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buildings
Article
The Inﬂuence of Panel LayUp on the Characteristic
Bending and Rolling Shear Strength of CLT
Conan O’Ceallaigh 1ID , Karol Sikora 2and Annette M. Harte 1, *ID
1College of Engineering & Informatics & Ryan Institute, National University of Ireland Galway,
University Rd., Galway, H91 HX31, Ireland; conan.oceallaigh@nuigalway.ie
2Faculty of Engineering and Information Sciences, University of Wollongong in Dubai, UAE;
karolsikora@uowdubai.ac.ae
*Correspondence: annette.harte@nuigalway.ie; Tel. +353(0)91492732
Received: 28 June 2018; Accepted: 16 August 2018; Published: 21 August 2018
Abstract:
The objective of this study was to characterise the behaviour of cross laminated timber
(CLT) panels and the inﬂuence of the panel layup on the failure strength. Three different panel
conﬁgurations of thickness, 60 mm, 100 mm, and 120 mm, were loaded in the outofplane direction.
The 60 mm and 120 mm panel conﬁguration comprised three layers of equal thickness, and the
intermediate 100 mm thick panel comprised ﬁve layers of equal thickness. The mean and characteristic
bending and rolling shear strength of the panels were examined. The results show that the mean
bending and rolling shear strength decrease with the panel thickness. The characteristic results have
shown that there is an inﬂuence because of the number of boards within the panel. The characteristic
bending strength values for the ﬁvelayer 100 mm thick panel were found to be higher than that of
the threelayer 60 mm panel. The characteristic rolling shear values decreased in the ﬁvelayer panels,
however, the increased number of layers subjected to the rolling shear results in a reduced variability
in the rolling shear strength.
Keywords:
bending strength; cross laminated timber; engineered wood products; rolling shear
strength; sitka spruce
1. Introduction
The construction industry has seen an increased movement towards more sustainable solutions,
leading to a requirement for more environmentally friendly, low carbon, thermally insulating, and less
labourintensive materials in the construction of buildings. Cross laminated timber (CLT) is one of
the most promising materials meeting these requirements. CLT is a multilayer engineered wood
panel product, manufactured from at least three layers of boards by glueing their surfaces together
with an adhesive under pressure. At present, the mechanical properties of CLT are regulated in
industryproduced technical approvals and there are currently steps being taken to collate these
technical approvals into the harmonized standard. CLT technology was developed in Europe
approximately 20 years ago, and is largely responsible for the increase in timber construction
throughout the world. CLT buildings are typically erected using platform or balloon construction
methods, connecting the walls and ﬂoors with angular metal brackets in combination with screws
or nails.
The ﬁrst activities to standardise CLT in Europe began in 2008 and the ﬁrst European product
standard for CLT, EN 16351 [
1
], is currently in effect. This product standard details the requirements
for CLT production, relating to the material and geometric constraints, manufacturing procedures,
and tolerances, in addition to test protocols. EN 16351 [
1
] deﬁnes the board width and thickness.
The board width and thickness should be between 40–300 mm and 12–45 mm, respectively. The boards
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Buildings 2018,8, 114 2 of 15
used for the manufacture of CLT are generally conditioned to a moisture content (u) of 12
±
2% and
are visually or mechanically strength graded. The common strength classes according to EN 338 [
2
]
are C24 for a homogeneous layup, C24 for longitudinal layers, and C16/C18 for the transverse layers
in a combined layup. In general, the guidelines and requirements of the adhesive manufacturers must
be followed in the manufacture of CLT panels. It must be stated that some parameters, such as the
bonding pressure, the quantity of applied adhesive, and the moisture content of adherends, are based
on experience with glulam production [3–7].
Many studies have examined CLT and its uses in the timber construction industry. Its excellent
inplane and outofplane strength, rigidity, and stability have allowed larger and taller buildings to be
built [
3
,
8
–
12
]. In Europe, spruce is largely used when manufacturing CLT, however, many other timber
species have been investigated worldwide. Fortune and Quenneville [
11
] aimed to establish the use of
CLT in New Zealand using locallygrown Radiata pine, bonded using resorcinol adhesive. In Japan,
Sugi timber CLT panels were manufactured by Okabe et al. [
13
]. In the United States, Hindman and
Bouldin [
14
] produced CLT using Southern pine, which was tested to establish the bending strength,
bending stiffness, and shear strength. The CLT panels made from Scottish Sitka spruce was investigated
by Crawford et al. [
15
]. Four CLT panels for each panel conﬁguration (three and ﬁvelayers) were
prefabricated using Sitka spruce boards, graded to C16, and bonded with polyurethane (PUR) adhesive.
