STRENGTH PROPERTIES OF THE MOST FREQUENT
CORNER AND MIDDLE JOINTS OF UPHOLSTERED
FURNITURE FRAMES CONSTRUCTED WITH BEECH AND
POPLAR SOLID WOOD
Vasiliki Kamperidou1 and Vassilios Vassiliou2
Aristotle University of Thessaloniki, Faculty of Forestry and Natural
Environment, Department of Harvesting and Forest Products Technology
1. Wood technologist, MSc, email:vkamperi@for.auth.gr
2. Wood technologist, PhD, Associate Professor, email:vass@for.auth.gr
Thessaloniki 2010
Abstract
This study was carried out to evaluate the strength of the four most frequent joints in
the upholstered furniture frames, made of beech and poplar solid wood. The research
included the following joints: Mortise and Tenon, double Dowel, Corner Blocks and
double Gusset Plates, which were constructed and tested both in corner and middle
joints. In the corner joints we examined the compression strength and the modulus of
elasticity of the joints and in the middle joints the tension strength was thoroughly
investigated. The results of the first test (compression strength) indicated that the
strongest corner joint was the double Dowel joint of beech wood (1179.6 N). The
strength of Mortise and Tenon joint was proved to be less powerful (1063.6 N) and
the joint with wooden Corner Blocks resulted in even weaker values (683 N). The
joint with double Gusset Plates appeared to have the lowest strength (529.6 N). The
highest modulus of elasticity proved to have derived from the joint with wooden
Corner Blocks. The double Dowel joint resulted in slightly lower elasticity values and
the joint with double Gusset Plates in even lower modulus of elasticity, whereas the
Mortise and Tenon joint resulted in the lowest elasticity values of all. On the other
hand, the highest tension strength appeared to have been a feature of the Mortise and
Tenon joint (5438.4 N). Lower tension strength values were measured in the joint
with Corner Blocks (4974.5 N) and even lower tension strength proved to have come
from the double Dowel joint (4542 N). The last one in tension strength was the joint
with double Gusset Plates (3863.5 N). Generally, in most cases beech wood resulted
in stronger joints compared to poplar wood.
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Key words: Corner Block, corner joint, Dowel, Gusset, middle joint, Mortise and
Tenon, upholstered furniture
INTRODUCTION
The upholstered furniture includes a wide range of furniture, which, apart from the
wooden frame, also comprises other groups of materials, such as those for investment
and support. These materials provide the user with comfort, support of the human body
and functionality of the furniture. The upholstered furniture contributes greatly to the
human needs and also represents an integral part of the human every-day life. The
strength and stability of this type of furniture depend on the strength and stability of its
joints (Eckelman, 2003). There is a high interest in the research of the strength of the
wooden frame and particularly the strength of the joints of this furniture. The strength
and stability of a piece of furniture really depend on the design of its pattern and its
joints, in other words, the way in which two or more wooden parts are put together and
connected (Efe et al. 2004, Wang 2007 ).
The present research deals with the investigation of the strength of the four most
substantial and frequent corner and middle joints of the upholstered furniture, which
are: Mortise and Tenon joint, double Dowel joint, joint with wooden Corner Blocks
and the double Gusset Plates joint. The Mortise and Tenon joint and the double Dowel
joint represent two main and traditional types of joints of upholstered and other sorts of
furniture, which have been studied by many researchers (Hill and Eckelman 1973,
Paulenkova 1984, Smardzewski 2002, Tankut 2006, Zhang and Eckelman 1993), while
the strength and the mechanical behavior of the other two types of joint of this
research, the one with Corner Blocks and the double Gusset Plates joint have not been
thoroughly studied so far and are relatively innovative types of connection in furniture
(Kazal et al. 2006, Erdil et al. 2003). The knowledge and understanding of the factors
that affect the joint strength can be used for joint improvements in upholstered
furniture construction.
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MATERIALS AND METHODS
Experiments were carried out with beech wood (Fagus sylvatica) and poplar wood
(Populus sp.), both of Greek origin and naturally desiccated for one year. Half of the
samples were constructed in corner form (L-shape), and the rest in middle form (Tshape). The corner joint samples were tested in compression (Bending Capacity and
Modulus of Elasticity), and the middle joint samples were tested in tension (Tension
strength).
