The Lovell Telescope at Jodrell Bank in Cheshire is an icon of the Space Age. The largest steerable dish radio telescope in the world when it was completed in 1957, it has kept a watchful eye on everything from cosmic rays to Russian satellites.
Its 76m-diameter bowl has provided the backdrop to Tom Baker’s final moments as Dr Who and a 1994 photoshoot by up-and-coming local band Oasis, as well as appearing on stamps everywhere from Haiti to Liechtenstein.
So it made perfect sense that it should also inspire the dome-like form of Jodrell Bank’s new visitor centre, the First Light Pavilion. “If you took the dish out of the Lovell Telescope and put it face down on the ground,” explains project architect Gary Collins of Hassell Studio, “it would have exactly the same circumference as the dome of the First Light Pavilion.”
The pavilion is the largest reinforced concrete dome in the UK, and the first to be built in this country for 25 years. The curving slab is 50m in diameter, rising up 8.2m at its apex, and rests on two rings of reinforced concrete: the building’s 300mm-thick outer wall and a secondary 225mm-thick wall, 4m into the space. Elliptical cut-outs create the entrance and cafe terrace.
Excavated earth is banked against the perimeter wall, drawing the dome smoothly down to the ground and expanding its diameter to a Lovell-matching 76m. Grass grows over these banks and the concrete structure above, embedding the pavilion in the landscape and inviting visitors to stroll over it.
The dome is shallower than the Lovell dish – partly to reduce the impact on the UNESCO World Heritage site – but the interior still feels expansive. “When you go inside, there's a tremendous sense of space and it does feel very lofty,” says Collins, adding that the exposed concrete structure lends a monumental quality. “The client said she wanted a cathedral to science,” says Collins, “and the solidity of the materials is reminiscent of a cathedral-type space. It's a raw sort of experience, but it still has the ability to inspire.”
At the heart of the building is a 150-seat auditorium, conceived as a dome within a dome with a 270˚ curving screen, and a large exhibition space, designed by museum specialist Casson Mann and built using original dish panels from the Lovell Telescope. The entrance foyer, cafe, offices and plant room occupy the outer rim of the dome, between the two ring walls.
With concrete domes such a rare building form in the UK, the structural design was a voyage of discovery. In particular, the low curvature of the dome presented a major challenge because it meant the structure worked partly as a shell and partly as a slab. It also had to support the load of a 300mm layer of soil and the possibility of people congregating on the grass above.
Structural engineer Atelier One carried out a complex material behaviour analysis, using two separate 3D modelling programs, to understand the relationship between slab and shell action and how this would be affected by the cracking behaviour and non-linearity of the material. The resulting design comprised a 200mm-thick slab with eight discrete reinforcement zones of radial and circumferential rebar.
When it came to making this design a reality, contractor Kier decided to cast the roof in a single pour to mitigate risks associated with day joints and the complexity of fitting waterbar to a shape curving in two directions. A dense forest of scaffolding, propping and curved beams was erected to support the 50m-diameter plywood mould – together with a viewing platform. “A lot of people involved with the project turned up to have a look because they thought this is not something you see very often,” says Collins.
“It's a special event seeing a concrete dome of that size being cast.” The continuous pour required 59 operatives and 55 wagons of concrete, and took 12 hours to complete. Special care was taken to ensure that the concrete didn’t slump to the edges of the dome – another factor that the reinforcement design had to take into account. The roof build-up was completed with 160mm of insulation, 75mm for root barrier, protection mat and waterproof membrane and then the soil.
As one might expect from a heritage site of international renown, sustainability and environmental impact were key considerations. The concrete was a 40% GGBS specification and has been left exposed on the internal walls to help moderate the temperature. Air tubes have been embedded in the banked earth slopes to provide passive cooling. And the dome itself – at just 200mm thick, with the shell working in pure tension and compression – is a reminder that shells can be an incredibly material-efficient form of construction.
The cut-outs from the dome are very precisely placed. The entrance faces due south and at an angle of 53˚, perpendicular to the celenial equinox. A narrow glass slot in the otherwise solid south wall allows a ray of light to enter the building, tracking the sun path through the day by its sweep, and the time of year by its length within the building. The cafe and services yard cut-outs are also positioned on significant radials, representing First Light and Pulsar CP1919 respectively.