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Abstract. Timber construction is nowadays becoming more and more favorable due to multiple advantages. In this
paper, the connection type and material of timber building were first highlighted. Then, the bolted joints with
slotted-in steel plates were systematically tested and analyzed. A typical slotted-in bolted glulam connection was
simulated using the software Abaqus. Furthermore, a few important design variables were measured and used to
numerically estimate the embedment strength, the shear force per plane, and the ultimate moment capacity of the
connections. At last, multiple configurations of joint design were compared in the paper, and a parametric design was
made. The results were systematically explained.
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
(http://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 303, 030 0 3 (2019) https://doi.org/10.1051/matecconf/2019303030 03
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Figure 1. UBC residential timber building, Figure 2. Tallest timber structure, Mjøstårnet, Norway [4].
Vancouver, Canada, 53 METRES [6].
lasts for about 100 years [7].
(shown in Figure 4). Glulam, which is the most commonly Moreover, the connection part is crucial for timber
used material currently, has a high strength-to-weight ratio. structures. The strength of the connection is highly relevant
Its shape can be changed into long a span bending form to the overall performance of the whole timber structure.
available for the construction of indoor stadiums. Better yet Different connections could be selected for different timber
more expensive materials, such as PSL were made of structures. For tall structures, the connections are desired to
strands that were clipped-veneer elements having a be stiff enough to hold the strong winds and/or earthquakes.
minimum dimension of 0.25 inches and an average length For many traditional timber buildings in China and the
of at least 300 times this least dimension. The Douglas-fir, Middle East, mortise and tenon connection is the common
southern pine, western hemlock, and yellow-poplar are type. The tenon, formed on the end of a member generally
commonly chosen for PSL because of their superior referred to as a rail, fits into a square or rectangular hole cut
strength [7]. Studies have shown that the tensile strength of into the other, corresponding member [14]. In modern
timber is much higher than that of concrete, which proves connections, fasteners are also commonly used as they
the strength and superiority of timber material [21]. could increase the strength of the connections. When
Furthermore, along with the invention of new timber fasteners have plastic deformation, ductile performance
materials, the likelihood of the inflammability of timber is occurs. In dowel-type fasteners (e.g. nails, screws, dowels
also significantly reduced. These large modern timbers and bolts), ductility is achieved by means of the plastic
covered with unburnt varnish. Even if the fire accidentally deformation in metallic connectors [13].
appears, a few centimeters of unburnt char will form, which
protects the timber inside (shown in Figure 3). Failure of
the beam or struct will occur only when the cross-sectional
area of the unburnt core becomes too small to support the
load [8]. Modern technologies also provide solutions
against termite, moisture, and temperature changes to
timber materials. Timber-made homes could last two to Figure 3. Unburnt portion of timber.
three times as long as the standard modern home, which
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The elastic performance of the bolted timber joints is 2.3. Specific parameters of model
usually decided by the embedment strength of glulam
material. To consider the ductile performance at the local Table 5. Specific parameters of model.
level, a common approach is to weaken the elastic modulus Variable Explanation Value
near the pre-drilled holes on the glulam elements. The Layer The Number of fasteners on the layer 2
elastic property and other properties of materials are Column The Number of fasteners on columns 2
summarized in Tables 1 to 5. ED The End Distance (Distance from fastener to the post) 120mm
VD The Vertical distance 85mm 110mm 85mm
Table 1. Isotropic steel material properties specified for steel
HD The Horizontal Distance 105mm
bolts.
Bsec The Width of the section 180mm
Elastic Plastic tsteel The Thickness of steel member 12mm
Young’s Poisson’s Ratio Yield Plastic G Specific gravity 400 kg/m3
Modulus Stress Strain d Diameter of the bolts 20mm
bh2 Flutes pitch of the bolts 2.5mm
1 200000 0.3 1 640 0
ft Tensile strength 600 N/mm2
2 640 2 fc,⊥ Compression strength perpendicular to the grain 2.5 N/mm2
Young’s Modulus Poisson’s Ratio fc,// Compression strength parallel to the grain 10 N/mm2
ρ Density of wood 399kg/m3
1 200000 0.3
k mod 0.8
γ mod 1.25
Table 2. Isotropic behavior of steel plates.
Hst Size of the pre-drilled holes on the side timber elements 22mm
Elastic isotropic behavior Plastic isotropic behavior
Hms Size of the pre-drilled holes on the middle steel elements 22mm
Young’s Modulus Poisson’s Ratio Yield Stress Plastic Strain
dBC Gap length between the beam and the column 3mm
fe,0 Embedment Strength parallel to the grain, unit in MPa Need to be calculated
1 200000 0.3 1 390 0 fe,90 Embedment Strength perpendicular to the grain, unit in MPa Need to be calculated
Mu Ultimate moment capacity of bolted joint, unit in kN`m
2 390 2
Hload How far the applied load to the rotational center is. 1337.5mm
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occurs at an earlier stage. Consequently, a normal yield FEM, the numerical model was created and the result was
point is commonly defined on a monotonic pushover curve. got. Assuming the ductile performance of the element, the
shear strength of a bolted connection is highly influenced
2.5. Simulation results by the embedment strength of wood. The Stress distribution
All model parts were loaded until they were fully damaged. of the deformed specimen is presented in Figure 8.
The damage condition is shown below. With the created
(a) (b)
Figure 8. (a) Stress; (b) Stress distribution of the deformed specimen.
