NEPAL NATIONAL BUILDING CODE
NBC 112 : 1994

TIMBER

Government of Nepal

Ministry of Physical Planning and Works
Department of Urban Development and Building Construction
Babar Mahal, Kathmandu, NEPAL

Reprinted : 2064

NBC112V1.RV420 May 1994

NEPAL NATIONAL BUILDING CODE
NBC 112 : 1994

TIMBER
This publication represents a standard of good practice and therefore
takes the form of recommendations. Compliance with it does not confer
immunity from relevant legal requirements, including bylaws
tTsflng >L % sf] ;/sf/ -dlGqkl/ifb\_ sf] ldlt @)^).$.!@ sf] lg0f{ofg';f/ :jLs[tGovernment of Nepal

Ministry of Physical Planning and Works
Department of Urban Development and Building Construction
Babar Mahal, Kathmandu, NEPAL

Reprinted : 2064

NBC112V1.RV420 May 1994

i
PrefaceThis Nepal Standard was prepared during 1993 as part of a project to prepare a National Building Codefor Nepal.
In 1988 the Ministry of Housing and Physical Planning (MHPP), conscious of the growing needs of
Nepal's urban and shelter sectors, requested technical assistance from the United Nations Development
Programme and their executing agency, United Nations Centre for Human Settlements (UNCHS).
A programme of Policy and Technical Support was set up within the Ministry (UNDP Project
NEP/88/054) and a number of activities have been undertaken within this framework.
The 1988 earthquake in Nepal, and the resulting deaths and damage to both housing and schools, again
drew attention to the need for changes and improvement in current building construction and design
methods.
Until now, Nepal has not had any regulations or documents of its own setting out either requirements orgood practice for achieving satisfactory strength in buildings.
In late 1991 the MHPP and UNCHS requested proposals for the development of such regulations anddocuments from international organisations in response to terms of reference prepared by a panel ofexperts.
This document has been prepared by the subcontractor's team working within the Department of
Building, the team including members of the Department and the MHPP. As part of the proposedmanagement and implementation strategy, it has been prepared so as to conform with the general
presentation requirements of the Nepal Bureau of Standards and Metrology.
The subproject has been undertaken under the aegis of an Advisory Panel to the MHPP.
The Advisory Panel consisted of :Mr. UB Malla, Joint Secretary, MHPP
Director General, Department of Building(Mr. LR Upadhyay)
Mr. AR Pant, Under Secretary, MHPP
Director General, Department of Mines & Geology(Mr. PL Shrestha)
Director General, Nepal Bureau of Standards & Metrology(Mr. PB Manandhar)
Dean, Institute of Engineering, Tribhuvan University(Dr. SB Mathe)
Project Chief, Earthquake Areas Rehabilitation &Reconstruction Project
President, Nepal Engineers Association
Law Officer, MHPP (Mr. RB Dange)
Representative, Society of Consulting Architectural &Engineering Firms (SCAEF)

NBC112V1.RV4ChairmanMemberMemberMemberMemberMemberMemberMemberMemberMember

20 May 1994

ii
Representative, Society of Nepalese Architects (SONA)
Deputy Director General, Department of Building,(Mr. JP Pradhan)

MemberMember-Secretary

The Subcontractor was BECA WORLEY INTERNATIONAL CONSULTANTS LTD. of New
Zealand in conjunction with subconsultants who included :Golder Associates Ltd., Canada
SILT Consultants P. Ltd., Nepal
TAEC Consult (P.) Ltd., Nepal
Urban Regional Research, USA
Principal inputs to this standard came from :Mr. AM Tuladhar, DoB, HMGN
Dr. AS Arya, Professor Emeritus, University of Roorkee, India
Dr. RD Sharpe, BECA (Team Leader)
Revisions and Updated to this code came from :Mr. Purna P. Kadariya, DG, DUDBC
Mr. Kishore Thapa, DDG DUDBC
Mr. Mani Ratna Tuladhar, Sr. Div. Engineer, DUDBC
Mr. Jyoti Prasad Pradhan, Ex. DDG, DOB
Mr. Bhubaneswor Lal Shrestha, Ex. DDG, DOB
Mr. Uttam Shrestha, Architect, Architects’ Module Pvt. Ltd.
Mr. Manohar Lal Rajbhandhari, Sr. Structural Engineer, MR Associates
Mr. Amrit Man Tuladhar, Civil Engineer, DUDBC

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iii

TABLE OF CONTENTS
Preface......... ................................................................................................................................................ i0

Foreword ....................................................................................................................................... v
0.1
0.2

1

Scope .............................................................................................................................................. 1
1.1
1.2

2

Design Aspect................................................................................................................... v
Related Codes ................................................................................................................... v

