Characterization of Splitting Behavior of Bamboo Culms
Derek Mitch, S.M.ASCE1; Kent A. Harries, Ph.D., M.ASCE2; and Bhavna Sharma, S.M.ASCE3
Abstract: Bamboo is viewed as a sustainable and highly renewable material that may be grown in any temperate climate zone. Interest
in the engineering properties of bamboo and the use of bamboo in engineered construction is growing. Although often having superior
mechanical properties than readily available sawn timber, bamboo, being largely an unidirectional fibrous material, is very susceptible to
longitudinal splitting. Despite being the dominant limit state in many applications, very little previous work has addressed the characterization of bamboo splitting failure. The present research focuses on the development of a split pin test method for characterizing the
splitting strength of bamboo culms. The proposed split pin test uses a full culm section test, thereby eliminating some of the complexities
of partial culm tests. Results of a pilot study of the split pin test having similar specimens with different split pin diameters yielded
consistent results and variations less than that of the previously standardized direct shear test. In order to demonstrate the validity of the
proposed test method, a series of tests was performed on samples of Tre Gai 共Bambusa stenostachya Hackel兲 bamboo.
DOI: 10.1061/共ASCE兲MT.1943-5533.0000120
CE Database subject headings: Test procedures; Wood; Cracking; Construction materials; Sustainable development.
Author keywords: Test method; Bamboo; Fracture.
Introduction
Engineering Properties of Bamboo
The principal objective of this work is the development of an
appropriate method of characterizing the splitting behavior of
bamboo culms. Splitting is the dominant limit state for bamboo in
structural applications 共Janssen 1981; Arce-Villalobos 1993兲 and
was documented by the authors in a 2008 field visit to Northeastern India 共Sharma et al. 2008兲. The proposed “split pin” test
method is founded on a fundamental fracture mechanics approach
and therefore can account, in a consistent manner, for the high
degree of variability present in geometric and material properties
of bamboo. While a number of test methods capable of assessing
splitting behavior are available in the literature, 共1兲 none presently
address full culm behavior; 共2兲 only one approaches the splitting
problem from a fracture mechanics perspective 共Amada and
Untao 2001兲; 共3兲 methods reported to provide the same material
property result in significantly different values; and 共4兲 none have
been standardized despite splitting being the dominant limit state
for many applications. Additional objectives of this work are that
the test method developed is 共1兲 sufficiently simple; 共2兲 requires
only basic equipment to perform; and 共3兲 is easily scalable to
assess a range of bamboo geometries 共structural bamboo ranges in
diameter from 50 to 250 mm兲. These criteria are based on the
need to use this test method in the field implying use in underdeveloped rural areas having little access to engineering test facilities.
1
Undergraduate Researcher, Dept. of Civil and Environmental Engineering, Univ. of Pittsburgh, Pittsburgh, PA 15261.
2
Associate Professor, Dept. of Civil and Environmental Engineering,
Univ. of Pittsburgh, Pittsburgh, PA 15261. E-mail: kharries@pitt.edu
3
Doctoral Candidate, Dept. of Civil and Environmental Engineering,
Univ. of Pittsburgh, Pittsburgh, PA 15261.
Note. This manuscript was submitted on December 29, 2009; approved on April 9, 2009; published online on May 8, 2010. Discussion
period open until April 1, 2011; separate discussions must be submitted
for individual papers. This technical note is part of the Journal of Materials in Civil Engineering, Vol. 22, No. 11, November 1, 2010.
©ASCE, ISSN 0899-1561/2010/11-1195–1199/$25.00.
The study of the engineering properties of bamboo is surprisingly
nascent. Janssen 共1981兲 explored the composition of a bamboo
culm. He developed a mathematical model of the culm and assessed different mechanical properties of bamboo including bending, shear, tension, and compression. He developed stochastic
models to establish which physical parameters are related to the
material properties of bamboo. Some of Janssen’s conclusions
relevant to the present study are as follows:
1. The angle that the microfibrils of bamboo make with the cell
axis has a large impact on the stresses and displacements
observed in mechanical tests.
2. Longitudinal shear stress 共splitting兲 is the cause of failure for
short spans tested in flexure; the limiting shear stress is much
lower than a typical shear test would indicate.
3. In bending, dry bamboo exhibits greater consistency in measurement and prediction of mechanical properties, strength
decreases with the height along the culm from which the
sample is taken, and ultimate bending stress and shear
strength are both related to density.
4. A shear test was needed to determine the correct shear
strength of bamboo. Janssen developed a four-plane shear
test which was later adopted by the present ISO guideline
关International Standards Organization 共ISO兲 2004b,c兴.
