As far as we are aware, this is the first gridshell to achieve the challenging feat of supporting a frameless glass roof.

Client

Chiddingstone Castle Trustees

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Architects

Peter Hulbert

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Engineer

Buro Happold

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Contractor

Carpenter Oak & Woodland

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More information

Andy Parker
Hall Farm
Thickwood Lane
Colerne, Chippenham
Wiltshire
SN14 8BE
United Kingdom

Tel: 01225 743089
E-mail Andy Parker

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08 August 2007 · Chiddingstone, Kent

Project Overview

The trustees at Chiddingstone Castle had an ambition to bring the orangery, a simple mock gothic ‘ruin’, back into service. Because it is only partially enclosed, the intention was to turn it into an external wedding venue. With an existing Grade I listed structure of questionable structural integrity, it was necessary to find a solution that limited the loads transferred to it.

The gridshell roof from above

Frame Design

• The roof had to be designed to sit within the confines of the existing structure
• Loading had to be transferred to the new foundation under the stone floor below, not to the existing structure
• Whilst similar to a number of other structures we’ve previously completed, Chiddingstone was unique in that it had to support a frameless glass roof
• To inform the design process, we carried out numerous tests on the cable and clamping system and bending tolerances of the lathe. This provided the data that dictated the curvature of the roof

 Cable and clamp testing in our yard

 Testing lathe to destruction and finger joint failure

• Our designers worked in close collaboration with the glass manufacturers to develop a node that would clamp the chestnut lathe, the cabling and support the 12mm toughened glass panels

The node clamp at the intersection of the lathe

Engineering

Buro Happold carried out the conceptual engineering prior to our involvement. Essentially, the structure comprises an elliptical rigid ring beam of ply, sandwiched with steel plate, supporting a lattice of chestnut lathe.

 The ring beam ‘sandwich’ being fabricated

A system of cables, anchored to the ring beam and running through the node clamps, provides triangulation to the structure.

The chestnut grid is constructed with four layers of lathe sandwiched together with blocking pieces that enable them to cross at the nodes. Each lathe is 40mm wide by 35mm deep and up to 12 metres long.

The four layers of lathe clearly visible

Fabrication

• Finger jointing of the chestnut lathe was carried out by a sub-contractor and delivered to us in lengths of 16 metres
• Steel plates for the ring beam were cut by laser to a CAD model provided by our designers and then sections of ring beam ‘sandwich’ were fabricated in our yard in temperature-controlled conditions
• Brackets were manufactured to create the exact angle of incidence for the apron so that each lathe could be secured to the ring beam between the apron layers

The apron brackets secured to the ring beam

• The ply apron was fabricated in several sections to an exact profile determined by the CAD modelling 

 The ply apron being fabricated

Erection

Erection on site was carried out within a tented structural scaffolding by TR Services (Bristol) Ltd. With the ring beam constructed and sitting on stools on the scaffold floor, the lathes were laid out as a flat lattice with the connecting nodes loose. The lattice was then lifted into shape over a number of days, allowing the lathes to slide through the nodes and shape to the required form. High ambient temperatures and low humidity were drying the lathe quicker than anticipated so this was counteracted by regular wetting down. Once the form had been achieved, the nodes were tightened and the cables secured around the apron.

The gridshell is levelled on stools to ensure accurate construction

The lathe being lifted into shape over the ring beam

The former from underneath

The scaffold floor was then removed and the roof structure lowered onto the steel stanchions that transfer the load to the new raft foundation under the stone floor.

 The gridshell sitting on its supporting stanchions