4.1
Scope
This Section relates to the use of bamboo in construction as structural elements,
nonstructural elements and also for temporary works in structures or elements of the
structure, ensuring quality and effectiveness of design and construction using
bamboo. It covers minimum strength data, dimensional and grading requirements,
seasoning, preservative treatment, design and jointing techniques with bamboo which
would facilitate scientific application and long-term performance of structures. It also
covers guidelines so as to ensure proper procurement, storage, precautions and
design limitations on bamboo.
4.2
Terminology
For the purpose of this Section, the following definitions shall apply.
4.2.1
Anatomical Purpose Definitions
BAMBOO
Tall perennial grasses found in tropical and sub-tropical
regions. They belong to the family Poaceae and sub-family
Bambusoidae.
BAMBOO CULM
A single shoot of bamboo usually hollow except at nodes
which are often swollen.
BAMBOO
CLUMP
A cluster of bamboo culms emanating from two or more
rhizomer in the same place.
CELLULOSE
A carbohydrate, forming the fundamental material of all
plants and a main source of the mechanical properties of
biological materials.
CELL
A fundamental structural unit of plant and animal life,
consisting of cytoplasm and usually enclosing a central
nucleus and being surrounded by a membrane (animal) or a
rigid cell wall (plant).
CROSS WALL
A wall at the node closing the whole inside circumference
and completely separating the hollow cavity below from
that above.
HEMI CELLULOSE The polysaccharides consisting of only 150 to 200 sugar
molecules, also much less than the 10000 of cellulose.
LIGNIN
A polymer of phenyl propane units, in its simple form
(C6H5CH3CH2CH3).
SLIVER
Thin strips of bamboo processed from bamboo culm.
TISSUE
Group of cells, which in higher plants consist of (a)
Parenchyma - a soft cell of higher plants as found in stem
pith or fruit pulp, (b) Epidermis - the outermost layer of
cells covering the surface of a plant, when there are several
layers of tissue.
4.2.2
Structural Purpose Definitions
BAMBOO MAT
BOARD
A board made of two or more bamboo mats bonded with
an adhesive.
BEAM
A structural member which supports load primarily by its
internal resistance to bending.
BREAKING
STRENGTH
A term loosely applied to a given structural member with
respect to the ultimate load it can sustain under a given set
of conditions.
BUNDLE-COLUMN A column consisting of three or more number of culm
bound as integrated unit with wire or strap type of
fastenings.
CENTRE
INTERNODE
A test specimen having its centre between two nodes.
CHARACTERISTIC
LOAD
The value of loads which has a 95 percent probability of
not exceeding during the life of the structure.
CHARACTERISTIC
STRENGTH
The strength of the material below which not more than
5 percent of the test results are expected to fall.
CLEAVABILITY
The ease with which bamboo can be split along the
longitudinal axis. The action of splitting is known as
cleavage.
COLUMN
A structural member which supports axial load primarily
by inducing compressive stress along the fibres.
COMMON RAFTER A roof member which supports roof battens and roof
coverings, such as boarding and sheeting.
CURVATURE
The deviation from the straightness of the culm.
DELAMINATION
Separation of mats through failure of glue.
END DISTANCE
The distance measured parallel to the fibres of the bamboo
from the centre of the fastener to the closest end of the
member.
FLATTEN
BAMBOO
Bamboo consisting of culms that have been cut and
unfolded till it is flat. The culm thus is finally spread open,
the diaphragms (cross walls) at nodes removed and
pressed flat.
FULL CULM
The naturally available circular section/shape.
FUNDAMENTAL
OR ULTIMATE
STRESS
The stress which is determined on a specified type/size of
culms of bamboo, in accordance with standard practice
and does not take into account the effects of naturally
occurring characteristics and other factors.
INNER DIAMETER Diameter of internal cavity of a hollow piece of bamboo.
INSIDE
LOCATION
Position in buildings in which bamboo remains
continuously dry or protected from weather.
JOINT
A connection between two or more bamboo structural
elements.
JOIST
A beam directly supporting floor, ceiling or roof of a
structure.
LENGTH OF
INTERNODE
Distance between adjacent nodes.
LOADED END OR
COMPRESSION
END DISTANCE
The distance measured from the centre of the fastener to
the end towards which the load induced by the fastener
acts.
MATCHET
A light cutting and slashing tool in the form of a large
knife.
MAT
A woven sheet made using thin slivers.
MORTISE AND
TENON
A joint in which the reduced end (tenon) of one member
fits into the corresponding slot (mortise) of the other.
