Feature

Songs in the stones

Concrete provides the perfect accompaniment to the Bath stone of Robert Smirke’s St George’s Bristol, finds Tony Whitehead

In 1976, St George’s Church in Bristol was repurposed as a concert venue. It became particularly favoured by classical and folk musicians who valued the superb acoustics, and in 2000 an upgrade installed a small bar in the crypt. More success followed, but with it came problems. There was a lack of facilities for high-profile artists and congestion problems for the audience who experienced long queues to get an interval drink.

An extension was clearly needed if St George’s was to realise its ambition to become one of the country’s foremost performance venues. But how to add to such a building? It was built in 1823 with columns and portico in the Greek Revival style and this, together with stony spaces in the crypt, gives it a rather ancient, almost mystical atmosphere.

“This is one reason we chose in-situ concrete,” says the extension’s designer, George Ferrari of architect Patel Taylor. “The church almost seems to grow out of the hillside where it is situated and concrete has this primary, of-the-earth feel to it – like the stone walls of the original crypt.”

Ferrari also noted the way in which the material mix of the original building changes – with roughhewn stone at ground level, but smoother finishes above. “Like a lot of buildings in Bristol and Bath, the original church has a combination of rough pennant stone with paler, smooth-finished Bath stone. We liked the contrast so we did something similar in going for an exposed in-situ concrete frame with some Bath stone walls and details.”

The new building comprises two floors. The lower is on the same level as the crypt and provides a much-needed cafe-bar able to accommodate a full house of 560 patrons. The upper storey is level with the performance hall and contains multipurpose rehearsal and event spaces, as well as back-of-house areas for management and performers.

“We don’t touch the existing church structurally,” explains Ferrari. “The connections are very light – such as the glazed roof which slants down from the church to the new building. It was also important to us that the new spaces, in the bar area especially, were open and welcoming, so we have very few shear walls and just two columns.”

This relative lack of support, and the fact that the building connects with but is not braced by the older building, created a challenge for the structural engineer. Essentially the extension is supported on the side away from the church by a long reinforced external concrete wall which is stiffened at 2m intervals by regular fins or piers that project out into the bar space. As well as their structural function, these piers have been put to work on the lower level, where the alcoves they create have been fitted with shelves for the bar. Budget constraints meant that there was little scope for fancy mixes or hi-tech formwork.

Smirke’s concrete legacy

If Robert Smirke is remembered at all these days, it is for designing the huge classical facade of the British Museum. But the architect of St George’s has an arguably more significant claim to fame: he is credited with reintroducing into the UK the use of concrete for foundations.

It was in 1816 that Smirke was called in to save the newly built Millbank Prison in London from collapse following severe structural problems. Together with the engineer John Rennie the Elder, Smirke recommended underpinning the entire building with concrete – thought to be its first use as a foundation material since the days of the Roman Empire.

It proved so successful that Smirke went on to deploy concrete for the foundations of the British Museum – but before that, it seems, he honed his technique at St George’s. Completed in 1823, the church predates his Bloomsbury masterpiece by several years.

This was only revealed when Buro Happold dug an investigative pit at the side of the building to establish how the structure was founded. “Much of the church is built directly onto rock, but because the rock-head has a dip in it, this is filled with concrete,” says Buro Happold director Claire Smith. “We were not surprised to find it – it’s what you expect foundations to be made of – but it would seem to be a very early example of its use.”

The mix is standard structural concrete and there is nothing special about the plywood board shuttering. Despite this, Ferrari wanted the finish to be as good as it reasonably could be. “The bar area, for example, is a very visible part of the building and we wanted the finish here to be quite fine,” he says. “Being among the earlier pours the contractor had not quite refined the technique, so some piers were a little too rough and had to be remade. What we have now is pretty good. I like the idea that, with the piers, the wall is thick enough to contain things – a bit like the inhabited walls you find in castle architecture.”

