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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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