Bending strength of large Japanese larch glued laminated timber

Wataru KAMBE Ph.D. Student Shinshu Univ. Nagano, Nagano, Japan t03h408@mail.shinshu-u.ac.jp

Graduate Student, System Development Engineering, Graduate School of Science and Technology (Doctor Degree Program), Shinshu University

Takuro MORI, Ph.D., Instructor, Wood Research Institute, Kyoto Univ., Uji, Japan Takeo HASHIZUME, Ph.D., Director, Nagano Prefectural General Forestry Research Center, Shiojiri, Japan Takashi TAKEDA, Ph.D., Assoc. Prof., Faculty of Agriculture, Shinshu Univ., Minamiminowa, Japan Akira SASAGAWA, Ph.D., Prof., Faculty of Engineering, Shinshu Univ., Nagano, Japan

Summary We conducted bending tests on large dimension structural glued laminated timber (glulam) composed of Japanese larch (Larix kaempferi, Carriere) lumber. The specimens were beams of two different dimension types. The first 7 beams were all 600[mm] deep and 150[mm] wide. The others of 7 were all 800[mm] depth and 150 [mm] wide. The difference between mean MOE of 24-ply and 32-ply glulams was very small (10.39 and 10.49 kN/mm2,). On the other hand, the mean MOR of 24-ply and 32-ply glulams was 28.2 and 31.8 N/mm2, respectively. The larger glulams were stronger than the smaller ones, and the test results for MOR were not consistent to so-called size effects (or depth effects) based on the weakest-ling theory. This phenomenon may be caused by the difference in failure modes between the two types. But the predicted values calculated with the prediction model proposed by Mori et al agreed with the tests results.

Keywords: Japanese larch, glued laminated timber (glulam), Bending Strength, Failure mode, Estimation Model

1. Introduction In Japan, the use for glulams has been increasing rapidly during the past decade. A glulam has been used to construct housing or large structure et al. For example, Odate-Juka-Dome is one of the largest structure to make from glulam built in the Akita prefecture in Japan. The large scale structure using glulam will be built in the future. Because the large wood structural techniques for building and manufacturing techniques for glulam have been improved. It is important that evaluate the strength of glulam precisely to build large wooden structure. Therefore the studies on the bending strength of glulam have been carried out before now (e.g. [1]). However it is not contented with the accumulation of data about the strength of large dimension glulam. It is now more important than ever to store the data of glulam strength. Our group therefore conducted bending tests with two types dimension glulam beams. This report presents bending strength (MOR), stiffness (MOE), and failure mode about these specimens. Additionally the estimating model proposed by Mori et al [3] ascertained whether calculate these specimens.

2. Experiments

2.1 Specimens Materials for static bending tests were large dimension structural glulams of symmetrical mixed –grade composition made of mechanically-graded Japanese larch (Larix kaempferi, Carriere). The tested beams were equivalent to E105-F300 specified in the Japanese Agriculture Standard for structural glued laminated timber (hereinafter referred to as “JAS”) [4]. We tested two types of beams: the first type was a 24-ply glulam that was 600[mm] thick, 150[mm] wide and 11,400[mm] long (hereinafter referred to as “h600”). The other type was a 32-ply glulam that was 800[mm] thick, 150[mm] wide and 15,000[mm] long (hereinafter referred to “h800”). Seven beams were prepared for each type.

2.2 Test Method The test method was recorded in JAS. Its outline is shown in Fig.1. For the bending experiment, we applied loads to two points on each beam. The load point was at a symmetrical position on either side. Each beam was supported on both ends. One end was supported by a pin support, and the other was supported by a roller support. The load point and support point on each beam was based on each beams thickness. As the figure shows, when that beam’s depth is “h”, the distance between the

two supports is 18h, the distance from one support to one load point is 7h, and the distance between two load points is 4h. As a result, the tests for “h600” had a support span of 10,800[mm] long. The tests for “h800” had a supports span of 14,400[mm]. To prevent the beam from falling over during the tests, we set jigs at two separate points to act as supports.

