The species of timber recommended for structural purposes are given in Table 1.
4.1.1 Grouping
Species of timber recommended for constructional purposes are classified in three groups on the basis of their strength properties, namely, modulus of elasticity (E) and extreme fibre stress in bending-tension (fb).
The characteristics of these groups are as given below:
a) Group A — E above 12.6 kN/mm² and fb above 18.0 N/mm².
b) Group B — E above 9.8 kN/mm² and up to 12.6 kN/mm² and fb above 12.0 N/mm² and up to 18.0 N/mm².
c) Group C — E above 5.6 kN/mm² and up to 9.8 kN/mm² and fb above 8.5 N/mm² and up to 12.0 N/mm².
NOTE — Modulus of elasticity given above is applicable for all locations and extreme fibre stress in bending is for inside location.
4.1.2 Timber species may be identified in accordance with good practice [6-3A(3)].
4.2 The general characteristics like durability, treatability and seasoning of the species are also given in Table 1. Species of timber other than those recommended in Table 1 may be used, provided the basic strength properties are determined and found in accordance with 4.1.1.
NOTE — For obtaining basic stress figures of the unlisted species, reference may be made to the Forest Research Institute, Dehra Dun.
4.3 The permissible lateral strength (in double shear) of mild steel common wire nail shall be as given in Table 2 and Table 3 for different species of timber.
4.5Sawn Timber
4.5.1 Sizes
Preferred cut sizes of timber for use in structural components shall be as given in Table 5, Table 6 and Table 7.
Table 5Preferred Cut Sizes of Structural Timber for Roof Trusses (Span from 3 m to 20 m) (Clause 4.5.1)
Sl No.
Thickness mm
Width mm
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
i)
20
40
50
60
80
100
—
—
—
ii)
25
40
50
60
80
100
120
140
160
iii)
30
40
50
60
80
100
120
140
160
iv)
35
—
—
60
80
100
120
140
160
v)
40
—
—
60
80
100
120
140
160
vi)
50
—
—
60
80
100
120
140
160
vii)
60
—
—
—
80
100
120
140
160
viii)
80
—
—
—
—
100
120
140
160
1 For truss spans marginally above 20m, preferred cut sizes of structural timber may be allowed.
2 Preferred lengths of timber: 1, 1.5, 2, 2.5 and 3m.
Table 6Preferred Cut Sizes of Structural Timber for Roof Purlins, Rafters, Floor Beams, etc (Clause 4.5.1)
Sl No.
Thickness mm
Width mm
(1)
(2)
(3)
(4)
(5)
(6)
(7)
i)
50
80
100
120
140
—
—
ii)
60
80
100
120
140
160
—
—
iii)
80
—
100
120
140
160
—
—
iv)
100
—
—
—
140
160
180
200
NOTE — Preferred lengths of timber: 1.5, 2, 2.5 and 3 m.
Table 7 Preferred Cut Sizes of Structural Timber for Partition Framing and Covering, and for Centering
(Clause 4.5.1)
Sl No.
Thickness mm
Width mm
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
i)
10
40
50
60
80
—
—
—
—
—
ii)
15
40
50
60
80
100
—
—
—
—
iii)
20
40
50
60
80
100
120
160
200
—
iv)
25
40
50
60
80
100
120
160
200
240
v)
30
40
50
60
80
100
120
160
200
240
vi)
40
40
—
60
80
100
120
160
200
240
vii)
50
—
50
—
80
100
120
160
200
240
viii)
60
—
—
60
80
100
120
160
200
240
ix)
80
—
—
—
80
100
120
160
200
240
NOTE — Preferred lengths of timber: 0.5, 1, 1.5, 2, 2.5 and 3 m.
4.5.2 Tolerances
Permissible tolerances in measurements of cut sizes of structural timber shall be as follows :
a) For width and thickness:
1) Up to and including 100 mm
:
+3mm-0
2) Above 100 mm
:
+6mm-3
b) For length
:
+10mm-0
4.6Grading of Structural Timber
4.6.1 Cut sizes of structural timber shall be graded, after seasoning, into three grades based on permissible defects given in Table 8:
a) Select Grade,
b) Grade I, and
c) Grade II.
PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER
Table 8 Permissible Defects for Cut Sizes of Timber for Structural Use (Clauses 4.6.1 and 4.6.2.2)
All dimensions in millimetres.
SI No.(1)
Defects(2)
Select Grade(3)
Grade I(4)
Grade II(5)
i)
Wane
Shall be permissible at its deepest portion up to a limit of 1/8 of the width of the surface on which it occurs
Shall be permissible at its deepest portion up to a limit of 1/6 of the width of the surface on which it occurs
Shall be permissible at its deepest portion up to a limit of 1/4 of the width of the surface on which it occurs
ii)
Worm holes
Other than those due to powder post beetles are permissible
Other than those due to powder post beetles are permissible
Other than those due to powder post beetles are permissible
iii)
Slope of grain
Shall not be more than 1 in 20
Shall not be more than 1 in 15
Shall not be more than 1 in 12
iv)
Live knots:
Width of Wide Faces of Cut Sizes of Timber, Max
Permissible Maximum Size of Live Knot on
Permissible Maximum Size of Live Knot on
Permissible Maximum Size of Live Knot on
Narrow Faces and 1/4 of the Width Face Close to Edges of Cut Size of Timber
Remaining Central Half of the Width of the Wide Faces
Narrow Faces and 1/4 of the Width Face Close to Edges of Cut Size of Timber
Remaining Central Half of the Width of the Wide Faces
Narrow Faces and 1/4 of the Width Face Close to Edges of Cut Size of Timber
Remaining Central Half of the Width of the Wide Faces
75
10
10
19
19
29
30
100
13
13
25
25
38
39
150
19
19
38
38
57
57
200
22
25
44
50
66
75
250
25
29
50
57
66
87
300
27
38
54
75
81
114
350
29
41
57
81
87
123
400
32
44
63
87
96
132
450
33
47
66
93
99
141
500
35
50
69
100
105
150
550
36
52
72
103
108
156
600
38
53
75
106
114
159
v) Checks and shakes:
Width of the Face of the Timber Max
Permissible Depth Max
Permissible Depth Max
Permissible Depth Max
75
12
25
36
100
18
35
54
150
25
50
75
200
33
65
99
250
40
81
120
300
50
100
150
350
57
115
171
400
66
131
198
450
76
150
225
500
83
165
249
550
90
181
270
600
100
200
300
4.6.2 The prohibited defects given in 4.6.2.1 and permissible defects given in 4.6.2.2 shall apply to structural timber.
4.6.2.1 Prohibited defects
Loose grains, splits, compression wood in coniferous species, heartwood rot, sap rot, crookedness, worm holes made by powder post beetles and pitch pockets shall not be permitted in all the three grades.
4.6.2.2 Permissible Defects
Defects to the extent specified in Table 8 shall be permissible.
NOTE — Wanes are permitted provided they are not combined with knots and the reduction in strength on account of the wanes is not more than the reduction with maximum allowable knots.
4.6.3 Location of Defects
The influence of defects in timber is different for different locations in the structural element. Therefore, these should be placed during construction in such a way so that they do not have any adverse effect on the members, in accordance with the accepted standard [6-3A(5)].
1) For working stresses for other locations of use, that is, outside and wet, generally factors of 5/6 and 2/3 are applied. 2) The values of horizontal shear to be used only for beams. In all other cases shear along grain to be used.
4.7Suitability
4.7.1 Suitability in respect of Durability and Treatability for Permanent Structures
There are two choices as given in 4.7.1.1 and 4.7.1.2.
4.7.1.1 First choice
The species shall be any one of the following:
a) Untreated heartwood of high durability. Heartwood if containing more than 15 percent sap wood, may need chemical treatment for protection;
b) Treated heartwood of moderate and low durability and Class ‘a’ and Class ‘b’ treatability;
c) Heartwood of moderate durability and Class ‘c’ treatability after pressure impregnation; and
d) Sapwood of all classes of durability after thorough treatment with preservative.
4.7.1.2 Second choice
The species of timber shall be heartwood of moderate durability and Class ‘d’ treatability.
4.7.2 Choice of Species for Load-Bearing Temporary Structures or Semi-Structural Components at Construction Site
The species shall be any one of the following:
a) Heartwood of low durability and Class ‘e’ treatability; or
b) The species whose durability and/or treatability is yet to be established, as listed in Table 1.