(3) The column element and the steel plates suffer no
Shown from the above study, some assumptions were damage;
made according to some preliminary estimation of the (4) The shear force is evenly resisted by all the n
moment carrying capacities for a type of timber-timber fasteners (shown in Figure 9(b)). That is, the horizontal
joints (shown in Figure 9(a)). The shear and rotational shear Fh,d = Hd/n, and the vertical shear Fv,d = Vd/n;
analyses were presented in the figure. (5) The rotational strength is dominated by the shear
Assumption: capacity perpendicular to the moment arm of each fastener
(1) The center of rotation is the geometric centroid of the Fm,d), whose direction is perpendicular to the line between
fasteners; the fastener and the geometric centroid (shown in Figure
(2) The deformations within the timber elements are 9(c));
small such that the relative locations of the holes do not (6) The dowel load is the vector addition of three forces,
change; i.e. Fm,d, Fh,d, and Fv,d.
y
y y
Fastener, A
Fi A
A Hd / n
Fastener, i ri
Vd / n rmax Fm,d,max
x x
Md
Hd
x
Vd
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f e,0
f e,90
k90 (4)
fe,0 FV FV Farthest bolt
fe,
h1
FM,1
k90 sin 2 cos 2 2 1 FM,2
h1
(5) FV FV Center of rotation
h2
θ1
θ2
2 1
4 3 r2 r1 F
h2
ᵝ h (1- ᵝ ) h
θ4 FM,4θ3
M,3
h1
k90 =1.35+0.015d
(6) x1 x2 r4 4 3
h1
r3
1
Fc x1 x2
h1
Center of rotation
θ1
θ2
2 1
θ1
r2
timber structure. Mu is influenced by the compression
1
r1 F
ᵝ h (1-ᵝ ) h
h2
1 θ4 F 4 θ 3 3
α1 r4 4 3
h1
r3
1
Fc x1 x2
n
Fc FM ,i cosi (9)
i 1
2 Fc h1
3
f bh
when Fc c ,/ / 1
2 (10)
Mc
1 1
2 2
f c ,/ / bh12 2
f c ,/ / bh
2 3
Figure 11. Key points defining the estimated moment-rotation
when f c ,/ / bh1 F f bh , = 2 Fc 1
2
c c ,/ / 1
f c ,/ / bh1 curves for the bolted timber joint.
Mu Mb Mc (11) 3.3. Compare and analysis
The FEM simulated result and the hand calculated result of
The shear force may have different effect on the timber the prototype bolted joint were compared. Shown in Figure
model if the angle of the force is applied differently. The 12.
rotation center will also determine the effect of shear force. When rotation occurs from rad 0 to rad 0.9, there is no
The following study simulated some hypothesized cases moment resistance for both the Abaqus model and
when timber structure joints under different shear force. hand-calculation model. This is because the size of the
For the cases when the joints were under pure shear, the pre-drilled hole is larger than the diameter of the fastener,
shear force is equally resisted by all the bolts (as shown in which creates a small gap between the hole and the timber.
Figure 10(a)). For the cases when the joint is under a pure In this gap, the rotation of structure will not result in
bending moment, a portion of the beam might touch and moment resistance because there is no contact, which
squeeze the column surface. This type of case is shown in creates the initial horizontal line.
Figure 8(b). For most of the cases, the shear force and the When timber contact with the surface of the fastener, an
moment are coupled together, which is shown in Figure increase in moment resistance occurs in both models.
8(c)) [1]. Because the stiffness and strength of both models are
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Moment (kN-m)
30
and the joint is broken, which means ductile. This joint is
plastic. The ideal ductility means that the joint still works
20
when the split is over 3%.
There might be two reasons for the plasticity of timber:
10 2x2
1. The large size of fastener. The fasteners take over 2x3
the most part of the surface, which makes the hole large. As 3x2
a result of that, the timber is easier to split. 0
0 0.02 0.04 0.06 0.08
2. The diameter of cross-section is small, only 130mm. Rotation (rad)
130mm is not enough for a normal beam in the real Figure 13. Moment-Rotation graphs of connections with different
construction. fasteners.
While in the FEM model, because the split of timber 2. Change column width
was not defined initially, the strength did not have an Once the width changed, the moment resistance of the
obvious reduction, shown in graph 12. connection changed significantly. Its ductility bond doubled,
as the width of the column doubled, which is shown in
Figure14.
On rad 0.026 to 0.033, the hand calculation model Figure 14. The change in width of column section.
estimates that the plastic deformation will occur, and
moment starts to reduce. The hand-calculating graph 3. Change fastener diameters
stopped at radian 0.033, because after 0.033, the hand The large size of fastener provides more moment
calculation model considers the joint not hold any moment resistance to the connection. However, it also makes the
resistance, which means it is broken totally. timber easier to split. According to Figure 15, the increased
Parametric Design: size of fastener brings more moment resistance, but also
Parametric design can be applied in Matlab with the makes the ductile performance occur more easily in less
hand-calculation equations, which would help to analyze rotational radians.
which type of connection is more suitable in real-world
construction.
1. Change fasteners
Besides 2×2, more fasteners could be added to the joints.
Accordingly, the diameter of the fastener was changed
smaller in order to adjust the new joint. 2×2, 2×3, 3×2,
were made in Matlab, and the graphs were given in Figure
13.
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MATEC Web of Conferences 303, 030 0 3 (2019) https://doi.org/10.1051/matecconf/2019303030 03
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