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Willmott Dixon’s Plymouth pilgrimage

9 January 2020 | By Will Mann

The cantilevered ‘floating box’

A new cultural centre under construction in Plymouth will mark the 400th anniversary of the Mayflower carrying the pilgrim fathers to America. Will Mann reports.

All eyes will be on Plymouth in 2020, as the Devon city marks the 400th anniversary of the Mayflower departure, when the pilgrim fathers sailed for America.

The ‘flagship’ of the celebrations will be The Box: a new 3,500 sq m cultural centre which is being created by revamping the existing City Museum and Art Gallery on Tavistock Place, along with St Luke’s Church opposite, and creation of a public piazza in between.

Willmott Dixon is main contractor for the £23m scheme. Challenges includes extensive heritage works on the Edwardian-era museum and 19th century church, plus upskilling of the local supply chain on a project where 79% of the spend comes from within the PL postcode.

Project factfile

Client: Plymouth City Council
Main contractor: Willmott Dixon
Value: £23m
Contract form: JCT traditional with bill of quantities

Programme:
August 2016: Preconstruction starts
June 2017: Main construction contract starts
December 2019: Main contract completed
Spring 2020: Completion of fit out

Heritage contractors:
Masonry: Westcountry Stonemasons
Drywall and acoustics: DDF
Stained glass windows: Glasswood
Lime plaster: MPC
Plaster mouldings: FH Crocker

The most striking visual feature is the new-build extension to the museum, particularly the 8.5m cantilever on the upper two storeys – a ‘floating box’ 10m above ground level, clad in stainless steel and aluminium panels. This 1,000 sq m building, standing 18m tall, is arranged over five split levels plus a basement and, unusually for a museum, its archive will be on the top floor.

“The location of the archive is key to the project and how it has been built,” explains Kristian Cartwright, Willmott Dixon project manager. “Museum archives are normally in basements or off site completely, but Plymouth City Council have an incredible historical archive, and wanted it to be visible at the heart of the new cultural centre.

“To achieve the right climatic conditions, we’ve had to build the archive as a 300mm-thick concrete box, and so the whole new-build extension is a reinforced concrete frame structure. Building this box has shaped our construction methodology.”

Cartwright began working on the project three years ago, at the start of a lengthy enabling works phase. Willmott Dixon was appointed through the SCF framework to the first stage tender in August 2016, signing a preconstruction agreement, and was then awarded the contract in June 2017.

The enabling works included considerable groundworks and demolition challenges [see box], and this phase eventually rolled over into the main contract.

The contract agreement includes four sectional handovers. The first two of these – one for the existing museum building, the other for the church – have centred on repairs to the ageing structures and heritage works (see box, p27).

Section three is the extension. The reinforced concrete frame structure sits on a roughly 10m grid, with beefy 600mm square concrete columns, to support the archive level. There are two cores, containing stairs and lift shafts, with shear walls which run all the way up to the roof.

“There are strict performance requirements for the archive,” says Cartwright. “The 300mm thick concrete walls, soffit and floor give a four-hour fire rating and an airtightness rating of less than 0.05 [cu m/hour/sq m].”

A new public piazza will be created

He notes that steel would not have been an option for the structural material: “It wouldn’t have given the fire integrity or airtightness.”

The cantilever extends to 8.5m at its maximum depth, tapering back into the building over 20m of the eastern elevation, facing the pedestrianised Tavistock Place. The cantilever also wraps around the building’s southern elevation, where it extends to 2.5m.

“The concrete in the cantilevered sections required higher than usual rebar,” explains Cartwright.

Although the concrete sections have been painted white, the designers were anxious to achieve a consistent finish.

“We used a special, self-compacting mix, with a 10mm aggregate to avoid day joints,” Cartwright says. “Peri’s plywood formwork was used, in sections up to 7.5m long, to ensure a fair-faced finish. Our concrete contractors Stephensons carried out the pour in a single day.”

Some 2,614 cu m of concrete – 6,273 tonnes – were used for the concrete structure.

Building on a bomb site

The legacy of Plymouth’s wartime bombing and post-war redevelopment complicated The Box’s enabling works phase.

“The post-war museum extension, rebuilt after the bombing, was constructed straight on top of bombed material, which at the time was classed as granular fill,” explains Cartwright. “So we had to put in temporary piling to create a backwall, which effectively acts as permanent formwork.

“The same work was required in the church, where all columns sat on the ground, so we installed pad foundations.”

The Box sits on shillet, a hard local stone, so the Odex piling system was used. “This piling design is popular in Devon due to the ground conditions,” says Cartwright. “It comprises a steel sheath, hammered into the ground, with every fourth pile an anchor pile.

“Caps and beams were added on top of the beams around the perimeter of the new build extension.

“We also changed the base of the tower crane design to suit the ground conditions, using Odex piles in two clusters of eight on one side, with a ground bearing slab on the other side.”

Demolition of the post-war extension also involved considerable asbestos removal, while an 18th century culvert, which was discovered under the site, required remedial works.

To the eye, the new build joins on to the existing 1905 museum seamlessly – both internally and externally – but it is structurally independent.

“At the north end of the new build, there is a 5m cantilever which takes each floor up to the wall of the existing library, and soft finishes then tie the two together,” explains Cartwright. Original windows from the old southern elevation of the museum have been retained, now providing a view through to the new extension.

For the cladding on the upper floors of the extension, Kingspan’s QuadCore insulated panel system has been used. Three-quarters of these are aluminium panels, the remainder mirrored Rimex stainless-steel panels, each measuring approximately 600mm by 600mm. The 2,500 panels, with four different finishes, have been arranged to represent the many pages of the archive inside.