Established bending strengths and stiffnesses in and outofplane were not dissimilar to commercially
available CLT panels, manufactured in Central Europe. Similarly, a sample of the CLT panels were
manufactured from Irish Sitka spruce by Sikora et al. [
9
]. Sikora et al. [
9
] tested six panels inplane
and 12 panels outofplane, to investigate the effect of the thickness of CLT panels on the bending
stiffness and strength. The tests performed on threelayer and ﬁvelayer panels showed a general
tendency that the bending strength decreased with panel thickness, but it was stated that further tests
are required to determine the characteristic strength values and conﬁrm the observed trend. In all
of the cases, the importance of proper quality control, especially for the bonding process, was stated
to be signiﬁcant in ensuring adequate bonding. Manufacturing defects have been shown to result in
delamination failure rather than bending or shear failure in isolated cases [9,13,15].
The shear behaviour of the CLT panels is often deemed as one of the most signiﬁcant factors
leading to the failure of the panel, because of the reduced strength in the transverse layers orientated
perpendicular to the major axis of the panel. The reason for this is the increased tension perpendicular
to the grain stresses in the transverse layers, which, together with rolling shear stresses, lead to a
remarkable decrease in resistance [
3
,
16
,
17
]. The comprehensive knowledge of the rolling shear modulus
and strength is therefore of the utmost importance in the design of CLT structures.
Ehrhart et al. [17]
investigated the effect of the sawing pattern and board geometry on the rolling shear behaviour for
different timber species. Tests were performed on individual boards and the results showed that
the shear modulus and strength were generally underestimated in the product approvals [
18
,
19
].
They stated that the higher rolling shear modulus results from current CLT practices, at least in
Europe, of using an increasing amount of boards taken closer to the pith than CLT products in the past,
which primarily used sideboards [
17
]. They observed a signiﬁcant relationship between the rolling
shear modulus and the annual ring pattern. Many researchers have performed experimental tests on
large scale CLT panels in accordance with EN 16351 [
1
,
8
,
9
,
11
,
15
,
17
,
20
–
22
]. Blaß and Görlacher [
23
]
established a characteristic value of 1.0 N/mm
2
for the rolling shear strength of European spruce,
independent of the strength class. The shear tests performed by Sikora et al. [
9
] on threelayer and
ﬁvelayer panels, have shown that rolling shear values decrease with the panel thickness, with the mean
reported values ranging between 1.0 N/mm
2
to 2.0 N/mm
2
. The geometric properties of the individual
boards have also been shown to inﬂuence the rolling shear behaviour [
3
,
16
,
17
]. As a result, for the
rolling shear stresses in the layers of the CLT loaded outofplane, a minimum board widthtothickness
ratio of four is proposed; otherwise, a reduced rolling shear resistance has to be considered [
1
].
Brandner et al. [
3
] state that, in accordance with practical applications, the characteristic rolling
strength values,
ƒr
,
CLT,k
, of 1.40 N/mm
2
and 0.80 N/mm
2
should be assumed for widthtothickness
Buildings 2018,8, 114 3 of 15
ratios greater than or equal to four and widthtothickness ratios less than four, respectively. According
to EN 16351 [
1
], if the boards are edge glued, a characteristic rolling shear strength of 1.10 N/mm
2
may
be applied. If edge glueing is not used, and a minimum board widthtothickness ratio is not achieved,
a characteristic rolling shear value of 0.70 N/mm
2
is recommended. The typical mean values of rolling
shear strength from the experimental results range from 1.0 N/mm
2
to 2.0 N/mm
2
, and while there
are characteristic design values proposed dependent on the ratio of the board widthtothickness, there
is no information on the inﬂuence of the number of layers and the position of the transverse layer
relative to the neutral axis of the CLT panel.
This study examines the bending and rolling shear strength behaviour of CLT panels and the
inﬂuence of the panel layup on the failure behaviour. Three panel conﬁgurations are manufactured
and tested in accordance with EN 16351 [
1
]. These comprise two threelayer panel conﬁgurations
(60 mm and 120 mm thick) and one ﬁvelayer panel conﬁguration of thickness 100 mm. A reduced
mean bending strength and mean rolling shear strength with an increased panel thickness has been
reported [
9
], with no consideration for the inﬂuence of the number of layers on the characteristic
failure behaviour. As characteristic strength values are used in the design of structural CLT
panels, the inﬂuence of the panel thickness and panel layup on the characteristic behaviour is
investigated here.
2. Materials and Specimen Manufacture
The manufacture of the CLT panels was carried out in accordance with EN 16351 [
1
].
Three different sizes and test conﬁgurations of CLT panels were manufactured in order to determine
the bending and rolling shear properties. Boards of C16 Irish Sitka spruce (Picea sitchensis) with
nominal crosssectional dimensions of 100 mm
×
35 mm (for panels comprising 20 mm thick layers),
and 150 mm
×
44 mm (for panels comprising 40 mm thick layers), were used to manufacture the
CLT panels. The boards were initially stored in a conditioning chamber at a relative humidity of
65 ±5%
and at a temperature of 20
±
2
◦
C, for a period of two months prior to specimen preparation.
