Matching Tests of Brittle Failure
of Bottom Rail Versus Tensile Strength Perpendicular to the Grain and Fracture
Energy in RT and TR Plane
Giuseppe Caprolu
ISSN 1402-1536
ISBN 978-91-7439-XXX-X (tryckt) ISBN 978-91-7439-887-8 (pdf) Luleå University of Technology 2014
Department of Xxxxxxxxxxxx
Division of Structural and Construction Engineering
Giuseppe Caprolu Matching Tests of Brittle Failure of Bottom Rail Versus Tensile Strength Perpendicular to the Grain and Fracture Energy in RT and TR Plane.
and Fracture Energy in RT and TR Plane
Giuseppe Caprolu
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
Division of Structural and Construction Engineering
Luleå 2014
www.ltu.se
1
Background ... 5
Aim ... 6
Limitations ... 6
TEST SETUP AND MATERIAL ... 6
Test specimen and Material Properties ... 6
Tensile strength perpendicular to grain and Fracture Energy ... 6
Bottom rail ... 7
Test program ... 7
Tensile strength perpendicular to grain ... 7
Fracture energy ... 9
Bottom rail ... 11
Correlation between the three experimental programs ... 13
Test Set-Up ... 16
Tensile strength perpendicular to grain ... 16
Fracture energy ... 17
Bottom rail ... 17
TEST RESULTS ... 18
Tensile strength perpendicular to grain ... 18
Load-displacement curves ... 18
Summary of test results ... 22
Fracture energy ... 23
Load-deflection curves ... 23
Summary of test results ... 26
Bottom rail ... 27
Failure modes ... 27
Load-time curves ... 28
Failure load ... 37
REFERENCES ... 41
APPENDIX A ... 42
Specimens tested in radial direction ... 43
2
Specimens tested with crack orientation TR ... 112
APPENDIX C ... 145
Series 1 – Anchor bolt at centre, 60 mm from the sheathing ... 146
Set 1 – Size of washer 40×40 mm ... 146
Pith down ... 146
Pith up ... 149
Set 2 – Size of washer 60×60 mm ... 152
Pith down ... 152
Pith up ... 155
Set 3 – Size of washer 80×70 mm ... 158
Pith down ... 158
Pith up ... 161
Set 4 – Size of washer 100×70 mm ... 164
Pith down ... 164
Pith up ... 167
Series 2 – Anchor bolt at 45 mm from the sheathing ... 170
Set 1 – Size of washer 40×40 mm ... 170
Pith down ... 170
Pith up ... 173
Set 2 – Size of washer 60×60 mm ... 176
Pith down ... 176
Pith up ... 179
Set 3 – Size of washer 80×70 mm ... 182
Pith down ... 182
Pith up ... 185
Series 3 – Anchor bolt at 30 mm from the sheathing ... 188
Set 1 – Size of washer 40×40 mm ... 188
Pith down ... 188
Pith up ... 191
Set 2 – Size of washer 60×60 mm ... 194
3
4 RT Crack orientation of the material. R indicates radial and T tangential
direction of the wood.
TR Crack orientation of the material. T indicates tangential and R radial direction of the wood.
PU Pith upwards (pith orientation of the bottom rail) PD Pith downwards (pith orientation of the bottom rail) Stddev Standard deviation
COV Coefficient of variation CV
0.05Characteristic value
f
t,90Tensile strength perpendicular to the grain [N/mm
2]
G
fFracture energy [N/m]
ω Moisture content [%]
ρ
0,ωDry density [kg/m
3]
5 anchored is under development in Sweden [1]. In this research an important focus has been put on the problem of the possible splitting of the bottom rail. In partially anchored timber frame shear walls there are not hold downs taking the vertical loads so the corresponding forces can be replaced by vertical loads from upper storeys, the roof or connection between shear wall and transversal wall. In this case the bottom row of rail transmits the vertical forces in the sheathing to the bottom rail (instead of the vertical stud) where the anchor bolts will further transmit the forces into the foundation. The bottom rail is then subjected to tensile load perpendicular to the grain, which can be often causes a splitting failure [2].
