On a congested site on the edge of the City of London contractor Gilbert-Ash is building the new headquarters for the Royal College of Pathologists.
It’s fair to say that contractor Gilbert-Ash has earned itself a reputation for delivering high-end architectural one-off projects. It has a RIBA Stirling Prize-winning project under its belt as well as a clutch of nominations.
But could its latest scheme – the construction of a new headquarters for the Royal College of Pathologists (RCoP), just a stone’s throw from the Square Mile in the City of London – be another to add to its portfolio of award-winning projects?
One thing’s for sure: the project team has its work cut out.
Delivering a complex and architecturally detailed scheme on such a small site means contractors are working on top of one another, burdening the construction process with various inefficiencies compared with less cramped projects.
“We can’t work in the way you would normally expect,” says Gilbert-Ash project director John Davis. “For example, there can’t be any drilling through concrete for services. All the conduits must be cast in. We had the M&E team in while we were concreting.”
The building will be seven storeys in total with a basement providing catering facilities for the offices and conference suites on the ground floor through to level six. Plant rooms and further office and accommodation space are located on the seventh floor. It is only at this level that the building’s cast in-situ reinforced concrete frame gives way to the structural steel that forms the frame for the roof, plant room and office. It is a landmark building for RCoP. “As the headquarters for the Royal College of Pathologists there are meeting rooms and conference facilities for its members as well as offices for its own staff,” Mr Davis says. “There is also office space that it plans to let out to others. It is a very flexible building.”
Ultimate recycling act
The Gilbert-Ash team moved onto site in February 2017 with demolition work already carried out to basement slab level. Unusually, instead of breaking out the concrete and installing new foundations, the team has
retained this existing basement slab and has launched the new frame from it.
It is the ultimate act of recycling. “The raft is large,” Mr Davis says. “It is 2-2.5 m thick, covering the entire site footprint. It was nice to work from it. It meant we didn’t have to go into the ground.”
Input from architect Bennetts Associates on the project is obvious across the project, and the confidence that client RCoP has in its work is just is clear.
That confidence runs through the site team and the quality of the detailing in the stairwells and internal concrete faced walls. Where the sawn timber board marked walls tie in exactly with the adjoining courses of bricks is of particular testament to the expertise of the concreting team.
To create the imprint, the team nailed 75 mm sawn boards to the standard Peri formwork systems it used. Even the board nails and sawn joints were painstakingly lined up.
The result is a superb example of reinforced concrete architecture. The team used London Concrete to supply its material and visited the batching plant to make sure its staff understood the importance of using the same aggregate source throughout the project.
“We needed to make sure that everything was exact, precise and that the aggregates were thoroughly washed and cleaned,” Mr Davis explains. “We explained that the concrete would be exposed and that there couldn’t be any staining.”
Springing straight from the slab meant the team had to be careful when breaking out the slab for the lift pits, of which there are three – two passenger and one goods lift. Checking the location of the existing reinforcement bar in the slab was vital, particularly when tying in the starter bars for the columns, which are on a 5 m grid in the basement. Above basement level, the building becomes more open-plan, largely thanks to the design of the reinforced concrete coffered beams that form the floor slabs and span lengths of up to 13 m.
Essentially the system is an elongated waffle slab with 1 m-wide beams and void formers used to create the dimple. The slab is 150 mm thick at the coffer, thickening to 800 mm deep at the beam.
It is this coffered design that enables the team to install such wide-spanning reinforced concrete beams; the void formers grant a reduction in overall weight of the beam without any loss in structural performance.
“It did make the casting process complicated,” Mr Davis says. “The void formers are like polypropylene boat hulls. They had to be located and fixed into position before the reinforcing steel bars are placed. The service openings and ducts were then set out and fixed to the bars before we could finally start casting.”
Unlike a London environment
With exposed concrete soffits the quality of the workmanship is pivotal and the team from west London-based specialist Oliver Connell & Son has not disappointed.
Even with pours of more than 200 m3, the casting is so crisp and clean it looks at first glance like it was carried out in the controlled environment of a precasting yard, instead of the challenging conditions of a London building site.
The building’s walls are a mixture of exposed concrete and brickwork. The concrete walls feature a sawn timber marking finish formed by the 75 mm-thick boards that line the formwork. These tie in exactly with the brick courses where the two facings meet.
“It is important that we ensure that everyone involved in the project understands the exacting standards and the quality of workmanship that we expect here,” Mr Davis says.
The distinctive red / blue / grey bricks form piers on the building’s front and rear elevation and are bedded using lime mortar.
Precast concrete spandrels sit on the brickwork piers on the front Alie Street elevation, while those on North Tenter Street at the rear are fixed back to the concrete frame.
The Alie Street elevation features 2 m x 5 m single sheet glazed windows which weigh in at 480 kg.
Inside the building there is a bespoke blackened steel feature staircase that runs in three sections from ground to first-floor level, first to second-floor and then third to fourth-floor.
Fabricated by Hull-based specialist CSI, the team started its installation in January 2018 and worked rapidly to get the building as weatherproof as possible.
“It is a mild steel staircase which we couldn’t install without the building being substantially weathertight otherwise it would rust,” Mr Davis says. “It arrived in sections but even so was difficult to manoeuvre into position and bolt and weld on site.”
The building is due to be handed over in October 2018, which Mr Davis admits will be tight.
But good things come to those who wait.
Steel caps things off
The top floor is a structural steel frame pavilion, which is set back from the front line of the building and clad in zinc. Here the steel was lifted into position in small sections thanks to the three-tonne capacity of the tower crane that served the site.
“It is quite a complex steel structure. There are lots of cantilevers and trusses. It is quite a lightweight structure but it is working hard,” Mr Davis says.
The crane itself was worked hard during the time it was in place. With its steel ballast bolted down to the basement slab, the tower has been busy moving all materials onto site from the two car-parking bay suspensions that provide the project’s delivery space. Only the concrete which was pumped into position missed out on the tower crane.
“Management of materials and deliveries has been very important,” Mr Davis says.
“The only storage is the working space itself, so it can get quite congested on there. Alie Street is a one-way street and we only had enough room to bring in one lorry at a time. Although North Tenter Street at the rear elevation is two-way, it is too narrow and there is a primary school so the parking suspension on Alie Street was the only real option we had.”