WOOD Week 11 Lecture 1 Tuesday andrea wheeler
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ARCH 542 Week 11 Lecture 1 An introduction to wood as a construction material.

ARCH 542 Week 11 Lecture 1 An introduction to wood as a construction material.

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WOOD Week 11 Lecture 1 Tuesday andrea wheeler WOOD Week 11 Lecture 1 Tuesday andrea wheeler Presentation Transcript

  • WOOD Andrea Wheeler ARCH 542 Sci-Tech 2 Week 11
  • Week 11 WOOD Week 12 MASONARY Week 13 STEEL Week 14 CONCRETE
  • THIS WEEK’S CONTENT (Lectures Tues and Thurs) 1. Physical properties of wood – introduction to understanding wood as a structural and constructional material. 2. Lumber (timber) and the production and supply of timber for the construction industry 3. Wood composites, laminated timber and new materials (e.g. OSB). 4. Wood and moisture: movement, rot and insects 5. Construction uses for wood and timber framing (US). 6. Fastenings, and connections 7. Sustainable use 8. Case studies of timber in structure and construction
  • Labs: 4 groups GROUP RESEARCH THEMES 1. Is wood a sustainable construction material? The case for new innovative wood based construction materials. 2. Why have we seen a development in structural connections and fastenings in timber: From the Old English Barn, log structures to Shiguru Ban? 3. The structural limits of timber structures: How high can we go? 4. Designing with wood – new materials and new methods of engineering, design and construction.
  • Attendance question ? Name four advantages and four disadvantages of wood in construction … ARCH 542
  • Advantages Disadvantages Strong Natural material, not precise, not perfectly straight and subject to flaws (growth defects) Light Affected by moisture – will size, and shape (will change dimensionally through seasons). Easily worked and shaped Can split and warp. Grading prevents this. Fastened quickly and economically Burns easily (Oak is more resistant in a fire than exposed steel as it produces a charred layer and burns slowly) Recyclable Decays (hardwoods are more naturally durable than softwoods). Some species very durable – Greenheart. Biodegradable and nontoxic (preservatives added to timber are toxic) Subject to insect damage (keeping timber dry and detailing well will prevent this) Renewable (it can regenerate and it can be properly managed and harvested) Some rare timbers should not be used.
  • Approximately 90% of buildings constructed in the US each year are framed with wood, whilst that percentage is only 15 – 20 percent in the UK. The use of green oak is becoming more common in the UK as a structural material than any other country. Green oak means freshly cut oak, drying (which will normally take place before construction) takes place in service (over 20 – 30 years). Green oak frames are subject to movement.
  • Freshly cut timber has a high moisture content, medieval carpenters used “green oak” but also developed a vocabulary of structural connections that allowed movement to happen.
  • Strategies to allow for movement in green oak structures
  • Six Story Office Building Design by Shigeru Ban, Switzerland http://www.designboom.com/architecture/shigeru-ban-tamedia-office- building-in-zurich-completed/
  • Physical properties of wood • Strong in tension, weak in compression and shear • Steel and concrete are isomorphic materials, wood is not – orthotropic /directional (evident where parallel to the grain 100% stronger than perpendicular to the grain). • Movement – sensitive to humidity, absorbs water from the air. • Durable. Above 13% moisture content, subject to rot, insect infestation.
  • Because wood is a natural material and the tree is subject to many constantly changing influences (such as moisture, soil conditions, and growing space), wood properties vary considerably, even in clear material. The variation of strength between different directions can be attributed to the fine structure of the wood cells.
  • Directions of Wood • Longitudinal direction: parallel to the long axis of the stem. • Radial direction: perpendicular to both the growth rings and the long axis of the stem. • Tangential direction: tangent to the growth rings.
  • Wood composition The anatomy of wood
  • Electron microscope images of porous wood cell structure. It is the long cells (20m in pine) with tough cellulose bound by lignin walls that provide the strength of the woods. Spring wood and summer wood – spring wood cells and denser/ summer wood cells larger and less dense. WOOD – material structure = “A bundle of straws”