Prior to fabrication, all of the sides of the boards were planed to the desired board thickness (20 mm
and 40 mm) and width (75 mm and 150 mm). The natural frequency of each board was recorded using
the MTG acoustic grader in order to determine the modulus of elasticity parallel to grain. A mean
value of 8098 N/mm
2
was observed. All of the panels were manufactured using a onecomponent
PUR adhesive (PURBOND HB S309), with a spreading rate of 160 g/m
2
, and a pressure (face bonding
only, no edge bonding) of 0.6 N/mm
2
. These manufacturing parameters are based on the adhesive
qualiﬁcation testing undertaken by Sikora et al. [
4
]. The pressure was applied using steel plates,
tightened with M20 steel bolts to provide the required compressive force, and maintained for a period
of 120 min. The manufactured panels were then stored in a conditioning chamber for a period of one
month, prior to experimental testing.
3. Experimental Programme
3.1. Introduction
The test panel conﬁgurations are presented in Table 1, which gives the number of layers, layer
thickness, panel thickness, panel width, and the test span of the panel. The span of the panel was
determined from the geometrical constraints outlined in EN 16351 [
1
]. These geometrical constraints
result in different failure modes, namely bending and shear failure modes, allowing the bending and
shear strengths to be determined, respectively. For each test conﬁguration, shown in Table 1, eight
replicates were tested, giving a total of 48 CLT panels.
Buildings 2018,8, 114 4 of 15
Table 1. Test panel conﬁguration and determined properties.
Specimen
Label
Number of
Layers
Thickness
of Layer
(mm)
Panel
Thickness
(mm)
Panel Width
(mm)
Span in
Bending
(mm)
Test Properties
B320 3 20 60 584 1440 Bending strength and stiffness
B520 5 20 100 584 2400 Bending strength and stiffness
B340 3 40 120 584 2880 Bending strength and stiffness
S320 3 20 60 584 720 Shear (rolling) strength
S520 5 20 100 584 1200 Shear (rolling) strength
S340 3 40 120 584 1440 Shear (rolling) strength
3.2. Bending Test and Rolling Shear Test
The experimental testing of the strength and stiffness properties of the CLT panels was in
accordance with EN 16351 [
1
]. This standard speciﬁes fourpoint bending tests over spans of 24–30
times the thickness for the bending strength and stiffness determination, and 12 times the thickness
for the (rolling) shear strength determination perpendicular to the plane. The distance between the
load points was equal to six times the panel thickness for all of the test conﬁgurations. The CLT
panels were simply supported, as seen in Figure 1. A load was applied across the whole width of the
CLT specimens using steel spreader beams and plates of a width, not greater than half of the panel
thickness. The load was applied at a constant rate of displacement, adjusted so that the maximum load
was reached within 300
±
120 s. The bending test setup for one ﬁvelayer panel over a test span of
2400 mm can be seen in Figure 1.
Figure 1. Fourpoint bending test setup on a ﬁvelayer cross laminated timber (CLT) panel (B5207)
over a span of 2400 mm.
Initially, each specimen was loaded up to 40% of the estimated maximum load, in order to
determine the global and local bending stiffness. The local displacement was measured by linear
variable differential transformers (LVDTs) over a central gauge length of ﬁve times the panel thickness,
in accordance with EN 16351 [
1
]. The global displacement was measured using two LVDTs at
the midspan on either side of the test specimen. The mean value of these two values was used
to calculate the global deﬂection. The specimen was then loaded to failure at a constant rate of
displacement, and the maximum load and vertical global displacement of the test were recorded.
The recorded local and global deﬂection measurements allowed the local and global elastic moduli (E
L
and E
G
, respectively), and the local and global stiffnesses (E
L
Iand E
G
I, respectively) of each panel to
be calculated.
The theoretical maximum bending stress (
σmax
) and rolling shear stress (
τ
r
max
) in the specimens
loaded perpendicular to the plane were calculated using the maximum test values of the bending
Buildings 2018,8, 114 5 of 15
moment and the shear force, respectively. The following theoretical methods were applied to evaluate
the stresses in the timber: layered beam theory, Gamma beam theory, and shear analogy theory.
A comparison of these approximate veriﬁcation procedures for CLT has been comprehensively
investigated by Bogensperger et al. [
24
] and Li [
25
]. For the stress calculations in this study, the mean
value used for the modulus of elasticity parallel to the grain was 8098 N/mm
2
, in line with the test
results on the modulus of the elasticity of the Sitka spruce boards. The normal and shear stress
distributions in a ﬁvelayer panel are illustrated in Figure 2a,b, respectively. As the modulus of
elasticity perpendicular to the grain (E
90
) is very low for timber, the contribution of the transverse
layers to the bending performance was excluded from the calculations. For the Gamma beam theory
calculations, a constant value of 50 N/mm
2
was used for the rolling shear modulus, G
R
, in accordance
with Bogensperger et al. [
20
]. In Figure 2b, the distinction between the maximum shear (
τmax
) and
maximum rolling shear (τrmax) strengths can be seen.
Figure 2.
CLT element loaded out of plane. (
a
) Normal stress distribution through a ﬁvelayer panel
assuming E90 = 0 N/mm2; (b) shear stress distribution assuming E90 = 0 N/mm2.