The problem of the splitting failure in the bottom rail has been studied in two experimental programs [3, 4], where two brittle failure modes were found for the bottom rail subjected to tensile load: (1) a crack opening from the bottom surface of the bottom rail and (2) a crack opening from the edge surface of the bottom rail along the line of the sheathing-to-framing joint. The compliance method, based on the fracture mechanics theory, have been used to derive two formulas, one per failure mode, in order to calculate the load carrying capacity of bottom rail in partially anchored timber frame shear walls [5, 6, 7]. One of the parameters in the formulas is the tensile strength perpendicular to grain. It should be noted that the orientation, and consequently the value, for f
t,90varies in basis to the failure mode of the bottom rail: tangential for failure mode (1) and radial for failure mode (2). Another parameter in the formulas is the fracture energy. Also in this case it should be noted that the crack orientations, and consequently its value, varies in basis to the failure mode of the bottom rail:
TR for failure mode (1) and RT for failure mode (2). Since in the literature different values
were found and not distinction was made between the directions of the applied tensile load, it
was decided to carry out two experimental programs, one for the tensile strength
perpendicular to grain and one for fracture energy, respectively, in order to have the right
value to tests these formulas and compare them with results of the experimental programs on
the load carrying capacity of the bottom rails. Finally, a third experimental study will be
presented about test on bottom rails. This experimental study was not planned at the
beginning but then it was decided to do it and cut the specimen for the tensile strength
perpendicular to grain and fracture energy tests from the same board of the bottom rail in
6 The aim of this report is to present the results of three experimental studies: tensile strength perpendicular to the grain in radial and tangential direction, fracture energy with TR and RT orientations and bottom rail. The experimental programs have been conducted at two different periods and places: bottom rail tests at Umeå University in October 2012 and tensile strength perpendicular to grain and fracture energy at SP in Stockholm in June 2013.
Limitations
In EN 408 [8] and NT BUILD 422 [9] the requirements to respect for the determination of tension strength perpendicular to grain and fracture energy are given, respectively. They have been used as much as possible but, due to test purpose, few requirements, i.e. the dimensions of the specimens, could not have been respected.
TEST SETUP AND MATERIAL Test specimen and Material Properties
Tensile strength perpendicular to grain and Fracture Energy
The specimens were built by hand using two different dimensions according to the direction tested. They have been glued one month before testing (week 20 of 2013) and kept in the climate controlled chamber with relative humidity RH 65%. The density of the specimen was measured before gluing; however for few specimens, due to carelessness, it was not measured. Once that the specimens were glued to the two timber pieces, the surface were accurately prepared to ensure that they were plane. This was made with an electronic planer.
The details of the test specimens were as follow:
• Specimen: Spruce (Picea Abies), C24 according to EN 338 [10]. For f
t,90test the dimensions of the specimen were as follow: 45×70×45 mm and 45×70×120 mm for radial and tangential direction, respectively. For G
ftest the dimensions of the specimen were as follow: 45×45×45 mm for both TR and RT orientation, with a notch length of 0.6×45 mm and a width of 2 mm. In order to have a stable curve after the peak load, a further notch, with a length of 3 mm, was made with a blade.
• Glue (two different glues were used for the specimens):
7 Bottom rail
• Bottom rail: Spruce (Picea Abies), C24 according to EN 338 [10], 45×120 mm.
• Sheathing: Hardboard, 8 mm (wet process fibre board, HB.HLA2, EN 622-2 [12], Masonite AB).
• Sheathing-to-timber joints: Annular ringed shank nails, 50×2.1 mm (Duofast, Nordisk Kartro AB). The joints were nailed manually and the holes were pre-drilled, only in the sheet, 1.7 mm. The centre distance between nails was 25 mm, cfr. pag. 13 and 18.
• Anchor bolt: Ø 12 (M12). The holes in the bottom rails were pre-drilled, 13 mm.
Test program
Tensile strength perpendicular to grain
A total of 48 specimens, according to Figure 1, were tested: 15 for the radial direction and 33 for the tangential direction. For both the directions, few trial tests were made. These specimens, when failed in the right way, are included in the test program. The dimensions of test specimens differ according to the direction tested. For the radial direction the specimen had dimensions according to Figure 1a and 1b while for the tangential direction the specimen had dimensions according to Figure 1d and 1e. The width “a”, the thickness “b” and the depth
“h” are also defined in Figure 1b and 1e.
