Lecture 3

Timber as a structural material

Structure II (AR-106 G)

BY- AR. KHURRAM ALI Asst. Professor Gateway College of architecture Sonipat, Haryana

Content

Introduction Classification of timber Advantages of timber over other materials Requirements of a good timber Characteristics to find the suitability of timber Structure of timber Defects in timber and their influence on the

strength of timber Density of timber Strength of timber

Introduction

Wood is considered as one of the best engineering material.

Despite the extensive use of other structural material like steel and concrete, many engineering concerns like buildings, Railways, mining etc. depend much upon wood.

Unlike steel and concrete, timber is not made of consistent structural properties. It does not behave equally in all directions.

Classification of timber

Structured timbers are classified on the basis of their Modulus of elasticity(E) and extreme fiber stress in bending (fb) as under-

Group A- ‘E’ > 12600 N/mm² and ‘fb’ >18 N/mm²

Group B- ‘E’ > 9800 N/mm² and ‘fb’ >12 N/mm²

Group C- ‘E’ > 5600 N/mm² and ‘fb’ > 8.5 N/mm²

Table of safe permissible stress in timber

Cont’d

Strength parallel to grain is vastly different to strength and stiffness normal to the grain.

The wood used for building or other architectural purpose is called timber.

Advantages of timber over other materials

Stronger than other materials of construction in use when compared in proportion to their weight.

Can be worked easily to any size and shape.

Can be jointed to the required form easily.

Structural connections can be easily made in timber work.

Timber construction is economical as the wastage is minimum.

Cont’d

Structural member made of timber are light in weight.

It is quite durable, if properly seasoned and preserved.

Re-sale value is good.

Non-conductor of heat and sound. The house built in timber will be warm in winter and cool in summer.

Requirements of a good timber

Should have uniform color and regular annual rings.

Should have straight fibers.

Should be sweet to smell when freshly cut. A disagreeable smell indicates decay in timber.

Should be sonorous.

Should be well seasoned.

Should be free from natural defects.

Should not be affected by fungi and other insects.

Should have firm adhesion of fibres.

Should be durable to resist the atmospheric effects.

Should be tough to resist the shocks.

Should not be split when nail is driven into it.

Cont’d

Characteristics to find suitability of timber

• Strength

• Toughness

• Elasticity

• Resistance to shear

• Hardness

• Fire resistance

• Retention of shape

• Durability

• Workability

Structure of timber

Defects in timber and their influence

Shakes- The partial or complete separation between adjoining layers of tissues are called shake. They lower the resistance to shear.

Defects in timber and their influence

Rind gall- The curved swelling resulted from the growth of sapwood layer on wound, left by a falling branch or cut off branch in an irregular manner is called rind gall. The new growth does not unite properly with the old wood and leaves the cavity where decaying action may set in.

Cont’d

• Knots- The dark hard pieces indicating places from where branches have been cut off or broken are called knots.

knots are the source of weakness as they break the continuity of wood fibres. Further, knot tend to weaken the timber in tension but may improve its strength in compression. Generally knots make the working of wood difficult.

Cont’d

Cont’d

• Upsets- The defect caused by crushing or by

injury in wood fibres is known as upsets. It

occurs usually during the growth. This makes

wood weak and unfit for any important work.

Upsets

Cont’d

• Twisted fibres- The defect due to the fibre of

tree which get twisted when young in age, by

force of wind is called twisted fibres. Twisted

fibre make the wood weak which cannot be

used for structural purposes.

Twisted fibres

Density of timber

Hardwood- Vary from1230 kg/m3 to 190 kg/m3

Softwood- Vary from 720 kg/m3 to 370 kg/m3

Strength of timber

Timber is strong along the grain but weak across it.

Timber is weak in shear parallel to the grain.

Typical minimum grade stress for a well seasoned & defect free timber

• Bending parallel to grain = 5.3 N/mm2

• Tension parallel to grain = 3.2 N/mm2

• Compression parallel to grain = 6.8 N/mm2

• Compression normal to grain = 2.2 N/mm2

• Shear parallel to grain = 0.67 N/mm2

• Modulus of Elasticity (Mean) = 8800 N/mm2