Arce-Villalobos 共1993兲 extended the work of Janssen by providing a more in-depth examination of the tensile properties of
bamboo both parallel 共along culm兲 and perpendicular 共transverse兲
to the primary orientation of the fibers. He also proposed a relationship between different mechanical properties; his most relevant conclusions are as follows:
1. Transverse tension capacity and density are not correlated
whereas longitudinal tension capacity and density are.
2. The tensile modulus of elasticity, E, in the transverse direction is about 1/8 that measured in the longitudinal direction.
3. There may be a universal maximum transverse strain that
bamboo may experience before transverse rupture. Three dif-
JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / NOVEMBER 2010 / 1195
Fig. 1. Split pin test setup
ferent species exhibited similar values during testing, approximately 0.0012 mm/mm.
The present understanding of the material properties of bamboo, as promulgated in the International Standards Organization
共ISO兲 共2004a兲 standard and the National Building Code of India
共2005兲, for instance, stems largely from the work done by Janssen
and Arce-Villalobos. While these standards are a start, there are
many areas that still require further refinement. Janssen 共1981兲
quoted several researchers who claimed that “the collapse of the
bamboo was always sudden and the material was split into pieces
parallel to the longitudinal axis.” Arce-Villalobos 共1993兲 concluded that “Bamboo culms do not fail in compression, in bending, or shear but do fail when a maximum tangential tensile stress
共splitting兲 is reached.” Despite these acknowledgments, the splitting behavior of bamboo has not been adequately addressed in
present standards.
More recently, the fracture properties of bamboo have been
investigated using notched tension specimens 共Amada and Untao
2001兲 and notched four point flexural tests 共Low et al. 2006兲.
These studies have demonstrated that behavior of bamboo is
highly dependent on fiber volume fraction 共ratio of fibril area to
total section area兲 and therefore is similar to the behavior of other
fiber reinforced materials. This work also indicated that crack
deflection and crack bridging are significant energy dissipation
mechanisms for bamboo. Low et al. 共2006兲 clearly showed that
longitudinal “splitting” initiating at a notch tip 共defect兲 was the
primary mechanism for flexural failure of bamboo.
Proposed Split Pin Test
A proposed test for determining the Mode I fracture toughness of
bamboo transverse to the fiber direction is required for two reasons. First, such a test would enable engineers to assess the relative performance of different bamboo species, preservative
treatments, and joining methods. Second, the test method addresses a critical behavior of bamboo 共splitting兲 in a consistent
manner. Due to different test orientations, previous studies have
reported values of fracture toughness that vary by an order of
magnitude. Amada and Untao 共2001兲 reported the behavior of
longitudinal tension tests, while Low et al. 共2006兲 reported what
amounts to a modulus of rupture test, although both reported their
results as Mode I fracture toughness. While both are correct
共based on their specimen orientation兲, the confusion arising from
inconsistency in reporting will hinder future research and adoption of bamboo. In order to address these requirements, the proposed test must be easy to perform, yield stable crack growth
rates, and yield a result that may be consistently reported and be
easily understood.
In order to determine an appropriate specimen geometry, the
writers considered many different test methods, assessing each
based on available literature and reported test results in terms of
the criteria noted above 共Mitch 2009兲. Geometries considered and
rejected included 共1兲 the short beam method proposed by ArceVillalobos 共1993兲; 共2兲 a double cantilever beam specimen; 共3兲 a
wedge penetration test; 共4兲 a notched flexural specimen 共Low et
al. 2006兲; 共5兲 a number of variations of a direct tension test orienting the culm so that the load is applied transversely; and 共6兲
the split pin test eventually selected 共see below兲.
The writers contend that an important feature of any bamboo
splitting test is that it utilizes the entire culm section. This eliminates eccentricities resulting from semicircular or arc-shaped
specimens. Such eccentricities result in through-thickness bending of the specimen. Additionally, using full culm sections permits the inclusion or exclusion of nodes as desired. The crack
arresting or promoting properties of the nodal regions can therefore be investigated. In order to utilize a full culm section as well
as produce a stable crack growth rate, this study pursued the split
pin test geometry shown in Fig. 1.
The determination of the fracture toughness of the split pin
specimen is determined using fundamental linear elastic fracture
mechanics. For the test geometry shown in Fig. 2共a兲, Mode I
stress intensity factor, K1, is given as 共Griffith 1921兲
K1 = ␴冑␲a
冋 冉 冊册
2w
␲a
tan
␲a
2w
1/2
共1兲
where ␴ = tensile stress perpendicular to the fibers over the gross
1196 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / NOVEMBER 2010
Split Pin Specimens and Test Protocol
Fig. 2. Split pin test specimen geometry
area; 2a = initial length of the crack; and 2w = specimen length.
