timber

Timber is a natural material which is not subjected to pre-use factory processing. As timber cannot be manufactured to specification, remember to check availability before specifying certain type or grade of timber.

Timber is converted mainly from the trunk.

Bark
The periphery of the trunk

Medullary ray
Groups of cells running radially

Heartwood
The older timber at the heart
Composed of dead tissue
Usually darker in color
Drier and harder than the outer layer of sapwood

Sapwood
The living outer layer
Contain more moisture than the heartwood
Not as strong in the green state as heartwood, but after seasoning, may be as dense and strong as heartwood
Sapwood is inferior to heartwood in respect of durability, containing starches which may attract insects and provide food for fungal growth. Sapwood, however, is very permeable and more easily impregnated with preservative and, where the conditions under which the timber must serve are such that treatment with a preservative is essential, it may be beneficial to use sapwood as a deliberate choice.

Tree classification:
Hardwood
Angiosperms or dicotyledons which have broad leaves shed in the autumn
Slower growth, more expansive, not grown in some countries
Mainly used for furniture and interior finish
Softwood
Gymnosperms or conifers which have needle-like leaves, broadly evergreen
Faster growth, normally cheaper
Used mainly for construction

Conversion of timber:
Conversion is the process whereby the felled truck is converted into marketable sizes of timber.
Different strength characteristics are possible with varying methods of sawing the trunk.

There are two main methods of converting timber:
Through and through (or Plain or Crown sawn) which produces tangential boards and
Quarter Sawn which produces radial boards.
The Quarter sawn is far more expensive because of the need to double (or more) handle the log. There is also more wastage. It is however more decorative and less prone to cup or distort. Note also there are two ways of sawing the quarter.
Through and through produces mostly tangentially sawn timber and some quarter sawn stuff. (see diagram) Tangential timber is the most economical to produce because of the relatively less repetitive production methods. It is used extensively in the building industry.
There are other ways but they are all variations of tangential and radial cuts to obtain the best or most economical boards for the use it is to be put. These basic cuts are not always able or need to be, on the exact tangent or radius of the trunk. The cuts, that fall between, crown and quarter are called 'rift' and between 'rift' and 'quarter' are identified as 'figured' - see below for explanation. Boxing the heart refers to eliminating the heartwood from the boards that would otherwise produce shakes, juvenile wood or may even be rotten.

Tangential boards (crown, plain or flat sawn) are used extensively for beams and joists. They are stronger when placed correctly edge up with the load in the tangential axis. These type of boards suffer from 'cupping' if not carefully converted, seasoned, and stored properly. Annual growth rings form an angle less than 45 degrees.

Radial boards (radial, figured or quarter sawn) are typically cut on 'the quarter' and produce a pattern of the medullary rays especially in quartered oak. Such timber is expensive due to the multiple cuts required to convert this board. The radial face of the board is slightly stronger and stiffer than the tangentially face but the cross section and condition of the timber has more effect on strength. Annual growth rings form an angle greater than 45 degrees.
Crown sawn is obtained by sawing tangentially to the annual rings. It is also referred to as 'Plain Sawn' or 'through and through'.
Rift sawn is the cut which falls between crown and true quarter sawn. It is straight grained and in oak, does not reveal any 'silver ribbon' grain features. Quality floor boards are prepared from rift sawn timber because it wears well and shrinks less. Annual growth rings form an angle between 30 and 60 degrees.
Quarter sawn boards are radial cut from the centre of the tree. It produces the distinctive silver ribbon effect (in oak) across the whole board. Annual growth rings form an angle greater than 45 degrees. True quartered boards producing the best features will have the angle on or very much closer to 90 degrees.
'Figured' - is the cut between 'rift' and 'true quartered'. It has varying degrees of 'silver ribbon' (in oak) showing through but not the full figured effect found in true quarter sawn boards.
Different species have their best features enhanced by choosing the best cut appropriate to their species.


Billet sawing

Commercial quarter sawing

True radial sawing

Strength of timber:
The strength of timber is affected by:
Species
Variability within a species (density, rate of growth, position in trunk etc.)
Defects (knots, decays, insect attack etc.)
Moisture content
The strength of timber increases with decrease of moisture content, and vice versa
Duration of loading
Strength decrease with duration of loading, may be up to 35%
Grain
The tensile & compressive strength in the direction parallel to the grain are much higher than in the direction perpendicular to the grain; the shearing strength along the grain is much lower than that across the grain

Moisture content:
moisture content= (wt.of moisture contained)/(dry wt.of timber)
The condition when all the cells are dry but the walls are still saturated is referred to as the fibre saturation point, usually between 23 to 27% moisture content. Below the fibre saturation point, all properties of timber improve with decease in moisture content except toughness or the shock resistance.

Seasoning of timber:
The purpose of seasoning is to expel or dry up the sap and water in the wood to improve its strength and other properties.
Air of natural seasoning
Air or natural seasoning is affected by stacking the timber in such a way that air can circulate freely around all surfaces. Air seasoning may take weeks or even months to complete.
Kiln or artificial seasoning
Kiln or artificial seasoning is carried out by subjecting the timber in a kiln or chamber to a current of hot air. Kiln seasoning may take a few hours to a few days.

