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STRUCTURAL CHARACTERISTICS OF BEAM-COLUMN
CONNECTIONS USING COMPRESSED WOOD DOWELS AND
PLATES
Zhongwei Guan*1, Kohei Komatsu
2, Kiho Jung
2 and Akihisa Kitamori
2
ABSTRACT: Steel dowels and plates are widely used in modern timber joints, which are difficult to be recycled. Also
due to large difference on stiffness between the steel and the timber, there is a poor compatibility in the joint that
produces less integrity. Therefore, a desired joint should be made with non-metallic fasteners, ideally timber fasteners.
In this paper, 3-D nonlinear finite element models have been initially developed to simulate structural behaviour of
beam-column connections subjected to racking loading conditions. All models developed are validated against
experimental results. Then using validated models, characteristics of the beam-column connections are thoroughly
investigated in terms of load carrying capacities of individual dowels made of compressed wood. In the numerical
models, timber and compressed wood are modelled as orthotropic linear elastic materials in tension, and as elasto-
plastic materials in compression in the embedding areas. Various contact conditions within the joints are modelled.
Moment-rotation relationships of the joints are simulated with reasonably good correlation to the corresponding
experimental results. Based on the structural characteristics obtained, recommendations are given on dowel patterns and
geometrical conditions of the constituent members.
KEYWORDS: compressed wood, dowel, joint, finite element, non-metallic fastener timber.
1 INTRODUCTION 123
Although steel dowels and plates have been used in
modern timber joint systems for a few decades, some
problems are still remained, i.e. it is difficult to recycle
them. More importantly, due to large difference on
stiffness between the steel and the timber, there is a poor
compatibility in the joint that leads less integrity.
Therefore, a desired joint should be made with non-
metallic fasteners, ideally timber fasteners. The idea to
use wood fasteners in construction is not new. There are
still some outstanding traditional temples and shires
using wood fasteners in China and Japan which were
built a few hundred even a thousand years ago.
However, the way to produce such traditional wood
fasteners is complex, which needs high skills and is not
mass productive. Also, hardwood used in the traditional
wood fasteners has high stress relaxation feature. This
1 Zhongwei Guan, Department of Engineering, Brodie Tower,
University of Liverpool, Liverpool L69 3GQ, UK. Email:
[email protected] 2 Kohei Komatsu, Research Institute for Sustainable
Humanosphere, Kyoto University, Uji, Kyoto, Japan. Email:
[email protected] 2 Kiho Jung, Research Institute for Sustainable Humanosphere,
Kyoto University, Uji, Kyoto, Japan. Email:jungkiho@
rish.kyoto-u.ac.jp 2 Akihisa Kitamori, Research Institute for Sustainable
Humanosphere, Kyoto University, Uji, Kyoto, Japan. Email:
leads joint loose so that joints need to be fastened on a
regular basis. The use of compressed wood fasteners
could overcome above problems, as compressed wood is
an engineered product which has much better
mechanical properties by controlling density and
moisture content through manufacturing processes.
There are major advantages to use compressed wood
fasteners against steel fasteners, such as reduction of
CO2 emission, good environmental impact, better
compatibility, tight fitting, better recycle ability, etc.
Although traditionally produced compressed wood could
offer higher density and better mechanical properties,
however such enhancement was not stable and of short-
term duration. An unfortunate reason is that untreated
compressed solid wood and veneer tend to undergo
irreversible “springback” or recovery from compression
when exposed to moisture. This prevented it to be used
to produce structural members. To eliminate springback
wood should be pressed in conditions that cause
sufficient flow of the lignin. There have been research
developments since 1960s to tackle the problem. Series
of breakthrough research outcomes have been produced
since 1990s. A compressed wood product without resin
treatment is Staypak [1], which is produced by
compressing wood at a moisture content equal to or
below the service one. However, due to the
thermoplastic nature of the lignin, also because the
moisture content of the wood is only slightly less after
compression than prior to pressing, there is considerable
springback on the product if it is removed from pressing
while still hot [2]. This prevented Staypak from
industrial uses. Therefore, stabilisation of compressed
wood becomes a key to open a door for its broad
industrial applications, particularly structural uses.
