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Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 15
BEDIG AD MODULUS OF ELASTICITY PROPERTIES OF TE
LESSER-USED TIMBER SPECIES I GHAA USIG STRUCTURAL
DIMESIOS
E. Appiah-Kubi
1
, C. K. Kankam
2
and M. Adom-Asamoah
2
1
CSIR-Forestry Research Institute of Ghana, University Post Office Box UP 63, KUST, Kumasi Ghana
2
Department of Civil Engineering, Kwame krumah University of Science and Technology, Kumasi,
Ghana
Email: eappiahkubi@csir-forig.org.gh
ABSTRACT
For structural use, the properties of large size specimens are preferred to those of small clear specimens
because of unavoidable defects such as knots and shakes found in wood. The objective of this study was to
assess the bending strength, modulus of elasticity properties and failure behaviour of ten Lesser-used
Species (LUS) by use of structural size dimensions (50mm x 120 mm x 2000 mm). The ten species were
Albizia ferruginea, (Gulland Perr) Beuth., Sterculia rhinopetala,(K. Schum), Blighia sapida,(Koenig),
Canarium schweinfurthii (Engl.), Petersianthus macrocarpus, (P. Beauv.) Liben, Sterculia oblonga,
(Mast.) Cola gigantea, (A.Chev.) Celtis zenkeri, (Engl.) Antiaris toxicaria (Lesch.) and Amphimas
pterocarpoides (Harms.). The moisture contents of the 10 timber species used ranged from 16.1%
(Antiaris toxicaria) to 51.0% (Albizia ferruginea). It was observed during loading under flexure that the
elastic stiffness and rate of increase in the strength capacity of the beams did not change after three
cycles of loading and unloading. All the beams failed in a form of tension rupture. The breaks were
usually splintering tension failure or brittle (brashness) tension failure or a combination of the two modes
of failure. Sterculia rhinopetala exhibited the highest bending strength with a 5
th
percentile bending
strength of 56.8 /mm
2
and a mean local modulus of elasticity of 15,973 /mm
2
. Sterculia oblonga was
also found to be the 2
nd
best in terms of material properties with a 5
th
percentile bending strength of 52.1
/mm
2
and a mean local modulus of elasticity of 16,408 /mm
2
. Celtis zenkeri which had the highest
mean local modulus of elasticity, however, had the 6
th
best 5
th
percentile bending strength of 39.9 /mm
2
.
A good linear correlation (69.6-91.3%) was established between mechanical strength properties for
average density, average bending strength, average local modulus of elasticity and average global
modulus of elasticity.
Keywords: Lesser-utilized species, bending strength, modulus of elasticity, density
ITRODUCTIO
The exploitation of timber in Ghana and indeed in
many parts of Africa is limited to a few of the over
300 known species (Oteng-Amoako, 2006). Some
of the popular species are Pterygota macrocarpa
(Koto), Milicia excelsa (Odum), Khaya ivorensis
(Mahogany), Triplochiton scleroxylon (Wawa),
Terminalia ivorensis (Emire), Aningeria altissima
(Asanfina) and esogordonia papaverifera
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 16
(Danta). The demand for these species nationally
and internationally with excellent properties in
terms of their strength and the quality of their
finishes have led to over-exploitation. Although
there are many other timber species, their
properties are less known. The available data on
the mechanical properties of such species has
generally been attained with tests on small clear
specimens (either 2 x 2 x 30 inches or 1 x 1 x 16
inches as specified by ASTM D 143-52 (1994).
For structural use, the properties of large size
specimens which are generally quite different
from those of small clear specimens because of the
unavoidable defects such as knots and shakes,
need to be determined. In Ghana, the bending
strength, modulus of elasticity and failure
behaviour of most Lesser-used Species (LUS) in
structural size dimensions have not yet been
determined.
Within the framework of timber as a construction
material, a distinction is made between primary or
commercially accepted species and lesser known
or lesser-used species. For several reasons, the
viability of timber in the context of construction is
dependent on lesser known timber species rather
than commercially accepted species. Freezaillah
(1990) defines Lesser-known species (LKS) as a
commercially less accepted species left in the
forest after a logging operation. But, as stated by
Hansom (1983), a better definition is that it is a
species that is not being put to best advantage. The
list of commercial species has lengthened to some
extent because of advances in technology and
promotion and because of a growing scarcity of
the more desired species. There has been
considerable discussion about the fuller utilization
of tropical forests with particular reference to the
LKS, but the problem has remained intractable
and little has been done (Freezaillah, 1990).