8
9
a b h
1 Radial 70 45 45 18
1)2 Tangential 70 45 120 34
2)1)
15 tests were planned but the three trial tests have been added
2)
33 tests were planned but one of the three trial tests has been added
Fracture energy
A total of 48 specimens, according to Figure 2, were tested: 15 for RT orientation and 33 for
the TR orientation. For both the directions, few trial tests were made. The dimensions of test
specimens were chosen according to Figure 2a and 2b. The width “c”, the thickness “d” and
the depth “c” are also defined in Figure 2a and 2b. The dimensions of the notch are defined in
Figure 2b. The two different orientations tested are shown in Figure 2c and 2d, while in
Figure 2e and 2d details of the test set-up are shown.
10
11
orientation
c d
1 RT 45 45 15
2 TR 45 45 33
Bottom rail
A total of 54 specimens, according to Figure 3, were tested. The specimens were divided into
three different series, where each series was divided into different sets. The series were
subdivided with regard the washer size and the position of the anchor bolt with respect to the
width “b” of the bottom rail (Fig. 3c). Knowing the anchor bolt position and the washer size,
the distance between the washer edge and the edge of the bottom rail at the loaded side, s, as
shown in Fig. 3c, is defined. The depth of the bottom rail is defined as h, as shown in Fig. 3c.
12
Figure 3 Test set-up and boundary conditions of sheathed bottom rails subjected to single-sided vertical uplift. a) Boundary conditions: a hinge is created that allows the specimen to rotate; b) View from above of the specimen; c) Lateral view of the specimen. The distance s is the distance between the washer edge and the loaded edge of the bottom rail; d) The connection between the specimen and the steel bar connected to the hydraulic piston
The test program is specified in Table 3.
13
PD PU [mm] [mm]
1
1 3 3
b/2
60 mm from
sheathing
40×40×15 40
2 3 3 60×60×15 30
3 3 3 80×70×15 20
4 3 3 100×70×15 10
2
1 3 3
3b/8 45 mm from
sheathing
40×40×15 25
2 3 3 60×60×15 15
3 3 3 80×70×15 5
3
1 3 3
b/430 mm from sheathing
40×40×15 10
2 3 3 60×60×15 0
1)
Distance from washer edge to loaded edge of the bottom rail.
Correlation between the three experimental programs
The specimens for the three experimental programs were cut from the same board, in order to have comparable results from timber with the same characteristics, as density and moisture.
The board had a cross section of 120×45 mm and a length of about 5 m.
Each board was cut in four parts; then two parts were used to build bottom rail specimens
with possible failure mode 1 and the other two to build bottom rail specimens with possible
failure mode 2, according to Figure 4. Figure 5 was used to foresee which kind of failure
mode would have occurred in the bottom rail. Figure 5 refers to Girhammar and Juto study
[3]. In that study, the distance between nails in the timber to sheathing joint was 25 mm. This
distance was chosen; despite it is not a realistic distance, in order to avoid failure due to
yielding and withdrawal of the nails. For the bottom rail experimental program, as given in
Table 3, the same characteristics as in study [3] were used. The plan was to test 15 specimens
for G
ftests for side crack and 33 for bottom crack, and 15 specimens for f
t,90in radial direction
and 33 in tangential direction. According to Figure 5 we could suppose that bottom crack
failure mode would have been occurred for rails in Series 1, Set 1, 2 and 3 and dominant side
crack failure mode for rails in Series 2, Set 3 and Series 3, Set 1 and 2. For bottom rail with
possible failure mode 2, it was decide to test fracture energy in both RT and TR orientation
14
Figure 4 Scheme of how to cut and select the boards for the specimens (PU = Pith upwards, PD = Pith downwards). *Size of washer [mm]. In this case, according to Figure 5 regarding series 1, we should have failure mode 1. These boards were then selected and specimens for test for G
fin TR direction and f
t,90in tangential direction were cut. **Size of washer [mm]. In this case, according to Figure 5 regarding series 2 and 3, we should have failure mode 2. These boards were then selected and specimens for test for G
fin RT direction and f
t,90in radial direction were cut.