In a ﬁvelayer panel, the maximum shear stress occurs in the middle layer, which is orientated
similarly to the outermost layers (E
0
). The maximum rolling shear stress is the greatest in the transverse
layers (E90) either side of the middle layer (E0). In a threelayer panel, the maximum shear stress and
maximum rolling shear stress both occur in the central transverse layer of the CLT panel.
The theoretical panel bending stiffness (EI
CLT
) based on composite theory is used for comparison
with the experimental results. This is calculated using Equation (1).
EIC LT =E0
n
∑
i=1Ii+Aiy2
i(1)
where:
E0= mean MOE of the batch of timber boards used to manufacture the CLT panels.
Ii= second moment of area of the board, i.
Ai= area of the board, i.
yi= distant from the neutral axis of the panel to the centroid of the board, i.
n= total number of boards, orientated parallel to the span.
Statistical methods have been implemented to examine the distribution of the experimental
bending and the shear test results. This has been applied to examine the differences in the panel
conﬁguration (Table 1). The characteristic or ﬁfth percentile bending and shear values were determined
Buildings 2018,8, 114 6 of 15
in accordance with EN 14358 [
26
], taking into account the adjustment factor, k
s
, to account for the
sample size.
4. Experimental Results
4.1. Introduction
This section details the experimental test results for the CLT panels subjected to fourpoint ﬂexural
tests in accordance with EN 16351 [
1
]. The bending and shear test conﬁgurations have allowed for the
respective strength and stiffness values to be determined. The statistical methods have been utilised to
establish the characteristic bending strength, and shear and rolling shear values. The inﬂuence of the
number of layers in each panel conﬁguration has been examined.
4.2. Bending Test Results
The bending test results for all of the panel conﬁgurations are presented. All of the specimens
failed in tension in the bottom lamination, because of bending as expected. The bending failure of a
ﬁvelayer panel can be seen in Figure 3after testing to destruction.
Figure 3. Bending failure on the bottom tensile face of the panel (B5207).
The total deﬂection to the failure of each panel was recorded using the mean measurement from
two LVDT’s located at the midspan on each side of the panel. The deﬂection to failure from the
bending tests on the ﬁvelayer panels (conﬁguration B520) can be seen in Figure 4. The maximum
moment has been calculated for each test and the analytical models previously mentioned have been
used to determine the theoretical maximum bending strength.
Figure 4. Panel conﬁguration B520: deﬂection to failure load behaviour.
Buildings 2018,8, 114 7 of 15
Tables 2–4present the individual global and local modulus of elasticity (E
G
and E
L
, respectively)
and bending stiffness (E
G
Iand E
L
I, respectively) results of each panel of the threelayer B320,
ﬁvelayer B520, and threelayer B340 conﬁgurations, respectively. The maximum theoretical bending
strengths calculated using the layered beam theory, gamma method, and shear analogy method are
also presented in addition to the failure mode. The average, standard deviation, and ﬁfth percentile
values are also tabulated.
When examining the maximum bending strength, the behaviour of each panel conﬁguration
is relatively consistent with the exception of panel B3205. Panel B3205 failed in the tension as
expected, but failed at a relatively low load of 29.52 kN, due to a series of large knots in adjacent boards
on the bottom tension face of the panel.
The mean bending strength results of the 60 mm B320 panel conﬁguration from the layered beam
theory, gamma method, and shear analogy method are 35.98 N/mm
2
, 35.71 N/mm
2
, and 35.93 N/mm
2
,
respectively. These values are similar to one another with the almost identical results from the
layered beam theory and the shear analogy method, and a slightly reduced value for the gamma
method. The comparative values for the 100 mm B520 conﬁguration are slightly reduced with
bending strength values of 34.43 N/mm
2
, 34.35 N/mm
2
, and 34.08 N/mm
2
, respectively, and the
120 mm B340 panel conﬁguration values are further reduced with bending strength values of
29.47 N/mm2, 29.25 N/mm2, and 29.43 N/mm2, respectively, from the layered beam theory, gamma
method, and shear analogy method.
The mean characteristic bending strength results for the 60 mm B320 panel conﬁguration
were found to be quite high for the C16 timber, with values from the layered beam theory, gamma
method, and shear analogy method of 24.11 N/mm
2
, 23.98 N/mm
2
, and 24.08 N/mm
2
, respectively.
The characteristic bending strength for the 100 mm B520 panel conﬁguration were found to be
greater than that of the 60 mm B320 panel conﬁguration, with comparative characteristic values
of 26.38 N/mm
2
, 26.34 N/mm
2
, and 26.13 N/mm
2
for the layered beam theory, gamma method,
and shear analogy method, respectively. While the mean maximum bending strength results are lower
than that observed in the panel conﬁguration B320, the characteristic strength values are higher.
