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International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 8, August 2018, pp. 771–780, Article ID: IJCIET_09_08_078

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=8

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

EXPERIMENT INVESTIGATION OF FLEXURE

RIGIDITY OF GLUE LAMINATED TIMBER

BEAMS CONSISTING OF COCONUT WOOD

(COCOS NUCIFERA) AND SENGON WOOD

(ALBIZIA FALCATARA) BASED ON FOUR-

POINT BENDING TEST

Kusnindar*

Civil Engineering Department, Brawijaya University,

MT. Haryono street 167 Malang 65145 East Java, Indonesia

Civil Engineering Department, Tadulako University,

Soekarno-Hatta street km.9 Palu, Central of Sulawesi, Indonesia

Sri Murni Dewi



Agoes Soehardjono



Wisnumurti



*Corresponding Author E-mail: [email protected]

ABSTRACT

This paper was intended to describe the performance of mix-glue laminated timber

beams (mix-glulam) compose of coconut wood (Cocos nucifera) and sengon wood

(Albizia falcatara) in the outer zone and core zone, respectively. Mix-glulam system is

an alternative to utilizing a low-grade timber as structural member of building.

Through the analysis of four-point-bending-test results of each of five mix-glulam

beams (BLG) and five sengon-solid beams (SDL) with span (L) = 2750 mm, height (d)

= 155 mm and width (b) = 55 mm was obtained that the mix-glulam system produces

a good performance. The analysis result showed that the flexural rigidity (EI) of BLG

is 1.66 times higher than SDL. Likewise, the maximum bending stress (σmax) of BLG is

Kusnindar, Sri Murni Dewi, Agoes Soehardjono and Wisnumurti


1.6 times higher than the σmax of SDL with tension failure in bending. Generally, the

failure of the beam that occurs is a brittle and sudden tension failure.

Key words: sengon wood (Albizia falcatara), coconut wood (Cocos nucifera), mix-

glulam, flexure rigidity, timber beam.

Cite this Article: Kusnindar, Sri Murni Dewi, Agoes Soehardjono and Wisnumurti,

Experiment Investigation of Flexure Rigidity of Glue Laminated Timber Beams

Consisting of Coconut Wood (Cocos Nucifera) and Sengon Wood (Albizia Falcatara)

Based on Four-Point Bending Test. International Journal of Civil Engineering and

Technology, 9(8), 2018, pp. 771-780.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=8

1. INTRODUCTION

The potential availability of sengon wood (Albizia falcatara) and coconut wood (Cocos

nucifera) in Indonesia is very adequate. In 2013, there are approximately 64 million sengon

trees that can be harvested. This amount is equivalent to 13.9 million m3 of sawn timber [1].

In addition, the potential availability of coconut wood in Indonesia is also quite adequate.

This is a necessity of the extent of the Indonesian coconut plantation which is equivalent to

approximately 26% of the total coconut plantations in the world [2].

Unfortunately, sengon-wood and coconut-wood have not been widely used as structural

elements of building, because in both there are deficiencies in terms of mechanical and

physical properties. Sengon wood has a low-grade modulus of elasticity that is only about

3,881 MPa. While coconut wood has a high variation in the density. The low grade of the

modulus of elasticity causes the carrying capacity of the wood to be low, and the density

variation causes the limitations of sawn timber dimension to be obtained. Approximately only

one-third of the coconut trunk (35 cm diameter) can be utilized to produce good quality sawn

timber with an average elasticity modulus value of 11.414 MPa [2, 3].

In order to utilize sengon and coconut wood for structural elements of building, the mix-

glue laminated timber method can be considered. Mix-glue laminated timber beams is one

type of laminated timber structure product. Mix-glue laminated timber beams consists of two

or more species of timber with different mechanical properties. Timbers that have higher

mechanical properties are placed as outer lamination and vice versa as core elements of the

beam.

Glue lamiated timber (glulam) is still very competitive in modern construction material. In

terms of structural efficiency, the laminated wood can produce construction elements with

sufficient strength to density ratios. For example, the mix-glulam beam composed of

Eucalyptus grandis and Poplar laminations has a structural-efficiency value of 120 x 10-3

MPa.m3/ kg [4]. In this case the structural efficiency of timber structures is better than

concrete [5]. The use of a laminated wood system will produce a large construction elements,

regardless of the diameter of the tree. The cross-section depth of glulam beam is in principle

unlimited, but for the accomplished maximum depths are of the order of 2 m. The weakening

of structures caused by natural wood defects can also be reduced by a laminated wood system

[5].

Based on the above description, this research is aimed to determine the flexure rigidity

and strength of mix-glue laminated timber beam was compose of sengon and coconut wood.

In this case, sengon wood lamination is placed in the core zone of the beam and coconut wood

lamination in the outer zone.

