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Importance du bois mort

Il est particulièrement important de noter que chaque utilisateur de bois mort est associé à des caractéristiques de bois mort (espèce, statut, stade de décomposition et taille) qui lui sont propres (Harmon et autres, 1986; Berg et autres, 1994; Jonsson, Kruys et Ranius, 2005; Saint-Germain, Drapeau et Buddle, 2007). Dans bien des cas, différentes étapes du cycle de vie de ces utilisateurs (ex. : alimentation et reproduction) correspondent à différentes combinaisons de ces caractéristiques (McComb et Muller, 1983; Nappi et autres, en préparation). La figure 1 représente une succession d’espèces, surtout des oiseaux, mais pourrait aussi être adaptée pour d’autres groupes d’organismes (mammifères, insectes, champignons, etc.) associés aux différents stades de dégradation d’un arbre. Cette figure pourrait aussi s’appliquer pour les débris ligneux au sol.

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Matthaüs Merian the Elder.

St. Peter Square in Basel : 17th century engraving by Matthaäus Merian

1654

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Frédéric Epaud, historien et archéologue,

Sur la connaissance réciproque entre modes constructifs et formes de sylvicoles au Moyen-Âge dans le bassin parisien 

2019

«Contrairement à de nombreuses idées reçues, les bois utilisés dans les charpentes des cathédrales gothiques ne furent jamais séchés pendant des années avant d’être utilisés. 

Ils étaient taillés verts et mis en place peu après leur abattage. Chaque poutre était un chêne équarri (tronc taillé en section rectangulaire) à la doloire et conservait le cœur du bois au centre de la pièce.

La scie n’était pas utilisée au XIIIe siècle pour la taille des poutres. 

Pour cette raison, les chênes abattus correspondaient précisément aux sections recherchées, et leur équarrissage se faisait a minima au plus près de l’écorce, avec peu de perte de bois. 

Les bois ainsi taillés étaient indéformables, contrairement aux bois sciés. Les courbures naturelles du tronc étaient donc conservées à la taille, ce qui n’était en rien un handicap pour les charpentiers du XIIIe siècle. À l’encontre, là aussi, de certains préjugés, les bois provenaient à plus de 90% de chênes de moins de 60 ans, au fût de faible diamètre (23-28 cm) et élancé sur une grande hauteur (10-16 m),avec peu de branches. Ces arbres jeunes, fins et longilignes, provenaient de hautes futaies où la densité du peuplement était maximale, la forte concurrence entre les chênes les contraignant à pousser rapidement vers la lumière, en hauteur, non en épaisseur. Ces futaies, appartenant le plus souvent à l’évêché ou au chapitre, étaient gérées selon une sylviculture spécifique, reposant sur la régénération par coupe à blanc, par rejet de souche, et sur l’absence d’éclaircie, afin de produire rapidement et massivement des peuplements extrêmement denses, et donc des chênes adaptés à la construction et à la taille à la hache.»

Frédéric Epaud. Les forêts et le bois d’œuvre au Moyen Âge dans le Bassin parisien. Bépoix S. et Richard H. (dir.). La forêt au Moyen Âge, pp.142-153, 2019, 978-2-251-44988-3. ⟨hal-02401264⟩

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New materials and methods for timber construction

Building Culture PLA is developing new, environmentally and socially responsible methods and components for timber construction, appropriate for one-to three-story structures. Our innovations relate to how certain structural elements (posts and beams) are produced and how these elements are integrated with conventional construction methods and materials.

Our system is, in some ways, a response to standard approaches to timber construction. In many countries, including the United States, Canada, and Sweden, wood building products are commodities manufactured by capital- and energy-intensive, high-volume, export-driven industries. Every step of a commodity manufacturing process aims to overcome the variation that characterizes its feedstock. Just as a chicken, viewed within commodity production, is not a particular bird of the genus Gallus domesticus but a bundle of muscle protein, so a tree, for commodity production, is not a singular tree but a source of homogeneous wood fiber. The most effective way to homogenize a tree is to decompose it. Flaking, veneering, boiling, and crushing operations produce OSB, plywood, LVL, and other types of engineered wood products. Glulam, CLT, and dimensional timber are produced by plain-sawing the tree in parallel slices. Knots and other irregularities, as well as stresses induced by differential radial and circumferential shrinkage, introduce defects. These irregularities are overcome through sorting and finger-jointing. To produce glulam and CLT, finger-jointed timbers (lamellas) are glued together with petroleum-based binders and adhesives. Each step of the decomposition/re-composition process demands energy and capital. Along the way, the macroscopic characteristics of a tree—properties that make the tree a brilliant structural proposition—are destroyed.

