Use of Wood as a Building Material
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Published: Mon, 30 Apr 2018
Wood is quite unique when compared to most building materials used today given that its material makeup is a result of naturally grown biological tissue (ill.18). Thus, the material makeup and structure of wood is significantly different than that of most industrially produced, isotropic materials. Upon close examination, wood can be described as an anisotropic natural fiber composite. In contrast to isotropy, which constitutes identical properties in all directions of a material, anisotropy concerns the property of being directionally dependent. For instance, one can see this in the way that wood can bend easily in the tangential axis (ill.19) which is the direction perpendicular to its grain direction. When examining wood from any given angle, one can identify material characteristics and behaviours specific to that angle, relative to the material’s main grain orientation. That is to say, should one examine the material properties of wood at an angle 45 degrees to the main grain orientation, one will discover properties extremely different than those obtained from an angle 90 degrees to the main grain orientation.
The directionally dependent property of wood is a result of the horizontal or vertical orientation of the individual cells and the arrangements of growth layers in a tree. Throughout architectural history, this inherent heterogeneity of wood as well as its complex material characteristics have often been characterized as deficiencies by architects, engineers and members of the timber industry. This can be traced to the fact that most designs and construction methodologies used today require the use of materials bearing minimal variations in their properties and behaviours in order to satisfy the need for isotropic structures.
In contrast, this thesis views wood’s complex material makeup and its capacities as significant advantages rather than deficiencies. Furthermore, it aims to understand these interesting characteristics of wood and employ them through an informed design process.
In addition to these complex material properties, wood also presents many favorable characteristics including diversity, weight, strength, appearance, workability, cost and availability. Another factor that makes wood a very appealing material today concerns its overall ecological advantages. In light of the environmental challenges that the built environment is facing today, it is becoming increasingly recognized that very few building materials can rival wood’s environmental benefits. Wood is a natural, renewable material that holds a very low level of embodied energy. It is known for its ability to reduce carbon dioxide emissions by storing CO2 and also by substituting for materials with a high carbon content. In this manner, the use of wood actually produces a positive carbon footprint. Wood is also an extremely energy efficient building material in its production. For example, wood requires 50 times less energy in its manufacturing than steel to ensure a given structural stiffness as a whole.
Unlike many natural resources, forests consist of a renewable resource. With careful forest management, one can ensure that forests thrive and continue to provide the many benefits to which we have become accustomed. Foresters can calculate an ‘allowable cut’ of trees per year for any given forest area that will secure a stable harvest. Tree farming is yet another way of sustainably satisfying today’s demand for wood. Programs at Oak Ridge National Laboratory have engineered a breed of super trees that can grow at rapid speeds in order to create a substantial amount of bio mass in a single given acre. These engineered trees are being farmed at tree farms such as the Boardman Tree Farm LLC, and are redefining modern forestry (ill.20). The Boardman Tree Farm plantations are located in eastern Oregon, United States, where dry desert land has been transformed into a thirty thousand acre farm. This plantation currently has seventy million trees and is capable of producing half a million trees every year to satisfy demands. The plantation harvests five acres of trees every day in order to maintain this continuous cycle.
As a result of wood’s naturally-grown origin, its unique material composition accounts for most of its properties and characteristics. The aim of the thesis is to explore some of the potential ways of utilizing the material properties and specific material characteristics of wood in the design field. In order to do so, the heterogeneous structure of wood must first be understood in greater detail.
Wood can be defined as a low-density, cellular, composite material and as such, does not readily fall into a single class of material, but rather overlaps a number of classes. In terms of its high strength performance and affordability, timber remains the world’s most successful fiber composite. On the microscopic scale, one can describe wood as a natural fiber composite. (Ill.21)
Wood cells are comprised of layers, upon which cellulose microfibrils function like fibers embedded in a matrix of lignin and hemicelluloses, reinforcing the assembly as a whole. Due to this makeup at the microscopic level, wood shares a number of properties with materials like: synthetic composites, reinforced plastics, fiberglass, and carbon fiber. Similar to wood, these materials are characterized with relatively low stiffness in combination with relatively high structural capacity. In other words, wood contains innate elastic properties especially well-suited for construction methods that seek to employ elasticity in achieving complex lightweight structures from initially planar elements.
What follows is intended as a brief overview of the material composition of wood. Understanding the anatomical aspects of wood is imperative to the research and investigations that have been conducted.
In contrast to building materials that are specifically designed and manufactured to suit the needs of an architect or an engineer, wood is a result of the biological tissue functions that take place in a tree. Although there exists a wide variety of species of trees in the world, all trees, despite their diversity, share certain characteristics. Trees are all vascular and perennial which means they are capable of adding yearly growth to previously grown wood. The growth process of a tree occurs in the cambium, a thin layer of living cells between the bark of the tree and the inner stem structure. (Ill.22) Cambial cells have thin walls and divide themselves lengthwise to grow into two new cells. Following the cell division, one of the two cells enlarges to become another cambial mother cell while the other either matures into a bark cell or forms towards the inside of the cambium to become a new wood cell.
When the primary wood cells reach maturity and develop into their mature size, a secondary wall is constructed from long chain hemicellulose and cellulose molecules. The long chains of cellulose molecules are oriented in a direction parallel to the long axis of the cells and reinforced by lignin (ill.23). Lignin is an integral part of the wood’s cellulous structure because it provides support for the cells. It is also the material that gives rigidity to plants. The distribution and orientation of the cells along with the material structure of the cell walls determine most of the resulting characteristics and properties of wood.
