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Structural Characterization of Traditional Moment-Resisting Timber Joinery

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This paper investigates the structural performance of traditional moment-resisting interlocking timber joints for modern building construction. Gradually improved over generations, traditional Japanese and Chinese interlocking connections have proven structural integrity. In this paper, first, an overview of the development of historical Japanese and Chinese joinery techniques is presented. Second, state-of-theart numerical modeling techniques for solid contact-only timber connections are reviewed. A hypothetical case study is then used to contextualize a selection of four interlocking joints in a modern building structure. The structural behavior of these joints is analyzed by means of finite-elements considering material failure and geometric nonlinearity. The structural capacity and the rotational stiffness of the joints are investigated. The goal of this research is to initiate a classification of interlocking joint typologies based on their structural behavior.
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Proceedings of the IASS Symposium 2018
Creativity in Structural Design
July 16-20, 2018, MIT, Boston, USA
Caitlin Mueller, Sigrid Adriaenssens (eds.)
Copyright © 2018 by Julieta Moradei, Jan Brütting, Corentin Fivet, Nick Sherrow-Groves, Devon Wilson, Aliz
Fischer, Jingxian Ye, and Javier Cañada
Published by the International Association for Shell and Spatial Structures (IASS) with permission.
Structural Characterization of Traditional Moment-Resisting
Timber Joinery
Julieta MORADEI*a, Jan BRÜTTING*b, Corentin FIVETb,
Nick SHERROW-GROVESa, Devon WILSONa, Aliz FISCHERa, Jingxian YEb, Javier CAÑADAb
a ARUP, Structural Engineering Department, San Francisco, USA
julieta.moradei@arup.com
b SXL, Structural Xploration Lab, École Polytechnique Fédérale de Lausanne, Switzerland
jan.bruetting@epfl.ch
Abstract
This paper investigates the structural performance of traditional moment-resisting interlocking timber
joints for modern building construction. Gradually improved over generations, traditional Japanese and
Chinese interlocking connections have proven structural integrity. In this paper, first, an overview of the
development of historical Japanese and Chinese joinery techniques is presented. Second, state-of-the-
art numerical modeling techniques for solid contact-only timber connections are reviewed. A
hypothetical case study is then used to contextualize a selection of four interlocking joints in a modern
building structure. The structural behavior of these joints is analyzed by means of finite-elements
considering material failure and geometric nonlinearity. The structural capacity and the rotational
stiffness of the joints are investigated. The goal of this research is to initiate a classification of
interlocking joint typologies based on their structural behavior.
Keywords: moment-resisting, interlocking joints, wood, traditional technology, finite element analysis
1. Introduction
Timber construction plays an important role in architectural history, in modern construction methods
and in sustainable design, for which wood-only joinery techniques have been used for centuries.
Traditional joinery typologies are the product of successive, empirical improvements over generations
of artisans [1]. However, over the industrialization era, many traditional wood-only construction
techniques were abandoned in favor of more efficient and profitable modern connections with
mechanical fasteners [2]. Today, this comes along with the use of engineered materials such as glue-
laminated timber. This shift of practice lead to the development of connections with quantifiable
capacity but neglecting the intrinsic anisotropic properties of natural timber.
Conversely, in the past, the structural integrity of traditional joinery techniques was proven mainly
through in-situ de-facto testing. Today, building codes include little-to-no information about the
application of wood-only carpentry connections in comparison to those with mechanical fasteners [2].
However, examples of historic timber buildings with a life span of multiple centuries have demonstrated
many advantages of interlocking timber joints in severe loading cases, e.g. in the case of earthquakes in
Japan or China [2, 3, 4]. Another potential of contact-only connections is their non-destructive
disassembly for reconfiguration allowing for sustainable solutions through their replacement or repair
of structural elements [5, 6]. Remarkable examples include the Japanese Ise-Shrines, which are rebuilt
in a 20-year frequency for the past 1300 years while reusing or repairing the wood that has not decayed
[7]. Additionally, these ancient techniques have the capability of quicker construction through new
technologies. Recent developments in digital fabrication make possible custom and automated
manufacturing of traditional wood-only connections for new buildings [8].
