Preparation No: 1

Subject: Timber Design

Name: Shayne Olaguer Roquid

Year and Section: BSCE 5B

Timber as Structural Material

1. The structure of timber

Timber is a type of wood which has been processed into beams and planks. It is

also known as “lumber”, any wood capable of yielding a minimum dimensional size

can be termed as a timber or lumber. Timber are used for structural purposes. It has

the outer most layer where it is called bark. Bark protects the tree from extreme

temperatures, bad weather, insects and fungi. after the bark there is a soft tissue which

is called cambium. Cambium is a layer between the inner bark or phloem and the wood

or xylem, which produces new phloem on the outside and new xylem on the inside in

stems, roots, etc. If the bark is removed for any reason, this layer get exposed and the

cells resulting in the death of tree. Each year, the tree forms new cells, arranged in

concentric circles called annual rings or annual growth rings. Drought, excessive rain,

fire, insect plagues and disease epidemics, injuries, thinning, air pollution, all leave

their mark on a tree’s annual growth rings. The heartwood consists of old cells, it is

the inner annual ring surrounding the pith. This is the dead part of the tree that

nevertheless provides structural strength. If air could reach these cells, the heartwood

would rot quickly. Sapwood is the living wood in the tree through which the raw sap

rises from the roots to the leaves. The central core of the tree is the pith or medulla, it
varies in size and shape for different types of trees. Consist of cellular tissues and

nourishes the plant in its young age. When they become old, pith dies up and decays

and the sap is then transmitted by the woody fibers deposited round the pith. Medullary

rays is the thin radial fibers extending from pith to cambium, the function of the ray is

to hold together the annual ring of heartwood and sapwood. It is sometimes broken,

in some varieties, they are prominent.

2. Defects in timber

Defects in timber may be defined as, any undesirable character – natural or

artificial that lowers its strength, durability or quality.

Natural Defects in Timber

Most common natural defects are: knots, shakes, cross grain, crookedness, rind

galls, burr, and curl.

1. Knots – It is essentially a discontinuity in the timber and hence a place of weakness.

Knots of whatever type and shape are always to be considered as defects in timber.

They influence the quality of timber at least in two ways:

Firstly, they make the workability of timber quite difficult. They are harder and offer

greater resistance to planning and polishing. Secondly, they reduce the tensile

strength of timber.

2. Shakes – After knots, shakes are more serious and common defects in timber. They

are also called cracks or fissures. These natural cracks may result from:

1. Shrinkage on aging of the tree;

2. Due to movement caused by wind action in the growing tree;

Another effect is that shakes allow an easy access of moisture and air to the

interior of the tree along with insects and fungi. This reduces the durability of the

timber.

3. Cross Grain – Fibers of the wood in a normal tree are usually parallel to the axis of

growth (length of timber). This structure is called straight grained. In some cases,

however, these fibers are not straight; they make an angle or slope with the axis of the

tree, the structure is then called cross grained. In fact, a minor slope (of grain) may

be seen in most trees. It may be quite harmless. The slope of grain becomes harmful

when it exceeds specified limits. This is because the strength properties are closely

related to the direction of the grain.

4. Rindgalls – It is a highly thickened, enlarged wood cover developed over an injured

part of the tree. It is a defect in timber because it will have to be removed first of all for

conversion of the tree. Burr and Curl are generally developed as thickened overgrowth

on junction between the tree trunk and a branch or between one branch. Such

overgrowths commonly result in severe twisiting of fibers in the board cut from this

portion.

Artificial Defects in Timber

These include such defects of timber that develop in the timber after felling and

conversion to different sizes.

1. Checks – These are cracks or clefts developing longitudinally in the logs after their

felling. These cracks or checks are often cutting across the annual rings of the log. In

most cases, their origin is due to a simple cause of unequal drying. Thus, when a log
of wood remains exposed to the atmosphere only on one side, the upper side will lose

more moisture than the underside. This will result in a shrinkage in the exposed region

to such an extent that tensile forces sufficient to cause a crack or cracks in the wood

will be easily developed. The effect of checks on the quality of timber is that they

reduce their strength on the one hand and allow the moisture to seep in on the other

hand.

2. Warps – It is a deformation in the original plane surface of a board cut from the

timber. This is also attributed to unequal shrinkage. It is most commonly developed in

those boards that are cut from green or unseasoned timber.

3. Types of Timber

Timber can be divided into two categories: hardwoods and softwoods.

There are many types of trimber under these two categories. They are:

1. Bamboo – it is a natural organic material. Most timber producing bamboos are from

South Asia. The color of bamboo timber is pale yellow to almost gold. It is an industial

material also identified as the most promising building material. Bamboo timber is an

alternative to tropical hardwoods in recent years.

2. Birch – is a great craft-wood. It is an important source of hardwood timber. Birch is

of many types of like-yellow birch, white birch etc. Yellow birch is commonly used

which is also known as gray birch, or swamp birch. It is light reddish brown color. Birch

gives low natural luster.

