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Traditional flavour

Timber engineering Norwich Cathedral

English oak was the structural material of choice when Norwich Cathedral needed a new refectory within its mediaeval walls. Dave Parker reports on the engineering challenges involved.

Slotted into a 50m by 10m gap between the cloisters and an existing mediaeval wall, Norwich Cathedral's new Hopkins-designed refectory is a deceptively complex structure - with a 100 year design life. 'Essentially we're talking about a sizeable two storey building with the kitchen and services on the ground floor. The upper floor will be a dining area, at the same level as the main library above the cloisters next door', explains structural engineer Buro Happold group director Angus Palmer.

'But the roof pitch is untied and no loads can be put into the cloisters. And some of the forces involved are quite large.'

Solid wood would have been more traditional, but was soon ruled out. Columns sit on a 7.5m by 5.5m grid, and even in laminated hardwood reinforced by steel flitch plates diameters are 250m and lengths 8m. Finding solid oak members of these dimensions would have been almost impossible.

'We did manage to find enough English oak to do the job, however, ' reports Buro Happold project engineer Domenico Del Rey. 'Laminating up the columns was quite a tricky job, because the lighting conduits actually run down inside them.'

Coping with roof loads was one of the biggest design challenges.

The roof beams are 300mm deep steel channels back to back, with Kerto laminated veneer lumber (LVL) joists. Above this a stressed skin of plywood panels supports a traditional lead roof, while oak planks make up the soffit. Any connections between the roof structure and the columns would be highly visible, so aesthetic as well as structural imperatives had to be satisfied.

'Finger struts' were the preferred option. Two pairs of laminated oak struts sprout from complex stainless steel nodes at the tops of the columns and link up with simpler nodes bolted to the steel roof beams. 'These nodes need some rotation potential to take up tolerances, ' explains Palmer. 'Their final design is a combined effort between ourselves and the architect.'

One bay per floor per side is cross braced longitudinally with stainless steel tie rods. Transverse stability is provided at each end of the first floor by steel struts linking the column head nodes to reinforced areas of the floor structure.

'The ground floor service area is a simple self supporting box with no connection to the main structure, ' says Del Rey. 'Ground floor slab and foundations are relatively conventional - except that the foundations were actually excavated by archaeologists a year before we arrived on site.

'They didn't find anything outstanding, so the holes were backfilled with sand until we needed them.'

In the entrance lobby area loads are at their greatest. A 2m cantilever picking up stair loads was more than the standard 300mm deep floor joists could carry without steel reinforcement.

Columns here are also heavily reinforced. And fitting the glazed partition around the nodes and the structural elements involved some tricky detailing. Structural movement had to be allowed for while the fixings themselves remained invisible.

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