The first-floor slab is supported by 8m-long reinforced concrete downstand beams which extend from each pier across to a massive 525mm x 375mm in-situ concrete beam that runs parallel to the bar and is supported by two columns and the lift core. The slab reaches beyond this beam towards the church, but does not touch it – first-floor access to the performance area being via two glass “bridges”.

This arrangement of beams has a structural logic that is satisfyingly easy to read, the plain concrete has a textured yet consistent surface, and the whole effect is amplified by subtle architectural detail. The beam on top of the columns, for example, has a cast-in groove along its underside – a “go-faster stripe”, as Ferrari calls it. In fact, this detail both celebrates the beam’s length while breaking up the visuals of its hefty underside.

“It was also important for the beams to clearly look like they are sitting on top of the columns,” he adds, “so the two columns are a little wider than the beam they support. It helps to differentiate them as separate, monolithic elements.”

The upper floor is reached via two staircases. One is constructed from a combination of precast concrete treads set within a painted steel structure while the other, with similar treads, has a concrete structure. Mirroring the changes in the original building, the upstairs aesthetic is less structural and more finished. For example, in the bar area the downstands project beyond the ash baton acoustic timber panelling. Above, the panelling is flush with much shallower downstands. Where there are no beams, much of the flat concrete soffit is exposed and the board marks of the formwork clearly visible. At the slab edge, concrete and bath stone abut each other, creating a two-tone textural effect.

The structural puzzle

The structure of the St George’s extension presented a challenge for engineer Claire Smith, director of Buro Happold’s Bath office.

“This extension is built below ground at the back and to one side so needs to resist large unbalanced soil pressures,” she explains. “The building, however, has open, flowing space towards the front – so it was difficult to incorporate solid shear walls to resist these lateral forces. The structure is also separate from the original church so the lateral loads couldn’t go into the old building.”

Buro Happold’s approach was first to design a contiguous piled concrete wall to hold back the earth of the hill and to act as the foundation for the shorter rear wall of the extension. Smith then specified a very strong and stiff reinforced in-situ concrete wall along the longer, far side of the extension away from the church.

This wall is 350mm thick and features six 250mm-thick piers or fins which project out 920mm from the wall at 2m centres. These act as “mini” shear walls. “We did need quite a lot of ‘beef’,” says Smith.

Near to the church, two 400mm x 250mm columns support a large 525mm x 375mm beam running parallel to the piered wall. Downstand beams run some 8m from each pier across to and beyond this beam. These drop 300mm from the first floor in-situ slab, which is 150mm thick.

A four-sided pyramidal oculus is the stand-out feature of the upper floor. It is constructed from 200mm-thick in-situ concrete slabs set at angles. This rises some 3m from the ceiling soffit and, being asymmetrical, its sides vary in both length and gradient. “We conceived the roof design as a series of sculptural objects,” says Ferrari. “The lift core overrun forms one, the pyramid is another. It was a challenge – the formwork was difficult, the gradient made the pours tricky and it was important to vibrate the concrete well to avoid honeycombing.”

The end result is worth the effort. The plywood shuttering has been well arranged and the board marks and tie holes add interest to draw the eye up towards the light. The oculus also contributes to the building’s heating and ventilation strategy since it naturally collects rising warm air which can be vented from the top if necessary.

“One oculus soffit has the ash batons on it for acoustic reasons,” says Ferrari, “and in fact the acoustics created by the shape work well. The area is used by musicians for rehearsals and the feedback has been very positive.” The client too is happy with the design.

Trish Brown, St George’s operations manager, admits that there was initial nervousness about using concrete so extensively: “But Patel Taylor showed us some examples of how it could work really well. The great thing is that this doesn’t feel like a new space added on. The old and the new have been married together. It feels like one building.”

PROJECT TEAM
Architect
Patel Taylor
Structural engineer Buro Happold
Contractor Midas
Precast supplier Vobster

Photos: Peter Cook