Roofing: Southmead sees the light
Building a huge 280m long glass atrium complete with technically complex ‘wind catchers’ for Southmead hospital in Bristol required a head for heights and no amount of dexterity for Carillion and its roofing subcontractor Prater. Stephen Cousins reports.
A visit to hospital can be stressful and traumatic, but what if you receive treatment in a building that’s flooded with natural daylight via numerous internal courtyards, and a full height atrium as tall as the nave of Westminster Abbey?
The importance of daylight in speeding recovery is one of the design principles pursued in the £430m Brunel building at Southmead hospital in Bristol, which is set to become one of Europe’s leading acute care hospitals when it opens its doors in May.
North Bristol NHS Trust’s brief was informed by research showing that good design can improve patient wellbeing, recuperation rates and the job satisfaction of staff. It therefore set PFI contractor Carillion strict design criteria that required the hospital to be flexible enough to adapt to changes in the way healthcare is provided; maximise access to views and light through efficient use of glazing and layout; and include excellent internal and external finishes designed to make patients feel like they are in a hotel or a private hospital environment.
The 280m long atrium divides the ward blocks and the treatment blocks and features three unusual “wind catchers”.
“This is my third major PFI hospital and it’s a one-off in terms of the high quality of design and the specification of materials,” says Keith Hutton, project director at Carillion. “The soaring atrium is sure to create a lasting impression on the public when they first enter hospital and there has been strong attention paid to the specification of joinery and other features throughout. We considered this the right thing to do to reduce maintenance and create longer term
value for money.”
It is also the most sustainable major acute hospital under construction in the UK and designed to achieve BREEAM “excellent” rating and a carbon footprint well below government targets. The design halves the amount of energy used per 100 cu m compared to the existing hospital, plus around 20% of the building’s energy will be provided by renewable sources. The atrium and wards will be naturally ventilated and many wards will face east to exploit the warmth of the morning sun.
The 115,000 sq m Brunel building is being built on a 27-hectare brownfield site within the existing Southmead hospital grounds. The sheer scale of the hospital means there are a total of around 40-50 separate roofs.
Lower level flat roofs on the ward block feature several green and brown roofs that have been landscaped to include intensive shrubs, trees and gravels. And the roofs of the clinical block have specialised stone paving designs, including ramps and other features designed for use by patients as part of their treatment.
The atrium runs from four storeys at the southern end (left). A glazed facade runs the full length of the clinical block (right).
The new hospital comprises three main elements: patient accommodation is in ward/bedroom blocks that form three connected U-shaped elements, while clinical facilities are in a treatment block to the west, including 24 operating theatres, critical care units, MRI and X-ray rooms. These two aspects of the hospital are positioned on either side of its signature 280m long full-height atrium, which is divided into three sections as it rises in stages from four storeys at the southern end to a seven storey-high main entrance to the north.
“We designed the atrium to create a distinct split between the ward block, which is focused on a patient environment with natural ventilation, views from bedrooms and landscaped areas outside, and the high-tech clinical block, which is mechanically-cooled and ventilated,” says Chris Green, a director of architect BDP, the project’s lead designer. It produced the design alongside structural engineer TPS, and M&E services engineer DSSR.
The cathedral-like atrium is designed to create a natural flow of foot traffic between the clinical and ward blocks via a continuous ground floor concourse and glazed footbridges at each floor level. “We considered designing a low level roof over the concourse, but this would not have given the same quality of internal light as the atrium and we wanted it to be the heart of the whole building,” says BDP’s Green.
It is essentially made up of three different spaces, divided by the interlinking footbridges and three public stair and lift cores that provide the main public route from the ground floor to patients in the wards. An internal glazed facade runs the full length of the clinical block and, as part of the wayfinding strategy, features green, blue and red glazing panels to define the three different sections of the atrium.
The roofs of the clinical blocks feature stone paving designs.
