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:

zguan@liv.ac.uk 2 Kohei Komatsu, Research Institute for Sustainable

Humanosphere, Kyoto University, Uji, Kyoto, Japan. Email:

kkomatsu@rish.kyoto-u.ac.jp 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:

kitamori@rish.kyoto-u.ac.jp

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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