The Compression Test and the Bending Test of Wooden Structural

Material of Traditional Houses of Batak Karo, North Sumatera

Sarwidi

, F. D. Artha

, Y. P. Prihatmaji

, Widodo

Graduate Program of Civil Engineering, Universitas Islam Indonesia, Yogyakarta, Indonesia

Study Program of Architecture, Universitas Islam Indonesia, Yogyakarta, Indonesia

Keywords: Traditional House, Batak Karo, Wood Mechanical Properties, Compression Test, Bending Test, Earthquake

Resistant.

Abstract: Traditional houses of Batak Karo, North Sumatra are known as the Traditional House of Siwaluh Jabu. The

structure of the buildings uses wood materials, where such buildings has been proved to be more earthquake

resistant. These traditional houses use several types of wood, namely Haudolok wood, Ingul wood, Pengki

wood, and Simartolu wood. To know the mechanical properties of the wood structures, then a series of tests

were done. This paper only is objected to discuss the results of compression and bending tests of the wood.

The compression test was conducted to find out the relationship of the density with the elasticity modulus and

the equivalent yield stress. Three-point and four-point bending tests were conducted to understand the

relationship of the density with the elasticity modulus, the maximum bending stress, and the shear modulus.

Teak wood and Jackfruit wood from Java were selected as control specimens. The test results showed that, in

the compression test, the density has a very strong correlation with the modulus of elasticity and the equivalent

of the melting stress. In both three-point and four-point bending tests, it shows that the density has a very

strong correlation with maximum bending stress and strong correlation with shear modulus.

1 INTRODUCTION

Wood is one of the most common structural materials

used in the construction of traditional earthquake

resistant houses (Prihatmaji et al., 2011; Prihatmaji et

al., 2012; Suara Merdeka, 2005). Wood materials are

capable of deforming and tend to be stable when

earthquake shaking as well as having a large damping

and relatively light (LPMB, 1985; Mardikanto, 2011;

Miller, 1999).

One of the buildings that use wood material as the

main structure is the traditional house of Siwaluh

Jabu, Batak Karo, North Sumatra. The selection of

materials to build the traditional house of Siwaluh

Jabu and the construction process that does not use

nails, iron, or wire fastener, but using pegs and fiber

ropes add the uniqueness of Traditional House of

Siwaluh Jabu (Sembiring, 2010). Survey results show

that the main structure of this traditional house uses

four types of timber namely, Haudolok wood, Ingul

wood, Pengki wood, and Simartolu wood (Nurdiah,

2011; Sembiring, 2010). A series of laboratory tests

was done to know the mechanical properties of the

wood used in making the traditional house of Batak

Karo.

This paper is objected to discuss the results of the

laboratory tests that covers the compression and

bending tests of the wood. The compression test was

conducted to find out the relationship of the density

with the elasticity modulus and the equivalent yield

stress. Three-point and four-point bending tests were

conducted to understand the relationship of the

density with the elasticity modulus, the maximum

bending stress, and the shear modulus. Teak wood

and Jackfruit wood from Java were selected as control

specimens.

2 THEORY

The theoretical basis in this paper is limited to very

closely related issues, i.e. the physical properties of

wood, the mechanical properties of wood, and the

mechanical test of wood (Miller, 1999; Suhardjono,

1994; SNI, 1961; SNI, 2000; Tjondro et al., 2013;

Mardikanto, 2011; Yosafat, 2014; Yoshihara, 1998).

Sarwidi, ., Artha, F., Prihatmaji, Y. and Widodo, .

The Compression Test and the Bending Test of Wooden Structural Material of Traditional Houses of Batak Karo, North Sumatera.

DOI: 10.5220/0010043103610370

In Proceedings of the 3rd International Conference of Computer, Environment, Agriculture, Social Science, Health Science, Engineering and Technology (ICEST 2018), pages 361-370

ISBN: 978-989-758-496-1

361

2.1 Physical Properties

The physical properties of wood are hygroscopic,

density, and specific gravity.

2.1.1 Hygroscopic

All of the physical properties of wood are strongly

influenced by changes in the water content of wood.

