Evaluating rolling shear strength properties of CLT by torsional shear tests and bending tests

Abstract

This paper presents a study on evaluating rolling shear (RS) strength properties of cross laminated timber (CLT) using torsional shear tests and bending tests. The CLT plates were manufactured with Spruce-Pine-Fir boards and glued with polyurethane adhesive. Two types of layups (3-layer and 5-layer) and two clamping pressures (0.1 MPa and 0.4 MPa) were studied. For the torsional shear tests, small shear block specimens were sampled from the CLT plates and the cross layers were processed to have an annular cross section. Strip specimens were simply sampled from the CLT plates for the bending tests. Based on the failure loads, RS strength properties were evaluated by torsional shear formula, composite beam formulae as well as detailed finite element models, respectively. It was found that the two different test methods yielded different average RS strength value for the same type of CLT specimens. The test results showed that the CLT specimens pressed with the higher clamping pressure had slightly higher average RS strength. The specimens with thinner cross layers also had higher RS strength than the specimens with thicker cross layers.

Full text

EVALUATING ROLLING SHEAR STRENGTH PROPERTIES OF
CROSS LAMINATED TIMBER BY TORSIONAL SHEAR TESTS
AND BENDING TESTS
Minghao Li1, Frank Lam2, and Yuan Li3
ABSTRACT: This paper presents a study on evaluating rolling shear (RS) strength properties of cross laminated timber
(CLT) using torsional shear tests and bending tests. The CLT plates were manufactured with Spruce-Pine-Fir boards and
glued with polyurethane adhesive. Two types of layups (3-layer and 5-layer) and two clamping pressures (0.1 MPa and 0.4
MPa) were studied. For the torsional shear tests, small shear block specimens were sampled from the CLT plates and the
cross layers were processed to have an annular cross section. Strip specimens were simply sampled from the CLT plates for
the bending tests. Based on the failure loads, RS strength properties were evaluated by torsional shear formula, composite
beam formulae as well as detailed finite element models, respectively. It was found that the two different test methods
yielded different average RS strength value for the same type of CLT specimens. The test results showed that the CLT
specimens pressed with the higher clamping pressure had slightly higher average RS strength. The specimens with thinner
cross layers also had higher RS strength than the specimens with thicker cross layers.
KEYWORDS: Cross laminated timber, rolling shear strength, torsional shear tests, bending tests
1 INTRODUCTION
1
Rolling shear (RS) stress in wood is defined as the shear
stress in the radial-tangential plane perpendicular to the
grain direction. RS strength and stiffness of wood is much
lower than its longitudinal shear strength and stiffness.
According to the Wood Handbook (FPL, 2010), RS
strength normally varies between 18% and 28% of
parallel-to-grain shear strength based on limited test data.
In Eurocode 5 (2004), a characteristic RS strength of 1.0
MPa is used for wood independent of its strength class.
Therefore, in timber design, high RS stresses should
always be avoided due to the low RS capacity of wood.
Cross laminated timber (CLT) consists of crosswise
oriented layers of wood boards that are often glued by
adhesives. RS strength and stiffness are not major design
properties for timber. For CLT, however, RS strength and
stiffness must be considered in some loading scenarios due
to the existing cross layers. For example, when a CLT
floor panel is supported by columns, highly concentrated
loads in the supporting area may cause high RS stresses in
1 Minghao Li, University of Canterbury, Private Bag 4800,
Christchurch 8140, New Zealand, minghao.li@canterbury.ac.nz
Formerly, University of British Columbia, 2835-2424 Main Mall,
Vancouver, Canada
2 Frank Lam, University of British Columbia, 4041-2424 Main
Mall, Vancouver, Canada, frank.lam@ubc.ca
3 Yuan Li, University of British Columbia, 2843-2424 Main Mall,
Vancouver, Canada, ubcliy@interchange.ubc.ca
cross layers; the same concerns may arise for designing
short-span floors or beams. Therefore, there is a need to
evaluate the RS strength and stiffness properties of CLT
products to provide technical support for more robust
designs.
ASTM D2718-00 (2006) stipulates two test methods
(planar shear test and short-span bending test) to evaluate
shear properties of wood products. In the planar shear test,
shear loads are applied by two metal plates face-glued onto
the specimen. The short-span bending test is to load the
specimen with small span-depth ratios to encourage shear
failure mechanism. Norlin et al. (1999) used a short-span
bending test to study longitudinal and RS shear strength
properties of a laminated veneer product. Using non-
destructive bending vibration tests, Fellmoser and Blass
(2004) studied the influence of RS modulus on CLT
stiffness as well as the relationship between shear
deformations and the span-depth ratios. Mestek et al.
(2008) studied the influence of shear deformations in cross
layers on the load carrying capacity of CLT beams. Zhou
et al. (2014) used both planar shear tests and short-span
bending tests to study RS strength and stiffness properties
of CLT specimens made by black spruce.
The objective of this study is to evaluate the RS strength
properties of non-edge-glued CLT plates by two test
methods: torsional shear tests and short-span bending tests.
The CLT plates were manufactured mainly by mountain
pine beetle killed lodgepole pine, which is a major species