Proceedings of the IASS Symposium 2018
Creativity in Structural Design
2
1.1 Outline
This initial study on interlocking timber joints is part of a larger investigation of how this ancient
technology can be used in today’s building construction. This form of connection-design is critical in
modern construction as wood-only joinery has the potential to achieve increased sustainability by easing
recycling, long-term maintenance, disassembly and reassembly. First, through an extensive literature
review, a selection of Japanese and Chinese joinery typologies are presented alongside a discussion of
their specific use in the building context. Second, the paper investigates the mechanical behavior of these
joints with state-of-the-art modeling techniques. The final goal is to translate insight from these findings
to classify a library of interlocking joints based on their structural behavior.
1.2 Traditional Chinese timber joinery
China holds a millennial-old tradition in using timber for load-bearing structures, making carpentry the
most important building technology and profession. After more than 2000 years of evolution and
progression, this culture has formed a complete system of wood construction, which often exhibits
complex geometries, and the assembly of numerous wood pieces [4]. This was achieved through the
development of complex joinery geometry without the use of iron or steel fasteners. The oldest technical
manual on buildings in Chinese literature is the Yingzao Fashi, the “Treatise on Architectural Methods”
dating back to the year 1103. It reports a unified set of architectural standards for builders and artisans.
Several timber joineries have been recorded in hand-drawn illustrations, without specifying dimensions.
Since the Song dynasty (960 1279), timber joineries have been gradually simplified. In the Qing
Dynasty (1644 to 1911), the interlocking connection between columns and beams in wooden frames, as
well as the use of dovetail joints, gradually became conventional. During this period, another official
regulation, the Municipal Engineering Practice Rules was published (1734). The Construction
Technology of Greater Woodwork in the Qing Dynasty (1985) [9] was one of the first publications to
explicitly list the names of ancient timber joineries at that time. Ma Bing Jian, in the book The
Manufacturing of Greater Woodwork in the Qing (1991) [10], listed and classified the existing ancient
wooden joineries into five categories of greater woodworks, reporting a total of 21 types, with
illustrations and simple description of mechanical characteristics.
1.3 Traditional Japanese timber joinery
Because of its heavily forested volcanic islands, Japan developed the most advanced techniques of
timber construction over millennia [11]. In addition, adaptable and demountable timber systems were
preferred in response to natural disasters and a need for constant city relocations. Entire cities made from
timber were constructed for disassembly and transported [12]. In cultural and temple buildings of the
Endo era (1603 - 1867) in Japan, up to 100 different wood-joint typologies were used [2]. Unique joints
were selected based on their geometry and placement in the building. The knowledge for crafting at least
200 - 400 different joints was required before becoming master carpenter [2, 11]. The linear nature of
the wood material, combined with continual need for flexibility gave rise to hierarchical modular
arrangements of structural components for making post-to-beam frame construction the most common
construction system [11, 12]. In the last decades, Japanese firms have developed a highly efficient
technological adaptation of age-old carpentry techniques to make industrially pre-cut timber framing the
predominant residential construction method today [12].
1.4 Structural analysis and numeric modeling of timber joints
Up to now, a limited number of fundamental studies on the structural behavior of traditional Chinese
wood connections exist [4]. In 1992, Wang [13] published A preliminary study of statics in the
traditional wood frame, which pioneered the field of mechanical analysis on ancient wooden structures
in China. Modern building codes in Japan nowadays require the reinforcement of traditional carpentry
joints with metal fasteners [12]. Similarly, the European Norm EN 1995 for timber structures does not
cover wood-only carpentry joints: only connections with metallic fasteners or adhesives are favored.
Only some EU countries maintain standards for carpentry joints in their national appendices to EN 1995,
which are often limited to simple step-joints for roof trusses or perpendicular mortise and tenon joints.
Equivalently, the Swiss SIA norm only considers step-joints, where the analysis considers primarily the
verification of stress limits, without considering the stiffness of the joint.
Proceedings of the IASS Symposium 2018
Creativity in Structural Design
3
Because analytical methods are often limited to simplified geometries and basic assumptions on the
structural behavior, it is complex to apply these assumptions to geometrically challenging traditional
carpentry joinery. Recently, increasing interest in understanding the behavior of wood-only connections
has motivated several research studies involving their numerical simulation. Connections in traditional
European truss structures have received special attention from researchers. Parisi and Cordié [14] studied
traditional Mediterranean and Alpine roof truss joints, aiming to describe the joint behavior as the basis
for retrofitting. They developed analytical and finite element (FE) models and compared them to results
from experimental tests. The FE analysis was carried out with 2D elements, reducing the joint geometry
into one plane. The mortise and tenon system behavior has been investigated with FE models by Koch
et al. [15], Descamps et al. [16], and Kekeliak et al. [17]. The simulation of dovetail halved joints for
trusses were addressed by Drdácký et al. [18]. Sangree and Schafer [19] focused on the halved and tabled
scarf joint and the stop-splayed scarf joints used in American timber truss bridges, identifying their limit
states through experimental and FE analysis.
