Turkish Journal of Agriculture and Forestry

Turk J Agric For
(2014) 38: 291-297
© TÜBİTAK
doi:10.3906/tar-1211-74

/>
Research Article

Bending moment capacity of simple and haunched mortise and tenon
furniture joints under tension and compression loads
1,

1

1

2

3

Javane OKTAEE *, Ghanbar EBRAHIMI , Mohammad LAYEGHI , Mohammad GHOFRANI , Carl Albert ECKELMAN
1
Department of Wood Science and Technology, Faculty of Natural Resources, University of Tehran, Karaj, Iran
2
Department of Wood Science and Technology, Faculty of Civil Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran
3
Department of Forestry and Natural Resources, Purdue University, Purdie, Indiana, USA
Received: 28.11.2012

Accepted: 10.06.2013

Published Online: 27.01.2014

Printed: 24.02.2014

Abstract: A study was conducted to examine the effects of tenon geometry on the bending moment capacity of simple and haunched
mortise and tenon joints under the action of both compressive and tensile loads. The effects of tenon width (25, 37.5, and 50 mm), tenon
thickness (7.5, 10, and 15 mm), and tenon length (20, 25, and 30 mm) were examined. All of the joints were constructed of Turkish
beech (Fagus orientalis Lipsky) and were assembled with a 40% solid-content polyvinyl acetate. Optimum results were obtained with
joints constructed with 10-mm-thick tenons that were 37.5 mm wide by 30 mm long. Tenon length was found to have the greatest effect
on joint capacity, whereas tenon width was found to have a much smaller effect. Joints constructed with 37.5-mm-wide haunched tenons
had essentially the same moment capacity as joints constructed with 37.5-mm simple tenons. Optimum tenon width was 10 mm (1/3
of rail thickness); joints constructed with 10-mm-thick tenons had greater capacity than joints constructed with either 7.5- or 15-mm
thick tenons.
Key words: Bending moment capacity, haunched, furniture joints, mortise and tenon joints

1. Introduction
Several researchers have defined the factors that affect the
bending moment capacity of mortise and tenon joints.
For instance, it has been shown that the highest strength
is achieved when a close tolerance between mortise and
tenon is maintained (Tankut, 2007), and a close-fitting
shoulder can basically increase the strength of the joints
(Eckelman et al., 2006). Furthermore, to obtain the best
strength, the glue should be applied to both parts of the
tenon and the sides of the mortise (Dupont, 1963), and
the delay of the joints’ assembly from the machining time
should be minimized (Barboutis and Meliddides, 2011).
Tests (Tankut and Tankut, 2005) have also shown

that joints with square tenons have 15% greater capacity
than similar joints constructed with round tenons. Finite
element analyses have indicated that joints constructed
with round or square tenons should behave similarly in
terms of stress and deflection (Mihailescu, 2001). Finally,
tests have also shown that joint capacities regularly
increase with increases in tenon width and length (Ishii
and Miyajima, 1981; Tankut and Tankut, 2005), and in
loose tenon joints, length of tenon has a significant effect
on withdrawal force capacity of the joints (Derikvand et
al., 2013).
*Correspondence:

Haunched mortise and tenon joints are widely used in
chair construction, but their performance characteristics
have not been determined, although it is commonly
believed that haunched tenons provide greater capacity
than simple tenons.
Although mortise and tenon joints have been replaced
by other constructions such as dowel joints in furniture
construction, they are simple to manufacture and are
still widely used by both small and large manufacturers,
and hence there is a need to define the parameters that
define their performance. There is also a need to evaluate
the performance characteristics of variations of the joint,
specifically the performance of haunched mortise and
tenon joints.
Accordingly, this study was undertaken to investigate
and compare the bending moment capacities (in
comp geometric tenon factors considered (shape, length,

and thickness) had highly significant effects on the bending
moment capacity of the joints; moreover, their interaction
effects were significant in both tests (Tables 3 and 4).
Duncan’s multiple range test was applied to determine
whether there was a significant difference among groups.
The homogeneous groups emerging at the end of the test
are given in Tables 5, 6, and 7.

Load

Moment arm

Span/2

Span/2

a

b

Figure 4. Method of loading the joints in tension (a) and
compression (b).