Eddowes (1980), in discussing the technical
aspects of promoting the LKS in Papua New
Guinea, identified inadequate data on physical and
mechanical properties as one of the main problems
in promoting the LKS. Lesser-known species are
species yet to be exported, but are now being
promoted or have the potential to be promoted in
the local market.
Lack of adequate information (mechanical
properties) on the lesser-used species in Ghana has
led to the over-exploitation of the few commercial
species such as Milicia. excelsa, Khaya. ivorensis,
Milicia regia whose properties are well known
(Allotey, 1992). Prior to 1970, the characteristics
of timber were assessed from tests carried out on
small clear pieces of wood. However, following
the extraordinary pioneering work by Madsen
(1992), it was realized that this could be quite
misleading. This is because the strength of
structural size timber is much influenced by the
presence of natural defects such as knots, pith
flecks, etc. Alik and Nakai (1997a) noted in their
work that using the results from full size structural
timber was considered to be more reliable to
allocate design strengths.
In this research work, ten lesser-used timber
species were selected for the study. The species
were Albizia ferruginea, Sterculia rhinopetala,
Blighia sapida, Canarium schweinfurthii
(Bediwonua), Petersianthus macrocarpus (Essia),
Sterculia oblonga (Ohaa), Cola gigantea
(Watapuo), Celtis zenkeri (Esa), Antiaris toxicaria
(Kyenkyen) and Amphimas pterocarpoides (Lati).
The objective of the study was to determine the
bending strength, modulus of elasticity properties
and failure behaviour of the selected lesser known
species using structural size specimens.
MATERIALS AD METHODS
Materials
Two or three trees of each of the ten species were
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 17
extracted from three forest regions in Ghana. The
forests were located at Finaso Nkwanta (Moist
Evergreen ecological zone), Kubease (Moist
Semi-Deciduous – North-East Type) and Juaso
(Moist Semi-Deciduous – South East type). The
diameters of the trees at 1.3 m above ground were
at least 45 cm with an average diameter of 60 cm.
Clear boles of at least 25 m length were obtained
and conveyed to Modern Wood Technology and
Company Limited, Kumasi for processing. The
logs were converted on a horizontal Band Mill to
55mm thick boards. The boards were then stacked
for air-drying under a shed. Specimens were
prepared from the boards for the determination of
bending strength properties for each of the species.
Test specimens were prepared according to the EN
408 (1995) standard for the determination of some
physical and mechanical properties of structural
timber.
Basic density and moisture content tests
Specimens were cut for the determination of basic
density and ‘green’ moisture content. The standard
for density determination was EN 408:1995 whilst
that used for moisture content determination was
EN 13183-1:2002.
Modulus of elasticity
Beam cross sectional dimensions of 50 x 120 mm
with an effective span of 2500mm (Table 1) were
used for both local and global modulus of
elasticity tests. The European test method for the
determination of the modulus of elasticity (MOE)
in bending of structural timber, EN 408:1995 was
used in this work.
The EN 408 specifies two methods or forms of
determining modulus of elasticity; the local and
global. The local modulus of elasticity is in
principle based on pure bending deflection whilst
the global modulus of elasticity is influenced by
shear deflection (Solli, 1999). When measuring
the global modulus of elasticity, the total
deflection will be a combination of bending and
shear deflection. The contributory effect of the
shear deflection makes a fundamental difference
between the global and local modulus of elasticity
(Bostrom and Holmquist, 1999; Solli, 1999).
Table 1: Number of beams (50
x 120 x 2500 mm)
tested for each of the 10 lesser-used species
Species Number of
Beams tested
Botanical Name Local Name Symbol
Albizia ferruginea Awiemfosamina AF 7
Blighia sapida Akye BS 11
Canarium schweinfurthii Bediwonua CS 12
Celtis zenkeri Esa CZ 16
Petersianthus macrocarpus Essia PM 8
Sterculia oblonga Ohaa SO 10
Sterculia rhinopetala Wawabima SR 10
Cola gigantea Watapuo CG 10
Antiaris toxicaria Kyenkyen AT 11
Amphimas pterocarpoides Lati AP 12
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 18
The global modulus is not as sensitive to
inaccurate measurements as the local modulus
since the global deflection is about ten times the
local. A measurement of the global modulus
contains a higher number of possible sources of
error. The temperature within which the tests were
conducted ranged from 28 – 31
0
C. The relative
humidity was about 70%.