The stress, ␴, is determined by dividing the load at failure by
the gross area of the specimen at the failure plane
␴=
P
2共2wt兲
共2兲
where P = total applied load and t = average culm wall thickness.
The scalar “2” in the denominator of Eq. 共2兲 accounts for the
fact that the entire culm is tested and thus two wall thicknesses
are engaged.
An alternative formulation 共Kanninen and Popelar 1985兲
which empirically accounts for the shape of the drilled hole 共radius R兲 and horizontal crack initiators 共as opposed to the horizontal crack 2a only兲, but not the specimen length 共2w兲, yields K1
values 共for the specimens reported here兲 about 7% greater than
those calculated using Eq. 共1兲
冋
K1 = ␴冑␲a 1 + 2.365
冉 冊册
R
R+a
2.4
共3兲
Due to the curved nature of the bamboo specimen, both formulations of K1 are approximations. Since Eq. 共3兲 assumes an infinitely wide specimen, the K1 formulation given in Eq. 共1兲, which
assumes a finite specimen length, 2w, is adopted. Finally, the
strain energy release rate, G1, which is a measure of the toughness
of the bamboo, is proportional to the square of K1
G1 =
where E = Young’ s modulus.
K21
E
共4兲
The test specimens used in this study 共Fig. 1兲 had loading hole
diameter d = 38.1 mm 共1.5 in.兲, 28.6 mm 共1.125 in.兲, or 22.2 mm
共0.875 in.兲 with crack initiation points having length a
= 3.18 mm 共0.125 in.兲. To create the center hole, a drill press with
an appropriate hole saw was used. The hole was created on both
sides of the culm in one pass in order to ensure specimen symmetry. The use of a hole saw, rather than a twist drill bit, resulted
in a “clean” hole having no significant stress raisers. The crack
initiation points were cut using a fine-toothed hacksaw blade.
Smaller hole sizes 关14.3 and 12.7 mm 共0.563 and 0.500 in.兲兴 were
attempted; however the small diameter split pins proved too flexible to span the culm diameter in these cases. Thus, the test setup
and culm size tested interact to dictate the smallest possible hole
that may be tested. In this test program, the culm diameter-tospecimen length ratio was approximately 1.15. The smallest practical hole diameter was one quarter the culm diameter.
The test setup consists of two clevis assemblies which are
pulled in opposite directions. Each assembly is made up of 共1兲 the
split pin; 共2兲 connection bolts; 共3兲 connection arms; and 共4兲 loading bolt and block as shown in Fig. 1.
Once the specimens were installed in the test frame 关Fig. 1共c兲兴,
they were loaded to failure in displacement control with the cross
head moving at a rate of 0.005 mm/s 共0.0002 in./s兲. This speed is
approximately one-half of the speed prescribed for the shear and
compression testing of bamboo 关International Standards Organization 共ISO兲 2004b兴 and was chosen due to the fact that the fracture specimens are more brittle and have a much lower failure
load. All failures were brittle in nature. The bamboo generally
experienced failure along all four possible surfaces 共Fig. 3兲, although some failures of just two or three surfaces were encountered.
Experimental Program
The bamboo culms used in the experimental program were Bambusa stenostachya Hackel which is known colloquially as Tre
Gai. Imported from Vietnam by a U.S.-based supplier, the culms
were vacuum pressure treated with boric acid in order to preserve
them for transportation and use. The culms had been stored in
laboratory conditions 共22° C, 70% relative humidity兲 for a number of months prior to testing and were tested in their dry condition. It is important to note that both the geometric and material
properties obtained in this series of tests may be considerably
different from those obtained in other test regimes due to a variety
of factors. While material properties will certainly vary across
Fig. 3. Split pin test failure modes
JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / NOVEMBER 2010 / 1197
Table 1. Summary of Results of Split Pin Tests
Culm diameter D
Culm wall thickness t
共mm兲
Specimen length 2w
共mm兲
Ultimate load P
共N兲
Failure stress 关Eq. 共5兲兴
共MPa兲
K1 关Eq. 共1兲兴
共MPa· m1/2兲
Average
Std. dev.
COV
n = 10
16.8
3.2
0.19
Hole diameter= 38.1 mm
103.9
1.7
0.02
1,961.0
516.1
0.26
1.004
0.217
0.22
0.165
0.035
0.21
Average
Std. dev.