Wood-only connections of historic buildings in Japan, Korea and China are subjected to recent research,
some of which includes the development of FE models to assess their mechanical characteristics. Guan
et al. [20] studied the traditional beam to column Nuki joints in Japanese buildings. Special focus was
put on the contact formulations to correctly treat the effect of introduced wedges. Jeong et al. [21, 22]
studied traditional wood joints through FE modeling of the Korean sagae beam-to-column joint and a
beam-to-beam dovetail connection. Instead of traditionally using solid wood, they reproduced the joint
with glue-laminated timber. In addition, they conducted experimental tests to validate the FEM results.
Chen et al. [23] carried out a comprehensive study on two types of traditional Chinese beam-to-column
dovetail mortise-tenon joints that included experimental testing and FE modeling to obtain a trilinear
model of the moment-rotation behavior. Li et al. [24] focused on the effect of looseness in historic
Chinese dovetail mortise and tenon joint.
The literature on interlocking joints concentrates on the analysis of few different joint typologies. There
are limitations in literature in terms of analyzing a variety, or a library of ancient interlocking wood
joints to develop an extensive relative comparison between joints. From the review of the state-of-the-
art FEM techniques, it could be concluded that the anisotropic mechanical properties of the wood
material, the surface contact as well as the overall joint load-deformation behavior, can be appropriately
simulated with a careful setup of the numerical model. Eventually, a full FEM campaign could produce
relative comparisons within a library of joints.
2. Selection of joints
A case-study building, inspired by a field survey of a newly constructed three-story building in Qiantong
(Ninghai County, Zhejiang province, China), with a wooden through-type frame structure is used as a
reference system for the sizing of the beams and joints. Through-type frames combine columns and
girders as well as secondary beams spanning between them [4]. The case study building is constructed
on a 3-meter by 4-meter column grid. Girder ends are connected either at columns or run continuously
through them. Primary girders span between columns in the short building direction (Figure 1 (a), section
1-1) and secondary beams span between the girders (section 2-2). With a load case of dead and live
loads, a uniformly distributed design load of 6.2 kN/m2 is assumed. For this loading, the beams sections
have been dimensioned.
Proceedings of the IASS Symposium 2018
Creativity in Structural Design
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Figure 1. Case study Primary structural system: (a) ground plan, (b) schematic view
For this case-study building, a selection of Chinese and Japanese through-type joints is shown in the top
row of Figure 2. These joints were selected through careful investigation of the literature review and the
conclusion that these are commonly used joints in China and Japan in modern construction.
Joint A is situated at the roof level of buildings where beams are inserted into the column from above
forming a bridle joint. The two beams connected in joint A are halved and form a cross lap-joint. Further,
two typical Japanese connection details for through-type frames are studied. Figure 2 B shows a Nuki
joint where a beam is continuously running through the column. Two wedges allow for a continuous
contact between the column and the beam and guarantee a rotational stiffness [20]. Joint C connects two
beams with a long tenon through the column opening. The tensile connection at the beam top permits a
rotational stiffness [2]. Joint C is used when building dimensions exceed available beam lengths. In
Europe, an equivalent connection topology is completely unknown and would require the use of steel
fasteners [2]. However, rounded, CNC-cut versions of such long tenon connection between two beams
are commonly used in present building construction in Japan [2, 12]. The dovetail joint D describes the
common connection of secondary beams to the girders or to a column head [22]. This connection detail
is frequently used in Chinese, Japanese and European timber construction.
Figure 2. Selection of commonly used Chinese and Japanese joints.
Top row: analyzed joint models; Bottom row: variations of joints in actual building structures.
3. Numerical modelling assumptions
3.1 Material model
Wood is a naturally grown, anisotropic and inhomogeneous material made of longitudinal tubular cells.