Coverage .......................................................................................................................... 1
Limitation ......................................................................................................................... 1

Interpretation ............................................................................................................................... 1
2.1
2.2

General.............................................................................................................................. 1
Terminology ..................................................................................................................... 1
2.2.1

2.3

Definition of Defects in Timber......................................................................... 2

Symbols............................................................................................................................. 3

3

Materials........................................................................................................................................ 6

4

Moisture Content in Timber....................................................................................................... 6

5

Sawn Timber.................................................................................................................. ................ 6
5.1
5.2
5.3

6

Sizes……………… .......................................................................................................... 6
Tolerances......................................................................................................................... 6
Location of Defects .......................................................................................................... 6

Design Considerations ............................................................................................................... 10
6.1
6.2

General............................................................................................................................ 10
Loads ............................................................................................................................... 10
6.2.1

6.3

Load Combination ........................................................................................... 10

Flexural Members ......................................................................................................... 10
6.3.1 Effective Span ................................................................................................... 10
6.3.2 Form Factors for Flexural Members ............................................................ 10

6.4

Deflection........................................................................................................................ 11

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iv

7

Columns....................................................................................................................................... 11
7.1
7.2
7.3
7.4

8

Solid Columns ................................................................................................................ 11Long Columns................................................................................................................ 12
Slenderness Ratio ............................................................................................................ 12
Permissible Load ........................................................................................................... 12

Design of Common Steel Wire Nail Joints.............................................................................. 13
8.1

Dimensions of Members ............................................................................................... 13
8.1.1
8.1.2
8.1.3

8.2
8.3
8.4
8.5
8.6

Length of Members....................................................................................................... 13
Width of Members ........................................................................................................ 13
Spacing............................................................................................................................ 13
Lengthening-Joints........................................................................................................ 13
Specification and Diameter of Nails............................................................................ 14
8.6.1
8.6.2
8.6.3
8.6.4
8.6.5

9

Nails ................................................................................................................... 14
Nail Diameter................................................................................................... 14
Nail Length....................................................................................................... 14
Numbers of Nails............................................................................................. 14
Spacing of Nails ............................................................................................... 14

Design of Bolted Joints .............................................................................................................. 15
9.1
9.2
9.3

General............................................................................................................................ 15
Design Considerations .................................................................................................. 15
Arrangement of Bolts.................................................................................................... 15
9.3.1
9.3.2
9.3.3

10

Minimum Thickness....................................................................................... 13
Maximum Thickness ...................................................................................... 13
Minimum Depth.............................................................................................. 13

Spacing ............................................................................................................. 15
Staggering ........................................................................................................ 16
Bolt Holes ......................................................................................................... 16

Glue Laminated Timber ........................................................................................................... 16
10.1

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Subjected to Bending ...................................................................................................... 16

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v

0.

Foreword
0.1

Design Aspect
This standard covers the general principle involved in the design of structural timber inbuildings. It also covers the specifications for structural timber for use in buildings
including, classification of such timber into suitable grades, as well as nail joint timber
construction.

0.2

Related Codes
The requirement of this section shall be applied in conjunction with the Indian Standard
IS: 883-1970 Code of Practice for Design of Structural Timber in Building” (Third
revision) and IS: 2366-1983 Code of Practice for Nail-Jointed Timber Construction(First Revision).

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1

1

Scope
1.1

CoverageThis standard provides a Code of Practice for the structural design of timber inbuildings of Nepal.

1.2

Limitation
This section do not covers anti-termite timber, use of plywood, timber in pilefoundation.

2

Interpretation
2.1

2.2

General
2.1.1

In this standard the word "shall" indicates a requirement that is to be adopted inorder to comply with the Standard, while the word "should" indicatesrecommended practice.

2.1.2

Commentary clauses are prefaced by the letter C and the number of theappropriate clause subject to comment.

Terminology
The terminology given IS : 883-1970 in clauses 2.0 to 2.19 will apply except as statedhere below.
BOX COLUMN means a column made of four planks connected together so as to havehollow core inside. The core is blocked by the solid pieces of timber at its ends and at
least one intermediate point.
BUILD UP - SOLID COLUMN means those built up by spiking, nailing or bolting
together planks or square section. The planks must be fastened together at intervals not
greater than six times their thickness. The allowable stress on such a column isconservatively taken as 80 % of that of solid column.
CASE HARDENING means a condition of timber during seasoning in which thedifferent layers of wood are under stress, by being under compression across the grain.
DURATION OF LOADING means the period during which timber structure isstressed as consequence of the loads applied to it.
GREEN TIMBER means timber in which the moisture content is above its
fibre-saturation point.