The dowels used in the present research were made of steamed beech wood, 12 mm in
diameter, with dapple surface. The adhesive used was Polyvinyl Acetate PVAc Ν203,
class of D3 and its trade name is MERKOLA. The staples were of Greek origin and
construction, with length of 4.5 cm and 3.5 cm.
The configuration of the tested joints is shown in the following Figures from 1 to 8.
Fig.1 Double Dowel corner joint
Fig.2 Mortise and Tenon corner joint
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Fig.3 Corner Block corner joint
Fig.4 Double Gusset Plates corner joint
Fig.5 Double Dowel middle joint
Fig.6 Mortise and Tenon middle joint
Fig.7 Corner Blocks middle joint
Fig.8 Double Gusset Plates middle joint
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All the specimens consisted of two wooden parts, the horizontal and the vertical
one. The samples for the bending strength were constructed in dimensions as
follow: 150 mm length x 50mm width x 25mm thickness for the horizontal parts,
while the dimensions of the vertical parts were 125mm length x 50mm width x
25mm thickness. These dimensions refer to corner and also middle forms of
specimen and are based on the pre-existent research, because of lack of particular
international standards on the method of the examination of strength of corner and
middle joints.
Concerning the double Dowel joint, it must be mentioned that the dimensions of
the dowel were: 40mm length x 12mm in diameter and the space between the two
dowels was 13mm. In the construction of the Mortise and Tenon joint it was
determined to maintain the contact between the mortise and tenon only in the two
surfaces of the mortise, because the mortise cut is semi-cylindrical, whereas the
tenon has square cut. In regard to the construction of the Corner Block joint, corner
blocks with perpendicular sides of 30mm were used. Also, each of the corner
blocks was fixed by two staples. On the part of the construction of the double
Gusset Plates joint, two wooden gusset plates were used alongside the connection
place and each gusset plate was stabilized with the help of two staples.
The amount of the glue used was the same in each joint (about 4 cm3). In the
construction of the joints, pressure was not applied, except for those which
included the use of staples (joint with Corner Block and joint with double Gusset
Plates).
For every joint 10 specimens were tested, except for the Mortise and Tenon joint,
where 5 specimens were used. By using four types of joints, two shape of joints
and two wood species, a total of 190 specimens were prepared.
After their construction, the specimens were placed into a conditioned room at 20o
C temperature and 65% relative humidity and were allowed to reach a nominal
1340
equilibrium moisture content (EMC) of 10%. At the moment of the tests, the mean
density of the beech wood was measured as 0.702 g/cm3 and of the poplar wood
0.364 g/cm3, while the mean moisture content was 9.55 % for the beech wood and
9.29 % for the poplar wood.
All the tests were carried out on a Universal Testing Machine (SHIMADZU UH300kNA), and the rate of crosshead-movement was adjusted at 8 mm/min, so that
the maximum load was reached within 1.5±0.5 min throughout the test. The
loading continued until a break of the joint occurred (Figures 9 and 10).
Fig. 9 and 10. Loading in compression of the corner joint specimens (left),
Loading in tension of the middle joint specimens (right)
After the measurements, the bending capacity (N.mm), the modulus of elasticity
(MOE) (N/mm2) of corner joints and the tension strength (N/mm2) of the middle
joints were calculated, using the following equations:
¾ Bending Moment Capacity (N.mm) = Maximum Rupture Load (N) x
Moment Arm (mm)
¾ Modulus of Elasticity (N/mm2) = Load at the Elastic Limit (Ν) x Space
between the two holding points of the joint (mm) / Area of the transverse cut
surface (mm2) x Deformation at the Elastic Limit (mm).
1341
¾ Tension Strength (N/mm2) = Maximum Rupture Load (N) / Area of the
transverse cut surface (mm2)
RESULTS AND DISCUSSION
The results of the strength properties tested are indicated in the table below.
Table 1 Bending strength of the corner form joints
Maximum Bending
Stand.
