THE CURVE THEATRE Leicester’s new Performing Arts Centre turns the mystery and back stage wonder of a theatre on its head. The flagship £61m building is leading the redevelopment of the St George’s conservation area in the city centre. The client, Leicester Theatre Trust, and architect Rafael Viñoly wanted visitors to be acutely aware of the internal engine of a theatre and its operational community, in a bid to redefine the way that theatre is experienced by its audience. The idea was to blur the traditional boundary between the street and the theatre, between the front of house and the back of house. In this first of a kind building, both the stage and back of stage is visible, and even the rehearsals are on show. In this challenging approach to contemporary theatre, Viñoly purposefully made everything visible to the man on the street. This breaking of boundaries between stage and foyer brought many exciting structural challenges to engineers Adams Kara Taylor (AKT). The Curve Theatre features a four-storey glazed and louvered curved curtain wall that gives the building its name. The form is taken from the curve of the street that the building is built on. Hung from a vast steel truss, the glass hits the ground without interruption from structure, offering a continuous and unobstructed 4m high window at ground level revealing the 750-seat main auditorium and a 350-seat studio, and the production and administrative facilities behind. Conceived as islands within a public foyer, a central stage sits at street level between the two coloured volume auditorium pods. A system of massive movable steel shutters allows the audience to view performances in each auditorium from a variety of configurations. When the steel shutters and acoustic wall are raised, the whole space can be converted into a theatre in the round for 1,000 people. Meeting and seminar spaces and restaurants and bars, with a mezzanine incorporated into the façade at first floor level to provide an area for interval drinks are all clustered under a massive roof that measures one and a half times the size of a football pitch. An 800 tonne steel structure supports the 3300m2 expanse of roof that provides a single cover to the central stage and both auditoriums. The same truss also provides plant and technical gallery space between further storey-height trusses, some of which weigh up to 60 tonnes. Long spanning secondary beams connected to the chords of the roof trusses support the roof deck and the various floor structures. The huge steel trusses are supported by four main concrete access cores positioned both sides of each auditorium, with additional support provided by the proscenium walls and the shoulder block columns to the rear. Along the ‘public’ elevation of the building the roof structure cantilevers out up to 15m to support the 20 perimeter hanging columns to which fix the sweeping glazed façade. The 18m long fabricated steel columns all hang from the top chord of a perimeter truss that skirts along the profile edge of the roof, spanning between the primary cantilevering trusses. The sheer scale of the roof (the biggest trusses are 90m long) meant thermal movement of the steel was a potential problem. This issue was tackled by attaching the roof to the supporting cores using huge sliding bearings which allow the roof structure to move laterally - independently of the concrete structure below. The truss sections were fabricated in Glasgow in 25m lengths. It took two days and a police escort to get them to Leicester. Once delivered, some sections were welded together on site to form the complete truss, before being lifted into place. The cantilevered roof structure had to be built in a precise sequence, with only one temporary trestle used to erect the primary roof trusses in order to keep costs down. Costs were also trimmed by using mobile rather than tower cranes to lift the roof structure into place. The two 30 tonne L-shape shutters at the sides of the stage, at 20m high and 400mm thick, are the biggest shutters of this type ever built and installed in a theatre. The seal between the shutter and shutter box had to be installed to a tolerance of just 2mm. Despite their size, each shutter can be raised in just three and a half minutes. The complex requirements of the roof space predetermined the use of steelwork, as the consistency of the material meant that the structure could be analysed with a high degree of certainty, especially in determining how the structure would deform and the magnitude of defection would take place under various loading conditions. The ability to do this was critical to achieving the tight tolerances demanded by the design of both the façade and the 30 tonne side shutters. In addition to supporting the façade, the bottom tip of the inclined columns carry the leading edge of a first floor walkway that links the two ends of the shoulder block and envelopes the auditoriums. The internal edge of the walkway is suspended from the roof structure via a series of high yield tension rods, creating the dramatic effect of a floating walkway. As well as providing public access, the walkway deck acts as a long horizontal beam, offering lateral support to the bottom of the 110m long curved glazed façade. Beneath the walkway a four metre high glass wall runs the length of the ‘public’ elevation, giving clear views to the foyer and stage area from the street, creating the inside-out experience at the heart of the architectural concept. Each performance space acts in acoustic isolation. The potential problem of noise transmission from the auditoriums across the roof was solved by acoustically separating the cores from the rest of the structure. Noise transmission via the proscenium walls was dealt with through acoustic bearings placed where the walls meet the roof. This project presents innovation in steel design and erection in order to achieve a visually thought provoking building.