Compressed wood can now be produced with a density
up to 3 times of its original one and with desired
stiffness and strength. Due to greatly enhanced
mechanical properties of compressed wood with
necessary stabilised dimensions, the newly developed
compressed wood increasingly attracts researchers to
find its structural uses. For the past a few years, there
have been many studies on its mechanical properties, but
limited research on its structural uses as fasteners for
connecting soft timber components. Zhou et.al [3]
investigated about bending creep behaviour of hot-press
wood under cyclic moisture condition and found that the
thickness swelling increased with moisture cycle, which
led to increase in the dimension of hot-press specimen
by the end of cyclic moisture sorption. Studies were also
carried out on structural timber and glulam in
compression perpendicular to grain and the
corresponding stress states [4]. Heger et al. [5] studied
about mechanical and durability performance of thermo-
hydro-mechanically densified wood and found a
diminution of the mechanical properties of THM treated
wood at temperatures higher than 180°C, which might be
the maximum temperature practical for processing.
Kobujima et al [6] investigated the bending properties of
compressed Japanese cedar (Cryptomeria japonica D.
Don), the specimens being compressed in the radial
direction with ratios (the deformation to the initial
thickness) of 33% and 67%. Adlam [7] also studied
effects of relatively low compression ratios of 13% and
22% on the mean MOE and MOR of densified wood.
Yoshihara and Tsunetamtsu, [8] examined the bending
and shear properties of compressed wood and showed
that Young‟s modulus increased with increasing
compression ratio. They also used tension tests to
investigate the elastic properties of Sitka spruce (Picea
sitchensis Carr.) compressed wood with various
compression ratios [9].
Jung et al. [10] studied timber joint systems using
compressed wood fastener. The result shown that the
joint with compressed wood dowels and plates have
enhanced mechanical properties such as pull out strength
and rotational performance. Hassel et al. [11] undertook
studies on the performance of a wooden block shear wall
which utilizes compressed wood as a connecting element
in place of the traditional metal connector. After
absorbing moisture, the compressed connector recovered
partly its radial dimension and filled the gaps with
adjacent block. Jung et al. [12] carried out research on
applications of compressed wood (CW) made of
Japanese cedar, as a substitute for high density
hardwood, to make shear dowels. CW with its annual
ring radial to loading direction (0o) had a unique double
shear performance characteristic, and showed good
properties as a dowel material with its strength and rich
ductility. However, CW with its annual ring tangential to
loading direction (90o) and maple exhibited brittle
failure. When the density of base member increased, its
stiffness, yield load, and maximum load exhibited
proportional improvement with different inclination. To
date, there is hardly any numerical models being
developed to assist optimise structural performance of
timber connections using compressed wood fasteners.
Testing the proposed portal frame joints is very time
consuming, on top of material and manpower costs.
When conducting experimentally-based research, tests
need to cover as many scenarios as possible, such as
stacking configurations, materials, geometries, loading
and boundary conditions, etc. Therefore, the
optimisation process is likely to be very expensive and
hugely time consuming. In contrast, the development of
computer models using finite element (FE) analysis is a
relatively quick and inexpensive process, especially in
cases where there is access to supercomputer facilities.
In such circumstances, only a limited number of
materials measurement tests and structural tests are
required for validation purposes. Once computer models
are verified against typical tests covering extreme cases
and possible an intermediate case, systematically-
designed parametric studies can be undertaken using
validated numerical models [13-16].
In this study, 3-D nonlinear finite element models have
been initially developed to simulate structural behaviour
of beam-column connections subjected to racking
loading conditions. All models developed are validated
against experimental results. Then using validated
models, characteristics of the beam-column connections
are thoroughly investigated in terms of load carrying
capacities of individual dowels made of compressed
wood. In the numerical models, timber and compressed
wood are modelled as orthotropic linear elastic materials
in tension, and as elasto-plastic materials in compression
in the embedding areas. Strain hardening of the
compressed wood is taken into account in the modelling.