in the Spruce-Pine-Fir (SPF) group in Canada. Another
important motivation for this study is to help recover the
wood resource from the beetle attacked forests. This study
is also focused on understanding the influence of the
clamping pressure for curing adhesives and the thickness
of cross layers on the RS strength properties of the CLT
products.
2 METHOD
2.1 CLT plate specimens
As shown in Figure 1, 5-layer Spruce-Pine-Fir (SPF5)
plates and 3-layer S.P.F. (SPF3) plates were manufactured
for the experimental studies. For each layup, two clamping
pressures (0.1 MPa representing vacuum press and 0.4
MPa representing mechanic press) were used for the
pressing process. Thus, combining the layup and the curing
pressure, four types of CLT plates were studied. For each
type, three full-size plates were sampled. In the following
context, a SPF5-0.4 represents the 5-layer S.P.F. panel
pressed under 0.4 MPa. Similarly, a 3-layer S.P.F plate
pressed under 0.1 MPa is labeled as SPF3-0.1.
Figure 1: Full-size CLT plates
Table 1 shows the board grades, laminate thickness, and
dimensions of the CLT plates. In the SPF5 plates, No. 2 or
better boards were used for two face layers and the core
layer while stud grade boards were used for two cross
layers. In the SPF3 plates, No. 2 or better boards were used
for two face layers and stud grade boards were used for the
crosswise core layer. All the laminates were 34 mm x 138
mm boards except that the cross layers in the SPF5 plates
were 19 mm x 138 mm boards. The average moisture
content of the boards was about 13% with a coefficient of
variation (COV) of 0.18. Modulus of Elasticity (MOEs) of
the boards were also measured by transverse vibration tests
following a test standard (ASTM D6874-03, 2009) before
they were glued to the CLT plates. Table 2 shows the
vibration MOE results which showed that the No.2 and
better grade boards had higher stiffness than stud grade
boards.
Table 1: CLT layup and lamination grade
Type
Laminate Grade
Laminate
thickness
(mm)
Plate size
L×W×H
(mm)
SPF5-0.1 &
SPF5-0.4
No.2/Stud/No.2
/Stud/No.2
34/19/34/
19/34
3658×1219
×140
SPF3-0.1 &
SPF3-0.4
No.2/Stud/No.2
34
3658×1219
×102
Table 2: Vibration MOE of wood boards
Grade
MOE
Sample
size
Mean (GPa)
COV
No. 2 or btr.
11.43
16.4%
256
Stud
10.66
18.5%
280
2.2 Torsional Shear Tests
Figure 2 shows the schematics of a torsional shear
specimen, in which the mid-layer, i.e., the cross layer in a
CLT plate, has a milled-down annular cross section. The
reason to have such a cross section is to facilitate the RS
failure mechanism and reduce the stress concentrations
typically experienced by a square-shaped shear plane.
The specimens were processed in three steps. In the 1st
step, 3-layer blocks were sampled from the full-size CLT
plates. For the SPF5 specimens, two layers of wood
needed to be removed. In the 2nd step, the 3-layer blocks
were press drilled from the centre of the top face to the
bottom face; in the 3rd step, the blocks were further
processed by a CNC machine to achieve the annular cross
section. It should also be noted that for each torsional shear
specimen, the mid-layer was cut out from one piece of
wood in order to eliminate the influence of gaps on the
torsional shear stress distributions.
Figure 2: Torsional shear specimens
Figure 3 shows the test setup in which the torsional
moment was applied via a steel arm firmly connected with
one face layer of the specimen. The moment arm length
was 500 mm. The other face layer of the specimen was