Bending strength test
The test beam was symmetrically loaded in
bending at two points over an effective span of
2500mm as shown in Figure 1a. The test piece
was set up simply supported with typical
instrumentation as shown in Figure 1b. Small steel
plates of length not greater than one-half of the
depth of the test piece were inserted between the
piece and the loading heads to minimize local
indentations. Lateral restraints were provided at
the supports to prevent buckling. The restraints are
provided such that they permit the piece to deflect
without significant frictional resistance. The load
was applied by means of a hydraulic pump and
was applied at constant loading-head movement so
adjusted that maximum load was reached within
300 (+120) s. The load was increased at multiples
of 2 kN up to 6 or 8 kN (about 40% of maximum
expected failure load) and then reduced back to
zero. This was repeated and the beam was loaded
to failure at the third time of loading. The dial
gauge readings were recorded for each load
increment/decrement. The mode of fracture and
the growth characteristics at the fracture section of
each test piece was recorded.
The bending strength f
m
was calculated using the
equation:
f
m
= a F
max
/ (2W) 1
where W is the section modulus, F
max
is the
failure load and a is the distance between a
loading position and the nearest support in a
bending test.
Figure 1a: Schematic test arrangement for measuring local modulus of elasticity in bending
Steel plate
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 19
Figure 1b: Typical experimental set-up with instrumentation
RESULTS AD DISCUSSIOS
Moisture content and basic density
The moisture contents of the 10 timber species
used, ranged from 16.1% (for Antiaris toxicaria)
to 51.0% (for Albizia ferruginea) as shown in
Table 2. The timber was only air-dried for about
three months. For practical use, it is difficult to
dry structural timber in Ghana to 12% moisture
content by air-drying because of the very humid
climate especially in the middle and coastal belts
of the country. More so, if the recorded strengths
could be derived at such high moisture contents,
then higher strengths will be obtained when the
timber species are in use.
The average density of the 10 timber species
ranged from a minimum of 436 kg/m
3
for Antiaris
toxicaria to a maximum of 1007 kg/m
3
for
Sterculia rhinopetala. These average density
values indicate that the 10 species with the
exception of Antiaris toxicaria and Canarium
schweinfurthii could be classified as ‘medium-
heavy’ (575-725 kg/m
3
) to ‘heavy’ (725-900
kg/m
3
) according to ATIBT, 1990 and TEDB,
1994. These high density values suggest that the
timber species used in this study could be used for
heavy construction (Ofori et al., 2009a). High
density timber is expected to perform better in
bending than low density timber (Tsuomis, 1991;
Kollman and Cote, 1968; Davis, 1962). It is
generally accepted that the density of wood is a
good index of its properties as long as the wood is
clear, straight grained, and free from defects.
Density of wood is however affected by the
presence of gums, resins and extractives which
add to their weight and contribute little to
mechanical properties (Lavers, 1983; Green et al,
1999; Ofori et al, 2009b).
Load-deflection and failure behavior
The load-deflection curves of all the species
showed elastic load-deformation behaviour.
Test beam
Steel plate
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 20
Table 2: Moisture content and density of the ten lesser-used species tested
Species
Moisture Content Density
Mean (%)
Std. Dev.
Mean (kg/m
3
)
Std. Dev.
