Building Materials, Third Edition

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cambium layer are primarily cellulose and are commonly referred to as fibres because of their
needle-like shape. They are cemented into groups by lignin, which gives the strength to wood.
The comparative width of annual rings, the direction and the arrangement of the cells and
fibres are the causes of the grains of the wood. Rapidly growing trees having wide annual rings
produce coarse grained wood, while those of slower growth produce wood with narrow rings
or fine grain. The wood is said to be straight-grained when the wood elements are straight and
run parallel to the pith and cross-grained when the elements do not run parallel to the axis.
Cross-grain has a pronounced weakening effect on the strength of beams when the slope of the
grains is 1:15 or greater.
Timber should be felled as soon as it is matured. The best time is midsummer or midwinter,
when the sap is at rest. If it is felled, when the sap is vigorous in movement, the timber decays.
If the tree is cut young, it yields soft wood and if it stands too long, the decay starts.

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The terms timber and wood are often used synonymously, but they have distinct meanings in
the building industry. Wood is the hard, fibrous material that makes up the tree under the bark,
whereas timber may be defined as a wood which retains its natural physical structure and
chemical composition and is suitable for various engineering works. Following is the
classification of timber as per IS: 399, except the classification of timber based on grading which
is given in IS: 6534.

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ƒ— „ ˜ implies a living tree.

‚ „ ˜ forms a part of the felled tree.

g  „ ˜  v ˜ are logs of timber sawn into planks, posts, etc.

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All grading specifications are clearly distinguished between structural or stress grading, and
commercial or utility grading based on Indian Standard classification.

ƒ ™ —2— is also known as stress grading. However, there is a small distinction
between the two. Structural grading refers to the principle by which the material is graded on
the basis of visible defects which have known effects on the strength properties of the material.
Stress grading refers to the principle by which the material is graded by consideration of
maximum principle stresses to which it can be subjected.
Structural grading is further divided as:

1. Grading based on known effects of defects and estimating accumulative value.


2. Machine grading.

g™—2— also known as yard grading or utility grading refers to the principle by
which the material is graded by consideration of usefulness of the material and price factors.
Commercial grading is further divided in the following classes: