12. TIMBER

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Introduction

– The principle characteristics of timber are:

• strength,

• durability

• finished appearance – All of these are derived from natural characteristics

present in the growing tree.

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Strength

• Strength of timber is affected by;

– Density,

– moisture content,

– grain structure as well as by the various defects.

• The more dense the stronger it is.

• Density is an indication of strength.

• Dense timber has thicker cells walls (contribute to the strength of the timber).

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Cell structure

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Strength (cont-d)

• Strength decreases with increase of moisture content.

• Moisture: causes fungal growth or the attraction of insects.

• Grain structure and continuity are of significance in strength context.

• Any disruptions due to growth defects will induce a reduction in strength.

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Stress Grading

• In order to design a timber structure properly, following properties of timber should be known: – Permissible bending stress

– Permissible shear stress

– Permissible compressive stress perpendicular to grain

– Permissible compressive stress parallel to the grain

– Modulus of elasticity (stress/strain)

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Stress Grading (cont-d) • Grading of timber can be determined in two ways.

– Visual stress grading: • This take into account the size of defects, knots, slopes of

grains etc. • Can be carried out rapidly by experienced grader,

• weakness of the method: strength is completely disregarded.

– Mechanical stress grading: • The strength of timber can be directly obtained from the mechanical

stress grading.

• modulus of elasticity can be found out from bending test, by measuring deflection of a timber under a bending loading.

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Moisture Content • The moisture content (MC) is quantity of moisture contained expressed

as a percentage of the dry weight. – MC(%) = 100 x (Weight of specimen–dry weight of specimen) / Dry weight of

specimen

• In green timber, moisture is contained within cells and the cell walls.

• The moisture may be removed from the cells without any effect other than a reduction in bulk density.

• Fiber Saturation Point: The condition when all the cells are empty but the cell walls are still saturated is referred to as the fibre saturated point, (between 23 and 27 percent moisture content).

• Any further reduction in moisture content results in shrinkage of timber.

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T IMBER SEASONING (air & kiln)

Air Seasoning • The timber is stacked in open-sided sheds to promote drying without

artificial assistance.

• Layers are provided with air space by means of spacers or sticks.

• Quick drying softwoods may be stacked in spring or early summer.

advantages of air seasoning:

cheaper method with very little loss in quality of timber if done properly.

disadvantage of air seasoning:

both timber and space are immobilized for a long period.

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Kiln Drying

• Method employs a heated, ventilated and humidified oven.

• Must be used to reduce the moisture content below 17 percent.

• The outer parts of the timber tend to shrink - splitting might occur.

• It is necessary to humidity the circulating air in order to control the rate of evaporation.

• Different species of timber withstand different initial kiln temperatures.

• Air circulating should be uniform over the face of the timber.

• velocity of the air through the pile should be sufficiently high to be consistent with economic operation.

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The essentials of a kiln

• Heat; under proper control and sufficient to raise the temperature to the maximum required.

• Humidification; also under proper control and sufficient to meet all requirements.

• Air circulation; uniform and of sufficient velocity.

• Air interchange; exhaust some of the air from the system from time to time and replace it with fresh air from outside.

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Class i f i ca t ion o f T rees

(so f twoods & Hardwoods) Softwoods:

• trees have needles rather than leaves

• needles stay green through the winter and remain on the tree for two or more years.

• They are cheaper than hardwoods.

Hardwoods:

• trees have broad leaves, which are normally shed in the fall.

• densest, strongest and most durable timber.

• Some hardwoods contain resins and/or oils, which interfere with the hardening of paints.

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Manufacturing of Lumber

• In the sawmill logs may be quarter sawed or

slash sawed.

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Manufacturing of Lumber (cont-d)

• The obtained lumbers will be preceded either as kiln drying or air-drying.

• To manufacture kiln-dried lumber: – logs are sawed into rough lumber, which is first kiln dried

and then surfaced (planed or dressed) to final finished size.

• Lumber to be air dried is sawed and dressed oversize while green (so that after drying it will be equivalent in size to lumber which has been surfaced dry).

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Manufacturing of Lumber (cont-d)

Standard softwood lumber is classified according to;

– Species (fir (köknar), spruce pine, cedar(sedir)

– use,

– extent of manufacturing,

– size.

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Manufacturing of Lumber (cont-d)

Softwood lumber is classified by use into three groups: 1. Yard lumber:

for use in ordinary construction or generally building purposes.

2. Structural lumber:

Lumber is at least 5 cm in nominal width and thickness, to be used where working stresses, are required.

3. Factory and shop lumber:

Preliminary for remanufacturing purposes

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Softwood lumber is classified according to the manufacture as follows:

i) Rough lumber:

Lumber, which has not been dressed but has been sawed, edged and trimmed.

ii) Dressed (surfaced) lumber:

Lumber, which has been dressed by a planar in order to obtain a smooth surface and uniform of size.

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iii) Worked lumber:

Matched lumber:

has a tongue on one edge and a matching groove on the opposite edge, to provide a close-fitting joint between two pieces.

Shaped lumber:

has been rebated on both edges of each pieces, so that two pieces will fit together with a close-lapped joint.

Pattern lumber:

has been shaped to a pattern or molded form, in addition to being dressed, matched or shiplaped or any combination of these.

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Softwood lumber is classified according to size by nominal size

and by dressed size.

Nominal sizes include the following:

– Board:

• less than 5 cm in nominal thickness and 5 cm or more in nominal width.

– Dimension:

• from 5 cm to but not including 12.7 cm in nominal thickness and 5 cm or more in nominal width. Further classified as framing, studs, rafters, planks and joists.

– Timber:

• Lumber, which is 12.7 cm or more nominally in least dimension. Timber may be further classified as girder, stringers, beams, posts and purlins.

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Hardwood Lumber

• Produced from trees, those that drop their leaves each year including – ash,

– beech,

– cherry,

– elm,

– magnolia,

– maple,

– bak,

– poplar,

– walnut.

• Produced in a variety of categories (boards, dimensions lumber, finishing lumber, and structural timbers) which are used for many purposes, such as;

– flooring, wall panelling, furniture, plywood etc.

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Plywood

• Made by bonding together thin layers of wood-veneers-in pairs about a central core.

• The outside veneers are called the faces or backs;

the center layer, the core, and where more than three plies are used, those in between crossbands.

• Generally, the same species or species group is used throughout.

• The veneers are united under high pressure and temperature with a high strength glue to produce panels of various sizes and thickness.

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Manufacturing of Plywood

• Selected logs are cut into 2.6 m or 3.2 m lengths, called blocks.

• Blocks are placed in a lathe and rotated at a constant surface speed against a long steel blade, which peels the veneer from the block in a continuous thin sheet.

• a veneer goes to a clipper, which cut the strip into various widths.

• The material is then dried to approximately 5% moisture content and graded into three grades (A, B, and C) based on the presence of defects, knots etc.

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• The strips are then glued.

• The cross band veneers are coated with a water proof resin glue and laid at right angles to the adjacent sheets on either side.

• The veneer sandwiches then go to a hot press (200 psi at 1490C).

• When the glue is cured, panels are cut usually (120240) cm.

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Air drying

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Kiln Drying

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Kiln Drying

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Manufacturing of lumber Methods of cutting

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Lumber

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Plywood

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Blockboard and Chipboard

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OSB (Oriented Strand Board)

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OSB