Table 1. Mean ultimate bending moment capacities of the mortise and tenon joints with their coefficients of variations (COVs) under
tension loading.
Tenon
width
(mm)
25


37.5
(simple)

50
37.5
(haunched)

Tenon
thickness
(mm)

Tenon length (mm)
Mean (Nm)

COV (%)

Mean (Nm)

COV (%)

Mean (Nm)

COV (%)

7.5

103.63

7.60


88.42

0.04

157.01

6.29

10

108.23

26.87

173.06

5.70

181.70

4.97

15

103.13

19.39

150.13


2.00

148.91

9.69

7.5

104.39

28.12

170.34

18.87

292.20

7.81

10

166.88

6.42

194.50

17.32


272.475

14.83

15

152.48

12.79

197.21

2.53

218.64

13.15

7.5

133.98

6.68

147.52

2.10

158.88


14.81

10

125.80

6.86

128.43

23.10

243.95

6.48

15

107.39

9.77

149.67

6.37

171.64

20.04


7.5

133.10

7.90

205.39

14.70

252.60

10.13

10

168.64

16.04

160.28

7.82

191.57

13.98

15


138.49

5.79

139.89

13.60

189.14

16.52

20

25

30

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OKTAEE et al. / Turk J Agric For
Table 2. Mean ultimate bending moment capacities of the mortise and tenon joints with their coefficients of variations (COVs) under
compression loading.
Tenon
width
(mm)
25

37.5

(simple)

50
37.5
(haunched)

Tenon
thickness
(mm)

Tenon length (mm)
Mean (Nm)

COV (%)

Mean (Nm)

COV (%)

Mean (Nm)

COV (%)

7.5

280.00

3.30

324.89


14.00

374.40

8.82

10

273.77

16.16

342.76

14.76

437.64

1.84

15

250.29

7.04

421.12

7.25


431.83

10.85

7.5

297.10

22.73

248.43

10.71

486.5

8.84

10

376.57

2.17

499.47

5.27

529.45


8.64

15

311.19

11.19

345.89

9.04

475.60

2.11

7.5

393.68

5.81

345.00

9.67

375.06

0.64


10

235.45

12.01

430.33

9.87

535.54

5.32

15

230.30

3.33

328.11

2.31

356.67

5.62

7.5


286.20

8.29

355.34

12.45

492.89

9.82

10

379.00

8.91

486.89

3.27

481.81

2.67

15

307.39


6.51

425.81

14.87

452.10

9.32

20

25

30

Table 3. ANOVA results for tension.
Source of variance

Sum of square

df

Mean square

F-value

P-value


Between shapes

58,911.484

3

19,637.161

43.423

0.000**

Between thicknesses

8055.538

2

4027.769

8.906

0.000**

Between lengths

110,643.168

2


55,321.584

122.329

0.000**

Shapes × thicknesses

11,824.909

6

1970.818

4.358

0.001*

Shapes × lengths

12,258.435

6

2043.073

4.518

0.001*


Thickness × lengths

7357.895

4

1839.473

4.068

0.005*

Shapes × thicknesses × lengths

33,752.548

12

2812.712

6.220

0.000**

Error

32,560.883

72


452.234

Total

3,205,603.531

108

*: Significant at P < 0.01.
Table 4. ANOVA results for compression test.
Source of variance

Sum of square

df

Mean square

F Value

Level of significance

Between shapes

66,222.149

3

22,074.050


19.037

0.000*

Between thicknesses

84,944.432

2

42,472.216

36.628

0.000*

Between lengths

408,992.136

2

204,496.068

176.357

0.000*

Shapes × thicknesses


65,369.649

6

10,894.942

9.396

0.000*

Shapes × lengths

23,299.173

6

3883.196

3.349

0.006*

Thickness × lengths

51,024.093

4

12,756.023


11.001

0.005*

Shapes × thicknesses × lengths

105,664.221

12

8805.352

7.594

0.000*

Error

83,488.023

72

1159.556

Total

1.631E7

108


*: Significant at P < 0.01.