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2

MOISTURE CONTENT means the weight of water contained in wood, expressed asa percentage of its oven dry weight.
NOTE :The moisture content of timber is generally determined by theoven-dry method.

STRUCTURAL TIMBER means timber used or intended for use in buildings wherestrength is the primary consideration.
PERMISSIBLE STRESS means the basic stress as modified by factors reflecting thetimber’s defects, its location and the particulars of design.
SPACED COLUMN means two column sections in which the parts are adequately
connected together by glue, bolts, screws or similar.
2.2.1

Definition of Defects in Timber
CHECK means a crack crossing radially the annual rings of the wood.
This defect reduce the strength in shear directly in proportion to its size.
COMPRESSION WOOD means abnormal wood which is formed on
the lower sides of branches and inclined stems of coniferous trees. It is
darker and harder than normal wood but relatively low in strength for its
weight. It can be usually identified by wide eccentric growth rings withabnormally high proportion of latewood growth.
DEAD KNOT means a knot in which the layers of annual growth are
not completely intergrown with those of the adjacent wood. It issurrounded by pitch or bark. The encasement may be partial orcomplete.
DECAY OR ROT means the disintegration of wood issues caused byfungi (wood destroying) or other micro-organisms.
DECAYED KNOT means a knot softer than the surrounding wood andcontaining decay.
DIAMETER OF KNOT means the maximum distance between thetwo points farthest apart on the periphery of a round knot, on the face on
which it becomes visible. In the case of a spike or a splay knot, themaximum width of the knot visible on the face on which it appears shallbe taken as its diameter.
DISCOLORATION means a change from the normal colour of thewood which does not impair the strength of the wood.
KNOT means a branch base or limb embedded in the tree or timbers by
natural growth.
KNOT HOLE means a hole left as a result of the removal of a knot.

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3
LIVE KNOT means a knot free from decay and other defects, in which the fibres are firmly
intergrown with those of the surrounding wood.
LOOSE GRAIN (LOOSENED GRAIN) means a defect on a flat
sawn surface caused by the separation or raising of wood fibres along
the growth rings.
LOOSE KNOT means a knot that is not held firmly in place by growthor position, and that cannot be relied upon to remain in place.
MOULD means a soft vegetative growth that forms on wood in damp
stagnant atmosphere. It is the least harmful type of fungus, usuallyconfined to the surface of the wood.
PITCH POCKET means an accumulation of resin between growthrings of coniferous wood as seen on the cross section.
SAP STAIN means a discoloration of the sapwood mainly due to fungi.
SAP WOOD means the outer layer of a log which, in the growing tree
contain living cells and food materials. The sapwood is usually lighterin colour and is readily attacked by insect and fungi.
SHAKE means a partial or complete separation between adjoining
layers of tissues as seen in end surfaces.
SLOPE OF GRAIN means the inclination of the fibres to thelongitudinal axis of the member.
SOUND KNOT means a tight knot free from decay, which is solid
across its face, and at least as hard as the surrounding wood.
SPLIT means a crack extending from one face of a piece of wood toanother and running along the grain of the piece.
TIGHT KNOT means a knot so held by growth or position as toremain firmly in position in the piece of wood.
WANE means the original rounded surface of a tree remaining on apiece of converted timber.
WARP means a deviation in sawn timber from a true plane surface or
distortion due to stresses causing departure from a true plane.
WORM HOLES means cavities caused by worms.
2.3

Symbols

The symbol are used as per IS: 883 - 1970 clause 3.0 unless inconsistent with the either context,
the following letters, symbols are used :A

NBC112V1.RV4Area of cross section

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4b

Width of the beam, in mm.

D

Depth of beam, in mm.

D1

Depth of beam, at the notch, in mm.

dDimension of least side of column in mm.

d1

The least overall width of box column in, mm.

d2

The least overall dimension of core in box column, in mm.

d3

Diameter of bolt.

E

Modulus of elasticity in bending, in N/mm2.

fab

Calculated bending stress in extreme fibre, in N/mm2.

fac

Calculated average axial compressive stress, in N/mm2.

fat

Calculated axial tensile stress, in N/mm2.

fb

Permissible bending stress on the extreme fibre, in N/mm2.

fc

Permissible stress in axial compression, in N/mm2.

fcn

Permissible stress in compression normal (perpendicular) to grain, in N/mm2.

fcp

Permissible stress in compression parallel to grain, in N/mm2.

fcoPermissible compressive stress in the direction of the line of action of the load, in
N/mm2.

ft

Permissible stress in tension parallel to grain, in N/mm2.

H

Horizontal shear stress, in N/mm2.