The material properties highly depend on the orientation of the grain against loading directions, which
can have a significant influence on the capacity and stiffness of the joint. For this study, the mechanical
model of timber is simplified to a transversely isotropic material that distinguishes strong directional
properties parallel to the grain from those of the transverse plane unifying tangential and radial grain
directions. Further, the material model considers an elastic-plastic behavior of the wood, as well as
(a)
4.00 m
1
1
22
(b)
3.00 m
Proceedings of the IASS Symposium 2018
Creativity in Structural Design
5
material failure, hardening and post peak softening [25]. Douglas fir, being one of the most typical
timber materials in North American wood construction, has been selected for the following analyses.
The considered mechanical properties listed in Figure 3 (a) are those of grade 1 Douglas fir at 20 °C
with a wood moisture content (WMC) of 12 % [25].
3.2 Joint geometry and modeling
As Figure 3 (b) illustrates, typical cross section sizes are used: columns are 12×12 nominal (11.25” by
11.25”, or 285.75 mm by 285.75 mm) and beams are 4×12 (3.5” by 11.25”, or 88.9 mm by 285.75 mm).
To remain consistent with traditional joint geometries, dimensions (e.g. tenon widths and lengths, or key
sizes) maintain sizing ratios retrieved from field-surveys or reported in literature. The interlocking region
of the joints and the beams up to 1.5D away from the column face are modelled with solid finite
elements. This considers the main region where stresses are concentrated. The strong direction (grain)
of the transversely isotropic material model is aligned parallel to the local x-axis of the solids. To
simplify the analysis and to reduce computational complexity, the solid beam portions are coupled to
linear beam elements that extend until mid-span, where a symmetry boundary condition is applied (see
Figure 3). The interface between all solid joint parts is considered with surface-to-surface contact models
(coefficient of friction μ = 0.6).
Figure 3. (a) Mechanical properties of Douglas fir, (b) sizing and modeling of the joints, here shown for joint A.
3.3 Loading and analysis
Each joint is analyzed by means of FEM using the commercial software LS Dyna®. A displacement
driven, material and geometric non-linear analysis is used to load the joints until failure. The
displacement is applied at the beam mid-spans as shown in Figure 3. To simulate the asymmetric loading
condition of the building structure, the primary beam-ends are displaced with a higher magnitude (u1)
than the secondary beams (u2).
4. Results
The goal of the analysis was to extract the relative capacity and stiffness of each joint to understand the
behavior of ancient interlocking wood joint techniques. Figure 4 illustrates the principle stress
distribution (parallel to grain axis) and the deformed geometries (10x magnified) for joints A and C at
different states of incremental loading until failure. Figure 4 (a) to (c) indicates that joint A fails in
tension at the top fibers of the section. The analysis showed a brittle failure of joint A through cracking
of the whole section (see Figure 4 (c)). In contrary, as illustrated in Figure 4 (d) to (f), joint C fails
through an opening of the joint, i.e. the joint pieces disconnect before material failure is reached.
u
2
u
2
u
1
u
1
D
1.5D 1.07 m
D/2
D
Beam elements
(b)(a)
Solid elements
W
Symmetry
boundary
conditions
x
z
y
D
W285.75 mm
88.9 mm
E
0
16500
E
90
960
G
0
810
G
90
350
f
t,0
57.0
f
t,90
1.8
f
c,0
36.9
f
c,90
7.2
f
v,0
6.9
f
v,90
9.7
y500
WMC
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[MPa]
[kg/m
3
]
12 %
Proceedings of the IASS Symposium 2018
Creativity in Structural Design
6
Figure 4. Deformed geometries of joints at different states of loading until failure, and corresponding principal
stress distribution states for: (a) to (c) joint A, (d) to (f) joint C.
To compare the capacity and stiffness of all joints, Figure 5 plots the bending moment against the
rotation of the primary beams. Moment and rotation are measured in reference to the cross-section of
the primary beam at the column face. Joint B shows the stiffest moment-rotation behavior (steepest
slope). This is due to the continuity of the beam that runs through the column opening without being
weakened. In contrast, joint A has a reduced cross section height due to the halved lap joint. Compression
forces are transferred through the secondary beam, perpendicular to its fiber direction, causing a less
stiff behavior. The weakening of the cross section of joint A further causes a brittle failure dominating
the behavior as shown in Figure 4 (c) and Figure 5. However, both joints, A and B, resist a nearly
equivalent ultimate bending moment of 27 kNm. This is approximately the same magnitude as the
maximum bending moment in main beams caused by the design load in the case study building.