2.3 Measurement Method The loads were measured with using the load cell of the test-machine, displacement using an electric formula displacement meter, and each layer’s strain using strain- gauge. The points at F.J., the points where gauges were used are shown in Fig.2. The location where strain gauges were used is each layer at the midspan, the location left from the midspan 380[mm], and 760[mm]. A picture showing the experimental apparatus is shown in Fig.3. The dimensions and test conditions were shown in Table1.

"h800"

"h600"

Center

Strain Gage

・・・・

layer24th

layer1st

380380 50

F.J.

32th

F.J. ・・・・・・・

layer

layer1st

Fig.2 Locations of gauges Fig.3 Experimental apparatus

Fig.1 Load configuration

Displacement

Jig

Jig

LoadOut-of-plane

Out-of-plane

A-A'

A

18h

7h4h7h

A'

h

1003

3. Results

3.1 The relation between the load and the at midspan-displacement The typical of the relation between the load and at midspan-displacement is shown in Fig.4. The relationships were linear in the most cases, but narrow plastic area (circled with a dotted line in the right of Fig.4) appeared after the linear straight line in some cases.

3.2 Failure modes There were 3 major failure modes. The first mode of failures occurred at the finger joints (hereinafter referred to as “F.J.”). The second mode of failures occurred at knot. The third and final mode of failures occurred at points on the beams where no knots or other irregularities could be observed. The examples of failure modes are shown in Fig.5-7 (Fig.5: failure at F.J., Fig.6: failure at knots, Fig.7: failure at a spot where no knots and other irregularities could be observed). In “h600”, 6 specimens failed at F.J., and the last one failed at a spot where no knots or other irregularities were observed. On the other hand, in “h800”, 3 specimens failed at the F.J., and the remaining 4 specimens failed at knots. The Table2 shows failure modes for all specimens. (In this paper we refer to the top layer as being the “1st layer”. The bottom layer of the “h600” is referred to as being the “24th layer. The bottom layer of the “h800” is referred to as the “32nd layer”.)

An example of the relation between the load and the strain is shown in Fig.8. It was observed that there was a relationship between the failure modes and the strain distribution. As is shown on the left of Fig8, (“h600”), strain on the F.J. on the 24th layer was much larger than that at any other layer. However, as is shown on the right side of Fig8, (“h800”), strain at the F.J. on the 32nd layer was almost equal to that strain found at any other layer.

Table1 Dimensions and test conditionsBeam

Length [mm]Beam Width

[mm]Beam Depth

[mm]

LaminaThickness

[mm]

Number oflayers

Distance BetweenSupports [mm]

Distance BetweenLoads Points

[mm]

Number ofbeams

h600 11400 600 24 10800 2400 7

h800 15000 800 32 14400 3200 7150 25

Fig.5 Failure at the F.J.

0

50

100

150

200

250

0 50 100 150 200 250 300

Load[kN]

Center-Displacement[mm]

h600_2 Pmax：111.50[kN] δmax：97.45[mm]

h800_2 Pmax：175.34[kN] δmax：152.35[mm]

Linear Distribution

non-LinearDistribution

Fig.4 An example of relation between the load and midspan-displacement

3.3 MOR and MOE The value of the MOR (S), the apparent MOE (E), the real MOE (E0), the shear module (G), and the ratio of the shear modules to the real MOE (G/E0) were shown in Table 3. The average of “S” was 28.20[kN/mm2] in “h600” specimens, and was 31.80[kN/mm2] in “h800”. The coefficient of variance was 17.0[%] in “h600”specimens and 10.5[%] in “h800”. The average of “E” was 10.39[kN/mm2] in “h600” specimens, and was 10.49[kn/mm2] in ”h800”. The average of “E0” was 11.76[kN/mm2] in “h600” specimens, and was 11.94[kN/mm2] in “h800”. The average of “G” was 0.39[kN/mm2] in “h600” specimens, and was 0.32[kN/mm2] in “h800”. The MOR, the apparent MOE, the real MOE and the shear module were obtained following formula (1) to (4)