I

Moment of inertia of a section, in mm4.

k1

Modification factor for change in slope of grain.

k2

Modification factor for change in duration of loadings.

k3|
k4|
|
k5|
k6|

NBC112V1.RV4Form factors.

20 May 1994

5

k7

Modification factor for bearing stress.

k8

Constant equal to0.702 E
fcp

k9

Constant equal to

π/ 2

k10

UE
5 qf cp

Constant equal to

0.702

2 .5 E
f cp

LSpan of beam or truss, in mm.

nShank diameter of the nail.

p1

Ratio of the thickness of the compression flange to the depth of the beam.

Q

Static moment of area above or below the neutral axis about neutral axis, in mm3.

q

A constant for particular thickness of plankq1

Ratio of the total thickness of web or webs to the overall width of the beam.

S

Effective length of solid and box columns in mm, distance between points of lateralsupport of spaced column, in mm.

t

Nominal thickness of planks used in forming box type column in mm.

UConstant for a particular thickness of the plankV

Vertical and reaction or shear at a section in kN.

Y

A factor determining the value of form factor k4

φAngle of load to grain direction.

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6
3

Materials
The provision in clauses 4.1 to 4.7.1.2 of IS : 883 – 1970 will be applicable in addition to thefollowing clause.
The general characteristics like durability and treatability 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
stress characteristics are determined.

4

Moisture Content in Timber
The moisture content of timber for various positions in buildings shall be as given in Table3.

5

Sawn Timber
5.1

Sizes
Preferred cut sizes of timber for use in structural components shall be as given in
Tables 4, 5, 6 and 7.

5.2

Tolerances
Permissible tolerances in measurements of cut sizes of structural timber shall be as
follows :
a)

b)
5.3

For width and thickness :1)

Up to and including 100 mm

+3 mm
-0 mm

2)

Above 100 mm

+6 mm
-3 mm

For length :+10 mm- 0 mmLocation of Defects
The influence of defects in timber is different for different locations in the structuralelement. Therefore these should be so placed during construction so that they do nothave any adverse effect on the members.

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7

Table 1 : Classification of Woods According to Characteristics
Hard
ness

Shear
StrengthStrength
as abeam

Stiffness
as a beam

Suitabili
ty as apost

Shockresistingability

Retention
shape

Decay
resistance

Freedomfromobjectionableknots

Freedomfromshrinkage

FreedomfromwarpingPaintholding
qualityNailholding
quality

Ease
of
Work
ing

S.No.

1

SAL (AGRAKH)

Shorea robusta

56

100

100

100

100

100

100

100

A

A

A

A

B

A

B

2

SISAU

Dalbergia Sissoo

50

79

83

86

82

77

97

133

A

A

A

A

B

A

B

3

KHOTE SALLA

Pinus longlfolla

33

30

47

59

73

64

55

177

C

C

B

B

B

B

B

4

GOBRE SALLA

Pinus excelsa

32

24

43

50

55

55

38

125

C

B

C

B

A

C

A

5

UTTIS (red)Alnus nepalensis

36

-

C

A

B

B

A

B

B

Ordinary furnitures,suitable tocurved leg.

6

UTTIS (white)

Aluns nitida

34

-

C

B

C

B

A

C

A

Partition boards,
boxes, furnitures etc.

7

CHAMP

Michella champaca

33

-

B

A

B

A

A

A

A

8

SATISAL

Dalbergia latlfolla

38

-

A

A

A

A

B

A

B

9

ASNA

Terminalla
Tomentosa

46

91

B

A

B

B

A

B

B

Doors, Windosn,
Cabinets' Furniture.

10

PHALAT

Quercus glauca

60

-

B

A

B

C

C

A

C

Handles for tools.

11

TOONI

Cedrella toona

37

39

B

C

A

A

A

B

A

12

SEMAL

Bombaxmalabaricum

25

21

13

OKHAR

Juglans regia

45

14

OAK

Quercus incana

64

15

KHAIR

Acacla catechu

60-

BIJYASAL

Pterocarpus
marsuplum

49

82

16

SCIENTIFICNAMESWeight(12%
moisturecontent)(lb/cft)

KINDS OF
WOODS

Note:
1. Strength properties taking SAL =100
2. Working behaviours and characteristicc
A = Relatively highB = Intermediate
C = Relatively low in particular respect listed
3. Source : Goverment of Nepal Dept. of Forestry

NBC112V1.RV4

77

95

100

86

90

108

67

50

59

55

41

108

37

C

A

C

A

-

A

B

A

A

A

A

C

-

A

C

C

C

A

A

C

B

A

B

B

A

A

B

Musical durms

B

A

B

B

B

B

A

Wooden vases,
tumblers of

86

38

93

150

Furnitures
Packing box, boards
match sticks,
plywood.
Fifle buts, Cabints forgrammo-phones, highclass furnitures.
Handles of
implements

C

86

41

Good furnitures,
Door Panels,
Carvings etc.
Hukka, smoking pipe,
susical instruments
etc.