  • SOFTWOODS – Evergreen trees. Douglas Fir, Southern Pine, fir. Typically less expensive than hardwoods. Structural and constructional use. Limited uses in finishes or as veneers.
  • Softwoods and hardwoods: Differences in the anatomy
  • Softwood (e.g. Pine) Hardwood (e.g. Oak)
  • Medullary Rays in Oak giving figure / interest to wood
  • Strong in tension, weak in compression and shear (but isomorphic) Strength is also affected by moisture content. Shear (splits) Compression (weak) Tension
  • Mechanical behavior Nails close to the end splits lumber easily because of the weak bonds between the fibers.
  • Water in wood • Wood is a hygroscopic material, i. e., it will absorb water vapor from the atmosphere • Moisture content in wood is expressed as a percentage of ovendry weight.
  • Water in wood - moisture and movement Water may be held in wood in two ways: • bound water • free water Bound water is held in the cell walls by adsoption force. • It is generally believed that bound water is not in the crystalline regions of the cell wall, but is absorbed in the amorphous regions • This has important implications for the volume changes associated with moisture changes. Free water is not held by any forces and is situated in the cell cavities known as lumin.
  • Moisture Content - Fiber Saturation Point (FSP) • The moisture content at which the cell wall is saturated with bound water and at which no free water is present is called the fiber saturation point (FSP) • The FSP varies from species to species, but it averages about 28% moisture content • Addition of removal of water below the FSP has a pronounced effect on practically all wood properties. • Addition or removal of water above the FSP has a almost no effect on any wood properties.
  • Shrinkage and swelling of wood • The variation of shrinkage between different directions can be attributed to the microstructure of wood. • The latewood cells dominate shrinkage since they absorb much more water and in the tangential direction there is an unbroken alignment of latewood. • The greater shrinkage in the tangential direction causes distortion in lumber with different orientations.
  • Class question A piece of wood containing moisture weighs 205g, and after oven drying to a constant weight, it weighs 110g. What is the percent moisture content? [slide 28] (205 – 110)/110 = 86.4%
  • Class question A piece of wood contains 18% moisture. What must its weight have been before oven drying if it has a constant weight of 140g after drying? [slide 28] (x-140)/ 140 = 0.18 x = 165.2g
  • Lumber (timber) and the production and supply of timber for the construction industry – from harvest to commercial product • Hardwoods and softwoods differ in the production for construction. • The production of wood for construction – from log to board: Sourcing, sawing/cutting, seasoning, planeing/ surfacing, preservative treatment, grading, and distribution.
  • Lumber production - SAWING • Maximum yield – cost efficiency • Varying grain pattern (especially in hardwoods) to maximize the figure of the wood (pattern of the grain)
  • Through cut
  • Lumber seasoning/ drying • Air seasoning: lumber dries naturally. Piles of lumber are stacked in a way that air can circulate freely. The process can take months. • Kiln seasoning: Warm air circulates through the pieces of lumber, controlled – defects can happen if too quick or too hot.
  • Why season or dry wood? • Equilibrium: when wood is used as a construction material, whether as a structural support in a building or in woodworking objects, it will absorb or desorb moisture until it is in equilibrium with its surroundings. • Drying causes unequal shrinkage in the wood, and can cause damage to the wood if equilibration occurs too rapidly. The equilibration must be controlled to prevent damage to the wood (i.e. seasoned) • Wet wood is subject to rot and beetle infestation. • Increases strength and stiffness • Reduces weight
  • Seasoning flaws • Warp – bow, crook, cup • Surface checks and splits • Stains (resins)
  • ESSENTIAL INFORMATION FOR ARCHITECT/ DESIGNERS
  • Labs: 4 groups GROUP RESEARCH THEMES 1. Is wood a sustainable construction material? The case for new innovative wood based construction materials. 2. Why have we seen a development in structural connections and fastenings in timber: From the Old English Barn, log structures to Shiguru Ban? 3. The structural limits of timber structures: How high can we go? 4. Designing with wood – new materials and new methods of engineering, design and construction.