NET SECTION
Section obtained by deducting from the gross cross-section
(A), the projected areas of all materials removed by
boring, grooving or other means.
NODE
The place in a bamboo culm where branches sprout and a
diaphragm is inside the culm and the walls on both sides
of node are thicker.
OUTER
DIAMETER
Diameter of a cross-section of a piece of bamboo
measured from two opposite points on the outer surface.
OUTSIDE
LOCATION
Position in building in which bamboos are occasionally
subjected to wetting and drying as in case of open sheds
and outdoor exposed structures,
PERMISSIBLE
STRESS
Stress obtained after applying factor of safety to the
ultimate or basic stress.
PRINCIPAL
RAFTER
A roof member which supports purlins.
PURLINS
A roof member directly supporting roof covering or
common rafter and roof battens.
ROOF BATTENS
A roof member directly supporting tiles, corrugated sheets,
slates or other roofing materials.
ROOF SKELETON
The skeleton consisting of bamboo truss or rafter over
which solid bamboo purlins are laid and lashed to the
rafter or top chord of a truss by means of galvanized iron
wire, cane, grass, bamboo leaves, etc.
SLENDERNESS
RATIO
The ratio of the length of member to the radius of gyration
is known as slenderness ratio of member. (The length of
the member is the equivalent length due to end
conditions).
SPLITS
The pieces made from quarters by dividing the quarters
radially and cutting longitudinally.
TAPER
The ratio of difference between minimum and maximum
outer diameter to length.
UNLOADED END
DISTANCE
The end distance opposite to the loaded end
WALL
THICKNESS
Half the difference between outer diameter and inner
diameter of the piece at any cross-section.
WET LOCATION
Position in buildings in which the bamboos are almost
continuously damp, wet or in contact with earth or water,
such as piles and bamboo foundations.
4.2.3
Definitions Relating to Defects
BAMBOO BORE/
GHOON HOLE
The defect caused by bamboo GHOON beetle (Dinoderus
spp. Bostychdae), which attacks felled culms.
CROOKEDNESS
A localized deviation from the straightness in a piece of
bamboo.
DISCOLORATION
A change from the normal colour of the bamboo which
does not impair the strength of bamboo or bamboo
composite products.
4.2.4
Definitions Relating to Drying Degrades
COLLAPSE
The defect occurring on account of excessive shrinkage,
particularly in thick walled immature bamboo. When the
bamboo wall shrinks, the outer layers containing a larger
concentration of strong fibro-vascular bundles set the weaker
interior portion embedded in parenchyma in tension, causing
the latter to develop cracks. The interior crack develops into
a wide split resulting in a depression on the outer surface.
This defect also reduces the structural strength of round
bamboo.
END SPLITTING
A split at the end of a bamboo. This is not so common a
defect as drying occurs both from outer and interior wall
surfaces of bamboo as well as the end at the open ends.
SURFACE
CRACKING
Fine surface cracks not detrimental to strength, However, the
cracking which occurs at the nodes reduces the structural
strength.
WRINKLED AND
DEFORMED
SURFACE
Deformation in cross-section, during drying, which occurs in
immature round bamboos of most species; in thick walled
pieces, besides this deformation the outer surface becomes
uneven and wrinkled. Very often the interior wall develops a
crack below these wrinkles, running parallel to the axis.
4.3
Symbols
For the purpose of this Section, the following letter symbols shall have the meaning
indicated against each, unless otherwise stated:
A
Cross-sectional area of bamboo (perpendicular to the direction of the
principal fibres and vessels),
d
D
, mm2
D
Outer diameter, mm
d
Inner diameter, mm
E
Modulus of elasticity in bending, N/mm2
fc
Calculated stress in axial compression, N/mm2
fcp
Permissible stress in compression along the fibres, N/mm2
I
Moment of inertia =
d
D
, mm4
l
Unsupported length of column, m or mm
M
Moisture content, %
r
Radius of gyration =
A
I
, mm
জ্থ
Modulus of rupture, N/mm2
W
Wall thickness, mm
Z
Section modulus, mm3
δ
Deflection or deformation, mm.
4.4
Materials
4.4.1
Species of Bamboo
In Bangladesh, four species are widely used, hence studied for the mechanical
properties as tabulated in Table 6.4.1-6.4.4 for top, bottom and middle positions.
Table 6.4.5 further summarize the average mechanical properties of 21 bamboo
species.
