Timber Structures

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<p>Timber StructuresINTRODUCTION</p> <p>Timber from well-managed forests is one of the most sustainable resources available and it is one of the oldest known materials used in construction. It has a very high strength to weight ratio, is capable of transferring both tension and compression forces, and is naturally suitable as a flexural member. Timber is a material that is used for a variety of structural forms such as beams, columns, trusses, girders, and is also used in building systems such as piles, deck members, railway sleepers and in formwork for concrete. There are a number of inherent characteristics that make timber an ideal construction material. These include its high strength to weight ratio, its impressive record for durability and performance and good insulating properties against heat and sound. Timber also benefits from its natural growth characteristics such as grain patterns, coloursand its availability in many species, sizes and shapes that make it a remarkably versatile and an aesthetically pleasing material. Timber can easily be shaped and connected using nails, screws, bolts and dowels or adhesively bonded together. STRUCTURE OF TIMBER Structural timber is sawn (milled) from the trunk of the tree, which provides rigidity, mechanical strength and height to maintain the crown. Trunk resists loads due to gravity and wind acting on the tree and also provides for the transport of water and minerals from the tree roots to the crown. Roots, by spreading through the soil and acting as a foundation, absorb moisturecontaining minerals from the soil and transfer them via the trunk to the crown. Crown, comprising branches and twigs to support leaves, provides a catchment area producing chemical reactions that form sugar and cellulose that cause the growth of the tree.</p> <p>TYPES OF TIMBER Trees and commercial timbers are divided into two types: softwoods and hardwoods. This terminology refers to the botanical origin of timber and has no direct bearing on the actual softness or hardness of the wood as it is possible to have some physically softer hardwoods like balsa from South America and wawafrom Africa, and some physically hard softwoods like the pitchpines. SOFTWOODS Softwoods, characterized by having naked seeds or as cone-bearing trees, are generally evergreen with needle-like leaves (such as conifers). Rays, present in softwoods, run in a radial direction perpendicular to the growth rings. Their function is to store food and allow the convection of liquids to where they are needed. Examples of UK grown softwoods include spruce (whitewood), larch, Scots pine (redwood) and Douglas fir SOFTWOODS CHARACTERISTICS Quick growth rate (trees can be felled after 30 years) resulting in low-density timber with relatively low strength. Generally poor durability qualities, unless treated with preservatives. Due to the speed of felling they are readily available and comparatively cheaper. HARDWOODS Hardwoods are generally broad-leaved (deciduous) trees, which often lose their leaves at the end of each growing season. Due to the necessity to grow new leaves every year the demand for sap is high and in some instances larger vessels may be formed in the springwood, these are referred to as ring-porous woods such as in oak and ash. When there is no definite growing period the pores tend to be more evenly distributed, resulting in diffuse-porous woods such as in poplar and beech. Examples of UK grown hardwoods include oak, beech, ash, alder, birch, maple, poplar and willow. HARDWOOD CHARACTERISTICS Hardwoods grow at a slower rate than softwoods, which generally results in a timber of high density and strength, which takes time to mature, over 100 years in some instances. There is less dependence on preservatives for durability qualities. Due to the time taken to mature and the transportation costs of hardwoods, as most are tropical, they tend to be expensive in comparison with softwoods</p> <p>PRODUCTION OF TIMBER SECTIONS</p> <p>The transformation from living forest tree to finished construction section involves four basic processing stages 1. Felling 2. Conversion 3. Seasoning 4. Stress grading 1. FELLING Felling is part of the whole forestry process. The forestry department ensures that the timber is sourced from well managed sustainable supplies. Felling of trees can be done by either chain saw or mechanical harvester 2. CONVERSION Conversion is the term used to describe the processes in which the felled trunk is sawn into marketable sizes of timber. After debarking at the sawmill, the logs are sawn lengthwise into the required sections using variations on the two basic methods of plain sawing and quarter sawing. The first step in most sawmill operations will start by scanning the log for the best alignment and cutting pattern for optimum return; then removing one or two wings (slabs) from the logs to give some flat surfaces to work from. The log, referred to as a cant, is turned on a flat face and sawn through and through to give boards (sections) of the required thickness.</p> <p>3. SEASONING When a tree is cut and its moisture content falls to around 27%, the only moisture left is the bound water, which is the moisture that is part of the cell wall (This state is referred to as fibresaturation point.) Wood, in general, is dimensionally stable when its moisture content is greater than the fibresaturation point. The process of drying (seasoning) timber should ideally remove over a third of the moisture from the cellwalls. Timber at this stage is referred to as seasoned with a moisture content of between 12 and 25%.