The amount of water contained in a piece of wood is

called the water content of wood (Ka). The weight of

wood on the dry state of the furnace is called dry

wood furnace (Wo). The weight of water present in

the wood is the difference between the weight of the

wood before it is dried (wet weight / initial weight =

Wb) minus the weight of the wood after it is dried by

the furnace. The above formula can be written as

follows.

𝐾𝑎











 100% (2.1)

2.1.2 Density

In this study, the weight used was dry weight of the

furnace / oven. So the wood density can be defined as

the ratio between the oven dry weight and the volume

of a piece of wood, i.e.:

𝑅

𝑊𝑜

𝑣

(2.2)

Where:

R : wood density (kg/m

)

Wo : oven dry weight (kg)

v : volume (m

)

2.1.3 Specific Gravity

Wood density is the ratio between wood density (on

the basis of dry weight of the furnace and volume

under various wood conditions) to the water density

at 4

C. The water has a density of 1 g / cm

or 1000

kg / m

at that standard temperature. Based on the

number, the density (R) and specific gravity (BJ) are

the same, but the specific gravity does not have units

because the specific gravity is a relative value which

can be determined by the following formula.

𝐵𝐽 

Densit

Water Densit

at 4 ͦ C

(2.3)

2.2 Physical Properties

The three wooden mechanical properties reviewed in

this paper are compression strength, elastic modulus,

and shear modulus.

2.2.1 Compression Strength

The compression strength of wood is the compression

force per unit area of compression. The formula can

be written as follows.

𝜎

𝑃

𝐴

(2.4)

where:

𝜎

: compression strength (kg/cm

)

P : compression force (kg)

A : compression area (cm

)

2.2.2 Elastic Modulus

The elastic modulus is a measure in which a material

or structure will be damaged and deformed when

placed under pressure. The elasticity modulus (E) can

be calculated by dividing the stress (𝜎) by strain (𝜀)

within the limit of linear elasticity on the part of the

stress-strain curve. The formula E can be written as

follows:

𝐸

𝜎

𝜀

(2.5)

2.2.3 Shear Modulus

The shear modulus (G) describes the tendency of an

object to deform at a constant volume when the object

is given opposing forces defined as shear stress to

shear strain. The shear modulus can be calculated

using the formula:

𝐺

𝐸

21  𝑣

(2.6)

where:

E : elastic modulus (N/mm

)

v : Poisson ratio

2.3 Mechanic Properties

Two kinds of test of mechanical properties in this

paper are compression test and bending test.

2.3.1 Compression Test

The compression test is carried out both fully and

partially in tangential and radial directions. To obtain

the elastic modulus (E) from the test applies Eq. (2.5).

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Technology

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2.3.1 Bending Test

There are two kinds of bending tests that are three-

point and four-point tests. A three-point bending test

is performed to obtain the elasticity modulus (E) and

MOR or maximum bending stress. To get the value

of E, first, sought the deflection in the middle of the

span by using the principle of the moment field as the

load (Figure 1).

Figure 1. Principle of the moment area as a pointed force

𝜎

 



3𝑃𝐿

2𝑏ℎ



(2.7)

δ 

𝑀

𝐸𝐼



𝑃𝐿



48 𝐸𝐼

(2.8)

𝐸 

𝑃𝐿



𝛿 48 𝐼

(2.9)

where:

δ = deflection

P = force

L = specimen length

E = elastic modulus

I = inertia moment

Then, a four-point bending test is performed to obtain

the elastic modulus (E) and MOR or maximum

bending stress. To get the value of E, first sought, the

deflection in the middle of the span by using the

principle of the moment field as the load (Figure 2).

The deflection formula can be seen in Eq. (2.8).

Figure 2. Sketch of loads for the four-point bending test

𝜎

 



𝑃𝐿

𝑏

ℎ





(2.10)



δ

𝑀

𝐸𝐼



𝑃. 𝑎. 𝑙



16𝐸𝐼





(2.11)



𝐸

𝑃. 𝑎. 𝑙



𝛿16𝐼





(2.12)

3 METHODS

In this research, wooden test specimens from

Sumatera are taken directly from Karo Regency,

North Sumatera, while the wooden test specimens of

Jackfruit wood and Teak wood using local wood

taken from Yogyakarta. The specimens were tested

using compression tests, three-point bending test, and

four-point bending test. All tests were performed on

specimens in the direction of radial and tangential

fibers.

