Structures | Maggie’s Centre, Bart’s

Bart’s hospital first opened its doors in the 12^th Century, and has occupied the same site ever since. Much of its impressive architecture is Grade 1 listed.Some dates back to medieval times and is classified as a Scheduled Monument. So slotting in an unashamedly avant garde building into such sensitive surroundings was always going to be a major challenge for designers and contractors.

Replacing an unremarkable brick clad 1960s building tacked onto one end of the hospital’s 18^th Century Great Hall is the latest in a long series of Maggie’s Centres. These offer free practical, emotional and social support to cancer sufferers and their families, usually in iconic buildings designed by leading architects. Virtually all of these are low rise: at Bart’s the restricted footprint of the Maggie’s Centre site required the architect to go for a three storey, plus single storey basement design.

On such a historic site it might be expected that basement and foundation design could be dictated by unpredictable discoveries below ground. Structural engineer Arup associate Derek Roberts, however, says there were no major obstructions.

“A 10 week archaeological investigation only revealed a Roman defensive ditch running diagonally across the site, and seven skeletons from the Roman period.

“Most of what was uncovered was the remains of Victorian cellars and medieval foundations.”

Of more immediate concern was the early 20^th Century service duct that crosses the site just outside the new building’s footprint. Proximity to one wing of the Georgian hospital buildings also posed an unusual challenge, Roberts adds.

“The building close-by houses a high-tech machine called a gamma knife, which is used to treat brain cancer. As a result, we had always to minimise vibration, particularly when driving the sheet pile basement walls.”

A Giken “press-in” piling rig was the solution to this particular challenge. The fully tanked basement is just above the water table, while the 500mm thick raft foundation sits on the interface between the underlying sands and clays.

Logistics were perhaps the biggest headache for the site team. “Everything has to come in through the Henry VIII gate, then take a sharp left around the church of Saint Bartholemew the Less, which was built in the 15^th Century,” explains Sir Robert McAlpine project manager Mark Love.

“There was never more than 100mm clearance around the crane as it went through the gate, and the piling rig was also a tight fit,” he says.

The narrow passage between the church and the Great Hall gives access to a very restricted site, with only around 50m² outside the new building’s footprint available for the crane and materials storage. Luckily, the project team was able to find a home in the Grade 1 listed gatehouse itself, which dates from 1702.

“We had to refurbish the office and welfare spaces,” Love reports. “In the process we discovered a 1m square hole in the roof – which explained the puddles on the floor.”

At first sight, the main structure of the new Centre is bafflingly complex. Asymmetrical curves and angles create what New York-based architect Steven Holl dubs “a branching concrete frame…that branches like a hand”. The design stems from an early-morning watercolour sketch by Holl, who envisioned the building as a “vessel within a vessel within a vessel.”

Translucent matt finished glass cladding with coloured glass inserts forms the outer “vessel”, while internally the fair face concrete structure remains exposed on the vertical elements. A solid bamboo balustrade wraps around the winding open internal insitu concrete staircase. Cantilevered from the main frame, this stair links all three stories.

Roberts describes the design as “geometrically challenging” – something of an understatement. He adds: “The floor to ceiling heights are up to 4.85m, and the roof itself was particularly difficult, with tricky twists and warps.”

Although floors were polished and aggregate exposed on the stair tread and risers, there was no requirement for an unusually high standard of finish on most of the exposed frame. A straightforward C40 mix was supplied by London Concrete and skipped into place. Forming the complex geometry turned out to be rather simpler than might be expected, says Love.

“Basically we used 4.5m high paper-faced ply formwork and created the shapes by box-outs. Wall thickness was a constant 350mm.”

He also pays tribute to falsework subcontractor Realtime Civil Engineering.

Linking the new centre and the Great Hall was fairly straightforward, Roberts reports. The two buildings will share a new lift and access stairs, with the shaft inside the Centre, despite a significant disparity in floor levels that presents users with more choices than usual. There will be new toilets for the Great Hall in the Centre’s new basement.

There is very little tolerance between the distinctive cladding panels, Love says. He adds: “But they’re getting on very well, probably because we constructed a full-scale mock-up on site right at the beginning, and learned a lot from that.”

When the new Centre opens next spring it will be topped by a roof garden, an oasis of tranquillity amidst the bustle of one of London’s busiest hospitals. By then its controversial architecture is likely to be much better accepted, and those who need its unique services are likely to be the most appreciative.