Figure 5 Recorded failure modes for the different test series and sets belonging to the Juto study []
(PD = Pith downwards, PU = Pith upwards). *Distance from washer edge to loaded edge of the bottom rail [mm], **Size of washer [mm], ***Bolt position
0 1 2 3 4 5 6 7 8 9
Set 1 (40)* PD Set 1 (40)* PU Set 2 (30)* PD Set 2 (30)* PU Set 3 (20)* PD Set 3 (20)* PU Set 4 (10)* PD Set 4 (10)* PU Set 1 (25)* PD Set 1 (25)* PU Set 2 (15)* PD Set 2 (15)* PU Set 3 (5)* PD Set 3 (5)* PU Set 1 (10)* PD Set 1 (10)* PU Set 2 (0)* PD Set 2 (0)* PU
40** 60** 80** 100** 40** 60** 80** 40** 60**
Serie 1 (b/2)*** Serie 2 (3b/8)*** Serie 3 (b/4)***
Mode 3
Mode 2
Mode 1
15
Table 4 Relation between the specimens for the three experimental programs (PU = Pith upwards, PD
= Pith downwards). From the same board were cut specimens for the three experimental programs.
Bottom rail test
Gf
test
Gftest
ft,90test
ft,90test Supposed failure
mode
Board
Specimen
ID Specimen ID Specimen ID Specimen ID Specimen ID
111 PU
Gf_TR_Test_1_V -
ft,90_Test_1_T_V - Vertical crack
Board 1 111 PD
Gf_TR_Test_2_V -
ft,90_Test_2_T_V -
231 PU
Gf_RT_Test_1_H G
f_TR_16_V f
t,90_Test_1_R_H f
t,90_16_T_V Horizontal crack 231 PD
Gf_RT_Test_2_H G
f_TR_17_V f
t,90_Test_2_R_H f
t,90_17_T_V
112 PU
Gf_TR_Test_3_V -
ft,90_Test_3_T - Vertical crack
Board 2
112 PD
Gf_TR_1_V -
ft,90_1_T_V -
232 PU
Gf_RT_Test_1_H G
f_TR_18_V f
t,90_Test_3_R_H f
t,90_18_T_V Horizontal crack 232 PD
Gf_RT_1_H
Gf_TR_19_V
ft,90_1_R_H
ft,90_19_T_V
113 PU
Gf_TR_2_V -
ft,90_2_T_V - Vertical
crack
Board 3
113 PD
Gf_TR_3_V -
ft,90_3_T_V -
233 PU
Gf_RT_2_H
Gf_TR_20_V
ft,90_2_R_H
ft,90_20_T_V Horizontal crack 233 PD
Gf_RT_3_H
Gf_TR_21_V
ft,90_3_R_H
ft,90_21_T_V
121 PU
Gf_TR_4_V -
ft,90_4_T_V - Vertical
crack
Board 4
121 PD
Gf_TR_5_V -
ft,90_5_T_V -
311 PU
Gf_RT_4_H
Gf_TR_22_V
ft,90_4_R_H
ft,90_22_T_V Horizontal crack 311 PD
Gf_RT_5_H
Gf_TR_23_V
ft,90_5_R_H
ft,90_23_T_V
122 PU
Gf_TR_6_V -
ft,90_6_T_V - Vertical
crack
Board 5
122 PD
Gf_TR_7_V -
ft,90_7_T_V -
312 PU
Gf_RT_6_H
Gf_TR_24_V
ft,90_6_R_H
ft,90_24_T_V Horizontal crack 312 PD
Gf_RT_7_H
Gf_TR_25_V
ft,90_7_R_H
ft,90_25_T_V
123 PU
Gf_TR_8_V -
ft,90_8_T_V - Vertical
crack Board
123 PD
Gf_TR_9_V -
ft,90_9_T_V - 6
313 PU
Gf_RT_8_H
Gf_TR_26_V
ft,90_8_R_H
ft,90_26_T_V Horizontal
16
f f t,90 t,90
Horizontal
crack 321 PD
Gf_RT_11_H
Gf_TR_29_V
ft,90_11_R_H
ft,90_29_T_V
132 PU
Gf_TR_12_V -
ft,90_3_R_H -
Vertical crack
Board 8
132 PD
Gf_TR_13_V -
ft,90_13_T_V -
322 PU
Gf_RT_12_H
Gf_TR_30_V
ft,90_12_R_H
ft,90_30_T_V Horizontal crack 322 PD
Gf_RT_13_H
Gf_TR_31_V
ft,90_13_R_H
ft,90_31_T_V
133 PU
Gf_TR_14_V -
ft,90_14_T_V - Vertical
crack
Board 9
133 PD
Gf_TR_15_V -
ft,90_15_T_V -
323 PU
Gf_RT_14_H
Gf_TR_32_V
ft,90_14_R_H
ft,90_32_T_V
Horizontal crack 323 PD
Gf_RT_15_H
Gf_TR_33_V
ft,90_15_R_H
ft,90_33_T_V
Test Set-Up
Tensile strength perpendicular to grain The test set-up is shown in Figure 1.