Trees are characterized into two types: softwoods and hardwoods (ill.24). The terms ‘softwood’ and ‘hardwood’ do not signify softness or hardness of wood. The two terminologies are related to the botany of the species and to the way in which a tree grows. The differences between the two types of wood can be seen in the cellular structure of the materials. In the relatively simple cellular structure of softwood, nine tenths of the wood volume consists of one cell type called “tracheid”, while the remainder consist of ray tissues. Tracheids are fiber-like cells and have a length-to-width ratio of 100:1, meaning that they are approximately one hundred times longer than they are wide. The tracheid cells are arranged parallel to the stem axis located in the radial layers of the tree and are responsible for the transport of water and minerals throughout the tree.
In contrast, a much greater variety of cell types and arrangement configurations are present in hardwoods. In addition to tracheids, hardwoods also contain vessels, rays and fiber cells. Vessel elements in hardwood have a large diameter and thin walls, containing no end-to-end walls. As a result, they are arranged in an end-to-end formation that is parallel to the stem axis of the tree, forming continuous channels that carry sap through the tree. Unlike vessels, fiber cells are much smaller in diameter and have thicker cell walls and possess closed tapered ends (ill.25). In both softwood and hardwood, the structure, distribution and orientation of cells are the determining factors of the anisotropic, structural, and hygroscopic characteristics of wood.
The anisotropic and hygroscopic characteristics of wood resulting from its internal cellular structure have traditionally been regarded as problematic in the practices of architecture and structural engineering, especially when compared to more homogeneous, stable, industrially produced isotropic materials like steel, plastic or glass. In design approaches within architecture, engineering and timber industries, knowledge of wood’s material composition and characteristics has mostly been employed to counterbalance its complex material behaviours. For instance, the development of engineered industrial wood products (ex: MDF, or cross-laminated-timber) came as a response to the heterogeneous composition of wood. These wood products are capable of producing a material that is much more homogenous and which provides isotropic material characteristics.
Unfortunately, the design opportunities that could be made possible using the innate heterogeneous characteristics of wood are too often overlooked in today’s construction projects. In fact, particularly in North America, the construction material of wood is often no longer referred to as such. Instead, wood is referred to as a dimensional building element, such as a ‘2×4’. The aim of this research is to propose an alternative approach to design which views wood’s complex material composition and related behaviours as advantageous rather than problematic. Such an integrated design approach can perhaps contribute towards a renewed appreciation for the behavioral capacities of wood and the rich design opportunities that can be realized thanks to the natural anatomy of this material.
Three-ply plywood and veneer are unmistakably industrially-produced materials. However, unlike other industrially-produced materials such as steel, glass, plastic, MDF or particle board, three-ply plywood and veneer are anisotropic materials. This signifies that the properties and behaviours of these materials vary significantly in relation to the fiber direction. For example, veneer and plywood encounter considerable differences in stiffness depending on the grain direction. The compressive strength of wood differs significantly depending on grain direction, as do most of its other mechanical and material properties. The following section details the manufacturing process of veneer and plywood in order to better understand the material exploration that will be presented in Chapter 3.
Plywood may appear to be a relatively new industrially-produced wood product, however its concept is in fact very old and can be traced back to more than 5,000 years. Before the word “plywood” was invented in the 1920s, the process was referred to as veneering. One of the earliest traces of plywood was found in the tomb of King Tutankhamun, an Egyptian Pharaoh who ruled around the year 1334 BC. The discovered pieces of plywood were remains of coffins made of six layers of wood, each 4mm thick and held together by glue and wooden pegs. The plywood remains were fabricated using the same fundamental techniques as today. Like modern plywood, the grains of the layers where arranged perpendicularly with each layer for strength (ill.26). From this period onwards, veneering techniques became increasingly widespread throughout the world. Thanks to the development of tools and technology over the years, veneer thicknesses were reduced and new adhesives (ex: glue made from bone, sinew and cartilage) were used to bond the layers together with heat.
Although plywood is made much in the same way today, modernized adhesion techniques and tools used in its production have improved significantly, making it one of the most affordable and easily-produced building materials. Both hardwoods and softwoods are used in the production of plywood. The typical sequence of operation involved in the production of plywood is as follows:
There exists a long standing discourse on the subject of sheet materials in architecture, in part because these are so ubiquitous in conventional construction. Expanding the understanding of these materials is valuable to the architectural profession, as it allows one to discover new potentials concerning materials which are already familiar. Being a sheet material, plywood thus offers many advantages as a subject of research and experimentation. Like other sheet materials, it can facilitate the creation of complex geometry using initially planar elements. Three-ply plywood is the material of choice for this thesis due to its ability to offer high amounts of flexibility in one direction, without compromising its strength. Three-ply plywood, as previously described, is made up of odd layers, two of which are oriented in one direction, while the center layer lies perpendicularly to the outer layers. Thus, due to the predominant fiber direction present in the two outer layers, three-ply plywood possesses a natural tendency to bend perpendicularly to this grain direction. The core of the assembly, otherwise known as the center layer, provides strength to the assembly by offering resistance to the predominant fiber direction. As a result, the plywood assembly is less likely to break or snap when being bent because it is reinforced by one interior sheet containing fibers running perpendicular to the outer layers.
Knowledge of the manufacturing process for plywood is important for this research because it provides an introduction to lamination techniques that can be further utilized in the material investigations and implementations that will follow. The process described above elaborates on the procedure involved in the mass-produced manufacturing of flat plywood sheets used in the building industry. However, the process of lamination need not strictly apply to planar surfaces, but also to the development of three-dimensional forms.
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