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BUILDING STUDY

Working details

The new four-storey college building contains a lecture theatre and teaching rooms on the ground floor and three floors of student accommodation above, reached by a series of bridges at first-floor level. The bridges lead to three staircases which give access to the upper floors.

The stairs have straight flights and fan-shaped half-landings. Each stair is set in an enclosure of loadbearing 105mm-thick semi-engineering brick walls which give support to the treads and risers. Independent blockwork walls, adjacent to the brick walls, support the concrete floor slabs; this arrangement allowed the main building shell to be completed rapidly, without being delayed by the more complex construction of the staircases.

The staircases follow the structural principles of Georgian stone staircases. The treads and risers, L-shaped in section, are made of quality polished precast concrete. The inner edges are built into the brick wall. The bottom of each upper riser interlocks, via dowels, with the back of the tread below it; this forms a vertical strut in the traditional Georgian manner which helps support the stair and direct most of the load down the flight on to the first-floor slab.

The fan-shaped precast landing units only just connect with the wall at their outer edges. They are held in position and supported by the locking action of the flights below and above.

The staircase was erected from within the enclosure. Each tread was lifted and grouted into position, using a special fabricated clamp which avoided holding the exposed precast faces. Only when the first lift had been erected was the brickwork above the stair built - proving that the stair does not have a cantilever action.

The central handrail has a continuous balustrade of toughened glass clipped into steel tubes which are fixed at roof level and tied back to the precast stair.

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