fully restrained onto the test table. The test to failure time
for was kept in 5 ~ 10 minutes for each specimen.
Figure 3: Test setup of torsional shear test
Figure 4 shows an example of RS failure mode observed in
the cross layer. Most of the specimens had brittle failures
and the cracks were developed at an inclined angle with
respect to the top or bottom face of the specimen. Load-
displacement curves from the actuator also indicated an
approximate linear relationship up to the peak load
followed with a sudden drop of the load due to failure.
Figure 4: Rolling shear failure mode
Figure 5 shows the cumulative distributions of the failure
torsional moments for four types of specimens. Table 3
also lists the mean and COV for each type. It was found
that the SPF5 specimens with 19 mm thick cross layers had
much higher torsional shear capacity than the SPF3
specimens with 34 mm thick cross layer although the
annular cross sections of these specimens are the same. On
average, the SPF3 specimens pressed with 0.1 MPa and 0.4
MPa had almost the same torsional capacity. The SPF5
specimens pressed with 0.4 MPa had slightly higher
torsional capacity.
Figure 5: Cumulative distributions of peak torsional
moments
Table 3: Summary of torsional shear results
CLT
Cross
layer
thickness
(mm)
Sample
size
Failure torque (N.m)
mean
COV
SPF3-0.1
34
23
85.1
17.8%
SPF3-0.4
34
25
84.6
16.6%
SPF5-0.1
19
28
101.7
21.1%
SPF5-0.4
19
31
104.0
16.4%
Assuming rigid glue line bonding and homogeneous
isotropic material properties, the maximum torsional shear
stresses on an annular cross section can be calculated by
the torsional shear formula:
(1)
where T is the peak torsional moment, ro and ri are the
outer radius and inner radius of the annular cross section.
In this study, finite element models were also developed in
finite element software ANSYS v14 (2011) to model the
torsional shear specimens in order to consider the glue line
shear stiffness and the orthotropic wood properties. Solid
elements were used to model the wood boards and linear
spring elements were used to model the glue line shear
stiffness. Table 4 lists the input wood material properties.
Poisson’s ratios of lodgepole pine were obtained from the
Wood Handbook (FPL, 2010). The parallel to grain
modulus EL were obtained from the transverse vibration
tests. Additional assumptions on other wood properties
were given as follows. The perpendicular to grain modulus
ET/ ER was assumed to be 1/30 of EL. The parallel to grain
shear modulus GLR or GLT was assumed to be 1/16 of EL.
The rolling shear modulus GRT was assumed to be 1/10 of
GLR or GLT. These assumptions are also consistent with
commonly adopted assumptions for CLT strength and
stiffness calculations in Europe (FPInnovations, 2011).
Table 5 lists the shear stiffness of glue lines which were

experimentally obtained by Schaaf (2010) using torsional
shear testing methods.
Table 4: Wood orthotropic properties for FE models
Grade
Elastic properties (GPa)
Poisson’s ratios
EL
ET/ER
GLR/
GLT
GRT
νLR
νLT
νRT
No.
2/ btr
11.43
0.381
0.714
0.071
0.316
0.347
0.469
stud
10.66
0.355
0.666
0.067
Table 5: Shear stiffness of glue lines
Species
Clamping pressure
(MPa)
Shear stiffness
(N/mm3)
S.P.F.
0.1
19.0
0.4
20.6
Figure 6 shows the meshed FE model for the torsional
shear specimen. The metal part of the test jig was also
modelled. Figure 7 shows the RS stress distributions in the
cross layers of the SPF3-0.1 and SPF5-0.1 specimens
loaded under the average peak torsional loads. Apparently,
the stresses were not uniformly distributed along the
perimeter of the annular rings due to the orthotropic wood
properties.
Table 6 lists the average RS strength for each type of CLT
specimen evaluated by the torsional shear formula and the
FE model simulations. It was found that the torsional shear
formula gives unreasonably high RS strength due to the
assumption of homogenous isotropic material property.
The FE models give rational evaluations which are in a
reasonable range of RS strength compared with available
test data.
Figure 6: FE models of torsional shear specimens
Figure 7: RS stress distributions in cross layers of SPF3-
0.1 and SPF5-0.1 specimens (Pa)
Table 6: Summary of average RS strength from torsional
shear tests (MPa)
CLT
Cross layer
thickness (mm)
Shear
Formula
FE
model
SPF3-0.1
34
3.54
2.33
SPF3-0.4
34
3.52
2.31
SPF5-0.1
19
4.23
2.40
SPF5-0.4
19
4.33
2.46
2.3 Short-span bending tests
As shown in Figure 8, a series of three-point bending tests
on strip specimens sampled from the full-size CLT plates
have also been conducted by Yawalata and Lam (2011)
following a test standard (ASTM, D198-05a, 2005). A
span-to-depth ratio of 6 was used to encourage the RS
failure mode. The wood fibres of the top and bottom layers
of the specimens were parallel to the beam span. Table 7
lists the dimensions and the test results in terms of the
mean and standard deviation of the failure loads. The
bending test results showed that for the specimens with the
same layup, the increased clamping pressure from 0.1 MPa