Albizia ferruginea 51.0 10.63 740 75
Blighia sapida 28.5 9.70 899 55
Canarium schweinfurthii 30.4 5.80 488 30
Celtis zenkeri 29.0 7.90 829 31
Petersianthus macrocarpus 28.8 6.98 859 63
Sterculia oblonga 32.5 8.20 821 58
Sterculia rhinopetala 43.6 11.30 1007 86
Cola gigantea 19.4 5.40 671 74
Antiaris toxicaria 16.1 0.64 436 17
Amphimas pterocarpoides 16.4 1.12 772 29
However, the test beams did not return to their
original positions after loading and unloading
during the test. Permanent deformations were
observed in the curves as presented in a typical
curve (Figure 2). This means the species were not
perfectly elastic. Cyclic loading, which is the
repeated application of loads, was used in the test
procedure. This is because many structures, such
as bridges are usually subjected to repeated
loading and unloading due to their usage. It has
been found that structural components subjected to
repeated loads may fail even though the associated
stress levels are well below the yield strength
(Bedford and Liechti, 2004). Basically, a small
amount of damage is produced each time a load is
applied. Although the amount of damage caused at
each repetition, or cycle, is insufficient to cause
failure, the damage can accumulate and eventually
result in failure. It is observed that the elastic
stiffness and rate of increase in the strength
capacity of the beams did not change after the
three cycles of loading (L1, L2 and L3) and two
unloading (U1 and U2). This is indicative that the
beam specimen did not undergo any stiffness
deterioration or strength reduction for the limited
cyclic loading.
The average failure loads for each of the 10 LUS
timber species is shown in Table 3. The highest
average experimental failure load of 22.8 kN was
measured for Sterculia rhinopetala beams whilst
the lowest of 10.7 kN was measured for Antiaris
toxicaria beams.
Sterculia rhinopetala had the
highest density of 1007 kg/m
3
whilst Antiaris
toxicaria had the lowest density of 422 kg/m
3
.
Even though the failure load of Sterculia
rhinopetala (22.8 kN) was slightly higher than that
of Sterculia oblonga (20.2 kN), the ultimate
deflection of Sterculia oblonga (2.5 mm) was
higher than that of Sterculia rhinopetala (1.57
mm). This could be explained by the fact that the
density of Sterculia rhinopetala (1007 kg/m
3
) was
higher than that of Sterculia oblonga (821 kg/m
3
)
as shown in Table 3. There was no particular trend
in the value of the ultimate loads so far as the
predominant failure loads were concerned other
than the effect of density. The highest average
ultimate deflection of 3.9 mm was measured for
Cola gigantea with an average ultimate load of
12.6 kN. All the average ultimate deflections were
lower than both the theoretical deflections and the
BS 5268 permissible design deflection (0.003 x
span). A maximum theoretical deflection of 5.8
mm was expected for Sterculia rhinopetala beams
which actually recorded 1.57 mm.
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 21
Figure 2: Typical load-deformation curve (SR31local modulus) of the species (L1 = First cycle loading; UI = First
cycle unloading; L2 = Second cycle loading; U2 = Second cycle unloading; L3 = Loading to failure)
This is indicative of the fact that Sterculia
rhinopetala beams exhibited the least deflection
per unit load for all the LUS timber species
studied.
A summary of the predominant failure mode of
the 10 LUS timber species tested in the laboratory
is given in Table 4. All the beams failed in a form
of tension rupture. The breaks were usually
splintering tension failure or brittle (brashness)
tension failure or in some instances a combination
of both modes of failure. Albizia ferruginea, Celtis
zenkeri, Antiaris toxicaria and Amphimas
pterocarpoides timber species failed in splintering
tension mode. Four (4) other species;
Petersianthus macrocarpus, Sterculia oblonga,
Sterculia rhinopetala and Cola gigantea timber
species failed in brittle (brashness) tension failure
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5
Deflection, mm
Load, KN
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 22
whilst the remaining two (2) species (Blighia
sapida and Canarium schweinfurthii) failed in a
combined splintering and brittle tension failure
modes. The failure modes of the timber beams
were observed to be related to the grain texture of
the wood.
The species which failed in splintering tension
were mostly medium to fine textured with straight
grains as reported in other publications (Ayarkwa
et al., 2012; Anon, 2000; TEDB, 1994). On the
other hand, brittle tension failure was observed in
wood with coarse grains (Farmer, 1972). Some of
the Sterculia rhinopetala beams had bearing
failure in addition to flexural tension failure. Even
though all the beams were within the dimensional
limits to avoid lateral buckling, the Canarium
schweinfurthii beams had the tendency to undergo
lateral buckling before failure. It was observed
that some of the timber beams (Sterculia oblonga
species) underwent excessive deflections before
failure.