This indicates less scatter in the experimental results and more predictable failure behaviour from the
ﬁvelayer conﬁguration. The characteristic bending strength for the 120 mm B340 panel conﬁguration
from the layered beam theory, gamma method, and shear analogy method were 19.52 N/mm
2
,
19.38 N/mm
2
, and 19.49 N/mm
2
, respectively. This is a low result when compared to the threelayer
conﬁguration B320 with a reduced board thickness of 20 mm, but the values are still relatively high
considering the raw material used to manufacture the panels was C16 grade.
Table 2.
Bending test results for threelayer B320 conﬁguration. E
G
—global elastic moduli;
EGI—global elastic moduli; EL—local stiffness; ELI—global stiffness.
Panel I.D EG
(N/mm2)
EGI
(Nmm2)
EL
(N/mm2)
ELI
(Nmm2)
Max. Bending Strength, (N/mm2)Failure
Mode
Layered Beam
Theory
Gamma
Method
Shear Analogy
Method
B3201 8840.9 8.26 ×1010 11,510.4 1.07 ×1011 40.44 40.08 40.39 Tension
B3202 7967.8 7.44 ×1010 11,785.4 1.10 ×1011 37.98 37.69 37.93 Tension
B3203 7342.6 7.48 ×1010 7771.9 7.92 ×1010 36.10 35.83 36.05 Tension
B3204 8326.2 8.29 ×1010 10,671.6 1.06 ×1011 44.34 43.98 44.29 Tension
B3205 6209.4 6.31 ×1010 9088.8 9.24 ×1010 24.60 24.45 24.57 Tension
B3206 6340.8 6.28 ×1010 6569.7 6.50 ×1010 32.03 31.83 31.99 Tension
B3207 6430.6 6.73 ×1010 10,164.6 1.06 ×1011 36.14 35.90 36.09 Tension
B3208 7100.9 7.57 ×1010 8854.8 9.82 ×1010 36.19 35.93 36.14 Tension
Average 7319.9 7.29 ×1010 9552.1 9.56 ×1010 35.98 35.71 35.93 
Std. Dev. 983.9 7.91 ×1091821.1 1.60 ×1010 5.84 5.77 5.83 
5th perc. 5417.2 5.70 ×1010 6030.9 6.27 ×1010 24.11 23.98 24.08 
Buildings 2018,8, 114 8 of 15
Table 3. Bending test results for ﬁvelayer B520 conﬁguration.
Panel I.D EG
(N/mm2)
EGI
(Nmm2)
EL
(N/mm2)ELI (Nmm2)
Max. Bending Strength, (N/mm2)Failure
Mode
Layered Beam
Theory
Gamma
Method
Shear Analogy
Method
B5201 6409.1 3.08 ×1011 10,511.9 5.05 ×1011 34.50 34.37 34.20 Tension
B5202 6389.0 3.07 ×1011 7322.5 3.51 ×1011 32.26 32.20 31.98 Tension
B5203 6823.0 3.28 ×1011 8130.5 3.90 ×1011 34.96 34.89 34.27 Tension
B5204 5806.1 2.69 ×1011 6688.3 3.10 ×1011 27.68 27.64 27.44 Tension
B5205 5730.3 2.82 ×1011 7433.6 3.66 ×1011 36.45 36.39 36.13 Tension
B5206 5588.9 2.65 ×1011 8223.8 3.91 ×1011 31.57 31.52 31.30 Tension
B5207 6623.2 3.16 ×1011 7874.2 3.76 ×1011 37.05 36.98 36.72 Tension
B5208 7112.3 3.11 ×1011 8140.7 3.56 ×1011 40.93 40.85 40.57 Tension
Average 6310.2 2.98 ×1011 8040.7 3.81 ×1011 34.43 34.35 34.08 
Std. Dev. 551.7 2.29 ×1010 1127.5 5.63 ×1010 4.00 3.99 3.96 
5th perc. 5183.7 2.51 ×1011 5971.1 2.78 ×1011 26.38 26.34 26.13 
Table 4. Bending test results for threelayer B340 conﬁguration.