Table 6.4.1:
Moisture content and specific gravity values of bamboo species
Species
Moisture content (%)
Specific Gravity
(based on oven dry weight and at different
volumes)
bottom
middle
top
Green volumes
Oven dry volumes
bottom middle
top
bottom middle
top
Kali (Oxytenanthera
nigrociliata)
0.48
0.49
0.51
0.66
0.69
0.74
Mitinga (Bambusa tulda)
0.54
0.58
0.61
0.75
0.79
0.83
Bethua (Bambusa
polymorpha)
0.55
0.57
0.61
0.79
0.81
0.54
Borak (Bambusa
balcooa)
0.57
0.64
0.74
0.79
0.84
0.85
Table 6.4.2: Shrinkages of wall thickness and diameter of bamboo species
Species
Shrinkage in wall thickness (%)
Shrinkage in diameter (%)
From green to 12% mc
From green to oven
dry condition
From green to 12% mc
bottom
middle
top bottom middle top
bottom
middle
top
Kali (Oxytenanthera
nigrociliata)
9.6
8.1
5.9
13.2
10.7
8.7
4.8
3.0
2.4
Mitinga (Bambusa tulda)
11.9
7.3
4.9
14.9
9.6
7.6
3.9
3.5
2.6
Bethua (Bambusa
polymorpha)
10.7
6.5
5.1
12.1
10.1
8.2
7.3
5.5
4.1
Borak (Bambusa balcooa)
11.1
7.6
4.8
13.7
11.1
8.4
4.2
3.4
2.5
Table 6.4.3: Compressive strength of bamboo species
Species
Compression parallel to the grain (kg/cm2)
Green
Air dry
bottom middle
top
bottom
middle
top
Kali (Oxytenanthera
nigrociliata)
Mitinga (Bambusa tulda)
Bethua (Bambusa
polymorpha)
Borak (Bambusa balcooa)
Table 6.4.4: Modulus of elasticity and modulus of rupture values of bamboo species
Species
Modulus of elasticity (1000 kg/cm2)
Modulus of rapture (kg/cm2)
Green
Air dry
Green
Air dry
bottom middle top bottom middle top bottom middle top bottom middle top
Kali
(Oxytenanthera
nigrociliata)
131 169
150 224
459 415
580 530
Mitinga
(Bambusa
tulda)
138 147
140 168
595 542
745 671
Bethua
(Bambusa
polymorpha)
426 373
468 414
Borak
(Bambusa
balcooa)
108 127
712 624
787 696
4.4.2
Grouping
Sixteen species of bamboo are suitable for structural applications and classified into
three groups, namely, Group A, Group B and Group C as given in Table 6.4.6.
The characteristics of these groups are as given in Table 6.4.6.
Species of bamboo other than those listed in the Table 6.4.6 may be used, provided
the basic strength characteristics are determined and found more than the limits
mentioned therein. However, in the absence of testing facilities and compulsion for
use of other species, and for expedient designing, allowable stresses may be arrived
at by multiplying density with factors as given in Table 6.4.5.
4.4.3
Moisture Content in Bamboo
With decrease of moisture content (M) the strength of bamboo increases
exponentially and bamboo has an intersection point (fibre saturation point) at around
25 percent moisture content depending upon the species. Matured culms shall be
seasoned to about 20 percent moisture content before use.
Table 6.4.5: Physical and Mechanical Properties of Bamboos (in Round Form)
Species
Properties
In Green Condition
In Air Dry Conditions
Density
kg/m3
Modulus of
Rupture
N/mm2
Modulus of
Elasticity 103
N/mm2
Maximum
Compressive
strength N/mm2
Density
kg/m3
Modulus
of Rupture
N/mm2
Modulus of
Elasticity 103
N/mm2
Bambusa auriculata
65.1
15.01
36.7
89.1
21.41
B. balcooa
64.2
7.06
38.6
68.3
9.12
B. bambos
(Syn.B.atwndinacea)
58.3
5.95
35.3
80.1
8.96
B. burmanica
59.7
11.01
39.9
105.0
17.81
B. glancescens
(Syn.B.nana)
82.8
14.77
53.9
্ত
্ত
্ত
B. nutans
52.9
6.62
45.6
52.4
10.72
B. pallida
55.2
12.90
54.0
্ত
্ত
্ত
B. polymorpha
36.6
6.0
31.4
40.6
5.89
B. tulda
53.2
10.3
39.5
65.8
11.18
B. ventricosa
34.1
3.38
36.1
্ত
্ত
্ত
B. vulgaris
41.5
2.87
38.6
্ত
্ত
্ত
Cephalostachyum
pergracile
52.6
11.16
36.7
71.3
19.22
Dendrocalamus giganteous
17.2
0.61
35.2
্ত
্ত
্ত
D. hamiltonii
40.0
2.49
43.4
্ত
্ত
্ত
D. longispathus
33.1
5.51
42.1
47.8
6.06
D. membranacaus
26.3
2.44
40.5
37.8
3.77
D. strictus
73.4
11.98
35.9
119.1
15.00
Melocanna baccifera
53.2
11.39
53.8
57.6
12.93
Oxytenanthera
abyssinicia
83.6
14.96
46.6
্ত
্ত
্ত
Oxytenanthera nigrociliata
40.70
11.7
25.2
51.98
12.85
Thyrsostachys oliveri
61.9
9.72
46.9
90.0
12.15
4.4.3.1 Air seasoning of split or half-round bamboo does not pose much problem
but care has to be taken to prevent fungal discoloration and decay. However, rapid
drying in open sun can control decay due to fungal and insect attack. Seasoning in
round form presents considerable problem as regards mechanical degrade due to
drying defects.