Complex contact conditions of the joints are simulated,
which cover those between the compressed wood dowel
and the compressed wood plate, the dowel and the
timber (column and beam), the timber and the plate, the
timber beam and the timber column. Different contact
algorithms are used to simulate small slide, finite slide
and possible separation between contact pairs. Structural
behaviour of the joints in terms of the moment-rotation
relationship is simulated with reasonably good
correlation with the corresponding experimental results.
Based on the structural characteristics obtained,
recommendations are given on dowel patterns and
geometrical conditions of the constituent members.
2 EXPERIMENTAL WORK ON THE
BEAM-COLUMN CONNECTIONS
In this research, E60-grade Japanese cedar glulam was
used for column and beam, with cross-sectional
dimensions of 120mm×120mm and 120mm×240mm,
respectively. Dowel was made by compressing Japanese
cedar (Crytomeria japonica D. DON) in the radial
direction until 30% of its original thickness was reached
at a temperature of 130℃ for duration of 30minutes to
obtain a density of about 1000 kg/m3. No fixation
treatments, such as steaming, chemical agent or resin,
were applied.
All boards selected for compressing had flat annual
growth rings and were without knots, split, and pith. The
initial moisture content (MC) was approximately 12%
prior to the compression process. For the fabrication of
the dowels, the initial dimensions of wood pieces were
15mm×15mm, which were then processed into round
shape with final diameter of 12mm. The processes to
make the compressed wood plates were almost the same
as those for the dowel, with only difference on the
compression rate. The dimensions of the plate were
80mm×580mm×14mm for the column-to-beam joints.
There are total 14 compressed wood dowels of 12 mm in
diameter used to link beam/CW plates and column/CW
plates.
Figure 1 shows the experimental apparatus of the
rotation test for the column-beam joint. A quadratic-link
steel frame system was used to load the joint with a
moment, which was connected by a steel pin with a span
of 1000 mm span. Each specimen was set up on the
frame and was jointed with a steel pin (Ø22 mm), as
shown in Figure 1.
The rotational deformation of the joint was applied by
this steel frame, which was controlled by a hydraulic
actuator. The loading schedule was determined through a
step displacement with angles of the steel frame of
1/300, 1/200, 1/150, 1/100, 1/75, 1/50, 1/30, and 1/15
rad. At each step, three loading cycles were applied. The
relative displacements between the plate and each
member were measured by displacement transducers for
estimating accurate rotational angles against the
corresponding applied load measured by a 50-kN load
cell.
#4
#5
500 500
100
500
500
100
120
500
120
Load Cell 50kN
#1
#2
#3
100
0
100
120
240
#6 #7
Figure 1: Apparatus for rotational test on CPD column-double-beam joint
Figure 2 shows typical failure mode of the joint, i.e.
splitting and embedding on the top and the bottom
boundary of the beam – column interface area.
Therefore, the numerical modelling developed needs to
simulate such failure features, on top the load-
displacement relationship.
Figure 2: A typical failure mode of the column – double beam joint
3 DEVELOPMENT OF FINITE
ELEMENT MODELLING
There are two different timber materials in the joint, i.e.
soft wood for the column and the beam and compressed
wood for the dowels and plates. For both materials in
tension, orthotropic elasticity would be an appropriate
constitutive relationship. Equation (1) shows such the
relationship used for all members in the tension zones.
(1)
However, for timber under compression, especially
under high contact stresses, elasto-perfect plasticity
would be a proper model [14, 17]. Commercial finite
element code Abaqus offers a good tool to tackle the
prescribed problems [18]. Material properties used in the
modelling were shown in Table 1. In order to simulate
the behaviour of compressed wood, elasto-plasticity with
strain hardening was used, which was corresponding to
the test results. Equation (2) shows elasto-plastic
relationship used in the modelling.
(2)
where is elasto-plastic matrix, which is dependent
on the elastic matrix , the yield function and the
hardening function (for perfect plasticity hardening is
zero).
The corresponding total stresses and plastic strains are
shown as follows.