Panel I.D EG
(N/mm2)EGI (Nmm2)EL
(N/mm2)ELI (Nmm2)
Max. Bending Strength, (N/mm2)Failure
Mode
Layered Beam
Theory
Gamma
Method
Shear Analogy
Method
B3401 7406.6 6.23 ×1011 8974.4 7.55 ×1011 22.39 22.22 22.36 Tension
B3402 7845.8 6.60 ×1011 9310.4 7.83 ×1011 26.17 25.96 26.14 Tension
B3403 7437.8 6.25 ×1011 10,780.4 9.07 ×1011 26.87 26.67 26.84 Tension
B3404 7252.2 5.56 ×1011 8212.9 6.29 ×1011 33.85 33.62 33.81 Tension
B3405 7254.4 5.86 ×1011 7568.2 6.11 ×1011 32.85 32.62 32.81 Tension
B3406 6005.5 4.90 ×1011 6925.0 5.65 ×1011 24.05 23.91 24.02 Tension
B3407 6641.7 5.50 ×1011 9065.1 7.51 ×1011 32.80 32.58 32.76 Tension
B3408 8420.2 6.94 ×1011 11,014.1 9.07 ×1011 36.75 36.44 36.70 Tension
Average 7283.0 5.98 ×1011 8981.3 7.38 ×1011 29.47 29.25 29.43 
Std. Dev. 726.5 6.57 ×1010 1429.3 1.29 ×1011 5.23 5.19 5.23 
5th perc. 5792.6 4.65 ×1011 6239.6 4.93 ×1011 19.52 19.38 19.49 
The theoretical panel stiffness has been calculated using Equation (1). The mean ﬂexural stiffness
results for different the CLT panel conﬁgurations using global and local deformations are presented and
compared to the theoretical values in Figure 5. The stiffness results obtained from testing are adjusted
to the per meter width of panel values for comparison with the theoretical results. The highest stiffness
of 1.03
×
10
12
Nmm
2
and 1.27
×
10
12
Nmm
2
using global and local deformations, respectively, was
recorded for the thickest threelayer panels of the 40 mm layers (B340) as expected. The corresponding
values for the ﬁvelayer panels (B520) were 5.19
×
10
11
Nmm
2
and 6.63
×
10
11
Nmm
2
, and for
the thinner threelayer panels (B320), the values were 1.28
×
10
11
Nmm
2
and 1.67
×
10
11
Nmm
2
.
As seen in Figure 5, the global stiffness results are in good agreement with the theoretical stiffness
results. The same cannot be said for the local stiffness results, but it should be noted that the values
calculated using the local deformation measurements presented greater scatter than those from the
global deformations. This is thought to be a consequence of local variabilities within test specimens
that inﬂuences the local deformations to a greater extent than measured globally. This phenomenon is
in agreement with the ﬁnding made by RidleyEllis et al. [
27
], who suggested that the primary reason
the for observed differences between the global and local deformations in timber is not necessarily
attributed to the shear deformations, but to the variation of modulus of elasticity within a specimen.
Buildings 2018,8, 114 9 of 15
Figure 5.
Mean bending stiffness results per meter width and the inﬂuence of panel conﬁguration
and depth.
The mean maximum bending strength results and the inﬂuence of the panel conﬁguration can
be seen in Figure 6. The analytical models examined in this study have been shown to give reliable
results for individual panel conﬁgurations. The analytically determined bending strength values,
calculated using the layered beam theory and shear analogy theory, were found to be very similar
for all of the specimens. For the ﬁvelayer panels, the Gamma beam theory produced similar results
to the layered beam theory and the shear analogy theory, but the values were lower by 0.5–3.5% for
the threelayer panel conﬁgurations. This is thought to be associated with the conservative value
of 50 N/mm
2
, assumed for the rolling shear modulus, G
R
[
9
]. When comparing the different panel
conﬁgurations, the highest mean values of approximately 36 N/mm
2
were obtained for the thinnest
samples (B320), and the lowest of approximately 29 N/mm
2
for the thickest (B340). Based on these
results, there is a general tendency that the thicker the CLT panel, the lower the bending strength.
Figure 6. Mean maximum bending strength results and the inﬂuence of panel conﬁguration.
The inﬂuence of the number of layers can also be seen to have an effect when examining the mean
and standard deviation associated with the ﬁvelayer panels (B520). There is a reduced scatter of
the results associated with this panel conﬁguration, when compared to the threelayer panels. This is
due to the increased number of layers and the homogenising effect or laminating effect, whereby
defects such as knots, seasoning checks, reaction wood, and sloping grain, which may be crucial in the
performance of a solid timber piece, are generally limited to one board in a CLT panel, greatly reducing
the effect on the structural integrity of the panel. The statistical distribution of the bending strength
Buildings 2018,8, 114 10 of 15
results (layered beam theory) for each panel conﬁguration are presented in Figure 7, and the inﬂuence
of panel thickness can be seen. The lines representing the mean and characteristic ﬁfth percentile
values are also presented, in order to examine the differences due to the panel conﬁgurations.
Figure 7. Bending strength distribution and inﬂuence of panel thickness and number of layers.
The variability and the shape of the distribution decreases in the ﬁvelayer panel with a thickness
of 100 mm. There is an increase in the characteristic bending strength in the ﬁvelayer panel to
a level higher than that of the threelayer panel of a 60 mm thickness. A small upward trend in
the characteristic bending strength is observed as the panel thickness increases from 60 mm to
100 mm, indicating the positive effect of an increased number of boards in the panel conﬁguration.
The observations of a reduced bending strength with an increased panel thickness are not valid when
examining the characteristic bending strength behaviour of the CLT panels.
4.3. Rolling Shear Test Results
The shear test results for all of the panel conﬁgurations are presented. Each panel failed in the
rolling shear as expected, with the exception of panel S5201, which delaminated prior to shear failure.
The shear and rolling shear test failure behaviour in a ﬁvelayer panel (S5205) and a threelayer panel
(S3407) can be seen Figure 8a,b, respectively. The failure mechanism can be seen in the transverse
layers of both of the panel types. The loaddeﬂection response of the threelayer panels (conﬁguration
S340) can be seen in Figure 9. The maximum moment has been calculated for each test specimen and
the analytical models have been used to determine the theoretical maximum shear and maximum
rolling shear stress.
