Building Materials, Third Edition

‡2—2‡2€

™ IPQ

IR* ‚—

—2  I* r—D22— ƒ—

D2‚2™—D2ƒ—D

 u—2˜D

P* h—˜



22

 h—

™





Q* g2—



IS* ƒ™—

 I* ƒ2—2—˜



 f—˜

P* p%

˜

Q* i—22,

IT* ƒ

 I* r—2—2—˜



 x——D2q——D

P* i—22, r—

—,



Q* h—˜



2

2



™





IU* ƒ

 D2f— I* r—D22— t—D2f—˜D2„—,D

 x——D2f—,D

P* h—˜



22— w——D2u—



—

IV* ‡2™˜ I* v—222— ‚2™—D2€—D2f——D

P* ƒ2—2

2

 f—˜D2q——D2w—

Q* i—22,

IW* w—™2f% I* w—2—22 u—

D2ƒ



RIS2€‚y€i‚„siƒ2yp2‡yyh

€"™— 2€0

h)2—2ƒ!™™2X2All the mechanical properties of clear wood are related to its
density, which varies directly with the apparent specific gravity. The true specific gravity of
wood is approximately equal for all species and averages 1.54, whereas the specific weight and
apparent specific gravity vary with density of wood. The percentage of moisture in the wood
has a very large effect upon the specific weight and hence true comparisons of this property can
only be made on dry specimens.

xX2ƒ™2222 2— 222 22—2 2
2 2
2—2—2 2D2—2
™
2

22—2—222
2
—2˜222—22 222™ 

™D2—2
—2— —222
 222222
222 22˜2

f  ) depends on the volume of pores and moisture content of the wood. For most
wood species, the bulk density is less than density. Bulk density value is used to determine the
quality factor which is the ratio of compressive strength to the bulk density. It is 0.6 for pine and
0.57 for oak.

w  &X Water is found in three portions of wood: (1) it constitutes over 90 per
cent of the protoplasm in the living cells; (2) it saturates the cell walls; (3) it fills, more or less
completely, the pores of the life less cells. Timber is liable to shrink or swell with the movement
of moisture. This movement is not the same in all the directions. Fig. 4.21 shows two pieces A
and B cut from a log. In piece A, the layers producing the annual rings run roughly parallel to

„—˜2RP @g
A

ƒ € ‚    x—    22 

x