Building the Gridshell – Carpentry Techniques
The End Frames
The end frames consist of a GLULAM arch and cross beam fabricated from Siberian larch which is slow growing and highly durable requiring only minimal surface treatment.
The infill framing is green oak joined with traditional pegged mortice and tenon joints identical to those used in many of the Museums’ historic houses.
The gridshell will be assembled from 600 component timber laths with a total length of 12000 metres, the longest lath is 37 metres.
Green oak is used in this structure as its high moisture content gives it great flexibility, which is essential during the forming process as the laths in the gridshell are curved in both directions. Once the laths are in their final position the natural drying will strengthen the structure. Oak also has the advantage that it is twice as strong as the equivalent size of other timber, meaning that smaller cross sections can be used, allowing them to bend to the required radius, and giving the whole structure a lighter look.
It is particularly fitting in the context of the Museum where so many of the historic buildings were built from green oak.
The oak was felled in October 2000 and sawn and machined to 50mm x 35mm
Every piece of timber was subject to visual inspection to identify knots and defects that could cause fractures when used in the building. The defective sections were removed and then the remaining lengths were joined using finger jointing which is barely detectable. The advantage of this approach is that very little timber is wasted in the process and the quality of the material is maximised. Some of the rejected timber will be used in areas of the structure where strength is not critical.
Six metre lengths were assembled in this way using a special finger jointing machine and special polyurethane glue working at a pressure of 40 atmospheres. The glue was selected as it will join wood with a high moisture content and resists the acidity found in green oak and cures rapidly. Furthermore the glue is pleasant to work with and is environmentally friendly.
Each 6 metre length is then proof tested by bending to a radius that exceeds the maximum bending radius to be found in the final structure.
The next stage in the process is to take the 6 metre lengths and join them using scarf joints, glued and screwed, to form the individual laths in the structure to a maximum length of 37 metres. This is an interesting contrast; the finger joints are the latest wood joining technology whereas the scarf joint has been used for centuries.
The completed laths are stored ready for the next stage when the grid will be assembled on the top of the scaffolding and the four layers loosely clamped to allow movement as the grid is lowered into its final position.
The laths will then be pinned to the deck and further horizontal laths which will fix the triangulation of the building and provide support for the exterior cladding will be attached. For the upper part of the building vertical laths will fix the triangulation and provide framing for the glazing.
When complete it is estimated that the roof will contain 12 tonnes of timber, whereas a conventional roof covering the same area might weigh 10 times that amount.
This project demonstrates that timber can be used in infinite lengths to form buildings with curved forms in much the same way that boat builders have used timber throughout the centuries. This surely points the way to the development of fascinating structures using traditional sustainable material and combining the oldest and the newest timber technology.