Figure 8. Rolling shear failure: (a) ﬁvelayer panel (S5205) and (b) threelayer panel (S3407).
Buildings 2018,8, 114 11 of 15
Figure 9. Panel conﬁguration S340: deﬂection to failure load behaviour.
Tables 5–7present the maximum rolling shear strength results of each panel of the threelayer
S320, ﬁvelayer S520, and threelayer S340 panel conﬁgurations. All of the panels failed in rolling
shear in the transverse cross layers, with the exception of panel S5201. Panel S5201 had minor
manufacturing defects, which resulted in delamination failure rather than shear failure, therefore, this
result was excluded from the average, standard deviations, and ﬁfth percentile calculations of shear
and rolling shear strength. There is no distinction made between the maximum shear and maximum
rolling shear strength in the threelayer panels, as they both occur at the same place (transverse layer)
and have the same value. There is a distinction made between the maximum shear strength (
τmax
)
and the maximum rolling shear strength (
τ
r
max
) in the ﬁvelayer panels (Figure 2b). This is due to the
maximum shear stress occurring in a board at the neutral axis, which is orientated in the longitudinal
direction (E
0
). The maximum rolling shear stress occurs in the adjacent transverse layers (E
90
) and is
accounted for and presented by analysing the entire cross section.
The mean shear/rolling shear strength from the layered beam theory, gamma method, and
shear analogy method for the 60 mm S320 panel conﬁguration are 2.19 N/mm
2
, 2.14 N/mm
2
,
and 2.22 N/mm
2
, respectively. The reduced mean rolling shear strength values were observed for
the 100 mm S520 panel conﬁguration, where the mean rolling shear strength from the layered beam
theory, gamma method, and shear analogy method were 1.40 N/mm
2
, 1.39 N/mm
2
, and 1.39 N/mm
2
,
respectively. By comparison, the mean rolling shear strength for the thickest 120 mm S340 panel
conﬁguration was further reduced with the rolling shear values from the layered beam theory, gamma
method, and shear analogy method of 1.33 N/mm
2
, 1.30 N/mm
2
, and 1.35 N/mm
2
, respectively.
The results indicated similar trends observed for the bending strength specimens, the thicker the CLT
panel, the lower its rolling shear strength.
The characteristic rolling shear strength values were found to follow the same trend as the
mean rolling shear strength values, with reduced mean/characteristic rolling shear strength values
with increased panel thickness. The maximum characteristic values were achieved by the 60 mm
S320 panel conﬁguration with values of 1.67 N/mm
2
, 1.63 N/mm
2
, and 1.69 N/mm
2
, respectively
from the layered beam theory, the gamma method, and the shear analogy method, calculated in
accordance to EN 14358 [
26
]. The characteristic rolling shear strength values for the 100 mm S520
panel conﬁguration from the layered beam theory, gamma method, and shear analogy method were
1.07 N/mm
2
, 1.06 N/mm
2
, and 1.06 N/mm
2
, respectively, and the characteristic rolling shear strength
for the 120 mm S340 panel conﬁguration from the layered beam theory, gamma method, and shear
analogy method are 0.90 N/mm2, 0.88 N/mm2, and 0.91 N/mm2, respectively.
Buildings 2018,8, 114 12 of 15
Table 5. Shear/rolling shear test results for threelayer S320 conﬁguration.
Panel I.D
Rolling Shear Strength (N/mm2)
Failure Mode
Layered Beam Theory Gamma Method Shear Analogy Method
S3201 1.90 1.85 1.92 Shear
S3202 2.03 1.98 2.05 Shear
S3203 2.30 2.24 2.33 Shear
S3204 2.42 2.37 2.46 Shear
S3205 2.03 1.99 2.06 Shear
S3206 2.43 2.37 2.46 Shear
S3207 1.88 1.84 1.91 Shear
S3208 2.55 2.49 2.58 Shear
Average 2.19 2.14 2.22 
Std. Dev. 0.26 0.26 0.27 
5th perc. 1.67 1.63 1.69 
Table 6. Rolling shear test results for ﬁvelayer S520 conﬁguration.
Panel I.D
Rolling Shear Strength (N/mm2)
Failure Mode
Layered Beam Theory Gamma Method Shear Analogy Method
S5201 0.65 0.65 0.65 Delamination
S5202 1.33 1.33 1.32 Shear
S5203 1.11 1.10 1.10 Shear
S5204 1.32 1.31 1.31 Shear
S5205 1.43 1.43 1.42 Shear
S5206 1.50 1.49 1.49 Shear
S5207 1.56 1.56 1.55 Shear
S5208 1.53 1.52 1.52 Shear
Average 1.40 1.39 1.39 
Std. Dev. 0.16 0.16 0.16 
5th perc. 1.07 1.06 1.06 
Table 7. Shear/Rolling shear test results for threelayer S340 conﬁguration.
