Hammerson’s West Quay Watermark development in Southampton, built on the city’s old foreshore, has posed some unusual structural challenges.
Project: West Quay Watermark, Southampton
Contract value: £70m
Main contractor: Sir Robert McAlpine
Construction manager: Gleeds
Structural engineer: AKT II
M&E subcontractor: Hoare Lea
Steelwork subcontractor: Hollandia
Start date: January 2015
Completion date: December 2016
The Watermark is billed as a vibrant new destination in the heart of Southampton, a leisure-led development to complement the adjoining West Quay retail centre.
But this new Hammerson project has required an unusually complex structural solution thanks to its planning conditions, site location and occupier needs.
“There have been several constraints that have shaped the scheme,” explains Hammerson development manager Guy Wells. “From a planning perspective, we have to maintain a viewing corridor from Catchcold Tower in the historic part of the city through to the waterfront, and create a new plaza at the northern end of the site, which will also provide a ‘buffer zone’ between the new-build Watermark building and the old city walls.
“The site itself is 7 m below the West Quay shopping centre and the city centre, so dealing with this drop posed some interesting design and engineering challenges.
“And then there are the Watermark tenants. The top two floors house a Showcase cinema, and because of their space requirements we’ve had to cantilever these floors out over the building’s footprint by as much as 25 m.”
The 950,000 sq ft scheme is situated immediately south of Hammerson’s West Quay shopping centre, an 820,000 sq ft development, which was completed by Sir Robert McAlpine – also main contractor on the Watermark – in 2000. This was a key factor in the contractor’s appointment for the Watermark on a negotiated basis, Mr Wells says.
The pull factor
Hammerson hopes the new development will help West Quay become more of a destination, through catering and leisure attractions.
“The market has changed with the advent of online shopping,” Mr Wells says. “Shops now need more attractions to draw in footfall. Catering is just 3-4 per cent of floor space in West Quay. Which is why we are building the Watermark.”
Work began on site in January 2015, after reserved matters planning approval was secured in June 2014. When completed, the five-storey, 30 m-high development will have a 10-screen, 275,000 sq ft multiplex cinema over the top two floors, a bowling alley and up to 20 restaurants and cafes on the lower floors, plus the public plaza and a link bridge to the West Quay multi-storey car park.
It is the first – and biggest – of two phases in the Watermark development. Phase one is being built on a parallelogram-shaped site, hemmed into the north by West Quay, to the east by the city walls, and to the west by a busy road.
To the south is the phase two site. “We designed the bisection between the two sites so it would form the sight line from the old city to the waterfront,” Mr Wells says.
“The market has changed with the advent of online shopping. Shops now need more attractions to draw in footfall. Catering is just 3-4 per cent of floor space in West Quay”
Guy Wells, Hammerson
The brownfield site was part of the old foreshore of Southampton, and only reclaimed in the 1920s. A Pirelli cable factory sat on the site until the mid-1990s, since when it is stood vacant, with the title held by Southampton City Council.
These poor ground conditions, the dramatic 7 m drop from the surrounding city centre, and the proximity of the proposed development to the city walls, have determined the design for the foundations – which also had to satisfy Historic England (formerly English Heritage) and the local council.
“Historic England set the 20 m exclusion zone around the walls and required 12 months vibration monitoring on the walls prior to construction to establish a baseline of any existing seasonal movement,” Mr Wells says. “Fortunately there wasn’t any. But we then had to maintain that baseline during construction works and Historic England also required the use of CFA piling.”
Gerry O’Brien, design director with structural engineer AKT II, picks up the story: “Piled foundations were required to support the building due to poor ground conditions. This, and the marrying together of the proposed development with the surrounding ground levels, meant that concrete was necessary for the substructure and retaining walls.”
The development sits on 536 CFA piles, each one 600 mm in diameter and running to depths up to 23 m, and a suspended concrete ground-floor slab. The lower three floors, which will house the catering areas, have a reinforced concrete frame arranged on a 10 m by 10 m grid.