Experiment Investigation of Flexure Rigidity of Glue Laminated Timber Beams Consisting of Coconut

Wood (Cocos Nucifera) and Sengon Wood (Albizia Falcatara) Based on Four-Point Bending Test


2. LITERATURE REVIEW

Mix-glue laminated timber beams of hardwood timber were composed of 50% of lower grade

lamination in the core zone and 50% of high-grade lamination in the outer zone can produce a

bending strength and stiffness is 16.5 MPa and 12.4 GPa, respectively. This suggests that the

mix-glue laminated timber beam design with this proportion is technically feasible to

implement [6].

Poplar and spruce combinations as mixed-glulam beams result in a more flexible or

pseudo-ductile behavior than individual poplar beams. Based on the load and deformation

relationships, the mix-glulam beams has a plastic behavior till failure [7]. Similarly, the

combination of poplar-larch and poplar-spruce. Both combinations have higher ductility

compared to solid poplar beams, which are 48% and 64% respectively. So, the performance of

mix-glulam-beams is determined by the mechanical properties of the wood species used,

especially the outer zone lamina [8]. Since the ductility of the structure is very advantageous,

it is appropriate that the structural system is designed to behave in a ductile manner. As stated

by [9] that ductility of timber beams can be improved by application of external fiber

reinforcement. This retrofitting method is easy to implement and at the same time reduces the

risk of failure due to defects.

Furthermore, the mix-glue laminated timber beam which is a combination of poplar and

eucalyptus was produced a higher strength and modulus of elasticity than individual poplar

beams. Modulus of elasticity of the mixed-glulam beams is 51% higher than the individual

poplar beam. Likewise, the flexural strength of the mixed-glulam beams was increased by

35% compared to the solid-poplar beams. In this case, poplar wood is placed in the core

lamination and eucalyptus as the outermost lamination, [4].

3. MATERIAL AND METHOD

3.1. Timber and Adhesive

The initial dimensions, mechanical properties and number of lumber prepared for the beam

manufacture are presented in Table 1. Both lumber materials were obtained from sawmills

industry in Malang, East Java, Indonesia. Before use, both are dried for about three months to

achieve air-dry conditions, ie at ± 12% of moisture content. The physical and mechanical

properties of them are obtained from the clear-specimens test according to [10].

Table 1 Mechanical properties of sengon and coconut wood

Properties in 12% of moisture content Sengon Wood Coconut Wood

Modulus of Elasticity (E12) 3881 MPa 10830 MPa

Modulus of Rupture (MOR12) = 20 MPa 20 MPa 39 MPa

Density (ρ12) 0.29-0.32 gr/cm3 0.6 gr/cm

3

Initial dimension :

width 70 mm;

dept 180 mm; length

3250 mm

width 60 mm;

depth 120 mm; length

3250 mm

Number of the lumber trunks : 15 10

Especially for the manufacture of laminated-timber beams required additional material ie

adhesive. The adhesive used is specifically for wood, ie urea formaldehyde (UF) resin in the

form of ready-made powder. This adhesive specification as issued by the manufacturer is

presented in Table 2.



Table 2 Properties of adhesive

Material Material Properties

Urea Formaldehyde Adhesive (UF)

shear strength of glue line = 45 kg/in2

Gelatin time = 5 hours

spreading ability = 2 m2/ kg

3.2. Geometry and the Beams Dimensions

The specimen consists of two series, namely sengon-wood solid beams (SDL) and glulam-

mixed beams (BLG). The dimensions and geometry of the cross-section of each beam are

presented in Table 3 and Figure 1. To ensure flexural failure without lateral buckling during

testing, it was determined that the high to span ratio of more than 14.0 [11] and the height to

width ratio less than 3.0 [12].

Tabel 3 The beams dimension.

Beams

Series

Number

of specimen

b

(mm)

d

(mm)

L

(mm) L/d d/b

SDL 5 55 155 2750 18 2.8

BLG 5 55 155 2750 18 2.8

Figure 1 Cross section of the beams

3.3. Beams Fabrication

The SDL series manufactured starts from the selection of five previously prepared sengon-

wood trunk as shown in Table 1. To ensure uniformity of density, a relatively similar weight

of sengon-wood trunk is selected. The sengon-wood trunk is then cut and finishing with a

planer machine until the net dimensions are reached as Table 3. Then each beam is labeled

SDL-1, SDL-2, SDL-3, SDL-4, and SDL-5. The total length of all the beams is made equal to

the length of the span (L) plus 0.5d to provide the placement area at the support during the test

[13].