Variance
rupture load capacity
Deviation
2
s
(Ν)
(Ν.mm)
s
1179.6
104164.5 30637879.2 5535.1
875.4
77297.8 29968126.4 5474.3
Joint Type
Wood species
Double Dowel
Beech
Poplar
Mortise and Tenon
Beech
Poplar
1063.6
374.4
93915.8
33059.5
28652011.3
19529650.0
5352.7
4419.2
683.0
745.6
60308.9
65836.4
30523958.0
4531205.9
5524.8
2128.6
Corner Block
Beech
Poplar
Beech in the parts
and Poplar in
corner block
653.6
57712.8
34907889.3
5908.2
Beech
Poplar
Poplar in the parts
and Beech in
gussets
529.6
499.8
46763.6
44132.3
22356629.1
21820549.6
4728.2
4671.2
483.4
42684.2
23176168.8
4814.1
Double Gusset
Plates
According to the table above, the corner joints of double Dowel, made of beech wood
excelled greatly in compression strength, compared to all the other joints of the
research, with maximum rupture load of 1179.6 N. A little lower values were measured
in the Mortise and Tenon joint, made of beech (1063.6 Ν). After that, the one that
followed in bending strength, was the double Dowel joint made of poplar (875.4Ν),
which means that this specific joint (double Dowel) exhibits satisfying strength
whether we use beech wood or poplar wood that undoubtedly is a material of lower
strength because of its lower density.
1342
Even lower values of compression strength were given by the Corner Block joint made
of poplar (745.6Ν), which were better than those of the same joint made of beech (683
Ν). The difference between the values can be explained, probably, because of the better
agglutination of the surfaces of poplar wood, because of its low density. A little lower
value (653.6N) was given by the Corner Block joint, whose parts were made of beech,
while the corner block was made of poplar. It seemed that the idea of using different
wood species in the corner block did not mark satisfying results, compared to the other
two versions of this joint.
Lower compression strength (529.6Ν) was demonstrated by the double Gusset Plates
joint made of beech, whereas the same joint made of poplar has shown even lower
strength (499.8N). In this case, the combination of two different wood species (poplar
in the parts and beech in gussets) did not reveal higher strength (483.4N). In general,
the joint with the double Gusset Plates did not give the expected high strength and
stability and maybe that is the reason, why it is chosen by the furniture manufacturers
only as a supplementary and additional way of reinforcement of other basic
connections.
Very low compression strength among all the joints appeared to be a feature of the
Mortise and Tenon joint made of poplar (374.4N). The Mortise and Tenon joint is one
of the most durable and stable joints, which is recommended to be used in
constructions of solid wood, but in the case of this research, the double Dowel joint
seemed to be of much higher strength, while the Mortise and Tenon joint made of
poplar measured the lowest strength of all the joints. That can be easily explained by
the fact that the construction of the Mortise and Tenon joint is made in such a way, that
the tenon has square cut, while the mortise has semi-cylindrical cut, which causes a
reduced contact between the two wooden parts and a reduction in the glued surface.
Table 2 Variance analysis result of compression strength of corner joints
ANOVA
Source of Variance
Sum of
Squares
D.F.
Mean
Square
F-ratio
Sign. Level
1343
Between Groups
3.723E10
9
4.136E9
Within Groups
1.943E9
79
2.460E7
Total
3.917E10
88
168.141
.000
According to variance analysis results, differences in groups were statistically
significant with a 5% error. It was recorded that there were statistically significant
differences among all the joints, except for the following cases: The Corner Block joint
made of beech gave similar values to the same joint made of poplar and the
combination of beech and poplar. Also, similar strength seemed to have been obtained
by the three versions of double Gusset Plates joint: the one made of beech, the one
made of poplar and the one made of the combination of the two species.
BENDING (COMPRESSION) STRENGTH OF CORNER JOINTS
BENDING MOMENT CAPACITY
Ν.mm
120000
100000
80000
60000
40000
20000
0
TYPE OF JOINT
Double Dowel Beech
Mortise-Tenon Poplar
Corner Block Beech-Poplar
Gusset Plates Poplar-Beech
Double Dowel Poplar
Corner Block Beech
Gusset Plates Beech
Mortise-Tenon Beech
Corner Block Poplar
Gusset Plates Poplar
Fig.11 Compression strength of corner joints
Table 3 Modulus of elasticity of the corner joints, measured in bending test
Joint Type
Wood species
Modulus of
Elasticity
(N/mm2)
Variance
s2
Stand.