The specimen was glued to two pieces of timber, according to Figure 1a and 1d. The dimension of the specimen and of the two timber pieces was varied according to Table 1 and Figure 1a and 1d, in basis to the direction tested. Due to the experimental study purpose, it was not possible to follow all the requirements given in the EN 408 [8]. The specimen was then connected to steel bars which in turn were connected to the testing machine by dowels, as shown in Figure 1g. The machine used was an universal testing Machine UTM “Alwetron”
TCT 50.
The tests were under displacement control and a tensile load with a rate of 10 mm/min until a
load of 20 N and then 0.5 mm/min until failure was applied by a hydraulic piston. The
displacement rate was decided according to the EN 408 [8], where it is suggested that it shall
be adjusted so that the maximum load is reached within (300 ± 120) s. Few trial tests were
performed in order to find the right displacement rate.
17 The reason for this was to have a part of the specimen with a smaller resistant surface and have the failure there. For the specimens tested in radial direction, since the tensile strength perpendicular-to-grain was found to be higher than that found for the tangential direction, the addition of the two half circles was not enough in order to get the failure in the specimen. The glued surface was then strengthening by addition of fiberglass (this was made also for few specimens in tangential direction), as shown in Figure 1c and 1e.
Fracture energy
The test set-up is shown in Figure 2.
The specimen was glued to two pieces of timber, according to Figure 2a. The dimension of the specimen and of the two timber pieces was chosen according to Figure 2a and 2b. The tests were made according to NT BUILD 422 [9]. The tests specimen was simply supported at both ends by two steel cylinders supported by two steel supports, as shown in Figure 2f, and loaded at midpoint through a cone connected to the load cell, according to Figure 2e. A 1 mm thick rubber layers were placed between the wood test specimen and the supports. The same was done between the wood test specimen and the cone connected to the load cell. The machine used was the same as for tensile strength perpendicular to the grain tests; an universal testing Machine UTM “Alwetron” TCT 50.
The tests were under displacement control and a compression load with a rate of 1.30 mm/min until failure was applied by a hydraulic piston. The displacement rate was decided according to the NT BUILD 422 [9], where it is suggested that it shall be adjusted so that collapse is obtained in about 3±1 minutes. Few trial tests were performed in order to find the right displacement rate.
During the trial tests the load vs. deflection curve was found to be unstable. As solution, the length of the notch was increased of 3 mm using a blade, according to Figure 2e.
Bottom rail
The test set-up is shown in Figure 3.
18 was chosen so that there would not arise any visible permanent deformations in the washers.
A hydraulic piston (static load capacity 100 kN) was attached to a steel bar, that was connected to the upper panel using C-shaped steel profiles and four bolts Ø16, according to Fig. 3d.
A hinge was created that allows the specimen to rotate, according to Fig. 3a.
The distance between the nails in the sheathing-to-timber joint was 25 mm.
A torque moment of 50 Nm was used to tighten the bolts. A tensile load was applied to the upper part of the panel with a displacement rate of 2 mm/min.
For each specimen the moisture content and density of the bottom rail were measured after the test, according to ISO 3130 [13] and ISO 3131 [14], respectively.