3. Cedar – is a timber of high quality. It comes from several different trees known as

cedars. Where both strength and appearance of the exposed wood beams are

important, cedar is the only savior and perfect fit for this. It is used in for landscape,
park, garden structure. Its moderate strength and softness make it unique. Cedar

offers the advantage of the natural advantage.

4. Cherry timber – is a grown as a tree plantation timber. Among different types of

cherry timbers, black cherry timber is the largest of the native cherries and the only of

commercial value. In cherry timber, the grain is generally straight. It is a hardwood. It

has medium density and moderately durable. And used for small pieces of furniture’s

veneer, handles, cabinets, scientific instruments, etc.

5. Mahogany – is a high-quality timber. It is widely used in the furniture and cabinet

building industry. It is made into plywood and all kinds of trim. It displays hard grain. It

is one of the softer of all the hardwoods they are commonly in use. The color of

mahogany timber darkens with age. The texture is medium and uniform.

6. Oak – is a type of hardwood from the oak tree. It has been used as timber for

thousands of years. It is straight grained. It is used for light construction most of the

time. Oak timber is used for homeware, wine barrels, firewood, etc.

7. Walnut – is a premium timber. It is a specialist hardwood. The grain is straight but

it can be irregular. Walnut timber is really decorative. It carries a lot of design for the

interior. Walnut timber is moderately durable. It is medium density.

8. Fir – is one of the world’s best-known timber species. It is a premier building product

for a wide range of applications. It is a softwood species. Fir has a very straight grain.

Fir has become the standard woods of choice for timber framing. Fir timber is used tin

housing, framing, flooring, lining fascias, bargeboards, and pergolas.

4. Physical Properties of Timber

can be ensured by investigating the properties of timber. It is also essential to ensure

that the timber is defectless. These properties are briefly discussed below:

1. Color – is a uniform property by which most trees are characterized as they

show variation from tree to tree. Light color indicates weak timber.

2. Appearance – smell is a good property as timbers for few plants as they can

be identified by their characteristics aroma. Fresh cut timber has a good smell.

3. Hardness – For the resistance of any kind of damage, hardness is an obvious

property.

4. Specific Gravity – Variation of timber in specific gravity is found. It depends

on pores present inside timber. The specific gravity of light material is less than that of

water. But in case of compact wood where pores are almost absent and become

heavier, their specific gravity increases.

5. Moisture Content – Timber are hygroscopic and gain water from nature. The

absorption of water or dehydration depends on atmospheric humidity. If timbers

moisture content is high that means the timber quality is low. Water content is the risk

of fungal attack.

6. Grain – On the grain structure, quality of timber varies.

1. Straight grain: Arrangement of vascular tissue (xylem and phloem) is

important which grow parallel to the length of the timber that is termed as straight grain.

2. Coarse grain: vascular tissue and fiber arranged broadly and widely.

3. Interlocked grain: Instead of parallel arrangement twisted, a spiral

arrangement may be found.

plant to plant. Some give higher percentage after drying. Shrinkage starts when cell

walls of timber start to release water. In moisture atmosphere timber swells when cell

walls absorb water. Good quality timber swell less. Timbers having thicker wall swell

more than thinner one.

8. Soundness – A good quality timber gives good sound.

9. Texture – The texture of good timber is fine and even.

10. Free of abrasion – Timber should not be damaged by the external environment. It

has to gain the ability to protect its skin.

5. Stress Grading of timber

Stress grading is needed to ensure timber is strong enough for a particular job.

It is the classification of a timber when used in structural applications. Stress grades

are derived from either visual grading or machine grading, which specify the stress

limits that apply to timbers used for structural applications.

Visual grading – it rules define the size, tyoe, and number of strength-

reducing characteristics allowed in each grade (for example, natural features such as

knots, wane and slope of grain, plus splits and shakes which may have developed

as a result of drying).

Machine grading – Stress grade determined by mechanical grading,

using a technique that correlates strength directly with stiffness. It is best suited to high

volumes of timber where the species and the cross section are not changed very often.

6. Strength Classes
 Strength Classes range from C14 to C50 for softwood and D30 to D70 for

hardwoods. The higher the number, the stronger the timber. The most common grade

for softwood carcassing is C16.

7. Design considerations (Factors affecting timber strength)

When properly designed, wood frames structures will resist damage by moisture

and living organisms. Recommendations for control of moisture and protection against

decay and insect infestation are contained in AWC’s Design of Wood Frame

Structures for Permanence. Protection of wood frame structures to provide maximum

service life involves four methods of control, which can be handled by proper design

and construction:

1. Control moisture content of wood.

2. Provide effective termite controls.

3. Use of durable materials such as naturally durable or preservative treated

wood.

4. Quality assurance.

8. Design of flexural members

Design of flexural members requires consideration primarily of bending and

shear strength, deflection, and end bearing.