“These units require only moderate structural spans,” Mr O’Brien says. “Use of a concrete frame minimises the structural zones and provides greater flexibility for routing services.”
Showtime for AKT II
The concrete frame extends up to the underside of the cinema, a height of 18 m, at third floor level. Which is where another structural challenge begins.
“The Watermark has a great deal more complexity than would be seen in most other cinemas of this type, because the cinema needed to occupy a larger footprint than that of the building below,” Mr O’Brien says.
“This was due to the building at ground level having to be pulled back to avoid protected views of the city walls.
“So the cinema structure involves a cantilever which extends up to 25 m beyond the building envelope directly below. But given the strain that this would place on supporting structural elements, it was paramount that we make the structure of the top storeys as lightweight as possible. Therefore we proposed a steel frame.”
“Use of a concrete frame minimises the structural zones and provides greater flexibility for routing services”
Gerry O’Brien, AKT II
As far as possible, AKT II has designed the cinema grid to align with the 10 m by 10 m concrete frame grid below, though the auditoria require much greater spans than at lower levels, another reason why steel was the obvious choice for the structure of the upper floors. The longest span is 17 m.
The result of this engineering solution is a hybrid concrete and steel frame, which, as Mr O’Brien puts it, “exploits the differences of the two materials”. “If this was a greenfield site, the structural frame would have been entirely steel,” Mr Wells adds.
The steelwork has been designed to work in concert with the building’s four concrete cores, which extend up from the lower floors, and also act as service risers and house the lifts.
“Cast-in elements pass load from the upper steel frame to the lower concrete frame,” Mr O’Brien explains. “The structure consists of large trusses which occupy the wall zones between the auditoria. We have used the height of the auditoria to maximise structural depth, which allows us to limit deflections and increase stiffness and vibration performance.
“This approach allowed us to construct the steelwork with the minimum amount of propping, building out from the support locations and benefiting from the ability of the trusses to span or cantilever large distances.”
To plan construction of the cinema levels, AKT II worked closely with Sir Robert McAlpine and its steelwork contractor Hollandia. Some 1,395 tonnes of steelwork is being used, fabricated over a 20-week period.
Craning for a view
The erection on site is expected to take 16 weeks, and has been dictated by the lifting capacity of the cranes – the three tower cranes can only lift up to 8 tonnes so they have been supplemented with two mobile cranes with capacities of up to 20 tonnes.
“The original plan was to pre-assemble large trusses on the ground before lifting them – complete – in to place,” Mr O’Brien says. “However, we have had to align the steelwork erection with the available lifting capacity. The scale of this building is huge and the magnitude of the cantilever is more commonly found in bridge building.”
The cantilever is deepest at the south-west corner of the development, where an aperture will provide spectacular views across to Southampton Water. It features the largest steel span on the project – 37 m between the corner and the lift core – and the design anticipates this section will deflect the most.
“The scale of this building is huge and the magnitude of the cantilever is more commonly found in bridge building”
Gerry O’Brien, AKT II
“It has required a huge amount of propping, including a temporary brace while the main girders are installed, and the unpropping process will be complex,” Mr Wells says. “This corner will have to be nigh on 100 per cent complete before the props can be removed. Then it has to deflect, and only then can the concrete floors be laid. The temporary works wouldn’t have supported the weight.”
The exterior of the cinema will have what Mr Wells describes as “a floating form”, with tubular, stainless steel cladding which “echoes the cables Pirelli used to manufacture on the site”.
The lower floors will be glazed, with precast plinth elements. Ramped promenades wrap around the building, rising on a gentle incline to bridge the 7 m drop between the Watermark site and the city centre.
To the east, the plaza will include a stepped amphitheatre for events plus a water feature, backed by the old city walls. To the north-west, a new 45 m-long bridge will provide a link to the West Quay multi-storey car park.
The Watermark is scheduled to open by Q4 2016, with fit-out of the individual units being completed by the tenants. The second phase of the development – which has planning permission for a residential tower with up to 240 apartments, offices, additional restaurant and cafes, and one or more hotels with up to 250 rooms – is expected to be ready by the end of 2018.