Commodity production has many advantages: resulting products are inexpensive, ubiquitous, and standardized. However, our research asks if there might be another, very different way to manufacture high-value timber building components—an approach that produces elements that may not be as cheap, universal, and uniform as commodities, but which are not overly expensive, not hard to obtain, and not difficult to integrate with standardized processes. We are developing production and construction methods that are less time- and skill-intensive than traditional craft methods, and less capital- and energy-intensive than conventional industrial methods. In order to match or offset the economic advantages, and overcome the environmental and social disadvantages, of standard industrial production methods for timber building products, we are applying the following ideas:

For some applications, use trees felled during forestry thinning operations. Today these trees are processed in a low-value way: pelleted, pulped, or flaked.

Instead of destroying an important structural property of the tree—the organization of stronger fibers at the outer circumference of the trunk, at the maximum distance from the neutral axis in bending—take advantage of this property in the final structural element.

Don’t plain-saw. Cut the tree in a way that minimizes stresses induced by differential radial and circumferential shrinkage. Work with a timber’s desire to shrink and bend, not against it.

Increase the percent of the feedstock—the debarked tree—that is retained as solid wood in the final building product. Trees are irregular tapered forms. In conventional processing operations, harvested trees are shaved or turned into a regular cylinder. We eliminate this operation and use the tree trunk in the shape it naturally is: a truncated cone.

Instead of chopping the tree’s continuous fibers, retain the maximum number of continuous fibers in the final building product. Locate the continuous fibers where they are the most structurally effective.

Reduce the number of manufacturing operations between felling the tree and shipping the finished element. Use inexpensive equipment.

Don’t process the tree into a perfect rectangular solid. Only machine a timber in places/directions where it needs to be regularized in order to facilitate manufacture, construction, and layout. In other places/directions retain the character of the wood. Reveal its beauty and origin as a tree.

Produce components that are longer than the industry limit for dimensional sawn timber (5.4 meters or 6.0 meters for finger-jointed timbers) and less expensive than conventional engineered wood products like glulam and LVL (which are readily produced in lengths greater than 6 meters). In many situations, posts, beams, joists, and studs longer than 6 meters are highly desirable. Longer timbers allow

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Using longer elements, the resulting structure is stiffer, stronger, and faster to assemble. Sweden’s transit regulations allow trucking of components 12 meters long. Standard glulam and LVL products can readily be used in such situations, but they are overpriced and over-engineered for many applications.

Instead of producing building products in high-volume, energy- and capital-intensive plants that draw their feedstock from a large geographic territory, produce them in local factories that draw upon locally sourced timber, use less energy, require less capital, and employ more people.

Instead of marketing these building components as a commodity, market them as components suited for certain applications. For example, the elements we are developing are highly compatible with small-scale prefabricated wall panels: building elements that could themselves be produced in local, small-scale, low-capital, energy-efficient factories. Production of the structural components we are developing could be vertically integrated, not only with production of custom prefabricated wall panels, but also with design and build services. An approach like this could introduce greater diversity and employment opportunity in the building industry. Sweden, where our research and development is being carried forward, has the most monopolized, energy-intensive, skill-intensive, and capital-intensive building industry in the world. In its Policy for the Designed Environment (prop. 2017/18:110), the Swedish government asks that “new and innovative material combinations and solutions be [developed and] promoted so that the use of sustainable materials can increase and that more sustainable buildings can be built… Increased industrial timber construction, based on sustainably produced forest raw materials, contributes to the development of a circular and bio-based economy and to new sustainable housing, and can create increased employment throughout the country.”

about – building culture PLA

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Louis Carmontelle Panorama transparent d'un paysage imaginaire 1790