Furthermore, the BLG manufactured starts from the preparation of lamination from the

source material as in the Table 1. The coconut-wood lamination will be made from six sawn

lumber. The six sawn lumber were subsequently cut into 24 sheets of board. Each of the board

has a thickness of 30 mm, width 60 mm and length 3250 mm. Each board was then leveling

with a planer machine so as to achieve a net of thickness is 26 mm. It also produces a cleaned

surface from dirt that can inhibit the gluing process. After obtaining 24 pieces of coconut-

wood lamination ready for use then selected 10 sheets of laminations that has a uniform

density of approximately 0.6 gr/cm3 (Figure 2).

For the preparation of sengon-wood laminations, the process is the same as the coconut-

wood lamination, only different amount. Seven sengon-wood sawn timber were used and

b

d

a. SDL

1

b

d

b. BLG

2

3

3

2

1

Legend:

a) Solid sengon wood beams (SDL)

b) Mix-glulam beams (BLG)

1 = coconut wood lamination (ρ12 = 0.6 gr/cm3)

2 = sengon wood lamination (ρ12 = 0.32 gr/cm3)

3 = sengon wood lamination (ρ12 = 0.29 gr/cm3)

∆d = lamnation tickness (26 mm)

∆d




produce 10 laminations having density ± 0.29 gr / cm3 and 10 laminations having density ±

0.32 gr/cm3 as Figure 2. The sengon and coconut wood laminations are then grouped

according to the beam formation to be made. The density uniformity control is necessary

because it determines the mechanical properties of wood in general, although there is a natural

wood factor that also determines [14].

Figure 2 Laminations are ready to be glued, pressure and clamping of the beams

The adhesive application was carried out using a manual brush spreader and the average

amount of 350 g/m2 [4]. The adhesive composition according to the manufacturer's

specifications is 0.5 kg water: 1kg UF-powder. Furthermore, the lamination arrangement is

given 2MPa pressure. This pressure is applied at 25 cm intervals along the beam as in Figure

2. After pressing at each point, immediately vertical steel clamps are fixed at that point and

attached for approximately 18 hours. The final process of mix-glulam beams fabrication is the

leveling of two sides of the beams to obtain a net width of 55 mm. These processes are not

applied on the top and bottom side of the beam, as it has been obtained 155 mm hight.

3.4. Method of Testing and Data Analysis

3.4.1. Test Methods

To evaluate the flexural rigidity and bending strength of the timber beams, four-point bending

tests were used as shown in Figure 4. In this test was recorded the load and deflection data of

each beam at three locations in the pure bending area [13]

Figure 4 Apparatus configuration of four-point bending test

3.4.2. Analisis of Moment-curvature Diagram

The load-deformation diagram of each beam is obtained by plotting of moment and curvature.

The curvature value is determined based on the experimental deflection data. The calculation

can use Equation 1 was refers to the beam theory.

κ � 2. ��2 � ��1��32� . s�2 (1)

Where:

κ = mid-span curvature of beam (mm-1

)



δ1 = deflection of LVDT-1 (mm)



s = distance between points of loading (mm)

Based on the moment-curvature diagram, then determined the value of elastic moment

(Me), elastic curvature (κe), maximum moment (Mmax) and maximum curvature (κmax). The

procedure of determining these magnitude value involves the initial deformation, elastic

deformation contribution, and permanent deformations [15]. On the other hand, the

determination of flexure rigidity (EI) and the bending strength (σmax) of each beam is based

on the beam theory using Equation 2 and 3 [16]. Furthermore, the ductility of beams is

determined using Equation 4 [3].

EI � P'.a

24.� �3L2-4a2� (2)

�max � 3Pmax . a

b . d2 (3)

μκ� κmaxκe

(4)

Where:

EI = flexure rigidity of beams (N.mm2)

P’ = linier stage load (0.4Pmax – 0.1Pmax) [15]

δ' = linier stage deflection at mid span (0.4δmax – 0.1δmax) [15]

a = distance between loading point to support (mm)

σmax = bending stresses of beam (MPa)

Pmax = maksimum load ( N )

L, b, d = span, width and depth of beams (mm)

µκ = curvature ductility

κmax = maximum curvature (mm-1

)

κe = limit of linear curvature (mm-1

)

4. RESULT AND DISCUSSION

4.1. Mechanical Parameters of Beams

The mechanical parameters of beams are determined based on the moment-curvature diagram

as Figure 5, Table 4 and Table 5. The moment-curvature diagram of each beam always begins

with a linear stage up to the elastic limit. This elastic load limit is further expressed by Me.

The experimental results show that the mean of elastic moments of mix-glulam beams (BLG)

and sengon solid beams (SDL) are 5.977 kN.m and 2.848 kN.m, respectively. This shows that

the elastic moment of mix-glulam beams is 2.25 times higher than the solid sengon beams.




Figure 5 The moment-curvature diagram at mid-span of beams

Similar to bending moment, the experimental results also show that the flexure rigidity of

mix-glulam beams is also higher than the solid sengon beam. Flexure rigidity of each beam is

represented by the tangent of the moment-curvature diagram in the linear stage as Figure 5. In

this case, the average flexure rigidity of mix glulam beams is 1.66 times higher than the solid

sengon beam.