COV
n = 10
18.2
4.3
0.24
Hole diameter= 28.6 mm
105.8
1.2
0.01
2,856.2
1151.0
0.40
1.161
0.264
0.23
0.191
0.044
0.23
Average
Std. dev.
COV
n=8
15.6
2.0
0.13
Hole diameter= 22.2 mm
105.7
0.8
0.01
2,458.2
818.8
0.33
1.023
0.219
0.21
0.165
0.036
0.22
1.065
0.238
0.22
0.174
0.039
0.22
All specimens
Average
16.9
105.1
n = 28
Std. dev.
3.4
1.6
COV
0.20
0.02
a
Not valid basis for comparison since 2w kept constant regardless of hole diameter.
species, it has also been shown that bamboo material properties
vary according to growing climate, height along culm, age, and
moisture content. As the culms are provided by a commercial
supplier, who does not record this information, there is no readily
available way to verify any of these parameters. As the intent of
this experimental program is to develop a test method, no effort
was made to account for these factors.
Twenty-eight test samples were cut from six different culms.
The average outside culm diameter 共two measurements at 180°
per specimen兲 of samples was 90.5 mm 共3.56 in.兲 having a coefficient of variation 共COV兲 of 4.4%. The average culm wall thickness 共four measurements at 90° per specimen兲 is given in Table 1.
In general, the variation of culm wall thickness is typical of thickwalled bamboo specimens—about 20% in this study.
a
the COV for ultimate compressive stress and direct shear stress
关International Standards Organization 共ISO兲 2004b兴 of the same
bamboo were 12% and 25%, respectively 共Mitch 2009兲. This degree of deviation is consistent with that measured for material
properties of both bamboo and other biological materials tested
by others 共e.g., Kretschmann 2008兲.
The variation in hole diameter had little effect on the failure
stress or K1. The K1 values were observed to be slightly higher for
the series of specimens having 28.6-mm holes. There is no obvious explanation for this; however it is also noted that variation in
culm wall thickness was considerably greater for this series than
for the other two 共Table 1兲.
Discussion
Experimental Results
Twenty-eight split pin specimens encompassing three different
hole diameters were tested; a summary of results is shown in
Table 1. Specimen length 共2w兲, crack length 共2a兲, and gross section stress 共␴兲 obtained from the failure load are summarized in
Table 1. The wall thickness of each specimen was measured at the
four different expected failure surfaces 共emanating from the crack
initiation points兲 and averaged. The stress used to calculate K1
关Eq. 共1兲兴 is given by Eq. 共2兲. This observation indicates that the
proposed split pin test method is more suitable for making comparisons between specimens rather than assessing absolute values
of fracture properties. The “failure stress” reported in Table 1 is
the tensile capacity of the bamboo in the direction perpendicular
to the culm calculated over the actual resisting area
␴failure =
P
2t共2w − 2a兲
共5兲
As can be seen in Table 1, the fracture test yielded similar results.
The COV was 22% for both failure stress and K1. By comparison,
In general, the material properties of bamboo compare favorably
to those of sawn timber. Compression, tension, shear, and flexural
modulus of Tre Gai bamboo, for instance, are reported 共ICC
Evaluation Service, Inc. 2006; Mitch 2009兲 to be two to four
times those of structural sawn timber 关U.S. Department of Agriculture 共USDA兲 1999兴. Although competitive with sawn timber in
most respects, the high susceptibility of bamboo to longitudinal
splitting is reflected in Mode I stress intensity factor, K1, with
values only about one half of those of sawn timber. This result is
more important with respect to designing connection regions than
it is to the structural layout itself. Arce-Villalobos 共1993兲 concluded “The majority of fittings based on some sort of penetration
normally used in construction 共nails, bolts, pegs兲 are not suitable
for bamboo because they create high tangential stresses.” These
observations simply reinforce the need for a reliable test method
for determining the splitting properties of bamboo.
The proposed split pin test yields the tensile capacity of the
bamboo in the direction perpendicular to the culm. To the authors’ knowledge this is the first study in which this value has
been reported. It is hypothesized that with sufficient experimental
1198 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / NOVEMBER 2010
data, a relationship between perpendicular tension capacity and
longitudinal shear could be established since these mechanisms
are mostly a function of the bamboo matrix material which is,
itself, largely isotropic. The advantage of correlation with longitudinal shear is that the standardized shear test 关International
Standards Organization 共ISO兲 2004b兴 requires minimal specimen
preparation and is a compression-based test and therefore easier
to adapt in a rural and/or underdeveloped environment due to the
simpler test frame.
