Structural Use of Bamboo Part 3 Design Values
Structural Use of Bamboo Part 3 Design Values
Structural Use of Bamboo Part 3 Design Values
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Structural use of bamboo: Technical note series in The Structural Engineer View project
All content following this page was uploaded by Sebastian Kaminski on 05 November 2018.
Sebastian Kaminski, MEng (Hons) ACGI CEng MIStructE, Senior Structural Engineer at
Arup Advanced Technology + Research London, Member of INBAR Task Force – Bamboo
Construction
David Trujillo, MSc DIC CEng MIStructE, Senior Lecturer at Coventry University, Chair of
INBAR Task Force – Bamboo Construction
Ian Feltham, MA CEng MICE MIStructE, Director and Fellow at Arup Advanced
Technology + Research London,
Luis Felipe López MSc, Head of Engineering Department at Base Bahay Foundation the
Philippines, Member of Colombian Earthquake Engineering Association (AIS)
Synopsis
Bamboo is a strong, fast growing and sustainable material, having been used structurally for
thousands of years in many parts of the world. In modern times it has the potential to be an
aesthetically-pleasing and low-cost alternative to more conventional materials such as timber,
as demonstrated by some visually impressive recent structures.
This Technical Note Series brings together current knowledge and best practice on the
structural use of bamboo, covering:
1. Introduction to bamboo
2. Durability and preservation
3. Design values
4. Element design equations
5. Connections
The series is aimed at both developed and developing world contexts. This third Technical
Note proposes: strengths and other properties for the scheme design of any bamboo species, a
method of calculating characteristic strength values from test data, and a method for
calculating design values of strengths for limit state design. It is believed to be the most up-
to-date guide for determining design values for bamboo elements.
Introduction
Bamboo typically has a strength similar to high grade (e.g. D40) hardwood. Testing for
strength will in most cases be required before detailed design as little reliable published data
is available. Some tests are more important than others, for example flexure, shear and
tension perpendicular are more important than compression and tension parallel since in most
structures it is rare for bamboo elements to be loaded close to their failure in the latter two
modes. For very simple structures it may be possible to use conservative design values
without any testing.
The methods proposed in this Note have been developed based on ISO 22156: Bamboo –
Structural Design (ISO, 2004a), ISO 22157: Bamboo – Determination of Physical and
Mechanical Properties (ISO, 2004b & ISO, 2004c), NSR-10 G.12: Colombian Code for
Seismically-Resistant Construction: Structures of Timber and Guadua Angustifolia Kunth
Bamboo (AIS, 2010), EN 384:1995 Structural timber – Determination of characteristic values
of mechanical properties and density (CEN, 1995) and EN 1995-1-1: Eurocode 5: Design of
Timber Structures (CEN, 2014). The methods should be used in conjunction with the
Eurocode suite of codes. Where there is any ambiguity, refer to EN 1995-1-1: Eurocode 5
(CEN, 2014), and use good practice timber design theory.
The values proposed assume rigour is applied to the process of testing the bamboo to obtain
the test data, and selecting bamboo that is of appropriate condition and quality for
construction, as outlined in Technical Note 1 (Kaminski et al. 2016a).
Nomenclature
The method described here adjusts the test data to Service Class 1 and 2 conditions, and also
includes adjustment factors for laboratory test conditions. Service Class 1 and 2 correspond to
a moisture content in the material corresponding to a temperature of 20oC and the relative
humidity of the surrounding air only exceeding 85% for a few weeks per year. This is
applicable to all air-conditioned spaces and most indoor/outdoor covered areas with normal
humidity (CEN, 2014). Service Class 3 corresponds to climatic conditions exceeding 1 and 2.
As outlined in Technical Note 2 (Kaminski et al., 2016b), bamboo should not be used outside
exposed to water or rain, therefore this Service Class 3 assumes that the bamboo is under
cover and protected from direct rain/water however in a very humid environment with a
relative humidity >85% – this scenario only exists in tropical countries. Bamboo must be
treated if in this environment as otherwise it is liable to rot in the high humidity.
To use the strengths for design, the characteristic values must then be factored by the
modification factors in Section 3. These may be conservative, but test data is limited.