TEST RESULTS
Tensile strength perpendicular to grain Load-displacement curves
The displacement was directly recorded by the testing machine. When plotting the load-
displacement curves, an initial consolidation for all curves was seen. This is illustrated by a
long initial horizontal part of the curve, as shown by the red dashed line in Figure 6. This
could be due to different factors but it is believed to be due to imperfect contact between
dowel and wood at the start of the test. However it has been deleted since it was not
meaningful.
19
Figure 6 Load-displacement curve of specimen 1_R_H. The red dashed line indicates the initial consolidation which has not been considered in the load-displacement curves. The black line indicates the part of the load-displacement curved used as representative of the tests
In Figures 7 and 8 the load-displacement curves for all specimens tested are shown, while the load-displacement curve for each singular test are shown in Appendix A. All curves show more or less the same stiffness and a brittle failure load, typical for timber loaded by a tensile load perpendicular to the grain.
0 1 2
0 0,5 1 1,5 2 2,5
L oad
Displacement [mm]
20
Figure 7 Measured load-time curves for specimens tested in tensile strength perpendicular to grain in radial direction
0 1 2 3 4 5
0 0,4 0,8 1,2 1,6 2
L oad [ k N ]
Displacement [mm]
Test_1_R_H Test_2_R_H Test_3_R_H 1_R_H 2_R_H
3_R_H 4_R_H 5_R_H 6_R_H 7_R_H
8_R_H 9_R_H 10_R_H 11_R_H 12_R_H
13_R_H 14_R_H 15_R_H
21
Figure 8 Measured load-time curves for specimens tested in tensile strength perpendicular to grain in tangential direction
0 1 2 3 4 5
0 0,5 1 1,5 2 2,5
L oad [ k N ]
Displacement [mm]
Test_3_T_V 1_T_V 2_T_V 3_T_V 4_T_V
5_T_V 6_T_V 7_T_V 8_T_V 9_T_V
10_T_V 11_T_V 12_T_V 13_T_V 14_T_V
15_T_V 16_T_V 17_T_V 18_T_V 19_T_V
20_T_V 21_T_V 22_T_V 23_T_V 24_T_V
25_T_V 26_T_V 27_T_V 28_T_V 29_T_V
30_T_V 31_T_V 32_T_V 33_T_V
22
Table 5 Results from testing of specimens in tensile strength perpendicular to grain in both radial and tangential direction.
Series
Direction Number of tests
Failure load Tension perpendicular f
t,90Mean density Mean
[kN]
Min. and Max [kN]
Stddev [kN]
COV
[%] CV
0.05Mean [N/mm
2]
Min. and Max [N/mm
2]
Stddev [N/mm
2]
COV
[%] CV
0.051 Radial 18 4.73 3.26 ÷ 6.45 0.83 17.4 3.12 2.28 1.54 ÷ 3.10 0.40 17.4 1.51 467
1)2 Tangential 34 3.63 1.98 ÷ 6.11 0.88 24.1 2.01 1.79 0.98 ÷ 2.84 0.39 22.1 1.06 463
2)1)
Result calculated with 9 specimens
2)
Result calculated with 31 specimens
23 by measuring continuously corresponding values of load, F, and deflection or cross head movement, u. For the test to be valid it is required that the load deflection response is stable.
In Figure 9 an example of stable load-deflection curve is shown.
Figure 9 Example of stable load-deflection curve (freely adopted from NT BUILD 422 [9]).
In Figures 10 and 11 the load-deflection curves for all specimens tested are shown, while the
load-deflection curve for each singular test are shown in Appendix B. Figure 10 shows curves
for specimens tested in the RT plane, where 6 curves were stable, 4 almost stable and 5
unstable.