23
13
12
33
22
11
23
13
12
3223113
3322112
3312211
23
13
12
33
22
11
G1 0 0 0 0
0 G1 0 0 0 0
0 0 G1 0 0 0
0 0 0 1
0 0 0 1
0 0 0 1
EEE
EEE
EEE
}{][}{ dDd ep
epD][
Total stress (Mpa): 22.0, 25.6, 29.9, 34.3, 39.0, 44.2,
49.1
Plastic strain (mm/mm):0.0, 0.0127, 0.0153, 0.0232,
0.0262, 0.0353, 0.0445
Table 1: Material properties
Component LE RE TE LTG
Beam&column 10100 160 390 470
CW dowel 25500 1170 2050 1860
CW plate 22600 918 2650 1480
LRG TRG LT LR RL
560 35 0.020 0.408 0.030
590 139 0.020 0.408 0.030
530 111 0.020 0.408 0.030
* 2mmN / for all modulus
Embedding strength of 6 MPa perpendicular to the grain
of compressed wood plate was implemented to the
specific compressive zones.
Mesh generation is shown in Figure 3, with loading and
boundary conditions. However, in order to model
racking behaviour of the joint accompanied by large
displacement, four rigid beams were attached to two
beams through pin joints. This is also shown in Figure 3.
Figure 3: Mesh generation of the joint
Since rigidity of the joint was underestimated by those
pin joints in contrast with the test setting shown in
Figure 1, some low stiffness springs (1 N/m) were
connected to the column and compressed wood dowels
to overcome numerical singularity problems. The most
challenging tasks are to deal with various contact surface
pairs, some with finite sliding and some with small
sliding. There are total 64 contact pairs in the modelling,
which are comprised from the following interfaces:
The compressed wood dowel – the timber,
The compressed wood dowel – the compressed
wood plate,
The beam – the column,
The compressed wood plate – the beam,
The compressed wood plate – the column
Due to various contact features in the joint, in some
contact pairs, such as those between the compressed
wood plate and the column, the beam and the column,
finite slippage was allowed. In contact pairs formed
between the dowel and the plate, the dowel and the
timber (beam and column), only small slide was
necessary.
4 RESULTS AND DISCUSSION
Here a series of numerical modelling results are
presented to assist evaluating structural behaviour of the
double beam – column joints. First, the predicted
moment-rotation relationship is plotted against the
corresponding experimental results, which is shown in
Figure 4. Very good correlation is obtained, in terms of
the initial stiffness, the peak load and overall
relationship. The deformation mode is also shown in the
same figure that demonstrates large rotational
deformations due to the horizontal racking.
Figure 4: Moment-rotation relationships and deformation mode
pinned pinned
pinned
pinned
Figure 5 shows the contour plot of the maximum
principle stress (Pa) and a failed specimen. It can be seen
that the predicted failure areas are coincident with the
failure areas obtained from experimental work, i.e. the
tensile splitting failure in the beam – column interface
areas opposite to the high embedding areas. The
maximum principle stress along the longitudinal
direction of the column has reached 36 MPa, which is
critical for Japanese cedar. Also large embedding
Figure 5: Comparison of the predicted failure mode and experimental failure mode
deformations are simulated reasonably well. In addition,
the opening between the beam and the column is
reproduced.
The longitudinal shear stress can be critical. Figure 6
shows such stress distributions on the central area of the
column. The contour plot demonstrates the maximum
shear stress of about 9 MPa, which is on the critical
value. If the area with such critical value is big enough
there may be shear failure occurred.
Figure 6: Longitudinal shear stress distribution on the column
Figure 7 shows the minimum principle stress
distributions on the CW plates , which displays high
contact stress regions. The maximum stress value is -63
MPa which is located adjacent to the high embedding
deformation areas between the column and the beam.
Deformed shapes of the dowels 7 and 8 are also shown
in the figure to view the deformation modes of the
mostly deformed dowels in the joint.
Figure 7: The minimum principle stress distributions on the CW plates and the dowels 7 and 8
The total contact force on the individual dowels is shown
in Figure 8, which combines both the normal and shear
interactions. It is clearly seen that the dowel 7 and dowel
8 bear the largest contact forces, i.e. 27.6 and 26.7 kN
respectively, whilst dowels 1, 3, 5, 10, 12 and 14 also
carry high contact forces ranging from 19.8 to 23.4 kN.
