Timber types

2.  Types of timber – Classification –
Allowable stresses – Design of beams
Flexure, Shear, Bending and Deflection
considerations – Design of columns –
Design of composite beam sections with
timber and steel .
2

3.  Earliest building material used
 From Engineering point of view, Timber is
different from Wood
 Timber is wood for building - the wood at
any stage after the tree has been felled
 Used for both temporary and permanent
structures
Scaffolding Formwork
Shuttering Purlin
Door Beam
 There are 100-200 types of timber
3
SAPWOOD HEARTWOOD
PITH
BARK
CAMBIUM

4.  Biological and natural material with
highly variable properties
 Hygroscopic – moisture content varies
with relative humidity of surroundings
 Timber is capable of transferring both
tensile and compressive forces
 Non corrosive and highly durable if
detailed properly
 Very high strength to weight ratio
 Physical & mechanical properties varies in
different directions with respect to fibre
orientation
4
Strength when loaded parallel to grain > when loaded perpendicular to grain

5.  Timber is viable to seasonal cracks and
warping
 Factor of safety depends on the exposure
conditions (inside, outside & wet) [Table 1]
 Well seasoned timber are less liable to
volume changes
 Green timber is weak
 There is a risk of biological degradation,
when exposed to high moisture
conditions. [Table 2]
 Defects in wood include knots, cracks,
wane, shake, dry rot, attack from termites,
white ants, wood borers etc.
5

6.  Typical Characteristics of Wood
 wood has higher strength per unit weight than most
other construction materials
 A non-homogeneous and anisotropic material
showing different characteristics not only in different
directions but also in tension and compression.
 Shrinkage of wood on drying is relatively large.
Joints loosen easily due to contraction in the
direction perpendicular to fibres. Therefore dry
wood shall be used with the moisture content less
than 20 %.
 The elastic modulus is small. Consequently, members
are apt to show large deformations
6

7.  Typical Characteristics of Wood…
 A notable creep phenomenon occurs under
permanent vertical loads. This is important
especially in snowy areas.
 Large deformation occurs due to compressive
force perpendicular to fibers. This influences the
amount of deformation of horizontal members
and chord members of built-up members.
 The defects and notches of wood influence
greatly its strength and stiffness. Consequently
it is necessary to select and to arrange
structural members considering their structural
properties.
7

8.  Typical Characteristics of Wood…
 Wood can decay from repeated changes of
moisture. Therefore seasoned wood should be
used in construction.
 Preservative treatment is necessary to avoid
premature rotting and insect attack.
 Wood is a combustible material. Precautions must
be taken to minimize the danger of fire.
 Lengths more than 3.5 m long and large size
timbers are difficult to obtain. This leads to splicing
through connectors or gluing.
 In view of its lightness, very easy workability like
cutting and nailing and safe transportability, timber
makes an excellent material for post-earthquake
relief and rehabilitation construction.
8

9.  Glued Laminated Timber
 Cross Laminated Timber
 Oriented Strand Boards
 Laminated Veneer Lumber
 Plywood
 Particle or Fiber Boards
9
Cross Laminated Timber Plywood
Glued Laminated
Timber
Laminated Veneer Lumber

10. Species of timber recommended for constructional
purposes are classified into 3 groups based on
their strength properties, Modulus of Elasticity (E)
& Extreme fiber stress in bending and tension (𝒇𝒃)
 Group A- E > 12.6 x 103
N/𝑚𝑚2
𝒇𝒃> 18 N/𝑚𝑚2
 Group B – 9.8 x 103
<E < 12.6x103
N/𝑚𝑚2
12>𝒇𝒃> 18 N/𝑚𝑚2
 Group C - 5.6 x 103
<E < 9.8 x 103
N/𝑚𝑚2
8.5 >𝒇𝒃>12 N/𝑚𝑚2
10
IS 883-1994

