WOOD “Crisscrossing light wood frame recalls Gothic and surrounding trees while lifting the mind skyward.” E. Fay Jones and Associates. • Strong & Stiff • Light • Easily worked / shaped • Fastened quickly & economically • Recyclable • Biodegradable • Renewable Resource Not perfectly straight nor precise Size & shape affected by moisture Contains growth defects Can spilt & warp Burns easily Decays Susceptible to Insect Damage Softwoods come from coniferous trees which are evergreen (do not lose their leaves), needle-leaved, cone-bearing trees. Softwoods also grow faster than hardwoods and are therefore less expensive. This type of timber is produced from broad leaf trees that lose their leaves in winter – a deciduous tree. Hardwood is more dense than softwood and scratches less easily, and usually is harder though not allways. For example balsa is used for modelling, it is a hard wood but it is one of the softest timbers to work with. Various defects which are likely to occur in timber may be grouped into the following three: (i) Due to natural forces (ii) Due to defective seasoning and conversions. (iii) Due to attack by fungi and insects. (i) Defects due to Natural Forces: The following defects are caused by natural forces: (a) Knots (b) Shakes (c) Wind cracks (d) Upsets (a) Knots: When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. Figure 1.9 shows some varieties of knots. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot. The essential difference between the wood from hardwoods and softwoods is the presence of vessels in hardwoods as shown in the above diagrams. These are continuous pipes running the length of the tree and serve as conduits for water and nutrients in the outer layers of wood in a growing tree. In hardwoods, the cells are closed and cannot function as conduits. In softwoods, the cells have openings to other cells. This means the cells are the nutrient conduits. The actual cells in the softwood species have the same function as the vessels in hardwoods. The open cell structure of softwoods makes them generally more receptive than hardwoods to preservative treatments to enhance durability. Shakes: The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes. Defects due to Defective Seasoning and Conversion: If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning. In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc. Fungal Attack causes wood to decay, resulting in loss of weight and strength Fungi are parasitic plants whose spores cause the cells to collapse Certain conditions are necessary for fungi to develop i.e. low cold temperature and especially a high moisture content. This is a sponge like fungus and is the most common in timber. It thrives in unventilated situations where there is a lack of circulated air. It is known as dry rot as it reduces the wood to a dry soft powdery condition. Dangerous in buildings because of the speed it can spread. Spread by wind and animals and even penetrate masonry. Timber subjected to alternate wetness and dryness begins to decompose. Outside wood is vulnerable The timber becomes spongy when wet and brittle when dry Unlike dry rot it does not spread so it can cut back and replaced. Generally affect warmer climates Termites Wharf borers or weevers which affect underwater installations. Pin hole borers which only attack certain types of timber in its green state. Common furniture beetle known Death Watch beetle Powder Post or Lyctus Beetle House houg horn beetle Wet Rot - This is a decomposition of sap and fibers of the living tree set up by the access of water through wounds in the barks. Dry Rot - This is a decomposition of converted timber induced by the commonest and most destructive dry rot fungi or weeping fungus. Due to fungal Attack – A fungus starts its life as a tiny seed or spore, one of millions produced at the fruiting stage. o Fungi are system plant organisms which live on and attack timber causes it to rot or decay. o Fungi can only exist in following condition: Air for their respiration Moisture Suitable Temperature Food Supplies Beetles 4 types of beetle Flour like powder Boarers BOA RS Graded by: Strength & Stiffness (Structural Lumber), or Appearance (Finish Lumber) Lumber sold by - Species and Grade Better Grade Higher price Scarce or Higher Quality Species Higher price Only major renewable structural material Forestry Practices Sustainable forestry Clearcutting & replanting Mill Practices – Lumber Recovery Factor (LRF) Transportation Embodies Energy Content