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OKTAEE et al. / Turk J Agric For
Table 5. Results of Duncan’s test with respect to the shapes of tenons.
Bending moment capacity (Nm)
Under compression

Under tension

Tenon shapes

Duncan group

Mean

Duncan group

Mean

A

358.90

A

151.92


Small

B

396.69

D

196.57

Medium

A

348.52

B

134.91

Large

B

407.49

C

175.46


Haunched

Table 6. Results of Duncan’s test with respect to the lengths of tenons.
Bending moment capacity (Nm)
Under compression

Under tension

Duncan group

Mean

Duncan group

Mean

Tenon lengths (mm)

A

301.74

A

128.84

20

B


369.63

B

158.62

25

C

452.46

C

206.56

30

Table 7. Results of Duncan’s test with respect to the thicknesses of tenons.
Bending moment capacity (Nm)
Under compression

Under tension

Duncan group

Mean

Duncan group


Mean

Tenon thicknesses (mm)

A

354.96

A

162.29

7.5

B

417.39

B

176.29

10

A

361.36

A


155.56

15

In both tension and compression tests, most failures
occurred due to glue line failure (Figure 5). In contrast,
joints with haunched tenons loaded in compression failed
owing to tension perpendicular to grain failure of the wood
at the top of the post (Figure 6), which tends to indicate
that the strength property of tension perpendicular to the
grain needs to be considered in the selection of woods for
haunched joints.
4. Discussion
Considering the width of tenons, the greatest bending
moment capacities were obtained with joints that had
37.5-mm-wide tenons. The capacity of joints with
37.5-mm-wide tenons was 29.4% greater than joints with
25-mm-wide and 46% greater than those with 50-mm-

wide tenons. It can be explained that the 50-mm-wide
tenons displayed the lowest strength as in these joints the
upper side of the mortise was open and thus the mortise
could not fully support the tenon. In this type of joint,
tenons are partially excluded from the mortise under
loading. According to Erdil (2005), joints with greater
width show more bending strength, which is in agreement
with the results of this study when comparing joints with
37.5-mm and 25-mm widths.
Analysis of the data for tension loading (for simple
tenons), in Table 7, indicates that the highest capacities

were obtained with 10-mm-thick tenons: joints with
10-mm-thick tenons had 8.6% and 13.3% greater capacity
than joints constructed with 7.5-mm- and 15-mm-thick
tenons, respectively. This result tends to confirm the

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OKTAEE et al. / Turk J Agric For

a

b

Figure 5. Mode of failure under tension loading (a) and compression loading (b).

Figure 6. Mode of failure in haunched tenon joints under compression loading.

convention that a tenon should be 1/3 the thickness of
the rail. Tenons with 7.5-mm thickness are thin and are
susceptible to failure under load. According to Eckelman
(2003), tenon thickness has an important effect on bending
moment of mortise and tenon joints, and with an increase
in tenon thickness, bending strength will successively
improve.
Tenons with 15-mm thickness have smaller shoulders
and, on the basis of Eckelman et al.’s (2004) studies, the
shoulders have great effect on the bending moment
capacity of the joints; thus, the size of the shoulders can
be a restrictive factor for increasing the tenon thickness.

Likewise, in the case of tenon length, the greatest capacities
were obtained with joints that had 30-mm tenons: joints
with 30-mm tenons had 61% greater capacity than those
with 20-mm tenons and 30.2% greater capacity than those
with 25-mm tenons. This result is in agreement with the
results reported by Tankut and Tankut (2005).
Overall, in the joints constructed with simple tenons,
the highest bending moment capacities were obtained

296

with tenon widths that were 3/4 the width of the rail.
Likewise, highest capacities were obtained with joints in
which tenon thickness was 1/3 the rail thickness. Joint
capacity was closely linked to tenon length; a 25% increase
in tenon length from 20 to 25 mm increased joint capacity
by 23%. Likewise, an increase in tenon length from 25 to
30 mm increased joint capacity by 30%. Haunched tenons
had only slightly greater capacity than comparable simple
tenons under compressive loads (which Duncan tests
showed to be insignificant) and less capacity (90%) under
tension loads.
Acknowledgments
The financial support of the University of Tehran is
gratefully acknowledged. This research was carried
out partially at the Department of Wood Science and
Technology at the University of Tehran and at the
Department of Wood Science and Technology at the
University of Shahid Rajaee, Tehran, Iran.



OKTAEE et al. / Turk J Agric For
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