A general observation has been that immature bamboo gets invariably deformed in
cross-section during seasoning and thick walled immature bamboo generally
collapses. Thick mature bamboo tends to crack on the surface, with the cracks
originating at the nodes and at the decayed points. Moderately thick immature and
thin and moderately thick mature bamboos season with much less degrade. Bamboo
having poor initial condition on account of decay, borer holes, etc. generally suffers
more drying degrades.
4.4.3.2 Accelerated air seasoning method gives good results. In this method, the
nodal diaphragms (septa) are punctured to enable thorough passage of hot air from
one end of the resulting bamboo tube to the other end.
4.4.4
Grading of Structural Bamboo
Grading is sorting out bamboo on the basis of characteristics important for structural
utilization as under:
(a) Diameter and length of culm,
(b) Taper of culm,
(c) Straightness of culm,
(d) Inter nodal length,
(e) Wall thickness,
(f) Density and strength, and
(g) Durability and seasoning.
One of the above characteristics or sometimes combination of 2 or 3 characteristics
form the basis of grading. The culms shall be segregated species-wise.
Table 6.4.6: Safe Working Stresses of Bamboos for Structural Designing(1)
Species
Extreme
Fibre Stress
in Bending
N/mm2
Modulus of
Elasticity
103N/mm2
Allowable
Compressive
Stress N/mm2
GROUP A
Barnbusa glancescens (syn. B. nana)
Dendrocalamus strictus
Oxytenanthera abyss inicia
20.7
18.4
20.9
3.28
2.66
3.31
15.4
10.3
13.3
Species
Extreme
Fibre Stress
in Bending
N/mm2
Modulus of
Elasticity
103N/mm2
Allowable
Compressive
Stress N/mm2
GROUP B
Bambusa balcooa
B. pallida
B. nutans
B. tulda
B. auriculata
B. burmanica
Cephalostachyum pergraci[e
Melocanna baccifera (Syn. M.
bambusoides)
Thyrsotachys oliveri
16.05
13.8
13.2
13.3
16.3
14.9
13.2
13.3
15.5
1.62
2.87
1.47
1.77
3.34
2.45
2.48
2.53
2.16
13.3
15.4
13.0
11.6
10.5
11.4
10.5
15.4
13.4
GROUP C
Bambusa arundinacea (Syn. B. bambos)
B. polymorpha
B. ventricosa
B. vulgaris
Dendrocalamus longispathus
Oxytenanthera nigrociliata
14.6
9.15
8.5
10.4
8.3
10.18
1.32
1.71
0.75
0.64
1.22
2.6
10.1
8.97
10.3
11.0
12.0
7.2
(1) The values given pertain to testing of bamboo in green condition.
Table 6.4.7: Limiting Strength Values (in Green Condition)
গড়ফঁষঁং ড়ভ জঁঢ়ঃঁৎব (জ্থ)
N/mm2
Modulus of Elasticity (E)
in Bending 103N/mm2
Group A
জ্থ>৭০
E>9
Group B
৭০≥ জ্থ>৫০
9≥E>6
Group C
৫০≥ জ্থ>৩০
6≥E>3
Table 6.4.8: Allowable Long-Term Stress (N/mm2) Per Unit Density (kg/m3)
Condition
Axial
Compression
(no buckling)
Bending
Shear
Green
0.011
0.015
্ত
Air dry (12%)
0.013
0.020
0.003
Note: In the laboratory regime, the density of bamboo is conveniently determined. Having
known the density of any species of bamboo, permissible stresses can be worked out using
factors indicated above. For example, if green bamboo has a density of 600 kg/m3, the allowable
ংঃৎবংং রহ নবহফরহম ড়িঁষফ নব ০.০১৫ ী ৬০০ = ৯ ঘ/সস২্থ.