11.  Group A- Mangrove, Dhaman, Bullet wood
 Group B – Babul, Ebony, Oak, Teak,
Eucalyptus
 Group C - Jack, Maple, Neem, Deodar,
Coconut, Rosewood, Walnut
Table 1
IS 883:1994
11

12. Classification for preservation based on durability
tests
 1 – Average life > 120 months
 2 – 60 months < Average life < 120 months
 3 – Average life < 60 months
12

13. Classification based on treatability grades
 a - heartwood easily treatable
 b – heartwood treatable, but complete penetration
not always obtained; least dimension > 60mm
 c – heartwood only partially treatable
 d – heartwood refractory to treatment
 e - heartwood very refractory to treatment,
penetration of preservative being practically nil
even from the ends
13

14. Classification based on seasoning behavior of
timber and refractoriness with respect to cracking,
splitting and drying rate
 A – Highly refractory (slow and difficulty to season
free from surface and end cracking)
 B – Moderately refractory (may be seasoned free from
surface and end cracking within reasonably short
periods, given a little protection against rapid drying
conditions)
 C – Non refractory (may be rapidly seasoned free from
surface and end cracking even in open air & sun)
14

15.  strength properties depend on:
1) Wood species
2) Direction of loading relative to the grain of wood
3) Defects like knots, checks, cracks, splits, shakes
and wanes
4) Moisture content or seasoning
5) Type of wood, such as sapwood, pith and wood
from dead trees
6) Location of use, viz. inside protected, outside,
alternate wetting and drying.
15

16.  The permissible stresses must be determined
taking all these factors into account.
 Table 6.1 gives typical basic stresses for
timbers placed in three groups A, B and C
classified on the basis of their stiffness.
 It is reasonable to increase the normal
permissible stress by a factor of 1.33 to 1.5
when earthquake stresses are superimposed.
16

17. 17
The permissible bearing stress depends on the
inclination of the direction of stress to that of the
grain, the length of the bearing area and its
distance from the free end of the member.
Larger the slope of grain more is the strength
reduction
 Select Grade 1 in 20
 Grade 1 1 in 15
 Grade 2 1 in 12
The strength transverse to grain is minimum

18. 18
 Cl:6.2 The permissible stresses of Groups A, B
and C (Grade 1) of different locations of use are
given in Table 1
Minimum permissible stress limits are given in
Table 3. The following conditions should be met.
a. Timber should be of high or moderate
durability, suitable treatment should be given
if necessary
b. Timber of low durability shall be used after
giving proper preservative treatment (IS 401-1982)
c. Loads should be continuous and permanent
IS 883-2016

19.  Cl:6.3 Permissible stresses of other grades
of timber (Table 1&3) should be
multiplied by the following factors
a. For Select Grade Timber 1.16
b. For Grade 2 Timber 0.84
 When low durability timbers are to be
used on outside location, the permissible
stresses for all grades of timber, arrived
by Cl 6.2 & 6.3 shall be multiplied by 0.80
19

20.  Modification Factors for Permissible
Stresses
a. Due to change in slope of grain
Timber with major defects, Permissible
stresses in Table 1 shall be multiplied by a
Modification factor 𝐾1(Table 4)
b. Due to duration of load
For different durations of design load, the
permissible stresses in Table 1 shall be multiplied
by a modification factor 𝐾2(Table 5)
20

21. 21
 Cl:7 All structural members, assemblies or
framework in a building, in combination with the
floors, walls and other structural parts of the
building shall be capable of sustaining, with due
stability and stiffness the whole dead and imposed
loads (as specified) without exceeding the limits of
relevant stresses specified.
 Cl:7.2 Worst combination and location of loads
shall be considered for design
 Cl:7.4.1 The net section shall be obtained by
deducting the gross sectional area of timber the
projected area of all material removed by boring,
grooving or other means of critical plane
IS 883-