to 0.4 MPa seemed to slightly increase the load-carrying
capacity on average.
Figure 8: Short-span bending test and RS failure mode
Table 7: Summary of bending test results
CLT
Span x depth
(mm x mm)
Sample
size
Failure loads
(kN)
mean
COV
SPF3-0.1
612x102
30
15.13
10.8%
SPF3-0.4
612x102
30
16.51
19.1%
SPF5-0.1
840x140
30
20.98
16.2%
SPF5-0.4
840x140
30
21.78
7.2%
Assuming rigid glue line bonding and material continuity
in the cross layers, the RS strengths of the bending
specimens were evaluated by three composite beam
theories: Layered beam (Bodig and Jayne, 1982); Gamma
beam method (Eurocode 5, 2004); and Shear analogy
method (Kreuzinger, 1999). Meanwhile, as shown in
Figure 9, considering orthotropic wood properties, gaps
between the wood boards in the cross layers and the glue
line shear stiffness, detailed FE models were also
developed in ANSYS v14 (2011) to study the RS stress
distributions under the failure loads. The wood properties
given in Table 4 and the glue line shear stiffness given in
Table 5 were used for the FE model simulations. Similar to
the torsional shear FE models, solid elements and linear
spring elements were used for the wood members and the
glue lines, respectively. Figure 10 shows the RS stress
distributions of the SPF5-0.1 and SPF3-0.1 specimens
under the average failure loads. It can be seen that the RS
stress distribution was not continuous in the cross layer
due to the gaps. The stresses in the vicinity of the gaps
were much smaller than those in the central parts of the
boards due to the shear stress release around the free
edges. This type of RS stress distribution also agreed with
the test observation that the RS failures of the specimens
tended to occur at a certain distance (comparable to the
thickness of the cross layers) from the location of the gaps.
Figure 9: FE model of a SPF5 bending specimen
Figure10: RS stress distributions in cross layers of SPF5-
0.1 and SPF3-0.1 specimens (Pa)
Table 8 lists the average RS strengths evaluated by the
composite beam theories and the FE modeling for each
type of the specimens. For the SPF5 specimens, the

calculations by the beam theories agreed reasonably well
with the FE results. However, for the SPF3 specimens, the
beam theories significantly over-estimated the RS strength
compared with the FE results. According to the composite
beam calculations, the average RS strength of the SPF3
specimens with 34 mm thick cross layers was about 18 %
higher than the SPF5 specimens with 19 mm thick cross
layers. However, the FE results showed that the average
RS strength of the SPF3 specimens was actually 8 % lower
than that of the SPF5 specimens. The FE models are
believed to be more accurate and the FE findings were also
consistent with the torsional shear test results. If one
considers the size effect on the RS strength of the cross
layers, it is understandable that thick cross layers tend to
have more volume of wood stressed under RS stresses than
thin cross layers. Therefore, thick cross layers will
normally have lower RS strength properties.
Table 8: Average RS strength calculated by beam theories
and FE models
CLT type
Layered
beam
Gamm
a beam
Shear
analogy
FE
model
SPF3-0.1
2.04
2.15
2.02
1.71
SPF3-0.4
2.22
2.34
2.20
1.87
SPF5-0.1
1.85
1.74
1.78
1.91
SPF5-0.4
1.93
1.81
1.85
1.98
3 CONCLUSIONS
In this study, RS strength properties of CLT plates
manufactured by S.P.F. boards and polyurethane adhesive
were evaluated by torsional shear tests and short-span
bending tests. The test results were analysed by torsional
shear formula, composite beam theories and detailed FE
modelling. Based on the test and modelling results, some
conclusions are drawn as follows:
 Two different test methods yielded very different RS
strength properties of the CLT specimens. Torsional
shear tests gave higher average RS strength range (2.31
~ 2.46 MPa) than that obtained from the short-span
bending tests (1.71 ~ 1.98 MPa). Besides the different
mechanism of the test methods, one reason might be
that the bending specimens had gaps between the
adjacent wood boards in the cross layers. However, the
torsional shear specimens eliminated the influence of
the gaps.
 On average, the SPF5 specimens with thin (19mm
thick) cross layers had about 7 % higher RS strength
than the SPF3 specimens with thick (34 mm) cross
layers although the cross layers consisted of the same
stud grade material. The thickness of cross layers
seemed to affect the RS strength properties for the CLT
specimens.
 For specimens with the same layup configuration, the
increased clamping pressure from 0.1 MPa to 0.4 MPa
increased the average RS strength of the CLT
specimens by 4% approximately.
 The torsional shear formula is not suitable for
evaluating RS strength properties of the torsional shear
specimens. The composite beam theories should also
be used with caution to evaluate the RS strength of the
bending specimens.
ACKNOWLEDGEMENT
The authors would like to thank NSERC strategic network
for engineered wood-based building systems for
supporting this research.
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