4.4.4.1
Diameter and length
4.4.4.1.1
Gradation according to the Mean Outer Diameter
For structural Group A and Group B species, culms shall be segregated in steps of 10 mm
of mean outer diameter as follows:
Special Grade 70mm<Diameter <100mm
Grade I 50mm<Diameter <70mm
Grade II 30mm<Diameter <50mm
Grade III Diameter <30mm
For structural Group C species culms shall be segregated in steps of 20 mm of mean outer
diameter
Grade I 80 mm < Diameter <100 mm
Grade II 60 mm< Diameter< 80 mm
Grade III Diameter <60 mm
4.4.4.1.2 The minimum length of culms shall be preferably 6 m for facilitating close
fittings at joints.
4.4.5
Taper
The taper shall not be more than 5.8 mm per metre length (or 0.58 percent) of bamboo in
any grade of bamboo.
4.4.5.1 Curvature
The maximum curvature shall not be more than 75 mm in a length of 6 m of any grade of
bamboo.
4.4.5.2 Wall thickness
Preferably minimum wall thickness of 8 mm shall be used for load bearing members.
4.4.5.3
Defects and permissible characteristics
4.4.5.3.1 Dead and immature bamboos, bore/GHOON holes, decay, collapse,
checks more than 3 mm in depth, shall be avoided.
4.4.5.3.2 Protruded portion of the nodes shall be flushed smooth. Bamboo shall be
used after at least six weeks of felling.
4.4.5.3.3 Broken, damaged and discolored bamboo shall be rejected.
4.4.5.3.4 Matured bamboo of at least 4 years of age shall be used.
4.4.6
Durability and Treatability
4.4.6.1 Durability
The natural durability of bamboo is low and varies between 12 months and 36
months depending on the species and climatic conditions. In tropical countries the
bio-deterioration is very severe, Bamboos are generally destroyed in about one to
ঃড়ি ুবধৎং্থ:রসব যিবহ ঁংবফ রহ:যব ড়ঢ়বহ ধহফ রহ পড়হঃধপঃ রিঃয মৎড়ঁহফ যিরষব ধ ংবৎারপব ষরভব
of two to five years can be expected from bamboo when used under cover and out of
contact with ground. The mechanical strength of bamboo deteriorates rapidly with
the onset of fungal decay in the sclerenchymatous fibres. Split bamboo is more
rapidly destroyed than round bamboo. For making bamboo durable, suitable
treatment shall be given.
4.4.6.2 Treatability
Due to difference in the anatomical structure of bamboo as compared to timber,
bamboo behaves entirely differently from wood during treatment with preservative.
Bamboos are difficult to treat by normal preservation methods in dry condition and
therefore treatment is best carried out in green condition.
4.4.6.3 Boucherie Process
In this process of preservative treatment, water borne preservative is applied to end
surface of green bamboo through a suitable chamber and forced through the bamboo
by hydrostatic or other pressure.
4.4.6.3.1 Performance of treated bamboo
Trials with treated bamboos have indicated varied durability depending upon the
actual location of use. The performance in partially exposed and under covered
conditions is better.
4.4.6.3.2
For provisions on safety of bamboo structures against fire, see Part 7.
4.5
Permissible Stresses
4.5.1
Factor of Safety
The safety factor for deriving stresses of bamboo shall be as under:
Extreme fibre stress in beams
Modulus of elasticity
4.5
Maximum compressive stress parallel
to grain/fibres
3.5
4.5.2
Coefficient of Variation
The coefficient of variation (in percent) shall be as under:
Property
Mean
Range
Maximum Expected
Value
Modulus of rupture
15.9
5.7-28.3
23.4
Modulus of elasticity
21.1
12.7-31.7
27.4
Maximum compressive stress
14.9
7.6-22.8
20.0
The maximum expected values of coefficient of variation which are the upper
confidence limits under normality assumption such that with 97.5 percent confidence
the actual strength of the bamboo culms will be at least 53 percent of the average
reported value of modulus of rupture in Table 6.4.5.
4.5.3
Solid bamboos or bamboos whose wall thickness (w) is comparatively more
and bamboos which are generally known as male bamboos having nodes very closer
and growing on ridges are often considered good for structural purposes.