A

95

41

Beams, posts, girders,rallway sleeper, door
frames etc.
Doors, Windows,
rallway carriages,
cart-wheels,
Furnitures etc.
Beams, rafter, post
for cheap houses,
boares, cheap
furnitures
Picture Frames,
Packing box, cheap
furniture, boards etc.

C

77

41

DISTINCTIVE
USES

125

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8

Table 2 : Structural Properties & Safe Working Stresses for Some Indian Timbers **

Trade
Name

Average
Density@12%M.C.
(kg/m2)Modulusofelasticity
All
grades(N/mm2)1

2

3Compression

Binding & Tension Along
Grains, fb, Ft. Extreme FibreStressShear Fs
Parallel to grain 'p'

Perpendicular to grain 'q'

In
Location((N/mm2)

Out
Location(N/mm2)Wet
Location(N/mm2)

Horizontal
in beams,
All
Locations(N/mm2)

Alonggrain All
Location2(N/mm )

InsideLocation(N/mm2)

OutsideLocation(N/mm2)Wet
Location(N/mm2)InsideLocation(N/mm2)InsideLocation2(N/mm )

Wet
Location2(N/mm )

Remarks

4

5

6

7

8

9

10

11

12

13

14

15

3

785.0
930.0
575.0
545.0
730.0

X10
10.6
11.2
9.6
9.3
11.8

17
16.5
8.2
10
15.2

15.1
13.7
6.9
8.6
12.8

12.2
11
5.9
6.9
10

1.5
1.2
0.6
0.7
1.4

2.2
1.7
0.9
1
2

11
9.2
6.3
7.7
8.9

10
8.2
5.5
6.9
7.9

7.8
6.9
4.5
5.5
6.6

6.4
5.4
2.2
2.6
4

4.9
4.2
1.7
2.1
3.1

4
3.4
1.4
1.7
2.6

III S2 P2 A1*III S2 P3 A1 D1III S1 P3 A2 D2
I S1 P3 A2 D2
S3 P2 A1 D1Shisham
Jack
Jaman
Mango

755.0
595.0
850.0
690.0

9.1
8
11
8.9

16.5
12.4
14.8
12.2

13.7
10.4
12.4
10

11
8.2
10
7.8

1.2
1
1.2
0.9

1.7
1.4
1.7
1.4

10.4
8.2
8.9
7.3

9
7.3
8.2
6.5

7.7
5.9
6.6
5.1

6.3
4.5
5.7
3

4.9
3.5
4.4
2.4

4
2.8
3.5
1.9

1 P1 A1 D1III S3 P2 A1III S2 P3 A2

3

850.0
865.0
850.0
787.0
640.0

10.9
12.5
8.4
8.7
9.4

15.1
16.5
13.1
14.9
13.7

12.8
13.7
11
12.4
11.4

10
11
8.6
10
9.2

1
0.9
1.2
1.2
1

1.5
1.3
1.7
1.7
1.4

9.2
10.4
8.2
9.2
8.6

8.2
9.2
7.3
8.2
7.7

6.9
7.7
6.3
6.5
6.3

5.4
4.4
4.9
4.5
3.9

4.1
3.4
3.8
3.5
3

3.3
2.8
3.1
2.8
2.5

II S3 P2 A1 D1I S3 P1 A1 D1I S2 P2 A1 D1
II
1 S3 P1 A1 D1Babul
Bakli1Chir
DeodarDhaman2

Sain
Sal
SandanSisso
Teak

SYMBOLS USED IN ABOVE TABLE
1 Axle wood
I for High durability Life, that is > = 120 months
II - for Moderate durability Life, that is 60 to 120 months
III - for Low durability Life, that is < 60 months
S1 - Scarcely refractory woods; Those which can be rapidly seasoned without seasoning defects.
If not rapidly dried, they develop blue stain and mould on the surface.
S2 - Highly refractory wood: Those which are slow and diffcult to season free from seasoning defects.
A1 - Ist choice for beams, rafters, purlins and trusses
stD1 - I choice for struts and columns.
P1 - Heartwood, non- durable and requires through preservative treatment.