</p> <p>The seasoning of lumber, the process of removing moisture from wood to bring the moisture content to an acceptable level, can be achieved through air drying or kiln drying. Air drying involves stacking lumber in a covered shed and allowing moisture loss or drying to take place naturally over time due to the presence of air. (Fans can be used to accelerate the seasoning process). Kiln drying involves placing timber pieces in an enclosure or kiln at significantly higher temperatures. The kiln temperature has to be strictly controlled to prevent damage to the wood members from seasoning defects such as warp, bow, sweep, twists, or crooks 4. GRADING Timber is usually cut from a tree log in the longitudinal direction, and because it is naturally occurring, it has quite variable mechanical and structural properties, even for members cut from the same tree log. Timber of similar mechanical and structural properties is grouped into a single category known as a stress grade. This simplifies the timber selection process and increases economy. The higher the stress grade, the stronger and more expensive the wood member is. The classification of timber with regard to strength, usage, and defects according to the grading rules of an approved grading agency is termed timber grading. DEFECTS IN TIMBER The various categories of defects in wood are natural, conversion, and seasoning defects. Some natural defects are knots, shakes, splits, and fungal decay. Conversion defects occur due to unsound milling practices, one example being wanes. Seasoning defects result from the effect of uneven or unequal drying shrinkage, examples being various types of warps, such as cups, bows, sweep, crooks, or twists. The most common types of defects in wood members include the following: 1. Knots. These are formed where limbs grow out from a tree stem. 2. Split or check. This occurs due to separation of the wood fibers at an angle to annual rings and is caused by drying of the wood. 3. Shake. This occurs due to separation of the wood fibers parallel to the annual rings 4. Decay. This is the rotting of wood due to the presence of wood-destroying fungi. 5. Wane. In this defect the corners or edges of a wood cross section lack wood material or have some of the bark of the tree as part of the cross section. This leads to a reduction in the cross-sectional area of the member which affects the structural capacity of the member.</p> <p>.</p> <p>DEFECTS IN TIMBER</p> <p>ADVANTAGES AND DISADVANTAGES OF WOOD AS A STRUCTURAL MATERIAL Some advantages of wood as a structural material are as follows: Wood is renewable. Wood is machinable. Wood has a good strength-to-weight ratio. Wood will not rust. Wood is aesthetically pleasing The disadvantages of wood include the following: Wood can burn. Wood can decay or rot and can be attacked by insects such as termites and marine borers. Moisture and air promote decay and rot in wood. Wood holds moisture. Wood is susceptible to volumetric instability (i.e., wood shrinks). Woods properties are highly variable and vary widely between species and even between trees of the same species. There is also variation in strength within the cross section of a tree log. TYPICAL STRUCTURAL COMPONENTS OF WOOD BUILDINGS Perspective view of a building cross-section</p> <p>1. Rafters These are usually sloped sawn-dimension timber roof beams spaced at fairly close intervals (e.g., 12, 16, or 24 in.) and carry lighter loads than those carried by the roof trusses, beams, or girders. They are usually supported by roof trusses, ridge beams, hip beams, or walls. The span of rafters is limited in practice to a maximum of 4m to 5m.</p> <p>2. Joists These are sawn-lumber floor beams spaced at fairly close intervals of 0.3m, 0.4m, or 0.6m that support the roof or floor deck. They support lighter loads than do floor beams or girders. Joists are typically supported by floor beams, walls, or girders. The spans are usually limited in practice to about 4m to 5.5m.</p> <p>3. Double or Triple Joists These are two or more sawn-lumber joists that are nailed together to act as one composite beam. They are used to support heavy concentrated loads or the load from a partition wall or a load-bearing wall running parallel to the span of the floor joists, in addition to the tributary floor loads. They are also used to frame around stair openings (see header and trimmer joists). 4. Header and Trimmer Joists These are multiple-dimension lumber joists that are nailed together (e.g., double joists) and used to frame around stair openings. The trimmer joists are parallel to the long side of the floor opening and support the floor joists and the wall at the edge of the stair. The header joists support the stair stringer and floor loads and are parallel to the short side of the floor opening.</p> <p>5. Beams and Girders These are horizontal elements that support heavier gravity loads than rafters and joists and are used to span longer distances. Wood beams can also be built from several joists nailed together. These members are usually made from beam and stringer (B&amp;S) sawn lumber, glued laminated timber (glulam) or parallel strand lumber (PSL), or laminated veneer lumber (LVL).Types of beams and girders</p> <p>6. Hip and valley rafters These are sloped diagonal roof beams that support sloped jack rafters in roofs with hips or valleys, and support a triangular roof load due to the varying spans of the jack rafters. The hip rafters are simply supported at the exterior wall and on the sloped main rafter at the end of the ridge. The jack or varying span rafters are supported on the hip rafters and the exterior wall. The top of a hip rafter is usually shaped in the form of an inverted V, while the top of a valley rafter is usually V-shaped. Hip and valley rafters are designed like ridge beams.</p> <p>7. Columns or posts These are vertical members that resist axial compression loads and may occasionally resist additional bending loads due to lateral wind loads or the eccentricity of the gravity loads on the column. Columns or posts are usually made from post and timber (P&amp;T) sawn lumber or glulam.</p> <p>8. Roof trussesThese are made up typically of dimension-sawn lumber top and bottom chords and web members that are subject to axial tension or compression plus bending loads. Trusses are usually spaced at not more than 1.2m on centers and are used to span long distances up to 36m. The trusses usually span from outside wall to outside wall</p> <p>9. Wall studs These are axially loaded in compression and made of dimension lumber spaced at fairly close intervals (typically, 0.3m, 0.4m or 0.6m). They are usually subjected to concentric axial compression loads, but exterior stud walls may also be subjected to a combined concentric axial compression load plus bending load due to wind load acting perpendicular to the wall.</p> <p>10. Header beams These are the beams that frame over door and window openings, supporting the dead load of the wall framing above the door or window opening as well as the dead and live loads from the roof or floor framing above. 11. Overhanging or Cantilever beams These beams consist of a back span between two supports and an overhanging or cantilever span beyond the exterior wall support below. They are sometimes used for roof framing to provide a sunshade for the windows and to protect the exterior walls from rain, or in floor framing to provide a balcony. For these types of beams it is more efficient to have the length of the back span be at least three times the length of the overhang or cantilever span.</p>