4.5.4
The safe working stresses for 18 species of bamboos are given in
Table 4.5.6
4.5.5
For change in duration of load other than continuous (long-term), the
permissible stresses given in Table 4.5.6 shall be multiplied by the modification
factors given below:
For imposed or medium term loading
1.25
For short-term loading
1.50
4.6
Design Considerations
4.6.1
All structural members, assemblies or framework in a building shall be
capable of sustaining, without exceeding the limits of stress specified, the worst
combination of all loadings. A fundamental aspect of design will be to determine the
forces to which the structure/structural element might be subjected to, starting from
the roof and working down to the soil by transferring the forces through various
components and connections. Accepted principles of mechanics for analysis and
specified design procedures shall be applied (see Chapter 11 Part 6).
4.6.2
Unlike timber, bamboo properties do not relate well to species, being
dependent among other factors, on position of the culm, geographic location and age.
The practice in timber engineering is to base designs on safe working stresses and the
same may be adopted to bamboo with the limitations that practical experience rather
than precise calculations generally govern the detailing.
4.6.3
Net Section
It is determined by passing a plane or a series of connected planes transversely
through the members. Least net sectional area is used for calculating load carrying
capacity of a member.
4.6.4
Loads
The loads shall be in accordance with Chapter 2 Part 6.
4.6.5
Structural Forms
4.6.5.1 Main structural components in bamboo may include roof and floor
diaphragms, shear walls, wall panellings, beams, piles, columns, etc. Both from the
point of view of capacity and deformation, trusses and framed skeletons are much
better applications of bamboo.
4.6.5.2 Schematization of bamboo as a structural material
This shall be based on the principles of engineering mechanics involving the
following assumptions and practices:
(a) The elastic behaviour of bamboo, till failure; (plastic behaviour being
considered insignificant);
(b) Bamboo culms are analysed on mean wall thickness basis as hollow tube
structure (not perfectly straight) member on mean diameter basis;
(c) The structural elements of bamboo shall be appropriately supported near the
nodes of culm as and where the structural system demands. The joints in the
design shall be located near nodes; and
(d) Bamboo structures be designed like any other conventional structural
elements taking care of details with regards to supports and joints; they shall
be considered to generally act as a hinge, unless substantiating data justify a
fixed joint.
4.6.6
Flexural Members
4.6.6.1 All flexural members maybe designed using the principles of beam theory.
4.6.6.2 The tendency of bamboo beams to acquire a large deflection under long
continuous loadings due to possible plastic flow, if any shall be taken care of.
Permanent load may be doubled for calculation of deflection under sustained load
(including creep) in case of green bamboo having moisture content exceeding
15 percent.
4.6.6.3 The moment of inertia, I shall be determined as follows:
(a) The outside diameter and the wall thickness should be measured at both ends,
correct up to 1 mm for diameter of culm and 0.1 mm for the wall thickness. (For
each cross-section the diameter shall be taken twice, in direction perpendicular to
each other and so the wall thickness shall be taken as four times, in the same
places as the diameter has been taken twice.)
(b) With these values the mean diameter and the mean thickness for the middle of
the beam shall be calculated and moment of inertia determined.
4.6.6.4 The maximum bending stress shall be calculated and compared with the
allowable stress.
4.6.6.5 For shear checks, conventional design procedure in accordance with Chapter
11 Part 6 shall be followed. The basic shear stress values (N/mm2) for five species of
bamboo in split form in green condition can be assumed as under:
Bambusa pallida
9.77
B. Vulgaris
9.44
Dedroculumus giganteous
8.86
D. humiltonii
7.77
Oxytenanthera abyssinicia
11.2
4.6.6.6 Forces acting on a beam, being loads or reaction forces at supports, shall act
in nodes or as near to nodes as by any means possible.
4.6.7
Bamboo Column (Predominantly Loaded in Axial Direction)
4.6.7.1 Columns and struts are essential components sustaining compressive forces
in a structure. They transfer load to the supporting media.
4.6.7.2 Design of columns shall be based on one of the following two criteria:
(a) Full scale buckling tests on the same species, size and other relevant
variables.
(b) Calculations, based on the following:
(i) The moment of inertia shall be as per Sec 4.6.6.3.
(ii) For bamboo columns the best available straight bamboo culms shall be
selected. Structural bamboo components in compression should be kept
under a slenderness ratio of 50.
(iii) The bending stresses due to initial curvature, eccentricities and induced
deflection shall be taken into account, in addition to those due to any
lateral load.