Table 4 Bending test result of solid sengon-wood beams.

Name of

Beams

EI Mmax σσσσmax κκκκmax/κκκκe Failure Mode x10

11 N.mm

2 kN.m MPa

SDL-1 1.466 4.145 21 4.270 Tension

SDL-2 1.536 5.317 23 2.773 Tension

SDL-3 1.029 4.804 25 2.951 Tension

SDL-4 1.107 2.425 12 1.419 Premature tension

SDL-5 0.525 4.913 24 2.211 Tension

Average 1.133 4.321 23.19 2.7 -

Table 5 Bending test result of mix-glulam beams

Name of

Beams

EI Mmax σσσσmax κκκκmax/κκκκe Failure Mode x10

11 N.mm

2 kN.m Mpa

BLG-1 1.264 8.018 35 2.431 Tension

BLG-2 1.432 7.073 30 1.810 Tension

BLG-3 2.668 7.912 40 1.048 Compression at load point

BLG-4 2.262 6.846 43 1.298 Tension

BLG-5 1.772 8.027 36 1.827 Tension

Average 1.879 7.575 36.8 1.7 -

After the limit of linear deformation is reached, the next deformation switches to a

nonlinear stage. This nonlinear deformation continues until the failure of the beam. This

becomes an indication that there has been a nonlinear stress in the compression zone [17] as

well as describing the ductility of the beam (κmax/κe) [3]. Ductility of sengon wood solid

beams is 1.6 times higher than mix-glulam beams. But in terms of a bending moment

achievement, it appears that the performance of mix-glulam beams is much better than sengon

solid beams. The average of the maximum value of bending moment of mix glulam beams is

1.75 times higher than the sengon solid beams.

Me

0

2

4

6

8

10

0 20 40 60 80 100

M (

kN

-m)

κ (x 10-6 mm-1)

SDL-1

SDL-2

SDL-3

SDL-4

SDL-5

Me

0

2

4

6

8

10

0 20 40 60 80 100

M (

kN

-m)

κ (x 10-6 mm-1)

BLG-1

BLG-2

BLG-3

BLG-4

BLG-5



Regarding the bending strength of the beam expressed by the maximum bending stress

(σmax), the mix-glulam beam has a better performance than the solid sengon beams. The

average of bending strength of mix-glulam beams and sengon solid beams are 36.8 MPa and

23.19 MPa, respectively, or it can be stated that the bending strength of the mix-glulam beams

is 1.6 times higher than the solid sengon beams.

4.2. Failure Mechanism of Beams

The SDL beams failure is characterized by a rupture on the tension side as shown in Figure 6.

This tensile failure is brittle and occurs suddenly. This corresponds to the nature of wood in

response to tensile stress. Cracks in the tensile side will then propagate toward the neutral axis

of the beam. This failure is identified at a time when the moment-curvature diagram begins to

switch to a non-linear stage. This means that the compression stress of the beam begins to be

a plastic stage, but the maximum compressive stress has not been reached. Such failure is

categorized as a second mode of failure [17].

Figure 6 Typical failure of SDL beams

In SDL-4 beams premature tensile failure occurred. The tensile failure is achieved at a low

magnitude of bending moment, and the value is less than the average of linear bending

moment limit of the SDL group. Premature failure is caused by defects in the wood caused by

insects attack (Figure 6). This defect is not detected when sorting from the material because it

is located inside the beam. This is one of the disadvantages of using wood for construction in

solid timber.

The BLG beams failure is characterized by a rupture on the tension side in the coconut

wood lamination. Furthermore, the crack propagation occurs horizontally following the fiber

orientation after it reaches the sengon wood lamination. At this time the total failure has

occurred. This total failure is suddenly with high damage intensity as shown in Figure 8.

However, different modes of failure occur on BLG-3. This beam failed at the contact-point

with the load spread. This failure mode is reached before the compression stress switches to a

nonlinear stage like Figure 9.

Figure 8 Typical failure of BLG beams




Figure 9 Local failure in the loading point contact of BLG-3

5. CONCLUSIONS

Mix-glulam beams consisting of one-third of the cross-sectional are coconut wood

laminations and the other two-thirds are wood-sengon laminations can produce a flexure

rigidity (EI) by 1.66 times higher than solid sengon wood beams. The placement of the

coconut wood laminations in the outermost of the glulam beams produces the bending

strength 1.6 times higher than the bending strength of the individual sengon beams. Although

there are consequences of decreasing the ductility index up to 40%. The failure mode that

occurs is similar for both types of beams. The failure mode that occurs in the form of sudden

tensile failure with crack propagation tends to adjust the fiber orientation.

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