However, the contact forces on dowels 2, 4, 6, 9, 11 and
13 are much lower, from 1.0 to 5.7 kN, but they would
pick up much more contact forces when racking the joint
in an opposite direction. Anyhow, contact forces on the
dowels 7 and 8 remain the highest. Therefore, both
dowels need to be reinforced by either increasing the
dowel size or number of dowels, subjected to dimension
increasing on the column. Also, it is interesting to see the
sequence order of the dowel getting fully engaged. The
dowels 7 and 8 pickup contact loads at 10% of the
loading (before the loading is taken by the beam-column
interface and the supports), then the dowels 1, 3, 5, 10,
12 and 14 get engaged at 25% of the loading, finally rest
of the dowels start to contribute load carrying capacity at
50% of the loading.
Figure 8: Contact forces on individual dowels
Figure 9 shows the contact shear forces carried by
individual dowels. As expected, the dowels 7 and 8 carry
the highest contact shear forces, about 4 kN, whilst
dowels 1, 3, 5, 10, 12 and 14 carry such forces in the
range from 1.5 to 2.1 kN. Again, when racking the joint
in an opposite direction the contact shear forces will be
swapped between the group of dowels 1, 3, 5, 10, 12, 14
and the group of dowels 2, 4, 6, 9, 11, 13. By subtracting
the contact shear forces from the total contact forces (see
Fig. 8), it can be seen that the normal contact forces play
much more important roles. However, this is related to
how tight fitting between the dowel and the column, the
beam and the CW plate. If CW dowels with much lower
moisture content than the ambient MC are inserted to the
joint, much the higher tight fitting will be anticipated due
to moisture-dependent swelling of the dowels. Therefore
more contribution from the contact shear force will be
expected.
Figure 9: Contact shear forces on individual dowels
Figure 10 shows the total contact force between the
beam and the column. There are 5 contact pairs on each
side of the column. The contact forces on the right of the
column are asymmetric to their counterparts on the left
Figure 10: Total contact forces on the beam – column interface areas
12 14 8 10 4 6 2
11 13 7 9 3 1 5
B-C-L1
B-C-L2
B-C-L3
B-C-L4
B-C-L5
B-C-R1
B-C-R2
B-C-R3
B-C-R4
B-C-R5
of the column in terms of contact force locations due to
rotation of the joint. Also from the figure, it can be seen
that the total contact forces on the beam – column
interface pick up at much earlier stage in comparison to
such contact forces on the dowels. It is understandable as
the beam gets into contact with the column quickly due
to racking force before the dowels get fully engaged.
The ratio of the contact area to the total contact surface
area for the dowels 1, 2 and 7 are shown in Figure 11.
The dowel 7 has the highest contact region as it bears the
highest contact force (see Fig. 7). Variation of the
contact areas on the beam and the CW plate is related to
their geometric conditions and material properties
(mainly stiffness). In general, contact force is dependent
upon the corresponding contact area in the joint, which is
reflected by Figs. 8 and 11.
Figure 11: Changing in contact area versus the loading percentage
5 CONCLUSIONS
Non-linear finite element models have been developed to
simulate structural behaviour of double beam – column
joints. Orthotropic material properties of all members
were implemented into the models. Both elasto-perfect
plasticity and elasto-plasticity with non-linear hardening
were used to model behaviour of soft wood and
compressed wood, respectively. Very good correlation
between the test results and the simulation in moment-
rotation relationship was obtained. The predicted failure
mode of the joint is also correlated well with the failed
specimen. The numerical models also produced
information on the total contact forces, the shear contact
force and the contact area of various contact pairs, which
are useful to analyse detailed characteristics of the joints.
The models developed may be used for further
parametric studies to optimise the joint systems.
ACKNOWLEDGEMENT
Authors sincerely thank the Research Institute for
Sustainable Humanosphere of Kyoto University to
support publication of the partial reserach output from a
joint research project.
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