22. 22

23. 23
 Cl:7.5 Flexural Members
7.5.2 Effective span = distance from supports
+ 2(half the bearing width)
For continuous beams, distance between center of bearings
7.5.3 Bending Stress
fab =
𝑀
𝑍
fb
fab – Calculated bending stress in extreme fiber
fb– Permissible bending stress on extreme fiber
7.5.4 Form factors shall be applied to bending stress
K3 Rectangular section
K4 Box & I beams
K5 Solid circular
K6 Square cross section
IS 883

24. 24
Cl 7.5.5 Minimum Width > 50mm OR
1/50 of span
Cl 7.5.6 Depth < 3 times width
If Cl 7.5.5 & 7.5.6 cannot be satisfied, lateral stiffening should
be provided to resist bending or buckling
 Cl 7.5.7 Shear
Cl 7.5.7.1 Maximum horizontal shear H
When load moves from support towards center &
load is at a distance of 3 to 4 times depth of beam
from support
Cl 7.5.7.2 Vertical end reaction or Shear at a section V
For concentrated & uniformly distributed loads
Cl 7.5.7.3 Deductions in load & Table 6 [Reduction
factors for concentrated loads]
IS 883
Greater

25. 25
 Cl 7.5.8 Bearing
Cl 7.5.8.1 Ends of flexural member shall be supported in
recesses which provide adequate ventilation to
prevent dry rot & shall not be enclosed
Cl7.5.8.3 Bearing Stress
V
𝐛𝐝
It is the vertical stress on the bearing area, should be less
than the permissible stress in compression perpendicular to
the grain fcn(Table 1) for a bearing length ≥ 150mm
Cl7.5.8.3.1 (c) For bearing length < 150mm & located
75mm or more from the end of the member, the
permissible stress shall be multiplied by
Modification factor K7(Table 13)
IS 883-1994

26. 26
 Cl 7.5.9.6 Deflection
Cl 7.5.9.6.1 Deflection of flexural members supporting brittle
materials < 1/360 of span
Other Flexural members < 1/240 of span
Cantilever 1/150 of freely hanging length
Cl7.5.9.6.2 Deflection δ
δ =
𝑲𝑾𝑳𝟑
𝑬𝑰
K values are given for different loading conditions
IS 883-1994

27. 27
 Solid beams
 Built up beam
Composed of vertical sections that are bolted together
firmly. Used for large spans and higher loads
 Flitched beam
It consist of 2 or more timber pieces
which is reinforced with steel plates
 Notched beam
Grooves are cut in the soffit of beams at supports or at
mid span
Notch
Notch

28. 28
Notched Beams
Built up Beams
Flitched Beam

29. Timber compression members may have solid rectangular or
circular cross section which may be uniform throughout the
length or tapering.
 Cl 7.6.1 Solid Columns
Short S/d 11
Intermediate 11 < S/d < K8
Long S/d > K8
The permissible compressive stress values for Solid
columns are given in Cl 7.6.1.1, 7.6.1.2 & 7.6.1.3
Cl 7.6.1.4 For solid timber columns, S/d shall not
exceed 50
The permissible compressive stress values for circular columns
andtaperedcross sectional columnsaregivenin Cl 7.6.1.6 & 7.6.1.7
29
IS 883
S –overall unsupported
length of the column

30. 30
 Cl7.6.2 Box & Built up Columns
Built-up columns are formed by spiking, nailing or
bolting together planks or square sections. Planks must be
fastened together at regular intervals (<6 times thickness)
Box columns are made by connecting planks together so
as to have a hollow core inside. The core is blocked by solid
pieces of timber at the ends and intermediate points. The pieces
are joined by screws, nails, bolts, glue or other connectors.
Slenderness ratio (Sr) is given by
𝑆
𝑑1
2 +𝑑2
2
d1 and d2 are least dimension of overall width and core width
respectively
IS 883

31. 31
Short columns Sr< 8
Intermediate Columns 8 > Sr > K9
Long Columns Sr > K9
The permissible compressive stress values for box columns
are given in Cl 7.6.2.2, 7.6.2.3 & 7.6.2.4
 Cl 7.6.3 Spaced Columns
The formulae for calculating permissible compressive stress of
short solid columns are applicable to spaced columns with a
restraint factor of 2.5 to 3
The permissible compressive stress values for intermediate and
long spaced columns are given in Cl 7.6.3.1 & 7.6.3.2