3.1 Compression Test

The specimens of compression tests are prepared in 4

(four) sizes with each size is 40 x 40 x 40 mm, 40 x

40 x 80 mm, 40 x 40 x 120 mm, and 40 x 40 x 160

mm. Each of these sizes is prepared for the specimen

with the direction of radial and tangential wood

fibers. Total test object used is as much as 120 pieces

of specimen. The test scheme is carried out fully and

partially in the tangential and radial directions as

shown in Figure 3 which is then followed by a tested

test using the test machine as shown in Figure 4

The Compression Test and the Bending Test of Wooden Structural Material of Traditional Houses of Batak Karo, North Sumatera

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(Prihatmaji et. al, 2012). Type A denotes a full urgent

test and type B denotes partial test. The push load is

imposed on wooden specimens of type A and type B.

For partial urgent test (type B), a steel plate of 40 mm

width is placed in the center of the wooden specimen.

The loading procedure at test is a static load at a speed

of 0.5 mm / min, applied up to 4 mm deformation.

The objective test is to obtain the elasticity modulus

value and the yield stress equivalent of the six types

of wood observed.

Figure 3: Specimens of compression test. Full (A40) Partial

(B80, B120, and B160)

Figure 4: Photos of conducting tests

3.2 Bending Test

Wood specimens for three-point bending test and

four-point bending test were prepared each with a size

of 20 x 20 x 380 mm. The test specimens were

prepared in the direction of radial and tangential

wood fibers. For three-point bending test and four-

point bending test, a total of 42 test pieces were used.

Wood specimens for four-point shear test are

prepared each with a size of 20 x 20 x 220 mm with a

total of 24 test pieces. The test scheme and the testing

process can be seen in Figures 5 and 6 (Prihatmaji et.

al, 2012). Flexural testing is performed to obtain the

elasticity modulus (E), maximum bending stress or

MOR, and Shear Modulus (G) values.

Figure 5: Three-point bending test (3 PB) and Four-point

bending test (4 PB)

Figure 6: Photos of conducting tests. 3 PB (top). 4 PB

(bottom)

4 RESULTS AND DISCUSSION

This part presents the results of the laboratory

experiments and the discussion, both compression

and bending tests.

4.1 Compression Test

From the graph of the compression test results in

Figures 7 and 8, it can be seen that in the direction of

the radial fibers and the direction of the tangential

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fibers of Haudolok wood has the highest urgency

force among the 6 types of wood present, either at full

test (type A40) or on partial testing (type B80, B120,

B160). Exceptions occur in testing the direction of

tangential fiber type B160 where the graphics results

are inconsistent compared to other graphs. Haudolok

wood has a lower pressing strength than wood of

Jackfruit and Pengki wood.

Figure 7: The results of the full compression test and the

partial compression test in the radial fiber direction

Figure 8: The results of the full compression test and the

partial compression test in the tangential fiber direction

4.1.1 The Relationship of Density and

Elastic Modulus

Figure 9 shows the relationship of density and

Modulus of elasticity (E) in the direction of the radial

fibers. From linear regression on each graph got value

average

= 0.870. To facilitate an interpretation of the

strength of the relationship between two variables,

Sarwono (2006) gives the following criteria: 0: no

correlation between two variables; > 0 - 0.25: the

correlation is very weak; > 0.25 - 0.5: enough

correlation; > 0.5 - 0.75: strong correlation; > 0.75 -

0.99: very strong correlation; and 1: perfect

correlation. The correlation coefficient value of

average

= 0.870 is at interval > 0.75 - 0.99 (very strong

correlation) so it can be seen that the density has a

very strong relationship with the modulus of elasticity

(E). Ingul wood has the lowest density and E value,

while Haudolok wood has the highest density and E

value.

Figure 9: Graph of the relationship of density and elastic

modulus in radial fiber direction

Figure 10 Graph of the relationship of density and Modulus

of elasticity (E) in the direction of tangential fibers.

