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

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.

Turkish Journal of Agriculture and Forestry Turk J Agric For (2014) 38: 291-297 © TÜBİTAK doi:10.3906/tar-1211-74 http://journals.tubitak.gov.tr/agriculture/ Research Article Bending moment capacity of simple and haunched mortise and tenon furniture joints under tension and compression loads 1, 1 Javane OKTAEE *, Ghanbar EBRAHIMI , Mohammad LAYEGHI , Mohammad GHOFRANI , Carl Albert ECKELMAN Department of Wood Science and Technology, Faculty of Natural Resources, University of Tehran, Karaj, Iran Department of Wood Science and Technology, Faculty of Civil Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran 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 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: oktaee@gmail.com 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 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 Load Moment arm Span/2 Span/2 a b Figure Method of loading the joints in tension (a) and compression (b) Table 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 30 293 OKTAEE et al / Turk J Agric For Table 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 ANOVA results for tension Source of variance Sum of square df Mean square F-value P-value Between shapes 58,911.484 19,637.161 43.423 0.000** Between thicknesses 8055.538 4027.769 8.906 0.000** Between lengths 110,643.168 55,321.584 122.329 0.000** Shapes × thicknesses 11,824.909 1970.818 4.358 0.001* Shapes × lengths 12,258.435 2043.073 4.518 0.001* Thickness × lengths 7357.895 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 ANOVA results for compression test Source of variance Sum of square df Mean square F Value Level of significance Between shapes 66,222.149 22,074.050 19.037 0.000* Between thicknesses 84,944.432 42,472.216 36.628 0.000* Between lengths 408,992.136 204,496.068 176.357 0.000* Shapes × thicknesses 65,369.649 10,894.942 9.396 0.000* Shapes × lengths 23,299.173 3883.196 3.349 0.006* Thickness × lengths 51,024.093 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 < 0.01 294 OKTAEE et al / Turk J Agric For Table 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 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 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 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 295 OKTAEE et al / Turk J Agric For a b Figure Mode of failure under tension loading (a) and compression loading (b) Figure 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 References Barboutis I, Meliddides T (2011) Influence of the time between machining and assembly of mortise and tenon joints on tension strength of T-type joints Ann WULS-SGGW For 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For a b Figure Mode of failure under tension loading (a) and compression loading (b) Figure Mode of failure in haunched tenon joints under compression loading convention that a tenon should be 1/3