Deviation
s
1344
Double Dowel
Beech
Poplar
42.5
26.9
86.3
35.3
9.2
5.9
Mortise and
Tenon
Beech
Poplar
33.2
28.7
50.9
148.8
7.1
12.2
Beech
Poplar
59.3
53.1
145.2
36.5
12.0
6.0
Beech in the parts and
Poplar in corner block
58.4
161.7
12.7
22.7
33.6
126.7
63.9
11.2
7.9
40.5
91.4
9.5
Corner Block
Double Gusset
Plates
Beech
Poplar
Poplar in the parts
and Beech in gussets
The Modulus of Elasticity (MOE) of each joint is recorded and calculated through the
relation of the load which acts on the joint, with the simultaneous deformation, which
is caused by that. According to the results, the highest MOE was measured in the
Corner Block joint made of beech (59.3 Ν/mm2), while the Corner Block joint made of
the combination of two wood species (beech in the parts and poplar in corner block)
marked 58.4 Ν/mm2. A little lower elasticity values (53.1 Ν/mm2) were recorded in the
Corner Block joint, made solely of poplar wood. So, it is concluded, that the Corner
Block joint, independently of the material it is made of, can give satisfying elasticity
values.
Lower Modulus of Elasticity was measured in the double Dowel joint made of beech
(42.5 Ν/mm2) and even lower elasticity characterized the joint of double Gusset Plates
made by the combination of two species (poplar in the parts and beech in gussets) with
40.5 Ν/mm2. Even lower elasticity was recorded in the same joint made solely of
poplar (33.6 Ν/mm2). At the same level of elasticity fluctuate the figures in Mortise
and Tenon joint made of beech with 33.2 Ν/mm2.
The Mortise and Tenon joint made of poplar appeared to have a little lower ΜOE (28.7
Ν/mm2) and even lower elasticity was present in the joint of double Dowel made of
1345
poplar (26.9 Ν/mm2). Consequently, the double Dowel joint is better to be constructed
of beech, particularly in the case of furniture construction, where high value of
elasticity is essential.
The double Gusset Plates joint, made of beech follows with MOE (22.7 Ν/mm2), which
is the lowest value of elasticity compared to all the other joints of this research.
Table 4 Variance analysis result of modulus of elasticity in compression strength of
corner joints
ANOVA
Source of
Variance
Between
Sum of
Squares
D.F.
Mean
Square
14525.055
9
1613.895
Within Groups
7238.600
77
94.008
Total
21763.655
86
Groups
F-ratio
Sign. Level
17.168
.000
According to variance analysis results, statistically significant differences were
observed between the following joints: The elasticity value of the double Dowel joint
made of beech differed significantly from the one of the same joint made of poplar, the
Corner Block joint, irrespectively of the material it was made of, and also the double
Gusset Plates joint made of beech. The modulus of elasticity of all the versions of
Corner Block joint presented statistically significant differences from the
correspondent value of the double Dowel joint made of poplar and the Mortise and
Tenon joint, independently of the material which it was constructed of. Furthermore,
the elasticity value of the Gusset Plates joint made of beech appeared to be statistically
different from the one of the same joint, made by the combination of poplar and beech.
1346
MODULUS OF ELASTICITY OF CORNER JOINTS IN COMPRESSION TEST
MODULUS OF
ELASTICITY (Ν/mm2)
70
60
50
40
30
20
10
0
TYPE OF JOINT
Double Dowel Beech
Double Dowel Poplar
Mortise-Tenon Beech
Mortise-Tenon Poplar
Corner Block Beech
Corner Block Poplar
Corner Block Beech-Poplar
Gusset Plates Poplar-Beech
Gusset Plates Beech
Gusset Plates Poplar
Fig.12 Modulus of Elasticity of corner joints in compression test
Table 5 Tension strength of the middle joints
Beech
Poplar
Maximum
rupture load
(Ν)
4671.0
4259.2
Tension
Strength
(Ν/mm2)
3.7
3.4
Beech
Poplar
5438.4
2334.8
4.3
1.8
0.1
0.1
0.4
0.3
4974.5
4663.0
3.9
3.7
0.3
0.3
0.5
0.5
5128.0
4.1
0.3
0.5
3863.5
3829.5
3.0
3.0
0.2
0.2
0.4
0.4
3990.5
3.1
0.2
0.5
Joint Type
Wood species
Double Dowel
Mortise and
Tenon
Beech
Poplar
Corner Block
Beech in the parts and
Poplar in corner block
Beech
Double
Poplar
Gusset Plates Poplar in the parts and
Beech in gussets
Stand.
Variance
Deviation
s2
s
0.1
0.3
0.3
0.5
The above table proves that the middle joint with the higher tension strength compared
to all the other joints of the research, is the Mortise and Tenon joint made of beech
with a mean value of tension strength 4.3 Ν/mm2.