Joint C reaches an ultimate moment capacity of 15 kNm. At a rotation of 5.0 mrad, joint C opens and
exhibits zero stiffness when further rotated. The dovetail joint D shows a limited moment capacity and
exhibits big rotations. This is due to the small contact area between the pieces and loading perpendicular
to the grain inside the dovetail pocket.
Figure 5. Moment-Rotation behavior of the four selected joints
Princ. Stress [MPa]
-30.00
-20.00
-10.00
0.00
0.00
10.00
20.00
30.00
35.00
40.00
45.00
50.00
55.00
(a) (b) (c)
(d) (e) (f)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
0.0 10.0 20.0 30.0 40.0 50.0 60.0
Bending moment [kNm]
Rotation [mrad]
ABCD
D
A
BC
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Creativity in Structural Design
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5. Discussion
This preliminary study allowed for a relative comparison between different joint typologies. It is
apparent that the geometry of the cross-section at the joint is a direct correlation to the capacity and
stiffness of the member.
Joint C connects two beams to a column and joints A and B consist of a continuous beam running
through or over a column. All three joint types could potentially be placed in the case study building at
beam-column or beam-beam connections depending on the archetype layout. The selection of the joint
depends on the required construction method and the structural demand. The continuous beams in joints
A and B allow higher bending moments and result in less rotation. If necessary, two beams can be
connected employing joint C while still achieving a certain rigidity. The opening of joint C, as indicated
in Figures 5 and 6, is primarily caused by a vertical sliding of the tenon out of the mortise. As e.g.
reported in [6], this behavior can be circumvented by additionally inserting a horizontal pin through
tenon and mortise. Joint D exhibits a very limited rotational stiffness and can be considered as a hinged
connection.
The considered strengths in compression and tension reported in [25] are higher than typical wood
design values, yet still permit a relative comparison of joint types. For the structural design of traditional
moment-resisting joints in today building construction, the strengths should be reduced to design values.
6. Conclusion and Future Work
This paper investigated traditional Japanese and Chinese interlocking wood connections through state-
of-the-art numerical modeling techniques. First, a literature review of the historical context of
interlocking wood joints was presented, and past studies of numerical modeling techniques for these
joints were reviewed. Then, the structural capacity and load-deformation behavior of the selected joints
were compared and contextualized. Future studies should diversify the joint topologies and conduct
experimental testing to confirm the results or update the assumptions. Furthermore, modernizing the
ancient geometries of the joints may be achieved through optimization techniques (e.g. varying tenon
lengths and widths) to increase or customize the rigidity of the connection.
The goal of this preliminary investigation is to initiate a library of interlocking wood joint typologies
based on their structural behavior. The comparative record of their capacity and stiffness will allow
academics and designers to push the capabilities of these ancient techniques while benefitting from
modern materials, construction techniques and fabrication tools.
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Creativity in Structural Design
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... Historical and structurally, laced timber or timber frame structures represent the most interesting case studies. These typologies are especially common around the Mediterranean Sea [10,11], Europe [12,13], and also China [14]. ...
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... In summary, the state of the art for analyzing interlocking joinery through experimental testing, mechanical analysis, and numerical simulation have yet to be synthesized and calibrated. This work particularly builds on own previous work by Fang and Mueller [7], Moradei et al. [16], and Fang et al. [17]. This work presents new results on digitally fabricated specimens made of glue-laminated timber as well as numerical results to compare with the analytical and experimental approaches. ...
Conference Paper
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This work fully investigates a specific timber joinery connection via experimental, analytical, and numerical methods. The selected joint is the Nuki joint: a mortised column with through-beam tenon. The experimental approach takes advantage of digital fabrication to reduce variations introduced by hand fabrication while the analytical approach builds on state-of-the-art embedment stress models. Material tests are used to calibrate the non-linear finite element model and analysis of the connection. Furthermore, the difference in behavior between prototypes of various beam thicknesses is examined across analysis approaches. This work not only sets out a workflow for digital fabrication, physical testing, and structural analysis for more complex joinery geometries, but also discusses the challenges and relevance of its application towards a reference library of joinery connections for modern timber construction.
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