4.6.7.3 Buckling calculation shall be according to Euler, with a reduction to 90
percent of moment of inertia, to take into account the effect of the taper, provided the
reduced diameter is not less than 0.6 percent.
4.6.7.4 For strength and stability, larger diameter thick walled sections of bamboo
with closely spaced nodes shall be used, alternatively, smaller sections may be tied
together as a bundle-column.
4.6.8
Assemblies, Roof Trusses
4.6.8.1 A truss is essentially a plane structure which is very stiff in the plane of the
members, that is the plane in which it is expected to carry load, but very flexible in
every other direction. Roof truss generally consists of a number of triangulated
frames, the members of which are fastened at ends and the nature of stresses at joints
are either tensile or compressive and designed as pin-ended joints [see Figure
6.4.1.(a)]. Bamboo trusses may also be formed using bamboo mat board or bamboo
mat-veneer composite or plywood gusset [see Figure 6.4.1(b)].
4.6.8.2 Truss shall be analysed from principles of structural mechanics for the
determination of axial forces in members. For the influence of eccentricities, due
allowance shall be made in design.
4.6.8.3 The truss height shall exceed 0.15 times the span in case of a triangular truss
(pitched roofing) and 0.10 times the span in case of a rectangular (parallel) truss.
4.6.8.4 For members in compression, the effective length for in-plane strength
verification shall be taken as the distance between two adjacent points of
contraflexure. For fully triangulated trusses, effective length for simple span
members without especially rigid end-connection shall be taken as the span length.
4.6.8.5 For strength verification of members in compression and connections, the
calculated axial forces should be increased by 10 percent.
4.6.8.6 The spacing of trusses shall be consistent with use of bamboo purlins (2 m to
3 m).
4.6.8.7 The ends in open beams, joists, rafters, purlins shall be suitably plugged.
Bamboo roof coverings shall be considered as non-structural in function. The
common roof covering shall include bamboo mat board, bamboo mat corrugated
sheet, bamboo tiles/strings, plastered bamboo reeds, thatch, corrugated galvanized
iron sheeting, plain clay tiles or pan tiles, etc.
4.7 Design and Techniques of Joints
4.7.1
Bamboo Joints
Round, tubular form of bamboo requires an approach different to that used for sawn
timber. Susceptibility to crushing at the open ends, splitting tendency, variation in
diameter, wall thickness and straightness are some of the associated issues which
have to be taken care of while designing and detailing the connections with bamboo.
4.7.1.1 Traditional practices
Such joining methods revolve around lashing or tying by rope or string with or
without pegs or dowels. Such joints lack stiffness and have low efficiency.
4.7.1.1.1 Lengthening joints (End Joints)
(a) Lap Joint
In this case, end of one piece of bamboo is made to lap over that of the other in line
and the whole is suitably fastened. It may be full lapping or half lapping. Full section
culms are overlapped by at least one internode and tied together in two or three
places. Efficiency could be improved by using bamboo or hardwood dowels. In half
lapping, culms shall preferably be of similar diameter and cut longitudinally to half
depth over at least one internode length and fastened as per full lap joint
(Figure 6.4.2).
(b) Butt Joint
Culms of similar diameter are butted end to end, interconnected by means of side
plates made of quarter round culm of slightly large diameter bamboo, for two or
more internode lengths. Assembly shall be fixed and tied preferably with dowel pins.
This joint transfers both compressive and tensile forces equally well
(Figure 6.4.3).
(c) Sleeves and Inserts
Short length of bamboo of appropriate diameter may be used either externally or
internally to join two culms together (Figure 6.4.4).
(d) Scarf Joint
A scarf joint is formed by cutting a sloping plane 1 in 4 to 6 on opposite sides from
the ends of two similar diameter bamboo culms to be joined. They shall be lapped to
form a continuous piece and the assembly suitably fastened by means of lashings.
Using hooked splays adds to the strength and proper location of joints (Figure 6.4.5).
4.7.1.1.2 Bearing joints
Bearing joints are formed when members which bear against one another or cross
each other and transfer the loads at an angle other than parallel to the axis.
(a) Butt Joint
The simplest form consists of a horizontal member supported directly on top of a
vertical member. The top of the post may be cut to form a saddle to ensure proper
seating of beam for good load transfer. The saddle should be close to a node to
reduce risk of splitting (Figure 6.4.6).
(b) Tenon Joint
It is formed by cutting a projection (tenon) in walls of one piece of bamboo and
filling it into corresponding holes (mortise) in another and keyed. It is a neat and
versatile joint for maximum strength and resistance to separation (Figure 6.4.7).