24
Figure 10 Measured load-deflection curves for fracture energy tests in RT direction 0
20 40 60 80 100 120 140
0 2 4 6 8 10 12 14 16 18
L oad [ k N ]
Displacement [mm]
RT_1_H RT_2_H RT_3_H RT_4_H RT_5_H
RT_6_H RT_7_H RT_8_H RT_9_H RT_10_H
RT_11_H RT_12_H RT_13_H RT_14_H RT_15_H
25
Figure 11 Measured load-deflection curves for fracture energy tests in TR direction 0
40 80 120 160 200
0 2 4 6 8 10 12 14 16 18
L oad [ k N ]
Displacement [mm]
TR_1_V TR_2_V TR_3_V TR_4_V TR_5_V TR_6_V
TR_7_V TR_8_V TR_9_V TR_10_V TR_11_V TR_12_V
TR_13_V TR_14_V TR_15_V TR_16_V TR_17_V TR_18_V
TR_19_V TR_20_V TR_21_V TR_22_V TR_23_V TR_24_V
TR_25_V TR_26_V TR_27_V TR_28_V TR_29_V TR_30_V
TR_31_V TR_32_V TR_33_V
26
Table 6 Results from fracture energy testing in RT and TR direction.
Series
Direction Number of tests
Failure load Fracture energy G
fMeand
density [kg/m
3] Mean
[N]
Min. and Max [N]
Stddev [N]
COV
[%] CV
0.05Mean [N/m]
Min. and Max [N/m]
Stddev [N/m]
COV
[%] CV
0.051 RT 15 98.0 60 ÷ 169 27.4 27.9 44.0 322 190 ÷ 476 86.7 26.9 149 455
1)2 TR 33 123 69 ÷ 192 29.7 24.2 68.0 303
2)196 ÷ 432 66.5 21.9 179 474
1)
Result calculated with 14 specimens
2)
Result calculated with 31 specimens
27 found during the others experimental programs related to bottom rail tests [3, 4], where a third failure mode, yielding and withdrawal of the nails in the sheathing-to-framing joints, was found. This failure mode did not happen in this experimental program probably due to the small distance between the nails, 25 mm.
1) Splitting along the bottom side of the rail according to Figure12a.
2) Splitting along the edge side of the rail according to Figure 12b.
Failure mode 1 is due to crosswise bending of the bottom rail introducing tension perpendicular to the grain. The failure is developed as a vertical crack propagating from the middle of the bottom side of the rail. When the anchor bolt is moved toward to the sheathing- to-framing joints or when big washer are used the crack usually appears closer to that edge.
Once the crack appears, it develops from one end to the other end of the bottom rail. Failure mode 2 is due to vertical shear forces in the nails of the sheathing-to-framing joints causing splitting failure at the edge of the bottom rail. The crack usually starts from the line of the nails propagating in the horizontal direction and finally changing to a more vertical direction.
The crack usually starts at one end and then propagates longitudinally along the bottom rail, but never reaches the other end. Sometimes a horizontal crack appears also in the other end but that is a separate crack independent of the first one.
a) Mode 1
28 In Figure 13, the number of observations of the two different failure modes is graphically shown for the series of the study. It is noted that the predominant failure mode is failure mode 1, splitting failure along the bottom side of the rail. It is also possible to note an influence between the distance s and the failure mode. For small values of distance s, failure mode 2 arises.
Figure 13 Recorded failure modes for the different test series and sets belonging to the experimental study (PU = Pith upwards, PD = Pith downwards). *Distance from washer edge to the loaded edge of the bottom rail [mm], **Size of washer [mm], ***Bolt position
Load-time curves
The displacements of the specimens were not recorded, but since the displacement was applied with a constant rate, it is possible to obtain fictitious load-displacement curves by plotting load vs. time. In Figures 14-31 the typical load-time curves for each set are shown.