(1)

(2)

(3)

Fig.6 Failure at Knots Fig.7 Failure at not irregularities Table2 The list of failure points

h600 h600_1 h600_2 h600_3 h600_4 h600_5 h600_6 h600_7

failure point at F.J. ofthe 24th layer

at F.J. ofthe 24th layer

at F.J. ofthe 24th layer

at F.J. ofthe 24th layer

at F.J. ofthe 24th layer

at F.J. ofthe 24th layer

at a spot ofnot knots or

F.J.h800 h800_1 h800_2 h800_3 h800_4 h800_5 h800_6 h800_7

failure point at knot ofthe 32nd layer

at F.J. ofthe 31st layer

at knot ofthe 32nd layer

at F.J. ofthe 32nd layer

at F.J. ofthe 32nd layer

at F.J. ofthe 32nd layer

at knot ofthe 32nd layer

0

20

40

60

80

100

120

140

160

0 0.05 0.1 0.15 0.2 0.25 0.3

at 24th layerat 23rd layerF.J. at 24th layerF.J. at 23rd layer

Load[kN]

Strain[%]

h600_1

F.J. at 24th layerat 24th layer

F.J. at 23rd layerat 23rd layer

020406080

100120140160180200

0 0.05 0.1 0.15 0.2 0.25 0.3

at 32nd laterat 31st layerF.J. at 32nd layerF.J. at 31st layer

Load[kN]

Strain[%]

h800_1

Fig.8 The relation between the strain and load(Left:h600_1,Right:h800_1)

2

)(5.1bh

sLPbS −=

3

22

8)22)((

ybhsLsLsLPE

∆−+−∆=

���

����

���

���

�

∆∆=

ILLPE h

48

2

0 δ

1005

(4)

Pb: max load, L: distance between two support points, s: distance between two load points, b: width of beam, h: depth of beam, I: geometrical moment of inertia, κ: 1.2 ΔP: difference in load between elastic are,Δδ: displacement in relation toΔP Generally, it is said that MOR declines as depth increases. It is so-called size effects (or depth effects) based on the weakest-theory. But our test results were found to be the opposite of this. The value of (G/ E0) is stated in the Standard for Structural Design of Timber Structures as being 1/15[5]. The average of (G/E0) in “h600” was 1/30.2, and the average in “h800” was 1/36.8. The values that we recorded in our tests were smaller than that in recorded regulations.

3.4 The conformability with estimation model When making our estimates, we made 1,000 pieces of pretend glulams using the Monte-Carlo simulation. The comparison between the estimate model which Isoda and Mori proposed and the bending strength in this experiment is shown in Fig.9. We have used a cumulative frequency distribution to show this. The average bending strength of the estimate model was 28.8[kN/mm2] in “h600”. The average bending strength of the test beams was 28.2[kN/mm2]. The standard deviation

of estimate model was 3.49.The standard deviation of test was 4.78. The average bending strength of estimate model was 29.3[kN/mm2] in “h800”. The average of tests was 31.8[kN/mm2]. The standard deviation of estimate model was 3.23. The standard deviation of tests was 3.07.We found that the value of estimated model was almost equal to that of tests results.