(c) Cross-Over Joint
It is formed when two or more members cross at right angles and its function is to
locate the members and to provide lateral stability. In case of the joint connecting
floor beam to post, it maybe load bearing (see Figure 6.4.8). Such joints are also used
to transmit angle thrust.
(d) Angled Joint
When two or more members meet or cross other than at right angles, angled joints
are formed. For butt joints, the ends of the members may be shaped to fit in as saddle
joints. Tenons would help in strengthening such joints (see Figure 6.4.9).
4.7.1.2 Modern practices
Following are some of the modern practices for bamboo jointing (Figure 6.4.10):
(a) Plywood or solid timber gusset plates maybe used at joint assemblies of web
and chord connection in a truss and fixed with bamboo pins or bolts. Hollow
cavities of bamboo need to be stuffed with wooden plugs.
(b) Use of wooden inserts to reinforce the ends of the bamboo before forming
the joints. Alternatively steel bands clamps with integral bolt/eye may be
fitted around bamboo sections for jointing.
4.7.1.3 Fixing methods and fastening devices
In case of butt joints the tie maybe passed through a pre-drilled hole or around
hardwood or bamboo pegs or dowels inserted into prefomed holes to act as horns.
Pegs are driven from one side, usually at an angle to increase strength and dowels
pass right through the member, usually at right angles.
4.7.1.3.1 Normally 1.60 mm diameter galvanized iron wire may be used for tight
lashing.
4.7.1.3.2 Wire Bound Joints
Usually galvanized iron 2.00 mm diameter galvanized iron wire is tightened around
the joints by binding the respective pieces together. At least two holes are drilled in
each piece and wire is passed through them for good results.
4.7.1.3.3 Pin And Wire Bound Joints
Generally 12 mm diameter bamboo pins are fastened to culms and bound by 2.00
mm diameter galvanized iron wire.
4.7.1.3.4 Fish Plates/Gusset Plated Joints
At least 25 mm thick hardwood splice plate or 12 mm thick structural grade plywood
are used. Solid bamboo pins help in fastening the assembly.
4.7.1.3.5 Horned Joints
Two tongues made at one end of culm may be fastened with across member with its
mortise grooves to receive horns, the assembly being wire bound.
4.7.1.4 For any complete joint alternative for a given load and geometry, description
of all fastening elements, their sizes and location shall be indicated. Data shall be
based on full scale tests.
4.7.1.5 Tests on full scale joints or on components shall be carried out in a
recognized laboratory.
4.7.1.6 In disaster high wind and seismic areas, good construction practice shall be
followed taking care of joints, their damping and possible ductility. Bracings in walls
shall be taken care of in bamboo structures.
Figure 6.4.1 Some typical configurations for small and large trusses in Bamboo
Figure 6.4.2 Lap joint in Bamboo
Figure 6.4.3 Butt joint with side plates in Bamboo
Figure 6.4.4 Sleeves and inserts for Bamboo joint
Figure 6.4.5 Scarf joint
Figure 6.4.6 Butt joints in Bamboo
Figure 6.4.7 Tenon joint
Figure 6.4.8 Cross over joints (Bearing joints)
Figure 6.4.9 Angled joints with integral tenons
Figure 6.4.10 Gusset plated joint
4.8 Storage of Bamboo
Procurement and storage of bamboo stocks are essential for any project work and
shall be done in accordance with Part 7 of this Code.
4.9
Related References
(১) ওঝ ৬৮৭৪: ১৯৭৩, ুগবঃযড়ফ ড়ভ ঞবংঃ ভড়ৎ জড়ঁহফ ইধসনড়ড়চ্, ইঁৎবধঁ ড়ভ ওহফরধহ
Standards, India, 1974.
(২) ওঝ ৯০৯৬: ১৯৭৯, ুঈড়ফব ড়ভ চৎধপঃরপব ভড়ৎ চৎবংবৎাধঃরড়হ ড়ভ ইধসনড়ড় ভড়ৎ ঝঃৎঁপঃঁৎধষ
চঁৎঢ়ড়ংবংচ্, ইঁৎবধঁ ড়ভ ওহফরধহ ঝঃধহফধৎফং, ওহফরধ, ১৯৭৪.
(৩) ঝধষবযঁফফরহ, অ. ই. গ., ুটহহড়ঃড় চড়ফফযড়ঃরঃব ইধংয ঝযড়হমৎড়শশযড়হ ড় ইধনড়যধৎচ্,
Bangladesh Agriculture Research Institute, 2004. Last modified on August 26, 2026
