all images:dalcoastalstudio.com
Cheticamp Farmer’s Market, 2014-15
The type of wood has been consistent through both the Coastal Studio gridshell projects: hardwood keeps the cross-sectional area of the wood lath manageable – we experimented with ash, but now use red oak. White oak would be better in weathering, but is only available in Quebec, an unsustainable thousand kilometres away. Our red oak lath is locally-sourced, green (not kiln-dried) and milled fresh, while it retains a high moisture content and so can bend. It is cost and energy efficient because it isn’t kiln- dried and we can get it from small local sawmills. There are many different types of grid shells. The project looked at here, the Cheticamp Farmer’s Market, was our first, and has led to the more complex Cape Breton Highlands National Park building still in construction. The farmer’s market tested the basic properties of the wood chosen to make the grid shell. The scale of the project – a pavilion that accommodates 20 merchant stalls – was workable for small groups of students. It consists of two concrete walls with a gridshell spanning between and has a crescent-shaped floor plan with open ends for optimal circulation. The orientation and form of the pavilion deflects wind and the cladding is strategically placed to reduce uplift. Such a
structure draws upon the strong local boat- building and woodworking culture and its use of locally sourced greenwood echoes the bentwood forms of the region. We tested a series of laths with different thicknesses and found that 3/4” was the thickest that a twelve-foot lath could be to loop back on itself without an explosive failure. This was an advance on structural tables that required 1/4” material to achieve the curvature we needed. 3/4” – three times that recommended by the tables – bent to failure at a curvature far in excess of what our project required and probably contributed to the ability to connect the lath as it twisted or planked 8 through some tight situations. In hindsight, even thicker wood lath would have been better; the tests could have been made more precise by investigating shallower curves, curves closer to the actual design. For structural integrity the laths are continuous from one support to another, meaning that a six-metre span is composed of diagonal laths up to twenty metres long. Because of practical problems with finger-jointing in thinner lath (green wood shrinks as it dries to its eventual moisture content), scarf joints were used. Several times during the project design
decisions were made based on a combination of factors: expense, remote location, available fabricators and the skill set of our crew. We were working with unusual applications, outside the experience of local or even North American practice. The glue, for example, specially formulated to bond with green wood, had to be imported from Europe and is normally used with large presses in factory conditions. Another issue was grading. During the milling process, each individual lath is inspected to ensure its integrity. An acceptable lath has a relatively straight grain pattern without too much deviation and little to no knots or cracks which would compromise the integrity of the entire system. Inspection is based on a structural ideal of a semi-isotropic material; an example of the organic having to conform to a standard. Grading is a convention that only accepts wood for which there are grading rules. In our case, there were no graders available for milled hardwood in Nova Scotia. As our millwright had softwood grading credentials, the engineer specified the softwood grade for the initial quality inspection. This kind of detail reveals the industrial and bureaucratic culture of wood construction that we conventionally work with and within.
On Site review 35 : the material culture of architecture
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