2 Indian Rosewood

3 Laurel

Note : Sapwood of all species requres preservative treatment.
** This table is reproduced from the book "Masonry and Timber structure including Earthquake Resistance Design" with due permission from the author Dr. Anand S Arya. (Permission taken by Mr. Amrit M.
Tuladhar)

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9

Table 3 : Data for Nailed Joints *With drawal
ResistanceWood

Specific
Gravity1
BabulBlack Siris
Chir
DeodarDhamanIndian Rose Wood
(Shisham)
Jack
Jaman
Mango
Mesua
Oak
Sain
Sal
Sandan
Sisso
Teak

(H)
(H)
(S)
(S)
(H)

2
0.835
0.735
0.575
0.56
0.755

(H)
(S)
(H)
(S)
(H)
(H)
(H)
(H)
(H)
(H)
(H)

0.755
0.595
0.85
0.655
0.969
0.865
0.88
0.800
0.865
0.77
0.625

Permissible Lateral Strength per nail in double shear in N
3.55 mm dia 80 mm
5 mm dia 150 mm
long
long

247
G5/2

3
157
114
62
58
122
122
67
164
86
222
171
179
141
171
128
76

Permanent
Construction
Lengthening
4
1500

Nod
e
5
1100

1100
960
1300

1000
400
500

Temp.
Construction
All
6
3400
1600
1500
2400

1500
1100
2600
1100
1600
1000
1700

1200
900
800
1100
1600
500
1100

2500
1600
4100
2700
2900
1900
1800

1400

800

1300

(H) = Hardwood, preboring necessary.

Permanent
Construction
Lengthening
7
2700
2750
1650

Nod
e
8
1350
1750
1500

Temp.
Constructio
n All

Gauge
of Nail

9

10
5300
4500
2400

1800
2250
3900

1450
1500
1550

3850
3200
5750

1950

1700

3700

1700
2800

1500
1300

4300
3003

(S) = Softwood, perboring not necessary.

* This table is reproduced from the book "Masonry and Timber structure including Earthquake Resistance Design"with due permission from the auther Dr. Anand S. Arya. (Permission taken by Mr. Amrit M. Tuladhar)

NBC112V1.RV4Nail Data

20 May 1994

Diamete
r

1
2
3
4
5

mm
11
727
700
640
589
538

6
7
8
9
10
11
12
113
4
1
18

488
447
406
366
325
294
264
234
293
162
122

Available Length

mm
12
228
228
178
152
127,152
102,111,127,15
2
89,102
75,89
68,76
51,57,68,76
51,67,63
38,44,51
38,44
25,32,38,44
25,32
19,25

10

6

Design Considerations
6.1

GeneralThe provisions in clauses 6.1 to 6.6.2 of IS : 883 – 1970 shall be applicable in additional
to following clauses :All structural members, assemblies or framework in a building in combination with thefloors, walls and other structural parts of the building shall be capable of sustaining,
without exceeding the limits of stress specified stresses, the worst combination of allloadings.

6.2

Loads
6.2.1

Load Combination
The worst combination and location of loads shall be considered for design.
Wind and seismic forces shall not be considered to act simultaneously. For
calculation of occupancy loads, refer to NBC 103 – Occupancy Loads.

6.3

Flexural Members
6.3.1

Effective Span
The effective span shall be taken equal to the clear span plus one half of therequired length of bearing at each end. For continuous beam, the effective span
may be taken equal to the distance between centres of bearings.

6.3.2

Form Factors for Flexural Members
For larger depth of single log beam, the allowable bending stress in compression
will be reduced by multiplying with a form factor K3 as given in the following
where D is the depth of beam in mm :K3 = 0.81

D2+89400
D2+55000*

For box and I - shape beams :K4 = 0.8 + 0.8Y

D2+89400
D2+55000-- 1*Reproduced from ”Masonry and Timber Structure including Earthquake-Resistant Design” by Dr. AS
Aryal, with the author’s permission.

NBC112V1.RV420 May 1994

11
Where :
Y=

p12 (6-8p1 + 3p12) (1-q1) + q1

p1 =ratio of thickness of compression flange to the depth of beam

q1 =ratio of thickness of web or webs to the width of the beam.

Note :i)

For solid circular cross sections, the form factor K5 shall be taken as 1.18.

ii)

For square cross-sections where the load is in the direction of diagonal, the form factor
K6 shall be taken as 1.414.

6.4

Deflection
The maximum deflection specified is 1/360 of span when the beam supports brittle
covering like asbestos cement sheets, earthenware, slates, gypsum, etc. Otherwise a
deflection of 1/240 of span may be taken. In the case of cantilevers, the deflection
should not exceed 1/180 of clear overhang.
It can be shown that, for uniform loads, the depth of a beam, D, which will give safe
deflection can be obtained from the following expression :For maximum deflection of 1/360 of the span :D ≥ (75 Fb/E) L
For maximum deflection of 1/240 of the span :D ≥ (50 Fb/E) L
where :
Fb is the actual bending stress in beams
L is the span of the beam

7

Columns
7.1

Solid Columns
These are classified into three categories - i) Short, ii) Intermediate, iii) Long-Solidcolumn depending upon their slenderness ratios (ratio of overall unsupported length l tothe dimension of least side, d). For circular section, least side d is taken equal to

NBC112V1.RV420 May 1994

12
the side of a square of equal area. Therefore, for a column of diameter D, the least sided will be equal to :( λ / 4 ) D 2 = 0.88

7.2

Long Columns
For long columns, the permissible compressive stress shall be calculated by using thefollowing formula :fc =0.329 E
( S / d )2

Where S/d is the slenderness ratio.