Structural grading rules
It is not economically justified to use defect free or clear timber for structural purposes. Instead of specifying that structural timber “shall be free from defects”, it is much more practical to accept defects, but to place definite limits on the size and number that will be permitted and to base working stresses on the strength of the piece of minimum acceptable quality.
Select grade
Weakest piece shall be 75% as strong as defect-free timber
Standard grade
65%
Common grade
40 to 50%

Defects in timber
Natural defects:
Cracks
Checks
Lengthwise across annual rings
Shakes
Lengthwise between annual rings
Knots
Part of a branch which becomes enclosed in a growing tree
Seasoning defects
Various defects in timber:
a = twist; b = cupping;
c = honeycomb checks;
d = bowing; e = checks;
f = end split;
g = compression failure;
h = behaviour of test sample from a case-hardended board;
i = spring.

Seasoning defects are due to distortion of timber during seasoning which may arise from excessive or uneven drying, exposure towind and rain, and poor stacking.

(1) A knot which is a portion of a branch and which has become incorporated in the body of a tree. All provisions of this chapter relating to the presence of knots apply only to the surface of the piece on which the knot appears, and all such provisions limiting the size of knots apply to the mean or average diameter as measured on the surface.
(2) Cross-grained wood in which the fibres are not parallel with the axis or longitudinal edge of the piece. It is expressed in this chapter as the slope of the grain with respect to the edges of the piece. For instance, one in 12 means that in a distance of 12 inches the grain deviates one inch from the edge. The presence in any surface of local discontinuity of grain or local deviations in the straightness of grain because of knots permitted in the piece shall be disregarded in applying the provisions of this chapter.
(3) A shake which is a separation along the grain, the greater part of which occurs between the rings of annual growth.
(4) A check which is a separation along the grain, the greater part of which occurs across the rings of annual growth.
(5) A pitch pocket which is an opening between the grain of the wood, containing more or less pitch or bark.
(6) Decay which is the destruction of the wood substance due to the action of wood destroying fungi.
(7) A cross-break which is a separation of the wood cells across the grain of the wood.
(8) A compression failure which is a deformation of the fibres due to excessive compression along the grain. This deformation takes the form of a buckling of the fibres.
(9) Compression wood, or proudwood, which is an abnormal growth occurring in conifers (softwood) and is characterized by relatively wide annual rings, usually eccentric and a comparatively large proportion of summerwood, usually 50% or more, which merges into the springwood without exhibiting a marked contrast in color.

Shrinkage
When timber is seasoning and it's moisture content (MC) is reduced below the Fibre Saturated Point (FSP) continued drying will cause dramatic change such as increase in strength but also distortion and shrinkage.
Shrinkage is the greatest tangentially over the radial direction with little loss along the length of the board, etc.

Cupping
Because of this varying shrinkage rates tangential boards tend to cup because of the geometry of the annual rings shown on the end grain. It can be seen that some rings are much longer than the others close to the heart. Therefore they will be more shrinkage at these parts than the others ~ cupping is the result.
In square section timber cut from the same place, diamonding is the result.



Knots
Knots are the result of the trees attempt to make branches in the early growth of the tree. They are the residue of a small twig, shoot, etc. that died or was broken off by man or an animal in the wood or forest. The tree subsequently continued its growth over this wood.
The knot may be live, sound, or tight or if it has become separated and is contained in residue of bark, dead.
Dead knots become loose and downgrade the appearance and stability of the board. Most grading systems uses the amount of knot area as an indication of its quality. The more knots the less the quality.
Dead loose knots are extremely dangerous to machinists. The cutters may pick these up and eject them rapidly towards the operator.

Splits
A separation of the wood fibres along the grain forming a fissure that extends through the board from one side to the other.
It is usual in end grain and is remedied by cutting away the defected area. All boards should have an allowance so that some end grain may be cut away because of possible shakes or splits.

Checks and end checking
A separation of the fibres along the grain forming a fissure which shows up on one face or at the end grain but does not continue through to the other side.


Uses of timber
Marine work
Wharves, piers, sheet piling, and cofferdams
Fenders
Requires high density, close grain, high strength timber
Timber has good resistance to impact
Problems with fungal and marine growth attack
Nowadays concrete or steel more popular for the main structure but timber is still used for denders

Heavy constructional work
Timber piles, bridges, gantries etc.
Once popular, nowadays concrete or steel used instead

Medium/light constructional work
Houses, roof trusses, partitions, floors etc.
Often prefabricated to reduce labor cost
With adequate maintenance, can last>30 yrs

Plywood
Plywood is an assembled product made up of piles glued together. The plies are “peeled” off a pre-boiled log by rotating it against a knife.
Plywood is manufactured in the form of flat sheets of various sizes. Thickness ranges from 3 to 25mm. Plywood is always built up of an odd number of plies, i.e. 3, 5 or 7 plies. In general, the plywood built up of a large number of plies is stronger and more stable, but it is more expensive to make.
The direction of the grain of each ply runs at right angles to that of the ply on each side so that strength properties are more uniform in the length and width of the boards than in solid timber.
The tendency of any ply to shrink transverse to the grain under change of moisture content is resisted on account of the direction of the grain of the adjacent plies.
Plies are not always of equal thickness and the inner ply or plies can be of lower strength than those on each side, provided the strength and moisture movement properties on each side of the centre are “balanced”

Fire resistance
Timber is easily ignited at 220 to 300℃. It easily becomes charred at the surface, but the unburnt interior of a timber member does not lose strength with a serious rise in temperature and there is no significant increase in length. The god thermal insulation of timber prevents a marked rise in the temperature of members on the side remote from the fire.