1347
A little lower stability and tension strength (4.1 Ν/mm2) proved to be present in the
Corner Block joint, made by the combination of two different wood species (beech in
the parts and poplar in corner block). The same joint, made of beech wood, follows in
tension strength with 3.9 Ν/mm2, while a little lower value of tension strength was
measured in the same joint made of poplar (3.7 Ν/mm2). Consequently, the Corner
Block joint proved to have the ability to ensure joints of quite high tension strength,
independently of the material which it is made of.
Similar tension strength was indicated by the double Dowel joint made of beech with a
mean of tension strength 3.7 Ν/mm2. The double Dowel joint made of poplar is the
following joint in strength, with 3.4 Ν/mm2.
Lower tension strength was also presented by the three versions of double Gusset
Plates joint. Among these three versions of the joint, the one made by the combination
of two different wood species (poplar in the parts and beech in gussets) proved to have
the highest tension strength with mean value of 3.1 Ν/mm2. The double Gusset Plates
joint, made solely of beech wood, follows with mean value of 3.0 Ν/mm2 and the same
joint made of poplar with 3.0 Ν/mm2, as well.
The lowest value of tension strength was measured in the Mortise and Tenon joint
made of poplar, with a mean value of 1.8 Ν/mm2. This kind of behavior of the specific
joint can be attributed to the limited glued surface between the mortise and the tenon,
and also the reduced strength of the poplar solid wood.
Table 6 Variance analysis result of tension strength of middle joints
ANOVA
Source of
Variance
Sum of
Squares
Between Groups
28.437
9
3.160
Within Groups
19.720
78
.253
Total
48.157
87
D.F.
Mean
Square
F-ratio
Sign. Level
12.498
.000
1348
According to variance analysis results, differences were statistically significant
concerning the following aspects: It was recorded that there was a significant
difference between the tension strength value of the Mortise and Tenon joint made of
beech from the respective value of the double Gusset Plates joint, regardless of the
material it was made of. Likewise, the Mortise and Tenon joint made of poplar seemed
to record statistically significant differences in its tension strength values, from all the
other joint values and particularly resulted in the lowest tension strength value of all
the joints. Finally, the tension strength of all the versions of the Corner Block joint
presented statistically significant differences from the strength of the double Gusset
Plates joint regardless of the material which it was made of.
TENSION STRENGTH OF MIDDLE JOINTS
TENSION STRENGTH N/mm2
5
4,5
4
3,5
3
2,5
2
1,5
1
0,5
0
Double Dowel Beech
Mortise-Tenon Poplar
Corner Block Beech-Poplar
Gusset Plates Poplar-Beech
TYPE OF JOINT
Double Dowel Poplar
Corner Block Beech
Gusset Plates Beech
Mortise-Tenon Beech
Corner Block Poplar
Gusset Plates Poplar
Fig.13 Tension strength of the middle joints
CONCLUSIONS
Conclusions from the bending test of the corner joints:
•
The corner joint of Double Dowel made of beech proved to be the most
durable joint of these four joints.
•
The corner joint of Mortise and Tenon made of beech resulted in very
satisfying strength, whereas the same joint made of poplar resulted in the
lowest strength.
1349
•
The corner joint with the Corner Block showed medium strength and the same
joint made by the combination of beech wood in the parts and poplar in the
corner block did not give durable joints. The last joint indicated the highest
modulus of elasticity of the four types of joint.
•
The corner joint with the wooden Gusset Plates gave the lowest strength,
irrespectively of the material it was made of, while, on the same occasion, the
combination of two different types of wood in the same joint did not provide
the expected strength. Also, this particular joint demonstrated the lowest
modulus of elasticity among the four types of joint.
Conclusions from the tension test of the middle joints:
•
The joint which showed the highest tension strength among the four types of
joint, examined in this research, proved to be the Mortise and Tenon joint
made of beech wood.
•
The joint with Corner Blocks proved to have quite high strength,
independently of the material it was made of and gave equal strength with the
double Dowel joint.
•
The joint of the lowest strength proved to be the one with the double Gusset
Plates, while the highest strength among the three versions of the joint
measured, was the version with the combination of poplar wood in the parts of
the joint and beech in the gusset plates.
•
The lowest tension strength was measured in the Mortise and Tenon joint made
of poplar solid wood.
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Products Journal. Vol.54, No.11.
1350
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Systems for the Use of OSB in Upholstered Furniture Frames. Faculty of Forestry,
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