7.3

Slenderness Ratio
In the case of solid columns of timber, S/d ratio shall not exceed 50.

7.4

Permissible Load
The permissible load on a column of circular cross-section shall not exceed that
permitted for a square column of an equivalent cross-sectional area.

NBC112V1.RV420 May 1994

13

8

Design of Common Steel Wire Nail Joints
8.1

Dimensions of Members
The dimension of an individual piece of timber (that is any single member) shall bewithin the range given below :8.1.1

Minimum Thickness
Minimum thickness of any individual member : 30 mm for main members inmono-chord construction; 20 mm for web members and 25 mm for chordmembers in split-chord construction.

8.1.2

Maximum Thickness
Maximum thickness (other than spacer blocks) : 100 mm. For spacer blocks, notmore than three times the thickness of the main elements.

8.1.3

Minimum Depth
Minimum depth of member : 75 mm for chord members and 40 mm for web
members of trusses in soft or hard woods.

8.2

Length of Member
Length of members : 4.5 to 5 m maximum in single piece, because longer members are
difficult to obtain and are costly.

8.3

Width of Members
Width of any individual piece : not to exceed about eight times its thickness.

8.4

Spacing
The space between two adjacent pieces of timber shall be restricted to a maximum of
three times the thickness of the individual piece of timber of the chord member. In caseof web members, it may be greater for facilities jointing.

8.5

Lengthening Joint
It is preferable that no lengthening-joint shall be located at a panel point. Generally, notmore than two, but preferably not more than one, lengthening-joint shall be permitted
between the two panel joints of a member.

NBC112V1.RV420 May 1994

14

8.6

Specification and Diameter of Nails
8.6.1

Nails
Common wire nails shall be made of mild steel having a minimum tensile
ultimate strength of 550 N/mm2.

8.6.2

Nails Diameter
Nail diameter shall be between 1/11 and 1/6 of the least thickest of the members
to be connected.

8.6.3

Nails Length
The length of a nail shall be at least 2.5 times the thickness of the thinnest
member and it shall penetrate the thicker member by 1.5 times the thickness ofthe thinner member, whichever is further.

8.6.4

Number of Nails
The number of nails in a group should not exceed 10 in one row in the directionof the force.

8.6.5

Spacing of Nails
The minimum spacing of nails having a shank diameter n in lengthening-joints
should be that given below:

No.

Spacing of Nails

Type of Stress in
the Joint

Minimum
spacingi)

End distanceTensionCompression

12 n
10 n

ii)

In the direction ofgrain

TensionCompression

10 n
5 n

iii)

Edge distance between
row
of
nails
perpendicular to thegrain

TensionCompression

5 n
5 n

Table 2:

Minimum Nail Spacing

Note :The 5 n distances between rows perpendicular to the grain may be increased
subject to the availability of width of the member keeping edge distanceconstant.

NBC112V1.RV420 May 1994

15

9

Design of Bolted Joints
9.1

GeneralFor total prefabrication, bolt-jointed construction most befits structural timbercomponents. Bolt-jointed construction units give better facilities with respect toworkshop ease, transport convenience and re-assembly at site of work site. This
technique is base-suited for defence purposes for semi-permanent structures (e.g.,sheds) which are required to be erected at high altitudes and in remote locations. Mass production of structural components in factories can thus be made far more rationally.

9.2

Design Considerations
Where a number of bolts are used in a joint, the allowable load in withdrawal or lateralresistance shall be the sum of the allowable loads for the individual bolts.

9.3

Arrangement of Bolts
The following spacings in bolted joints shall be followed :a)Spacing of bolts in a row :For loading parallel and perpendicular to grain loading : 4 d3.

b)

c)

9.3.1

Spacing between rows of bolts :1)

For loading perpendicular to the grain: 2.5 d3 to 5 d3 (2.5 d3 for t/d3 ratioof 2 and 5 d3 for t/d3 ratio of 6), where t is thickness of main member
and d3 is the diameter of the bolt used.

2)

For parallel-to-grain loading: At least (N-4) d3, with a minimum of 2.5
d3, where N is total number of bolts. The spacing is also governed by net
area at the critical section which should be 80 percent of the total area inbearing under all bolts.

