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.
Trang 1http://journals.tubitak.gov.tr/agriculture/ (2014) 38: 291-297
© TÜBİTAK doi:10.3906/tar-1211-74
Bending moment capacity of simple and haunched mortise and tenon
furniture joints under tension and compression loads
Javane OKTAEE 1, *, Ghanbar EBRAHIMI 1 , Mohammad LAYEGHI 1 , Mohammad GHOFRANI 2 , Carl Albert ECKELMAN 3
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
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)
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Although mortise and tenon joints have been replaced
by other constructions such as dowel joints in furniture construction, they are simple to manufacture and are still widely used by both small and large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 compression and tension) of several configurations of mortise and tenon joints constructed with a) simple and b) haunched tenons One purpose of the tests was to investigate whether joints with haunched tenons have greater bending moment capacity than joints constructed with the more commonly used simple rectangular tenons
A second purpose was to determine the effect of tenon and mortise geometry on the bending moment capacity of
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
Received: 28.11.2012 Accepted: 10.06.2013 Published Online: 27.01.2014 Printed: 24.02.2014
Research Article
Trang 2joints constructed with both type of tenons, when loaded
in either compression or tension
2 Materials and methods
2.1 Plan of study
Four groups of joints were constructed with tenon widths
of 25, 37.5 (simple), 50, and 37.5 (haunched) mm For
each tenon width, 3 sets of specimens were constructed
with tenon widths of 7.5, 10, and 15 mm For each of these
tenon widths, 3 sets of specimens were constructed with
tenon lengths of 20, 25, and 25 mm Six joint replications
were constructed for each category so that the total number
of specimens constructed was 6 replications × 4 tenon widths × 3 tenon thicknesses × 3 tenon lengths, or 216 specimens Half (108) of these specimens were prepared for compression tests and half for tension tests
2.2 Preparation of specimens
The nomenclature of the specimens is given in Figure 1 and the dimensions of corner joints in Figure 2 Four different configurations of tenons in L-shaped corner joints that were investigated in this study are shown in Figure 3 All
of the specimens were constructed of defect-free, straight-H.L
T.T
T.L
M.T
M.L
50 mm 100 mm
30 mm
50 mm
20-25-30 mm 20-25-30 mm
20-25-30 mm
20-25-30 mm
a
d c
b
6-8-10 mm
Figure 1 Nomenclature of a haunched tenon.
Abbreviations: T.W., Tenon width; T.T., Tenon thickness; T.L.,
Tenon length; H.L., Haunched part length; M.L., Member length;
M.T., Member thickness; M.W., Member width.
Figure 2 Dimensions of corner joints used in this study.
Figure 3 Geometries of various experimental joints tested in this study: large simple
(a); medium simple (b); small simple (c); and haunched tenon joints (d).
Trang 3grained beech (Fagus orientalis Lipsky); moisture content
of the wood was a nominal 8% Joints were assembled with
a 40% solid polyvinyl acetate adhesive
Tenons were cut with a table saw, whereas mortises
were machined with a horizontal router operating between
end stops Tenons were then hand-sanded to provide an
average mortise/tenon clearance of 1 mm Both the tenons
and the mortise walls were coated with adhesive During
assembly, a bar clamp was used to fully seat the tenons in
the mortises and ensure full contact between the tenon
shoulder and the face of the mortised post Following
assembly, the joints were conditioned for 14 days at 22
°C and 65% relative humidity to bring them to an average
equilibrium moisture content of 12%
2.3 Test procedure
Tension tests were conducted as shown in Figure 4a and
compression tests as shown in Figure 4b All tests were
performed on a computer-controlled INSTRON machine (Model 4486) according to the method represented and used by Eckelman and Lin (1997) Rate of machine-head loading in both cases was 6 mm/min Ultimate load was taken as the point at which a nonrecoverable drop-off in load occurred
3 Results
Machine loads were converted into bending moments by means of the following expressions:
In tension loading:
M = P × L1/2 (1)
In compression loading:
M = P × L2 (2) Here, M refers to the bending moment capacity (Nm),
P refers to the machine load (N), L1 refers to the distance between reaction supports (0.07 m), and L2 refers to the distance from the line of action of the machine load
to the point of intersection of the centerlines of the rail and post (0.07 m) Average ultimate bending moments obtained from the tension are given in Table 1 and those for compression tests in Table 2
Standard analysis of variance (ANOVA) methods were applied separately to the tension and compression data The 3 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 Load
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 length (mm) Tenon
thickness
(mm)
Tenon
width
(mm) 20Mean (Nm) COV (%) 25Mean (Nm) COV (%) 30Mean (Nm) COV (%)
6.29 157.01
0.04 88.42
7.60 103.63
7.5
9.69 148.91
2.00 150.13
19.39 103.13
15
7.81 292.20
18.87 170.34
28.12 104.39
7.5
37.5
13.15 218.64
2.53 197.21
12.79 152.48
15
14.81 158.88
2.10 147.52
6.68 133.98
7.5
20.04 171.64
6.37 149.67
9.77 107.39
15
10.13 252.60
14.70 205.39
7.90 133.10
7.5
37.5
16.52 189.14
13.60 139.89
5.79 138.49
15
Trang 4Table 2 Mean ultimate bending moment capacities of the mortise and tenon joints with their coefficients of variations (COVs) under
compression loading.