Conclusions
A split pin test method has been proposed to assess the tensile
behavior of full culm bamboo perpendicular to the culm. The
behavior is associated with culm splitting which has been observed to be a dominant limit state for bamboo in structural applications. The proposed split pin test is a full culm section test
thereby eliminating some of the complexities and eccentricities of
partial culm tests. The small test specimen requires a length only
equal to the culm diameter. Based on limitations of the test setup,
however, the split pin diameter cannot be smaller than one quarter
the culm diameter. Results of a pilot study of the split pin test
having similar specimens with different split pin diameters
yielded consistent results and variation less than that of the previously standardized bamboo shear test.
Despite its propensity for longitudinal splitting failures, the
generally favorable material properties of bamboo, when compared to sawn timber, point to bamboo as having great potential
for structural applications, particularly in axial load carrying applications such as columns and trusses. Bamboo is viewed as a
sustainable and highly renewable material which may be grown in
any temperate climate zone. Therefore, with further confidence in
its reliability as a structural material, it is hoped that bamboo will
become be recognized as a sustainable alternative to sawn timber
in some applications.
Acknowledgments
The writers recognize the support of the Mascaro Center for Sustainable Innovation and the University Honors College, both at
the University of Pittsburgh. The contributions of Dr. Ian Robertson of the University of Hawaii, Ms. Gayatri Kharel, an engineer-
ing consultant in Kalimpong, India, and Dr. Khosrow Ghavami at
PUC-Rio are gratefully acknowledged. The writers also wish to
acknowledge the cooperation of St. Joseph’s School at Northpoint, Darjeeling, India.
References
Amada, S., and Untao, S. 共2001兲. “Fracture properties of bamboo.” Compos., Part B Eng., 32, 451–459.
Arce-Villalobos, O. 共1993兲. “Fundamentals of the design of bamboo
structures.” Thesis, Eindhoven Univ., Eindhoven, The Netherlands,
281.
Griffith, A. A. 共1921兲. “The phenomena of rupture and flow in solids.”
Philos. Trans. R. Soc. London, 221, 163–197.
ICC Evaluation Service, Inc. 共2006兲. ESR-1636-2006 Structural Bamboo
Poles, Whittier, CA.
International Standards Organization 共ISO兲. 共2004a兲 “Bamboo—
Structural Design.” ISO 22156, Geneva, Switzerland.
International Standards Organization 共ISO兲. 共2004b兲 “Bamboo—
Determination of physical and mechanical properties—Part 1: Requirements.” 22157-1, Geneva, Switzerland.
International Standards Organization 共ISO兲. 共2004c兲 “Bamboo—
Determination of physical and mechanical properties—Part 2: Laboratory manual.” ISO 22157-2, Geneva, Switzerland.
Janssen, J. 共1981兲. “Bamboo in building structures.” Thesis, Eindhoven
Univ., Eindhoven, The Netherlands.
Kanninen, M. F., and Popelar, C. H. 共1985兲. Advanced fracture mechanics, Oxford University, New York.
Kretschmann, D. 共2008兲. “Influence of juvenile wood content on shear
parallel, compression, and tension transverse to grain strength and
mode 1 fracture toughness for Loblolly Pine.” Rep. No. FPL-RP-647,
U.S. Dept. of Agriculture—Forest Products Laboratory, Madison, WI.
Low, I. M., Che, Z. Y., Latella, B. A., and Sim, K. S. 共2006兲. “Mechanical
and fracture properties of bamboo.” Key Eng. Mater., 312, 15–20.
Mitch, D. 共2009兲. “Splitting capacity characterization of bamboo culms.”
Honors College thesis, Univ. of Pittsburgh, Pittsburgh.
National Building Code of India. 共2005兲. “Part 6 structural design—
Section 3 timber and bamboo: 3B bamboo.” SP7, New Delhi, India.
Sharma, B., Harries, K. A., and Kharel, G. 共2008兲. “Field Documentation
and Survey of Bamboo Structures: Service Learning Opportunities in
Sustainability Research.” Proc., Association for the Advancement of
Sustainability in Higher Education Conf., American Association of
Engineering Education 共ASEE兲, Raleigh, NC.
U.S. Department of Agriculture 共USDA兲. 共1999兲. “Wood handbook.”
General Technical Rep. FPL-GTR-113, Forest Products Laboratory,
Madison, WI.
JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / NOVEMBER 2010 / 1199
Copyright of Journal of Materials in Civil Engineering is the property of American Society of Civil Engineers
and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright
holder's express written permission. However, users may print, download, or email articles for individual use.