“The panels fix on to mullions on the QuadCore system, installed manually using cherry pickers,” says Cartwright.

The lower levels of the facade feature 340 sq m (82 tonnes) of locally quarried Plymouth limestone, giving a marble effect, plus glazing where the three-storey atrium faces the new piazza. Here, the glazing panels are supported by horizontal steel beams which span between the vertical concrete columns.

The flat roof uses a polyroof system and is also where most of the M&E plant is located. Inside, all the structural columns and walls have been painted white, giving a clean, airy feel to the interior. Looking up, the archive floor is clearly visible through rooflights on level four, as intended by the client. The basement has a chiller and freezer for storage of film.

Some internal fixings have been required for the exhibits, including steel hanging points in the atrium ceiling for ship figureheads.

“These are attached to the concrete slab above through the 600mm services void in the ceiling,” says Cartwright. “The largest, King Billy, is 5m tall – it was the figurehead for 19th century warship HMS Royal William.”

The interior fixtures and finishings include frameless glass balustrades and polished concrete floors and Cartwright says quality in the detailing has been a top priority.

“At the start of the project, we had to run thorough quality training to upskill all our suppliers on our standards and protocols, devoting the same amount of time we’d spend on health and safety,” he explains. “Where necessary we have put suppliers on our own internal management courses.

Above: Whitewashed columns and walls give an airy feel to the interior Below: The archive is visible through rooflights on level four.

“The client’s mantra for the project is ‘by Plymouth for Plymouth’, but there is a limited trade and labour pool down here, so it was harder finding suppliers who could work to our usual standards.

“We could have gone around the country finding specialists with heritage skills, but we wanted to enrich the local supply chain, upskilling where necessary.”

Cartwright adds that Willmott Dixon has provided 843 weeks of onsite training for 35 work experience placements and 28 apprentices.

Willmott Dixon’s contract completed in December. Final works included the new piazza, with a further 19 tonnes of local limestone used for planters. Installation of exhibits in the new extension by the client’s fit-out contractor will follow, a process already underway in the Edwardian-era museum and the church. These include a scale model of the Mayflower itself.

The archive, which houses documents such as Sir Francis Drake’s privateer commission from Queen Elizabeth I, will be open for public tours monthly. The Box is scheduled to open in spring 2020.

Aluminium and mirrored stainless steel panels make up the cladding

Conserving heritage

The Box project has involved extensive heritage works to the Edwardian museum building and St Luke’s Church.

Heritage works to the existing museum and St Luke’s Church have been a key part of The Box project.

The post-war extension was demolished, but the Edwardian facade of the museum has been retained, which meant erection of a temporary works structure over a section of about 10m.

“Because of the tight site constraints, this had to be integrated into the scaffolding system – a structure of I-beams and scaffold poles – and allow space for the demolition behind it,” explains Cartwright.

“A problem was where to thread the I-beams, as there were only ‘tax windows’ on this elevation, and we had to avoid aesthetically important masonry such as the key stones. We had to select stones that were not such an important heritage consideration and drilled through these for the I-beams.”

The glass roof of the original library had to be removed and reconstructed with structural steel, and a temporary roof was constructed to protect features such as the wood panelling in the interior.

“We had to build up a birdcage scaffold from ground level, up 18m to where the steelwork starts for the new roof, back propped through every floor,” explains Cartwright.

The restored Edwardian interior

Conservation work on the roof included new lead parapet capping, relining of gutters, fitting bird nesting deterrents and repointing of the stonework with lime mortar. “Cementitious material had been used in previous repairs,” says Cartwright.

Inside, the M&E systems had been externally mounted, causing damage to the decorative plaster and flooring.

“The plaster mouldings were assessed by a conservation surveyor, and where possible, kept in situ,” says Cartwright. “Some mouldings had rotted completely and these were replaced, moulded by a local company.”

New services installations have been fitted, partially concealed through use of voids. The parquet and terrazzo floors were sanded back and resealed.

The interior walls were covered in a stretched hessian canvas. This was removed along with the damaged tongue-and-groove batons and timber plugs which held it in place. The walls were skimmed, with new batons and 4,000 proprietary anchor fixings used to fix the hessian back to the masonry.

“The plaster mouldings were assessed by a conservation surveyor, and where possible, kept in situ. Some mouldings had rotted completely and these were replaced, moulded by a local company.”

Kristian Cartwright, Willmott Dixon

The church, which dates from 1829 but was deconsecrated in the 1960s, also required a temporary works structure during the roof repairs.“This was another large scaffold bird cage, which allowed us to remove the timber and plasterboard ceiling – added during past repairs – and install a new steel frame to support the existing trusses,” explains Cartwright. “Extra steel trusses were fitted for new art installations.”

A mezzanine floor was added, on top of the existing cast iron columns.

“To support the extra loading, we constructed new pad foundations under each column (there were none before), fitted a narrow steel reinforcement cage inside the hollow columns, then filled up each column with grout,” says Cartwright. “This allowed the mezzanine load to go directly down into foundations.”

During the structural work, ceiling and timber mouldings were removed, logged, and stored on site, to be re-installed on completion of the refurbishment works. The stained glass windows were also taken out to be repaired and re-leaded.

“Externally, we hacked off and re-rendered the principal facade – the decorating mouldings had to be replaced,” adds Cartwright. “Again, cementitious material had been used in past repairs, causing condensation problems. This was replaced with a lime mortar which will allow the facade to breathe.”

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