1.2
,
= + +,- ,. .1 − 5 Eq 1
34
,. = − 1.645( Eq 2
Alternatively, for samples of 20 or more specimens, the data can be ranked and the value
corresponding to the nth term in the rank may be used as the 5th percentile. The nth term would
be determined as: total sample/20.
For elements in axial compression only, where an element is formed from four or more culms
connected together such that they equally share the load (for example a column), the
characteristic value of strength for a whole population, fi,k, can be determined from the
following equation:
9.:; 1.2
,
= + +,- . − 5 .1 − 5 Eq 3
3< 34 3<
This modification takes Equations 1 and 2 and divides s by 3 , which is essentially using
standard statistical theory to say that the characteristic value of a number of samples selected
from a population is likely to be greater than the characteristic value of a single sample.
Alternatively, EN 1990:2002 Annex D can be used, adjusting for Cmois and Clab as above.
Table 1: Moisture content correction factor Cmois, as a function of the moisture content
at time of testing
Moisture content Flexure Shear Tension parallel to Compression parallel
(MC) (%) fibre to fibre
MC ≤ 12 1.0 1.0 1.0 1.0
12 < MC ≤ 18 Interpolate between above and below
MC > 18 1.2 1.2 1.2 1.2
Using “green” (unseasoned) bamboo for construction should be avoided at all cost. “Green”
bamboo is cheaper and carpenters will tend to push for it, because it is much easier to work
than dry bamboo, however as the bamboo dries and shrinks it is likely to split, weakening it
and failing the connections.
Table 3: Characteristic strengths, fi,k, for design of dry*, mature** bamboo, free of
visual defects (splits, decay etc.) and assuming a 10min test load (N/mm2)
Flexure Shear Tension parallel Compression parallel
(fm,k) (fv,k) to fibre to fibre
(N/mm2) (N/mm2) (ft,0,k) (N/mm2) (fc,0,k) (N/mm2)
Colombian grown Guadua 35-50 3-5 40 20
angustifolia Kunth
For scheme design, all species 30 2 40 20
C24 softwood 24 2.5 14 22
Note that some widely available published data on bamboo strengths is misleading, as the
form of the strength is not immediately obvious (e.g. characteristic, ultimate, average, design,
allowable etc.). Testing methodologies affect the interpretation of results too, hence the
recommendation to adhere to ISO standards. Strengths do vary between bamboo species,
however it is unlikely that they would be significantly different from the above. If published
strengths are found to be widely different from these then care should be taken to ensure that
the strengths are in the correct form. For example, it is commonly quoted that “bamboo is
stronger than steel”, which is misleading and can lead to bamboo being used in a structure
when it is in fact inappropriate.
* As outlined in Technical Note 2 (Kaminski et al., 2016b), bamboo should not be used
outside exposed to water or rain, therefore this Service Class 3 assumes that the bamboo is
under cover and protected from direct rain/water however in a very humid environment with
a relative humidity >85% – this scenario only exists in tropical countries. Bamboo must be
treated if in this environment as otherwise it is liable to rot in the high humidity.
then it is suggested that the allowable stresses provided in Table 3 are modified by a system
strength factor ksys of 1.1.
This is based on NSR G-12 (AIS, 2010) and EN 1995-1-1 cl. 6.6 (CEN, 2014). ksys should
only be applied if the characteristic stress obtained is as per Equation 1 and not Equation 3.
Some authors believe creep to be negligible (3-5% of the elastic deformation) (Janssen,
2000), however recent research suggests it could be as high as 50% of the initial deflection –
limited research has been conducted on this topic. Note that the values below are typical
values and like strength, there is likely to be a wide variation in stiffness depending on
species, origin, distance from the ground, etc.
Table 6: Typical moduli of elasticity E for bamboo at 12% and 19% moisture content
Moisture content (%) Average modulus E0.5 (N/mm2) 5th percentile modulus E0.05 (N/mm2)
12 10,000-17000 7500-13,000
19 8500-15,000 6700-8000
Summary
This Technical Note proposes strengths and other properties for the scheme design of any
bamboo species, a method of determining characteristic strength values from test data, and a
method for calculating design values of strengths for limit state design. Significantly more
research is still required for all species of bamboo to provide more accurate design values and
coefficients – current values are therefore likely to be conservative. Bamboo will be as well
understood as timber is, but we have some way to go before that happens.
The next paper in this Technical Note series will cover element design equations.
References