Edge Distance :1)

For parallel-to-grain loading 1.5 d3, or half the distance between rows ofbolts, whichever is greater.

2)

For perpendicular-to-grain loading: the loaded edge distance shall be atleast 4 d3

Spacing
For inclined members, the spacing given above for perpendicular and parallelto-grain of wood may be used as a guide and bolts arranged at the joint withrespect to loading direction.
The bolts shall be arranged in such that the centre of resistance of the bolts
passes through the intersection of the centre-of-gravity axes of the members.

NBC112V1.RV420 May 1994

16

9.3.2

Staggering
Staggering of bolts shall be avoided as far as possible in the cases where
members are loaded parallel to the grain of the wood. For loads acting
perpendicular to grain, staggering is preferable in order to avoid splitting due toweather effects.

9.3.3

Bolt HolesThe bolt holes shall be bored or drilled perpendicular to the surface involved.
Forcible driving of the bolts shall be avoided which may cause cracking or
splitting of members. As a guide, a bolt hole which is 1 mm oversize may be
used for pre-boring.

10

Glue Laminated TimberThis type of timber is often used for large span structures when both the available cross-sections
and lengths are much limited.
The limitation is overcome by building up large size member out of 25 to 50 mm thick planks
by gluing them together so that grain of all planks is in the same direction
This operation requires uniform application of glue while maintaining both the required
temperature and applying the necessary pressure.

10.1

Subject to Bending
For best use of laminated timber, high-strength species and select grade timber should
use near the extreme fibres of members subject to bending.
Curved members should ordinarily have a minimum radius of curvature of about 130 t
where t is the maximum thickness (in mm) of lamination used.

NBC112V1.RV420 May 1994

17

Zones
Use

TABLE 4 :

I

II

III

IV

Structural elements

12

14

17

20

Doors and windows

10

12

14

16

Recommended Percentage Moisture Content Values

Note :
Zones are defined by annual relative humidities :Zone I
less than 40 percent,Zone II
40 to 50 percent,Zone III
50 to 67 percent, andZone IV
more than 67 percent.

Thickness
(mm)
20
25
30
40
50
60
80
100

TABLE 5 :metres)Width (mm)
40
40
40
-

50
50
50
-

60
60
60
60
60
-

80
80
80
80
80
80
-

100
100
100
100
100
100
-

120
120
120
120
120
120
120

140
140
140
140
140
140
140

160
160
160
160
160
160
160

Preferred Cut Sizes of Structure Timbers for Roof Trusses (from 3 to 20

Note: 2Preferred lengths of timber : 1 m, 1.5 m, 2 m, 2.5 m and 3 m.

NBC112V1.RV420 May 1994

18

Thickness (mm)Width (mm)

60
80
100

TABLE 6 :

80
-

100
100
-

120
120
-

140
140
140

160
160
160

180

200

Preferred Cut Sizes of Structure Timbers for Roof Purlins, Rafters, Floor
Beams, etc.

Note :
Preferred lengths of timber : 2 m, 2.5 m, 3 m and 3.5 m.

Thickness (mm)
10
15
20
25
30
40
50
60
80

Table 7 :40-

Width (mm)
50
50
50
50
-

60
60
-

80
80
80
80
80
80
80
-

100
100
100
100
100
100
100
100

120
120
120
120
120
120
120
120

160
160
160
160
160
160
160
160

200
200
200
200
200
200

240
240
240
240
240
240

Preferred Cut Sizes of Structure Timbers for Partition Framing and
Covering

Notes:
Preferred lengths of timber : 0.5 m, 1 m, 1.5 m and 2 m.
While nailing, the area of the pre-bored hole shall not be taken into account forthis purpose.

NBC112V1.RV420 May 1994

19

Permissible Stresses (N/mm²)
Type

LocationGroup AGroup B

Group C

Bending and Tension Inside Outside
Wet
Along Grain

17.8
14.9
11.8

12.1
10.0
7.9

8.2
6.9
5.9

Horizontal, alllocations *

1.2

0.9

0.6

Along grain, alllocations**
Compression Parallel Inside Outside
Wet
to Grain

1.7

1.3

0.9

11.8
10.4
8.6

6.9
6.2
5.7

6.3
5.5
4.5

5.9
4.5
3.7

2.2
1.8
1.5

2.2
2.7
1.4

Shear

Compression
Perpendicular
to Grain

TABLE 8:Inside Outside
Wet

Permissible Stresses for Grade 1 Timber*

The values of horizontal shears to be used only for beams.

**

In all other cases, values for shear along grain to be used.

NBC112V1.RV420 May 1994