The glass atrium roof is made up of three huge, flat wing-shaped elements, each of which has a “wind catcher” positioned at its centre, designed to ventilate the space and extract smoke in case of fire, each one around three times the length of a double decker bus and twice the height. The individual glazing panels themselves, fabricated and installed by Novum Structures, typically measure 3.1m by 1.8m, weigh 380kg and sit within a framework of powder coated aluminium box sections.
“The most obvious challenge here was working at height, so a birdcage scaffold was provided across the entire atrium to enable the provision of a fully boarded working platform to reduce the risk of falling persons or materials,” says Carillion’s Hutton. “As a result it was like a forest in the atrium, so the sequencing of works in and around that area had to be very carefully considered.” The complex system of temporary supports and birdcage scaffolding reached some seven storeys above ground.
The complex three-dimensional geometry of the atrium roof also posed several engineering and installation challenges for roofing subcontractor Prater.
“The geometry of the main structural steel work for the roof [completed by steelwork fabricator Severfield Reeve], is all pitches and angles with hardly a right angle in it,” explains Philip Wood, operations manager at Prater. “This made it very awkward for us to gain access with materials, and meant designing special scaffolding that could cantilever over the top of the sloping windcatchers. The accurate sequencing of work was also critical to avoid blocking ourselves in!”
Kalzip lines the sides of the wind catchers.
The Brunel building’s structural frame comprises two different systems: the clinical block is erected around a cast in-situ post-tensioned reinforced concrete frame, and the ward block uses a structural steel frame, with waffle decking and cast in situ concrete slabs. A total 46,000 cu m of concrete and 7,000 tonnes structural steel will be used during construction.
“When doing healthcare you need all the room you can get above ceiling level for M&E and plumbing services and the clinical block’s concrete solution gave us flat soffits to maximise the available space,” says Hutton. “The ward block is not so densely serviced, so we could be more flexible in terms of ceiling height and so went for the steel frame option. From a procurement angle, splitting packages in this way lessened our risk and we were able to run the two in tandem fairly seamlessly.”
But a consequence of the different structural solutions used for the ward and clinical blocks was a relatively high degree of movement in the atrium roof that spans between them. Thus, the structural steelwork supporting the atrium roof is designed to accommodate +/-40mm of movement across the entire length of the building. The roof and the walls are fixed to separate steel structures that allow the roof steelwork to slide over the tops of the walls.
The wind catchers are nose-shaped and formed by two Kalzip roofs, each elevated at a 60 degree pitch from the main roof. An aluminium bullnose lip sweeps up and down each side of the wind catcher, then transitions into an identical bullnose that runs around the entire atrium roof. These transitions had to join seamlessly, but due to the complexity of the roof design and tolerance issues, Prater had to rethink its plan to use prefabricated elements, eventually electing to make them up by hand on site. In the end, it took around 10 weeks to complete each wind catcher.
One of the wind catcher elements of the atrium roof can be seen here, with the bull-nose edging visible to the front.
“Initially we struggled to design the wind catchers in 3D and they became exceptionally complicated,” says Wood. “We had all the bullnose elements pre-fabricated in three or four huge sections, but we couldn’t join them together on site due to the differences in tolerances between our structure and the roof structure. As it turned out, the only way we could make it work was to make them up again on site out of aluminium.”
But that only partially solved the problem, as the client refused to accept the spray finish Prater had applied to the bullnoses, so they had to be taken apart and sent to the factory to be powder coated, then returned and reassembled.
“It was a lot of hassle and cost a lot of money to put right, but the end result is fantastic and achieved exactly what the architect wanted,” says Wood. Prater also installed around 22,000 sq m of hot melt bitumen roofing under all the flat roofing, a huge task says Wood: “It was a challenge getting the sheer volume of materials into the job and up onto the roofs. The tight site made it difficult getting deliveries in and out and there wasn’t much footprint for storage.”
Carillion’s team is currently closing out the project, installing the final fixtures and fittings and hard and soft landscaping ahead of the late-March handover. The finished building will have gone through a lengthy three years and nine months of construction, but the benefits to visiting patients are expected to last for decades to come.