Figure 3: Typical splintering tension failure of an Albizia ferruginea beam
Table 3: Average failure loads and deflections of the 10 lesser-used species tested
Species
Experimental
Failure loads (kN)
Deflections (mm)
Experimental Theoretical BS 5268 limits
(0.003 x span)
Albizia ferruginea 14.7 1.44 3.7 7.5
Blighia sapida 17.6 1.69 4.0 7.5
Canarium schweinfurthii 17.2 1.47 2.6 7.5
Celtis zenkeri 18.6 1.02 4.6 7.5
Petersianthus macrocarpus
17.0 2.65 5.2 7.5
Sterculia oblonga 20.2 2.50 4.7 7.5
Sterculia rhinopetala 22.8 1.57 5.8 7.5
Cola gigantea 12.6 3.90 4.5 7.5
Antiaris toxicaria 10.7 1.43 3.3 7.5
Amphimas pterocarpoides 17.0 1.30 3.9 7.5
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 23
Table 4: Predominant failure modes of structural size timber of the ten lesser-used species
Species Predominant failure mode
Albizia ferruginea Splintering tension failure
Blighia sapida Combined splintering tension failure and brittle tension failure
Canarium schweinfurthii
Combined splintering tension failure, brittle tension failure and buckling of
beams
Celtis zenkeri Splintering tension failure
Petersianthus macrocarpus Brittle tension failure
Sterculia oblonga Brittle tension failure with excessive deflection in some beams
Sterculia rhinopetala Brittle tension failure and bearing failure in some beams
Cola gigantea Brittle tension failure
Antiaris toxicaria Splintering tension failure
Amphimas pterocarpoides Splintering tension failure
Modulus of elasticity and bending strength
The bending strength and modulus of elasticity are
shown in Table 5 for the 10 timber species. The
higher the density of a species, the higher the
modulus of elasticity and the higher the bending
strength. The order of decreasing characteristic
(5
th
percentile) bending strength (MOR) of the ten
(10) species was as follows: Sterculia rhinopetala,
Sterculia oblonga, Amphimas pterocarpoides,
Blighia sapida, Petersianthus macrocarpus, Celtis
zenkeri, Canarium schweinfurthii, Albizia
ferruginea, Cola gigantean and Antiaris toxicaria.
The corresponding order of overall decreasing
mean local modulus of elasticity (MOE) of the ten
species was as follows: Celtis zenkeri, Sterculia
oblonga, Sterculia rhinopetala, Amphimas
pterocarpoides, Albizia ferruginea, Blighia
sapida, Cola gigantea, Petersianthus
macrocarpus, Canarium schweinfurthii and
Antiaris toxicaria. Sterculia rhinopetala
(Wawabima) has a 5
th
percentile bending strength
of 56.8 N/mm
2
and a mean local modulus of
elasticity of 15,973 N/mm
2
. Sterculia oblonga was
also found to be the 2
nd
best in terms of material
properties. It had a 5
th
percentile bending strength
of 52.1N/mm
2
and a mean local modulus of
elasticity of 16,408 N/mm
2
. Celtis zenkeri which
had the highest mean local MOE, had the 6
th
best
5
th
percentile bending strength of 39.9 N/mm
2
.
Global MOE values were mostly lower than the
local MOE values for each species.
Solli (1999) investigated the differences between
the local and global modulus of elasticity (MOE)
in bending of structural timber. There have been
several discussions whether the local or the global
value is the most representative value of the
bending stiffness. Some researchers believe that
since the local MOE is the current system being
used and works well, there is no need to welcome
a new system of global MOE, whose possible
consequences are unknown. The local MOE is
well known in the European strength class system
(EN 338:2003), so with new system of values,
design engineers might be confused. It is worthy
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 24
of note that the limits of deflection given in the
European building regulations are based on design
by local MOE. It is also argued that the local
MOE is not the correct parameter for the
calculation of the deflection of timber floors. The
argument is that the local value as described in EN
408:1995 is based on the critical section and
therefore cannot be representative for a whole
span. In addition, the test procedure of global
MOE is easier and timesaving compared with the
corresponding local MOE test procedure.
Solli also concluded that global MOE is not as
sensitive to inaccurate measurements as local
MOE since the global deflection is about ten times
the local. The local MOE is in principle based on
pure bending deflection whilst global MOE is also
influenced by shear deflection. A measurement of
the global MOE contains a higher number of
possible sources of error such as the initial
twisting of test pieces during testing. If the
intended use of MOE is to estimate the
corresponding bending strength of a piece of
timber then the local MOE is the unrivalled
alternative of the two methods.