Figure 10 shows the relationship of density and

Modulus of elasticity (E) in the direction of tangential

fibers. From linear regression obtained value R

average

= 0.745. According to the criteria given by Sarwono

(2006), the correlation coefficient value R

average

0.7451 is at interval> 0.5 - 0.75 (strong correlation)

so it can be seen that the density has a strong

relationship with the modulus of elasticity (E). In the

tangential direction the Ingul wood has the lowest

density and E value, while the Haudolok wood has the

highest density and E value of type A40, B80 and

B120. Haudolok wood on type B160 has a different

behavior on the value of E, this is due to the

porousness in the test specimen used.

The Compression Test and the Bending Test of Wooden Structural Material of Traditional Houses of Batak Karo, North Sumatera

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4.1.2 The Relationship of Density and

Equivalent Yield Stress

Figure 11 shows the relationship between the density

and the equivalent yield stress of the radial fibers

direction. From linear regression on each graph got

correlation coefficient value R

average

= 0.912. The

correlation coefficient value of R

average

= 0.9121 is at

intervals> 0.75 - 0.99 (very strong correlation) so it

can be seen that the density has a very strong

relationship with the equivalent yield stress. Ingul

wood has the lowest equivalent yield stress, while

Haudolok wood has the highest equivalent yield

stress.

Figure 11: Graph of the relationship of density and

equivalent yield stress in radial fiber direction

Figure 12 shows the relationship of density and the

equivalent tensile stress to the tangential fiber. From

linear regression on each graph got correlation

coefficient value R

average

= 0.8365. The correlation

coefficient value R = 0.8365 is at intervals> 0.75 -

0.99 (very strong correlation) so it can be seen that

the density has a very strong relationship with the

equivalent of the melting stress. In the tangential

direction the Ingul wood has the lowest melting and

equivalent value of the melting stress, while the

Haudolok wood has the highest yield stress and

equivalent yield stress in the A40, B80 and B120

types. Haudolok wood on type B160 has a different

behavior on the equivalent value of yield stress, this

is due to the porousness in the test specimen used.

Figure 12: Graph of the relationship of density and

equivalent yield stress in tangential fiber direction

4.2 Three-point Bending Test

The following description will show the relationship

between density with elastic modulus, the equivalent

yield melting stress, and the shear modulus of the

three-point bending test results. Figure 13 shows the

photos of the conducting test.

Figure 13: Photos of the conducting tests

4.2.1 The Relationship of Density and

Elastic Modulus

Figure 14 shows the graph of the density relationship

and σ maximum flexure in the direction of radial

fibers. Linear regression on the graph yields a

correlation coefficient R value of 0.787. The

correlation coefficient value R = 0.787 is at interval>

0.75 - 0.99 (very strong correlation) so it can be seen

that the density has a very strong relationship with σ

maximum flexure. In the direction of Ingul wood

radial fibers have the lowest density and σ maximum

flexural value, while Haudolok wood has the highest

density and σ maximum flexural value. Figure 12b.

shows the graph of the density relationship and σ

maximum flexure in the direction of tangential fibers.

From the linear regression on the graph obtained

correlation coefficient value R of 0.589. The

correlation coefficient value R = 0.589 is at interval >

0.5 - 0.75 (strong correlation) so it can be seen that the

density has a strong relation with maximal flexure. In

the direction of Ingul wood tangential fiber has the

lowest density and σ maximum flexural stress, while

Pengki wood has the highest density, whereas wood

of Jackfruit has the highest maximum flexural stress.

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Figure 14: Graph of the relationship of density and elastic

modulus

4.2.2 The Relationship of Density and

Maximum Flexural Stress

Figure 15 shows the graph of the relation density and

σ bending of the radial fiber direction. From the linear

regression on the graph obtained correlation

coefficient value R of 0.787. The correlation

coefficient value R = 0.787 is at interval> 0.75 - 0.99

(very strong correlation) so it can be seen that the

density has a very strong relationship with σ

maximum flexure. In the radial fiber direction, Ingul

wood has the lowest density and σ maximum flexural

value, while Haudolok wood has the highest density

and σ maximum flexural value.

Figure 15: Graph of the relationship of density and

maximum flexural stress

Figure 15 shows a graph of the density relationship

and σ maximum flexure of tangential fiber direction.

