oculus, where the buses turn and park.
What makes the building intriguing is that the roof doubles as a 2-acre park. This is level with the surrounding urban realm on two sides, giving the impression that the oculus is sunken, or cut out like the top of a tissue box. On the other two sides, the park appears elevated, looking over the River Mersey and across to the town’s famous railway viaduct. A cycle path wraps around the roof, spinning off onto a ramp suspended over the water and, on the southern side, via a landscaped bridge to the nearby train station.
The concrete structure has to support all of this activity while also carving out plenty of room for buses to manoeuvre. Radial beams fan out at 6m centres from the edge of the oculus to the building perimeter. These are 1,050mm wide, up to 2.2m deep, and span as much as 22m between two rings of 750mm-diameter circular columns. Above this sits a 250mm-deep slab with a flat soffit, which extends 1.2m beyond the inner ring of columns. Here, it is stiffened with a 2.5m-high ring beam, framing the oculus in a loop of pale fair-faced concrete.
The park above combines grass areas with play zones, spacious paths and cycle lanes. There are 95 trees, large boulders for climbing, seating areas, and rain gardens designed to reduce runoff and boost biodiversity.
The build-up for the landscaping starts with a 300mm layer of polystyrene void formers on top of the slab. Spaces are left for the tree pits, which are up to 1m deep. This is followed by a profiled layer of lightweight recycled aggregate, up to 450mm deep, which creates the contours of the park. Finally, a 300mm layer of topsoil is added as a base for lawns, paths and flowerbeds.
Renaissance worked closely with landscape designer Gillespies to make sure the structure was as efficient as possible, and tailored to very localised loads. “We weren’t assuming the worst case for everything,” says Antony Dewar, principal engineer at Renaissance. “If there was a path, that’s what we designed for. If there was a tree pit, we designed for that.”
When Dewar’s team joined at design stage 3, the upfront embodied carbon of the baseline structure was about 4,000tCO2e. They cut it by 10% by rationalising the concrete grillage and removing a secondary ring of circumferential beams. This reduced the meterage of beams in each bay by half, as well as the amount of formwork needed. The optimised design saved 853 tonnes of concrete in the superstructure and 1,889 tonnes in the foundations.
Renaissance and concrete contractor Mayo Civils also increased the proportion of GGBS cement replacement in the concrete from a 25% baseline to an average of 60%. This lowered upfront embodied carbon by a further 16% to a final figure of 3,250tCO2e.
The high proportions of GGBS improved the structure’s resistance to freeze-thaw effects and de-icing salts – critical in a bus station. It also controlled shrinkage. Because of the large spans involved, the engineers were concerned that any movement could exert a huge strain on the slab. As a result, they split the structure into four independently stable quadrants separated by 60mm movement joints, and increased the GGBS to 66% to slow the curing process.
Even in the bus lanes, care has been taken over detailing and finishes. The circular columns are topped with a 150mm-deep indented shadow gap to negotiate the transition from the 750mm-diameter columns to the 1,050mm-wide beams. “It just makes the connection a bit more delicate,” says Dewar.
Likewise, in the back-of-house area, a double-height wall has been cast with no visible joints. “We worked very closely with Mayo on the construction sequence, making sure that the reinforcement details aligned with how they wanted to build it.” Timber formwork has been used throughout, helping to creating elegant curves in both the central oculus and the interiors.
The Interchange is the flagship infrastructure project in the wider £1bn regeneration of Stockport. It is hoped that schemes including the £145m Stockport Exchange business district and the £60m Weir Mill development will revitalise the town centre, introducing new neighbourhoods, social and economic opportunities.
Aerial photos of the Interchange tell a story of Stockport’s past. To the north and west, the Mersey and the huge brick viaduct are reminders of the town’s industrial heyday. To the east, the A6 roars past, slicing the town centre in two. Sitting between these main arteries, the Interchange suggests an alternative future – of integrated transport, active travel and walkable neighbourhoods. It is less a hole in the ground, more a new heart.