Tenon length (mm) Tenon
thickness
(mm)
Tenon
width
(mm) 20Mean (Nm) COV (%) 25Mean (Nm) COV (%) 30Mean (Nm) COV (%)
8.82 374.40
14.00 324.89
3.30 280.00
7.5
10.85 431.83
7.25 421.12
7.04 250.29
15
8.84 486.5
10.71 248.43
22.73 297.10
7.5
37.5
2.11 475.60
9.04 345.89
11.19 311.19
15
0.64 375.06
9.67 345.00
5.81 393.68
7.5
5.62 356.67
2.31 328.11
3.33 230.30
15
9.82 492.89
12.45 355.34
8.29 286.20
7.5
37.5
9.32 452.10
14.87 425.81
6.51 307.39
15
Table 3 ANOVA results for tension.
P-value F-value
Mean square df
Sum of square Source of variance
0.000**
43.423 19,637.161
3 58,911.484
Between shapes
0.000**
8.906 4027.769
2 8055.538
Between thicknesses
0.000**
122.329 55,321.584
2 110,643.168
Between lengths
0.001*
4.358 1970.818
6 11,824.909
Shapes × thicknesses
0.001*
4.518 2043.073
6 12,258.435
Shapes × lengths
0.005*
4.068 1839.473
4 7357.895
Thickness × lengths
0.000**
6.220 2812.712
12 33,752.548
Shapes × thicknesses × lengths
452.234 72
32,560.883 Error
108 3,205,603.531
Total
*: Significant at P < 0.01.
Table 4 ANOVA results for compression test.
Level of significance
F Value Mean square
df Sum of square
Source of variance
0.000*
19.037 22,074.050
3 66,222.149
Between shapes
0.000*
36.628 42,472.216
2 84,944.432
Between thicknesses
0.000*
176.357 204,496.068
2 408,992.136
Between lengths
0.000*
9.396 10,894.942
6 65,369.649
Shapes × thicknesses
0.006*
3.349 3883.196
6 23,299.173
Shapes × lengths
0.005*
11.001 12,756.023
4 51,024.093
Thickness × lengths
0.000*
7.594 8805.352
12 105,664.221
Shapes × thicknesses × lengths
1159.556 72
83,488.023 Error
108 1.631E7
Total
*: Significant at P < 0.01.
Trang 5In 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-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
Table 5 Results of Duncan’s test with respect to the shapes of tenons.
Bending moment capacity (Nm)
Tenon shapes Under tension
Under compression
Mean Duncan group
Mean Duncan group
Small 151.92
A 358.90
A
Medium 196.57
D 396.69
B
Large 134.91
B 348.52
A
Haunched 175.46
C 407.49
B
Table 6 Results of Duncan’s test with respect to the lengths of tenons.
Bending moment capacity (Nm)
Under tension Under compression
Tenon lengths (mm) Mean
Duncan group Mean
Duncan group
20 128.84
A 301.74
A
25 158.62
B 369.63
B
30 206.56
C 452.46
C
Table 7 Results of Duncan’s test with respect to the thicknesses of tenons.
Bending moment capacity (Nm)
Under tension Under compression
Tenon thicknesses (mm) Mean
Duncan group Mean
Duncan group
7.5 162.29
A 354.96
A
10 176.29
B 417.39
B
15 155.56
A 361.36
A
Trang 6convention 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
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
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.
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