Panel I.D
Rolling Shear Strength (N/mm2)
Failure Mode
Layered Beam Theory Gamma Method Shear Analogy Method
S3401 0.98 0.96 0.99 Shear
S3402 1.16 1.14 1.18 Shear
S3403 1.17 1.15 1.18 Shear
S3404 1.28 1.26 1.30 Shear
S3405 1.57 1.53 1.59 Shear
S3406 1.55 1.52 1.57 Shear
S3407 1.39 1.36 1.41 Shear
S3408 1.55 1.52 1.57 Shear
Average 1.33 1.30 1.35 
Std. Dev. 0.22 0.22 0.22 
5th perc. 0.90 0.88 0.91 
The established mean values of rolling shear strength are in agreement with the study by Blaß and
Görlacher [
23
], where rolling shear results ranged from 1.2 N/mm
2
to 2.1 N/mm
2
, and a characteristic
value of 1.0 N/mm
2
for the rolling shear strength of European spruce was proposed. The mean
characteristic rolling shear values of the tests were found to range from 0.88 N/mm
2
to 1.69 N/mm
2
.
The lowest values were observed in the thickest panel conﬁguration, S340. The boards used in this
panel conﬁguration had board widthtothickness ratios less than four. The two most common design
recommended values for such layers are 0.7 N/mm
2
, prescribed by EN 16351 [
1
], and 0.8 N/mm
2
,
prescribed by Brandner et al. [
3
]. In Figure 10, the inﬂuence of the panel thickness and number of
layers on the rolling shear strength distribution can be seen. When examining the rolling shear strength
distribution, there is a decrease in the mean and characteristic shear strength with the increasing panel
Buildings 2018,8, 114 13 of 15
thickness. The variability in the distribution of the rolling shear strength for the ﬁvelayer conﬁguration
is seen to be less than either of the threelayer conﬁgurations.
Figure 10. Rolling shear strength distribution and inﬂuence of panel thickness and number of layers.
5. Conclusions
The following conclusions can be formulated based on the investigations presented on the bending
and shear performance of the CLT panels manufactured from C16 timber.

The ﬁndings of this research project suggest that C16 timber is suitable for the manufacture of
CLT products.

The global bending stiffness results are in good agreement with the theoretical bending stiffness
results, regardless of the panel thickness or layup studied.

The mean and characteristic bending strength was higher than expected for the CLT panels
manufactured from C16 grade timber.

The test results generally indicated a decreasing bending strength with an increasing panel
thickness. The highest mean bending strength value, of approximately 36 N/mm
2
, was obtained
for the thinnest panels (B320), and the lowest, of approximately 29 N/mm
2
, for the thickest
(B340).

The characteristic bending strength was found to be inﬂuenced by the number of layers, with
the highest characteristic bending strength values being achieved by the intermediate 100 mm
ﬁvelayer panel conﬁguration. The homogenising or laminating effect resulting from the increased
number of layers in a CLT panel may be utilised to achieve an additional capacity in the structural
design of the CLT structures.

The mean rolling shear strength was found to reduce with the increasing panel thickness.
The mean values ranged between 1.30 N/mm
2
and 2.22 N/mm
2
, which is in line with the rolling
shear values presented by Blaß and Görlacher [
23
] of between 1.2 N/mm
2
and 2.1 N/mm
2
,
for European spruce.

The characteristic rolling shear strength of the panels was found to range from 0.88 N/mm
2
to 1.69 N/mm
2
. Given that the board widthtothickness ratio in this study was less than four,
the experimental results are greater than the characteristic rolling shear values of 0.7 N/mm
2
,
prescribed by EN 16351 [1], and 0.8 N/mm2, prescribed by Brandner et al. [3].

The results show that the mean bending and rolling shear strength decrease with the increasing
panel thickness, irrespective of the number of layers. The characteristic bending strength is also
inﬂuenced by the panel thickness, but the number of layers is found to have a signiﬁcant impact.
Buildings 2018,8, 114 14 of 15

To further examine these initial ﬁndings, tests are required on the panels of equal thickness
comprising a different number of layers to allow the inﬂuence of the layer thickness on the
bending and rolling shear strength to be assessed.
Author Contributions:
C.O.C. contributed to the conceptualization, experimental investigation and analysis of
the project in addition to writing, reviewing and editing of the ﬁnal article. K.S. was involved in the acquisition
of funding for this project in addition to experimental activities and writing, reviewing and editing of the ﬁnal
article. A.M.H. was involved in the acquisition of funding for this project, supervison of all experimental activities
and writing, reviewing and editing of the ﬁnal article.
Funding:
This work has been carried out as a part of the project entitled ‘Commercialisation of Irish Cross
Laminated Timber’ (project ref. 15/C/694), funded by the Department of Agriculture, Food, and the Marine of
the Republic of Ireland.
Acknowledgments:
The contribution of the technical staff of the College of Engineering and Informatics, National
University of Ireland Galway, in particular, Peter Fahy, Colm Walsh, and Gerard Hynes, is acknowledged.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).