Fig.9 Cumulative frequency of the bending strengths on model and tests

0

0.2

0.4

0.6

0.8

1

0 20 40

ModelTests

EstimationAve. : 28.8[N/mm2]Stdev. : 3.49

TestsAve. : 28.2[N/mm2]Stdev. : 4.78

Frequency

Strengh[N/mm2]

h600

0

0.2

0.4

0.6

0.8

1

0 20 40

ModelTests

EstimationAve. : 29.3[N/mm2]Stdev. : 3.23

TestsAve. : 31.8[N/mm2]Stdev. : 3.07

Frequency

Strengh[N/mm2]

h800

Table3 The list of strength

)43)((02

220

2

SLLEEEEhG

−−= κ

Specimen MOR (S) apparent MOE (E) real MOE (E0) Shear Modules(G) G/E0[N/mm2] [kN/mm2] [kN/mm2] [kN/mm2]

h600_1 26.0 10.45 11.73 0.30 1/39.1h600_2 26.0 10.36 10.79 0.80 1/13.5h600_3 26.8 10.23 11.16 0.38 1/29.4h600_4 27.1 10.45 11.16 0.51 1/21.9h600_5 25.5 10.48 - - -h600_6 26.8 10.32 13.38 0.14 1/95.6h600_7 39.0 10.40 12.31 0.21 1/58.6

Ave. 28.2 10.39 11.76 0.39 1/30.2h800_1 29.4 10.75 11.37 0.61 1/18.6h800_2 30.7 10.70 12.16 0.28 1/43.4h800_3 37.5 10.53 12.24 0.23 1/53.2h800_4 30.7 10.54 11.41 0.43 1/26.5h800_5 32.6 10.44 11.48 0.35 1/32.8h800_6 27.4 10.35 12.94 0.16 1/80.9h800_7 34.3 10.14 11.95 0.21 1/56.9

Ave. 31.8 10.49 11.94 0.32 1/37.3

3.5 Distribution of Strain An example of the strain distribution for at the midspan of “h600” was shown in fig.10. The value of the strain is in case of the Long-Range Load (“FB” in Fig.10) [5] and the maximum load (“max” in Fig.10). The average of Long-Range Load in “h600” is 37.4[kN], and the average in “h800” is 49.8[kN]. The figure shows that the value of the strain is in the straight line relation in case of the Long-Range Load. But the figure shows that disorder has occurred to the value of the strain in case of the maximum load.

4. Conclusions In this paper, we obtained the following results. (1) The difference between mean MOE of 24-ply and 32-ply glulams was very small (10.39 and

10.49 kN/mm2). (2) On the other hand, the mean MOR of 24-ply and 32-ply glulams was 28.2 and 31.8 N/mm2,

respectively. The larger glulams were stronger than the smaller, and the test results were not consistent to size effects phenomenon. The results should be affected by the differences in failure modes and stress distribution within the cross section.

(3) The prediction model for bending strength of structural glulams proposed by Isoda and Mori was applicable to the large dimension glulams.

Acknowledgment This study was supported by JSPS Grant-in-Aid for Scientific Research (Project # 14350299). References [1] For example: Isoda H., Mori T., and Sasagawa S., “The Resistance and Fracture Mechanisms of

Glulam Beam In Bending”, AIJ Journal of Structuctral and Construction Engineering, No.529, 2000, pp.13-20

[2] For example: Hayashi T. & et al, “Prediction of Bending Strength of Sugi Medium Dimension Glulam Beams by Monte-Carlo Simulation”, J. Soc. Mat. Sci.,Vol.51, No.4, pp.373-378, Apr. 2002

[3] Mori T., Isoda H., Sasagawa S., “The proposal of the model for estimating the flexural strength of the glulam beam and verification by experimental results”, AIJ Journal of Structure and Construction Engineering, No.541, 2001, pp.51-57

[4] Japan Plywood Inspection Corporation, “Japanese Agricultural Standard for structural glued laminated timber”, 2000

[5] Architectural Institute of Japan, “Standard for structural Design of Timber Structures”, MARUZEN CO., LTD., 2002

Fig.10 Example of strain distribution

0

100

200

300

400

500

600

-0.3 -0.2 -0.1 0 0.1 0.2 0.3Unit Strain[%]

Dis

tanc

e fr

om b

otto

m[m

m]

Load : FBLoad : max

h600_1