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Wednesday23 July 2014

Steel structure at the heart of Bennetts’ Royal Shakespeare Theatre in Stratford

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Steel plays both a functional and aesthetic role in the design Bennetts Associates’ RST auditorium.

After a three-year construction programme, the Royal Shakespeare Company’s reworked theatre complex in Stratford-upon- Avon was officially opened last month and stage productions have begun in the main auditorium.

This was the most complex space in the entire project, and the last to be completed. One of the priorities of Bennetts Associates’ design was to create a greater intimacy between the 1,040-strong audience and the actors by halving the maximum distance from seat to stage. This was achieved with a thrust stage and also through the use of a steel structure that plays both a functional and aesthetic role in the design of the auditorium.

Steel was a natural choice for the framework because it allowed a slimmer structure than concrete or wood. This assists sightlines and helps the architects to bring the audience closer to the stage.

CUTAWAY SECTION SHOWING STEEL CONSTRUCTION

Source: Buro Happold

CUTAWAY SECTION SHOWING STEEL CONSTRUCTION 1 Stage and flytower, 2 Existing 1930s flytower basement, 3 New thrust stage basement, 4 New lightweight roof to 1930s flytower, 5 Auditorium balcony structure, 6 Suspended technical lighting level, 7 Removable auditorium balcony sections, 8 Primary roof trusses, 9 Auditorium air handling units, 10 New foyer canopy roof, 11 New restaurant hung from roof, 12 Concrete stability walls and auditorium access ramps, 13 New steel prop to enable existing basement wall opening to be cut, 14 Existing Swan Theatre, 15 New basement plant rooms, 16 Existing art deco foyer

Bennetts was also keen to take a different approach in the main auditorium to the timber structure of the far smaller Swan Theatre at the RSC. Instead, a “scaffold” of exposed architectural steelwork columns and beams was used to support faceted balconies of seating around the auditorium. Above this are two further steel structures unseen by the audience: three levels of lightweight technical bridges suspended from giant roof trusses which span over the new flytower and rest on the concrete frame of the building.

“Steel achieves a delicacy of construction. Every millimetre counts in this space – it really does make a difference. The slimmest construction is absolutely essential,” says Bennetts Associates director Simon Erridge.

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The emphasis was on as lightweight a steel construction as possible in order to minimise the load and piling requirements, especially in a difficult waterside location. In total, the auditorium contains approximately 650 tonnes of steel.

Overall, the theatre complex is expected to have a 20% smaller carbon footprint than the original theatre, assisted by the use of cross-laminated timber floor panels instead of precast concrete.

Although the auditorium has been designed as a permanent structure, the steel structure could be dismantled to allow the whole space to be reconfigured.

“It’s a steel armature that can be changed, and the expression of the steel elements supports that,” says Erridge. “If you really wanted to, you could unbolt it in 30 years’ time and redo the auditorium.”

Auditorium structure

The auditorium seating is arranged on three levels around a 7.2m-wide, 10.25m-deep thrust stage that protrudes from the proscenium. The stalls seating rests on a concrete slab supported on a steel sub-frame.

Ten exposed cruciform columns support the light-weight steel structure that holds the circle and upper circle seating. The ring beam structure is fixed back to concrete cores behind the timber-clad walls of the auditorium.

The slender columns measure 10m high, including 1m concealed below auditorium floor level, and are set approximately 4.5m apart, breaking up the large audience into small pockets of approximately 30 seats.

In all, there are 24 types of seats, including five width variants from 450-555cm. The steel structure is left exposed, including the ribs of the underside of the circle balconies, which have painted plywood soffits.

To ensure the columns aren’t too prominent and don’t interfere with sightlines, they are positioned one row back from the front of the balcony and this front row is bolted on to the structure and cantilevered out.

An extra stipulation was that the two seating bays closest to the proscenium had to be demountable at circle, upper circle and technical area level to give production designers the flexibility to use large pieces of scenery near the proscenium. These wedge-shaped blocks can be unbolted from the rest of the structure if required.

In all, more than 500 components were manufactured for the auditorium, according to Paul Tierney, associate director of CMF, the steelwork contractor responsible for the exposed steelwork in the auditorium, which had a team of more than 40 working on the job. All welding was done off site, thus limiting the time spent constructing the structure in the auditorium where it was bolted together.

One of the biggest challenges was incorporating all the ventilation, fire, IT and electrical servicing within the slim auditorium, especially the need to bring large amounts of cabling to lighting bars at the front of the balconies.

Roof trusses

Four 24m-long steel trusses criss-cross the top of the space, resting on the concrete walls that form the sides of the auditorium. Weighing 20 tonnes each, these trusses support both the upper technical levels which are suspended from them plus 30 hoists for flying sets weighing up to a total of 25 tonnes.

They have to be stiff enough to keep deflections to a minimum in order to be capable of lowering scenery travelling at a rate of 2.5m per second with the utmost precision. The trusses are 3.4m deep, allowing technicians to walk through them as they work above the auditorium.

Steel beams fan out from the concrete walls.

Steel beams fan out from the concrete walls.

By modelling the effect of a fire on stage, Buro Happold was able to show that the trusses and other technical steelwork did not need a fire protective coating – which would have been problematic given that they are constantly handled and clamped when in use. In this way the need for a safety curtain between the proscenium and thrust stage was avoided. Large smoke extract fans are installed in the roof.

The original flytower has been refurbished and the old concrete roof replaced with a lighter weight steel and timber roof. This allows more of the structural load capacity to be given to supporting theatre services.

Untreated steelwork

The architect’s initial instinct was to paint the exposed auditorium steelwork, or to shot-blast it. Eventually, it was decided to leave it untreated, except for a clear lacquer to preserve the finish.

CMF similarly left the raised welding on the auditorium structure intact rather than grinding it off, as would be more usual, putting more pressure on the welders to make sure their welds all matched and lined up perfectly.

“Not to clean up the welds goes against the grain,” says CMF’s Paul Tierney. “We had to relay a few welds where the guys had naturally cleared them up.”

Project architect Alasdair McKenzie adds: “You get a sense of it being handmade and that’s deliberate. All the columns are very different – some are orangey, some blue.”

This ties in with the broader design decision to have rawer material finishes in the auditorium – rough sawn oak timber panels are visible on the rear walls and exposed brick around the proscenium – in contrast with the more highly finished foyer areas.

PROJECT TEAM

Architect Bennetts Associates, Engineer and transport consultant Buro Happold, Theatre consultant Charcoalblue, Construction management Mace, Acoustic consultant Acoustic Dimensions, Project management & strategic planning Drivers Jonas Deloitte, QS and planning supervisor Gardiner & Theobald, Steelwork contractors Billington Structures (primary steelwork); CMF (auditorium steelwork)

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