Table 5: Modulus of elasticity and bending strength of the ten lesser-used species
Modulus of elasticity (N/mm
2
) Bending Strength (N/mm
2
)
local global
Average Std. Dev 5
th
Percentile
Species
Average Std. Dev. Average Std. Dev
Albizia ferruginea 13847 2544 11238 1819 49.9 9.1 31.7
Blighia sapida 12686 3517 12078 1161 61.4 10.2 42
Canarium
schweinfurthii 10316 1281 9331 911 44 3.6 37.4
Celtis zenkeri 17422 2292 14273 1143 65.8 15.2 39.9
Petersianthus
macrocarpus 12021 2223 10494 1504 60.9 9.6 41.7
Sterculia oblonga 16408 3008 13004 1756 70 9.4 52.1
Sterculia rhinopetala 15973 2839 13382 1356 81.7 13.1 56.8
Cola gigantea 10219 2526 9679 1375 45.7 8.8 29
Antiaris toxicaria 9675 1105 8827 868 38.4 5 29
Amphimas
pterocarpoides 15595 2397 14220 917 63.6 9.7 46.1
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 25
Figure 4a: Relationship between average density and
average bending strength of the ten lesser-used species
Figure 4c: Relationship between average bending
strength and average local MOE of the 10 lesser-
used species
Figure 4b: Relationship between average global MOE
and average local MOE of the ten lesser-used species
Figure 4d: Relationship between avearge bending
strength and average global MOE of the 10 lesser-
used species
Bending and modulus of elasticity properties of 10 lesser-used timber species in Ghana E. Appiah-Kubi et al.
Ghana J. Forestry, Vol. 28 (1), 2012, 15 – 28 26
Correlation between mechanical strength
properties
Correlation between mechanical strength
properties have been shown in Figure 4a-d for
average density, average bending strength,
average local MOE and average global MOE. All
the properties were found to be linearly related to
each other. There was a good correlation (81.4%)
between average density values and average
bending strength values (Figure 4a). Average local
and global MOE values (Figure 4b) were highly
correlated with a correlation coefficient of 91.3%.
As shown in Figures 4c and 4d, average bending
strength values were correlated at 69.6% and
70.3% for average local and global MOE values
respectively.
COCLUSIO
In Ghana, the properties of structural size
specimens, which are generally quite different
from those of small clear specimens because of the
unavoidable defects such as knots and shakes,
have not yet been determined. In this study, the
mechanical properties of ten Lesser Known
Species (LKS) of timber were investigated to
assess their suitability for structural use. The
moisture contents of the 10 timber species used
ranged from 16.1% to 51.0%. The average density
of the species ranged from a minimum of 436
kg/m
3
for Antiaris toxicaria to a maximum of
1007 kg/m
3
for Sterculia rhinopetala. The mean
density values indicate that the 10 species could be
classified as ‘medium-heavy’ (575-725 kg/m
3
) to
‘heavy’ (725-900 kg/m
3
) according to ATIBT,
1990 and TEDB, 1994. It was observed during
loading under flexure that the elastic stiffness and
rate of increase in the strength capacity of the
beams did not change after the three cycles of
loading and unloading. This is indicative of the
fact that the beam specimen did not undergo any
stiffness deterioration or strength reduction for the
two cycles of loading. All the beams failed in a
form of tension rupture. The breaks were usually
splintering tension failure or brittle (brashness)
tension failure or in some instances a combination
of both modes of failure.
Sterculia rhinopetala exhibited the highest
bending strength with a 5
th
percentile bending
strength of 56.8 N/mm
2
and a mean local modulus
of elasticity of 15,973 N/mm
2
. Sterculia oblonga
was also found to be the 2
nd
best in terms of
material properties with a 5
th
percentile bending
strength of 52.1N/mm
2
and a mean local modulus
of elasticity of 16,408 N/mm
2
. Celtis zenkeri
which had the highest mean local MOE, however,
had the 6
th
best 5
th
percentile bending strength of
39.9 N/mm
2
. A good linear correlation (69.6-
91.3%) was found between mechanical strength
properties for average density, average bending
strength, average local MOE and average global
MOE.
ACKOWLEDGEMET
Many thanks to the Swiss Development Agency
(SDA) and the Swiss National Science Foundation
(SNSF) for providing financial support for this
study.
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