THE WEALD AND DOWNLAND GRIDSHELL

The historic scarcity of timber gridshells is understandable. They are very complicated to engineer and to build. One problem has been the high rate at which timber breaks this was a real problem at Frei Otto’s Mannheim building. Another has been the lack of wood technology adequate to produce laths long and strong enough to create effective structures.

Image Stuart Keegan and Elaine Duigenan

In the intervening twenty-five years between Mannheim and the Weald and Downland gridshell there have been parallel revolutions in glue and wood technology which has meant the project team could realistically contemplate laths with spans that Frei Otto could only dream of. At Weald and Downland, research identified a Swiss glue manufacturer, Collano's, whose recently developed polyurethane glue; super strong, yet environmentally sound adhesive could be used to form the long laths. This glue was used to bond six pieces of timber together into the impressively long 35 to 50 metre laths. At the same time French oak was sourced, chosen for quality and cost, and cut in

France, into much shorter, 6m laths, 35mm by 50 mm in section. Shipped to Britain, it was next transported to specialist timber processors, Grecon Dimter in Newcastle. There imperfections were cut out, leaving high quality sections ranging from 30Omm to 140Omm in length. These were finger jointed back together again to form the 6m 'improved timber’, before being returned to Sussex to form the extra-long laths.

As well as this wood and glue technology the computer revolution was a central influence on the evolution in engineering capacity, enabling the structural engineers, Buro Happold, to test the gridshell for weaknesses long before it was built. Buro Happold, linked in with the structural engineering department at Bath University, began applying off-the-shelf software modeling to explore the bending properties and behaviour of wood, hoping to ensure minimal breakages in the finger joints.

Image Stuart Keegan and Elaine Duigenan

Taken together the convergence of this computer modeling research from the synergies between wood, glue and materials technology, is a graphic example of how timber design is in the midst of a transformative revolution. This convergence is enabling the potential realisation of radically different, yet low energy, sustainable buildings, which were hitherto too impossibly complicated to construct.

At the same time as this modeling work was going on the project team brought in an open-minded frame carpentry business, the locally-based Green Oak Carpentry Company (GOCC), to carry out the physical construction of the gridshell. The convergence of hi tech with hands on carpentry is at the heart of how the building captured the imagination of press and public alike. The notion of the hi-tech new media engineers working in concert with in-the-body carpenters, guiding them to reposition the timber to micro-exact positions, like astronauts working on an earthbound, wooden space-station, stirs the mind, and aptly illustrates the fusion of traditional craft with twenty first century hi-tech. It was a gift for the carpenters as well, enabling them to show exactly what they could do, given half a chance. This in a building environment which for years has been heading in precisely the opposite direction, further and further into pre-fabrication and away from the uses of individual skill.

Image Stuart Keegan and Elaine Duigenan

Whereas prefabricated inorganic materials are built to the exact gridshell shape, wood is a living material, essentially a, albeit stiffer, fabric. At Weald and Downland, this meant the gridshell could be prepared on the ground flat as a two dimensional lattice grid mat, before being moved into its vertical position, where it changed, or deformed into a new, springy, and somewhat provisional shape. When this happened all sorts of surprise and unexpected shapings emerged, as the wood adjusted itself, its random living quality making even a computer model not fully equipped to entirely anticipate how things turn out. Indeed, knowing how the timber actually shapes out, and doing so without it being damaged in the process, turned out to a considerable dilemma. Previous gridshells have been pulled or pushed up from the ground. These resulted in many of the breakages. As a new experiment, this time the gridshell was constructed on a specially prepared and expensive, PERI scaffolding system, standing initially 7.5 m from the floor. The gridmat was constructed on the raised scaffolding, resting flat, high above ground level. Then section by section, the scaffolding was removed, and the laths slid into its shell-like form, each piece cajoled into place by the carpenters, a forest of jacks tightening into position of the intricate crisscrossing laths at the connector nodes, the elaborate whirling weave of the building's surface at last becoming clearly visible. The diamond latticing, comprising two layers of laths was pulled in different ways depending on their position in the grid. The outer layer was able to slide into larger lozenges, narrower and deeper over the domes, and flatter in the valleys. This surface form was originally anticipated by the computer's modeling, but once the structure was up, much fine-tuning was needed from human eyes, supported by repeated and careful measurement. The lower laths were tightened in relation to their upper partners. It was here that a lot of the fine-tuning was done by eye. The carpenters knocked the laths into position, pushed out flat areas and deepened those where the building needed greater volume. Once the final shape was reached, the whole building was measured again, comparing what was in front of the team with the shape originally modeled. Depending on your faith in the skill of the carpenters, amazingly, or not, the gridshell closely echoed the shape of the original digital model.

This carpentry ethic has extended from the large-scale gridshell, right across the building process. One example of the teamwork, involved the original plan which envisaged positioning slotted holes at the crossing points of the laths, but when this was taken to the carpenters, they pointed out how difficult the hole would be to make and the comparative cost involved. After a round of meetings, phone conversations, emails and faxes, a new and refined design solution emerged: the 'nodal connectors', were eventually designed, a combined effort of everyone in the team, from the hands-on carpenters to the engineers and architect. The result was practical in construction, while performing several engineering functions within the finished building and is also visually appealing.

Image Stuart Keegan and Elaine Duigenan

In a second example, some of the laths originally required rows of ten bolts in close proximity to meet the Euro Code 5 timber safety regulations, GOCC pointed out that green oak would split due to shrinkage and was likely to split, from the number of bolts. So again by returning to drawings, further computer modeling, and the hands-on work of the carpenters, a solution was arrived at which successfully transferred the load of the lath, whilst meeting the regs and maintaining the stress integrity of the oak.

Today the Weald and Downland latticed gridshell is a remarkable building to visit. It has been covered in locally sourced Western red cedar cladding, the roofing taking on something of a contemporary version of the layered cladding found on Norwegian and Russian churches. The costs are generally considered efficient, and if the environmental costs are factored in compares particularly well to high-tech buildings. Costing £1.6 million, the buildings planned life of sixty years. Given all this the potential of an emergent smart timber futurism may give the high eco-tech end of architecture community pause for thought. Although the gridshell is very much a showcase now, the hope is it will spur on further buildings of its form and type.

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