From the linear regression on the graph obtained

correlation coefficient value R of 0.589. The

correlation coefficient value R = 0.589 is at interval>

0.5 - 0.75 (strong correlation) so it can be seen that

the density has a strong relation with maximal

flexure. In the direction of Ingul wood tangential fiber

has the lowest density and σ maximum flexural value,

while Pengki wood has the highest density value,

whereas wood of Jackfruit has the highest maximum

flexural stress.

4.2.3 The Relationship of Density and Shear

Modulus

Figure 16 shows the graph of the relation density and

shear modulus (G) of the direction of the radial fibers.

From linear regression on the graph got value of

correlation coefficient R equal to 0.681. The

correlation coefficient value R = 0.681 is at interval>

0.5 - 0.75 (strong correlation) so it can be seen that

the density has a strong relation with G. In the

direction of Ingul wood radial fiber has the lowest

density and G value, while Haudolok wood has the

highest density value, while wood of Jackfruit has the

highest G value. Figure 14b. shows the graph of the

relation density and shear modulus (G) of the

direction of the tangential fibers. From the linear

regression on the graph obtained correlation

coefficient value R of 0.447. The correlation

coefficient value R = 0.447 is at interval> 0.25 - 0.5

(enough correlation) so it can be seen that the density

has a strong relation with G. In the direction of Ingul

wood the tangential value has the lowest density and

G value, while Pengki wood has the highest density

value, while Haudolok wood has the highest G value.

Figure 16: Graph of the relationship of density and shear

modulus

4.3 Four-Point Bending Test

The following description will show the relationship

between density with elastic modulus, the equivalent

yield melting stress, and the shear modulus of the

four-point bending test results. Figure 17 shows the

photos of the conducting test.

Figure 17: Photos of the conducting tests

4.3.1 The Relationship of Density and

Elastic Modulus

Figure 18 shows the graph of the relation density and

modulus of elasticity (E) the direction of the radial

fibers. From the linear regression on the graph

obtained correlation coefficient value R of 0.717. The

correlation coefficient value R = 0.717 is at interval>

0.5 - 0.75 (strong correlation) so it can be seen that

the density has a strong relationship with the modulus

of elasticity (E). In the direction of Ingul wood radial

fiber has the lowest density and E value, while

The Compression Test and the Bending Test of Wooden Structural Material of Traditional Houses of Batak Karo, North Sumatera

367

Haudolok wood has the highest density and E value.

Figure 16b. shows the graph of the relation density

and the modulus of elasticity (E) of the direction of

the tangential fibers. From the linear regression on the

graph obtained correlation coefficient value R of

0.272. The correlation coefficient value R = 0.272 is

at interval> 0.25 - 0.5 (enough correlation) so it can

be seen that the density has a fairly strong relationship

with the modulus of elasticity (E). In the direction of

Ingul wood tangential fiber has the lowest density and

E value, while Haudolok wood has the highest E

value, while Pengki wood has the highest density

value.

Figure 18: Graph of the relationship of density and elastic

modulus

4.3.2 The Relationship of Density and

Maximum Flexural Stress

Figure 19 shows the graph of the density relationship

and σ maximum flexure of each type of wood in the

direction of radial fibers. From linear regression on

the graph got value of correlation coefficient R equal

to 0.829. To facilitate an interpretation of the strength

of the relationship between two variables, Sarwono

(2006) gives the following criteria: 0: no correlation

between two variables; > 0 - 0.25: the correlation is

very weak; > 0.25 - 0.5: enough correlation; > 0.5 -

0.75: strong correlation; > 0.75 - 0.99: very strong

correlation; and 1: perfect correlation. The correlation

coefficient value R = 0.829 is at interval> 0.75 - 0.99

(very strong correlation) so it can be seen that the

density has a very strong relationship with σ

maximum flexure. In the direction of Ingul wood

radial fibers have the lowest density and σ maximum

flexural value, while Haudolok wood has the highest

density and σ maximum flexural value.

Figure 19: Graph of the relationship of density and

maximum flexural stress

Figure 19 shows a graph of the density relationship

and σ maximum flexure of tangential fiber direction.