32. 32
 Usually employed in trusses with nailed, bolted or disc
dowelled connections
 Spaced column consist of 2 or more wooden members
with their longitudinal axis parallel and joined at their
end and intermediate points by block pieces
 These members are separated from each other by means
of spacer blocks
 Thickness of spacer block ≥ thickness of individual
components
 Safe load carrying capacity
of spaced column is the
sum of safe load carrying
capacities of individual
members

33.  Wooden beam reinforced with steel strips form
composite beams.
 Flitched section are stronger than pure wooden
beams of same dimensions and are also
economical.
 reinforcing material should have a modulus of
elasticity greater than that of the reinforced
material.
 The steel plates are bolted or screwed to the
timber beams. The connection is made
perfectly such that there is no slipping between
them.
 The composite beam behaves like a single
33

34. 34

35.  The bending theory is valid when a constant
value of Young's modulus applies across a
section
 it cannot be used directly to solve the
composite-beam problems where two
different materials, and therefore different
values of E, exists.
 The method of solution in such a case is to
replace one of the materials by an
equivalent section of the other.

35

36.  Assumption
 In order to analyze the behavior of composite
beams, make the assumption that the materials
are bonded rigidly together so that there can be no
relative axial movement between them.
 This means that all the assumptions, which were
valid for homogenous beams are valid except the
one assumption that is no longer valid is that the
Young's Modulus is the same throughout the
beam.

36

37. 37

38.  Beam have stiffening plates
 The equivalent beam of the main beam
material can be formed by scaling the breadth
of the plate material in proportion to modular
ratio.
 the strain at any level is same in both
materials, the bending stresses in them are in
proportion to the Young's modulus.

38

39.  Strain Compatibility
 With two materials bonded together, both will
act as one, and the deformation in each is the
same.
 Therefore, the strains will be the same in each
material under axial load.
 In flexure the strains are the same as in a
homogeneous section, i.e. linear.
 In flexure, if the two materials are at the same
distance from the N.A., they will have the same
strain at that point because both materials
share the same strain diagram.
39

40. 40

41.  Strain Compatibility
 The stress in each material is determined by
using Young’s Modulus
 Care must be taken that the elastic limit of
each material is not exceeded. The elastic
limit can be expressed in either stress or
strain.
41

42.  Advantages
 Compatible with the wood structure, i.e. can
be nailed
 Lighter weight than a steel section
 Less deep than wood alone
 Stronger than wood alone
 Allow longer spans
 The section can vary over the length of the
span to optimize the member
 The wood stabilizes the thin steel plate
42

43.  Flexure Stress using Transformed Sections
 basic flexural stress equation, derived based on a
homogeneous section.
 Therefore, to use the stress equation one needs to
“transform” the composite section into a homogeneous
section
 For the new “transformed section” to behave like the
actual section, the stiffness of both would need to be the
same.
 Since Young’s Modulus, E, represents the material
stiffness, when transforming one material into another, the
area of the transformed material must be scaled by the
ratio of one E to the other.
 In order to also get the correct stiffness for the moment of
Inertia, I, only the width of the geometry is scaled. Using I
from the transformed section (ITR) will then give the same
flexural stiffness as in the original section.
43

44. 44

45. 45

46.  Calculate the Transformed Section, ITR
 Use the ratio of the E modulus from each material
to calculate a modular ratio, n.
 Usually the softer (lower E) material is used as a
base (denominator). Each material combination
has a different n.
 Construct a transformed section by scaling the
width of each material by its modular, n.
 Itr is calculated about the N.A. using the
transformation equation (parallel axis theorem)
with the transformed section.
 Separate transformed sections must be created
for each axis (x-x and y-y)
46

47. 47

48. 48