From the linear regression on the graph obtained

correlation coefficient value R of 0.713. The

correlation coefficient value R = 0.713 is at interval>

0.5 - 0.75 (strong correlation) so that it can be seen

that the density has a strong relationship with

maximal flexure. In the direction of Ingul wood

tangential fiber has the lowest density and σ

maximum flexural value, while Pengki wood has the

highest density value, whereas wood of Jackfruit has

the highest maximum flexural stress. Tests on radial

fiber direction with R correlation coefficient value of

0.829 has a stronger density and flexural relationship

is stronger than the test at the direction of tangential

fiber with the value of correlation coefficient R of

0.713.

4.3.3 The Relationship of Density and Shear

Modulus

Figure 20 shows the graph of the relation density and

shear modulus (G) of the direction of the radial fibers.

From the linear regression on the graph obtained

correlation coefficient value R of 0.717. The

correlation coefficient value R = 0.717 is at interval>

0.5 - 0.75 (strong correlation) so it can be seen that

the density has a strong relationship with G. In the

direction of Ingul wood radial fiber has the lowest

density and G value, while Haudolok wood has the

highest density and G value.

Figure 18b. shows the graph of the relation of density

and shear modulus (G direction of tangential fiber)

From the linear regression on the graph obtained

correlation coefficient R of 0. 272 that is at interval >

0.25 - 0.5 (enough correlation) so it can be seen that

the density has strong relation with G. In the direction

of Ingul wood tangential fiber has the lowest density

and G value, while Pengki wood has the highest

density value, while Haudolok wood has the highest

G value.

Figure 20: Graph of the relationship of density and shear

modulus

ICEST 2018 - 3rd International Conference of Computer, Environment, Agriculture, Social Science, Health Science, Engineering and

Technology

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5 CONCLUSIONS

The results and discussion in advance summarize the

following.

 At the radial fiber direction test it is found that

the density has a very strong correlation with

the elastic modulus (E), whereas in the

direction of the tangential fiber the density has

a strong correlation with the elastic modulus

(E). The density has a very strong correlation

with the equivalent of the melting stress in both

the radial and tangential fiber direction tests.

The higher the density value the higher the

value of E and the equivalent of the melting

stress. Haudolok wood has a density value,

elastic modulus (E), and the highest equivalent

of melting stress, whereas Ingul wood has

density value, elastic modulus (E), and lowest

melting equivalent. According to the

Indonesian Wood Construction Regulation

(PKKI) 1961 NI-5, based on σ maximum

bending of Haudolok and Jackfruit wood

entering in strong class I (very good), teak

wood entering strong class II (good), wood

Pengki enter in strong class III (enough), the

wood of Simartolu entered in strong class IV

(less), and Ingul wood entered in strong class V

(weak).

 In the 3-way bending test the radial fiber

density direction has a strong correlation with

the elastic modulus (E), whereas for the

tangential fiber direction the density has a fairly

strong correlation with the elastic modulus (E).

The density has a very strong correlation with

the maximum bending stress in the direction of

the radial fibers, whereas in the direction of the

tangential fiber the density has a strong

correlation with the maximum bending stress.

The density has a strong correlation with the

shear modulus (G) in the direction of the radial

fibers, whereas in the direction of the tangential

fiber the density has a fairly strong correlation

with the shear modulus (G).

 In the 4-way flexural test the radial fiber

density has a strong correlation with the elastic

modulus (E), whereas for the tangential fiber

direction the density has a strong correlation

with the elastic modulus (E). The density has a

very strong correlation with maximum bending

stress in the direction of the radial fibers,

whereas in the direction of the tangential fiber

the density has a strong correlation with the

maximum bending stress. The density has a

strong correlation with the shear modulus (G)

in the direction of the radial fibers, whereas in

the direction of the tangential fiber the density

has a fairly strong correlation with the shear

modulus (G).

ACKNOWLEDGEMENT

The authors thank to all persons and institutions that

have supported this study to become possible and

completed. The Study Program of Civil Engineering

and the Postgraduate Program of the Faculty of Civil

Engineering and Planning of Universitas Islam

Indonesia as well as partners from the Directorate of

Higher Education of the Republic of Indonesia and

the Laboratory of Structural Function of Kyoto

University, Japan have funded this work.

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