0 1 2 3
Set 1 - PU (40)* Set 1 - PD (40)* Set 2 - PU (30)* Set 2 - PD (30)* Set 3 - PU (20)* Set 3 - PD (20)* Set 4 - PU (10)* Set 4 - PD (10)* Set 1 - PU (25)* Set 1 - PD (25)* Set 2 - PU (15)* Set 2 - PD (15)* Set 3 - PU (5)* Set 3 - PD (5)* Set 1 - PU (10)* Set 1 - PD (10)* Set 2 - PU (0)* Set 2 - PD (0)*
40** 60** 80** 100** 40** 60** 80** 40** 60**
Series 1 (b/2)*** Series 2 (3b/8)*** Series 3 (b/4)***
Mode 2
Mode 1
29
Figure 14 Load vs. time curves for bottom rail with pith down of Series 1 – Set 1. Washer size 40×40 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
Figure 15 Load vs. time curves for bottom rail with pith up of Series 1 – Set 1. Washer size 40×40 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
0 2 4 6 8 10 12 14
0 100 200 300 400
L oad [ k N ]
Time [s]
Load vs. Time
111 N 112 N 113 N
0 2 4 6 8 10 12
0 100 200 300
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
111 U
112 U
113 U
30
Figure 16 Load vs. time curves for bottom rail with pith down of Series 1 – Set 2. Washer size 60×60 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
Figure 17 Load vs. time curves for bottom rail with pith up of Series 1 – Set 2. Washer size 60×60 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
0 4 8 12
0 100 200 300 400 500
L oad [ k N ]
Time [s]
121 N 122 N 123 N
0 4 8 12 16 20
0 100 200 300 400 500
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
121 U
122 U
123 U
31
Figure 18 Load vs. time curves for bottom rail with pith down of Series 1 – Set 3. Washer size 80×70 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
Figure 19 Load vs. time curves for bottom rail with pith up of Series 1 – Set 3. Washer size 80×70 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
0 4 8 12
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
131 N 132 N 133 N
0 4 8 12 16 20 24
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
131 U
132 U
133 U
32
Figure 20 Load vs. time curves for bottom rail with pith down of Series 1 – Set 4. Washer size 100×70 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
Figure 21 Load vs. time curves for bottom rail with pith up of Series 1 – Set 4. Washer size 100×70 mm, bolt position b/2 (b = 120 mm), 3 failure of type 1
0 4 8 12 16
0 100 200 300 400 500 600 700
L oad [ k N ]
Time [s]
141 N 142 N 143 N
0 4 8 12 16 20 24
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
141 U
142 U
143 U
33
Figure 22 Load vs. time curves for bottom rail with pith down of Series 2 – Set 1. Washer size 40×40 mm, bolt position 3b/8 (b = 120 mm), 3 failure of type 1
Figure 23 Load vs. time curves for bottom rail with pith up of Series 2 – Set 1. Washer size 40×40 mm, bolt position 3b/8 (b = 120 mm), 3 failure of type 1
0 4 8
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
211 N 212 N 213 N
0 4 8 12 16
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
211 U
212 U
213 U
34
Figure 24 Load vs. time curves for bottom rail with pith down of Series 2 – Set 2. Washer size 60×60 mm, bolt position 3b/8 (b = 120 mm), 3 failure of type 1
Figure 25 Load vs. time curves for bottom rail with pith up of Series 2 – Set 2. Washer size 60×60 mm, bolt position 3b/8 (b = 120 mm), 3 failure of type 1
0 4 8 12
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
221 N 222 N 223 N
0 4 8 12 16 20 24
0 100 200 300 400 500
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
221 U
222 U
223 U
35
Figure 26 Load vs. time curves for bottom rail with pith down of Series 2 – Set 3. Washer size 80×70 mm, bolt position 3b/8 (b = 120 mm), 3 failure of type 1
Figure 27 Load vs. time curves for bottom rail with pith up of Series 2 – Set 3. Washer size 80×70 mm, bolt position 3b/8 (b = 120 mm), 2 failure of type 1 and 1 failure of type 2
0 4 8 12 16 20
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
231 N 232 N 233 N
0 4 8 12 16 20 24 28 32
0 100 200 300 400 500
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
231 U
232 U
233 U
36
Figure 28 Load vs. time curves for bottom rail with pith down of Series 3 – Set 1. Washer size 40×40 mm, bolt position b/4 (b = 120 mm), 3 failure of type 1
Figure 29 Load vs. time curves for bottom rail with pith up of Series 3 – Set 1. Washer size 40×40 mm, bolt position b/4 (b = 120 mm), 2 failure of type 1 and 1 failure of type 2
0 4 8 12 16 20
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
311 N 312 N 313 N
0 4 8 12 16 20 24
0 100 200 300 400 500
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
311 U
312 U
313 U
37
Figure 30 Load vs. time curves for bottom rail with pith down of Series 3 – Set 2. Washer size 60×60 mm, bolt position b/4 (b = 120 mm), 2 failure of type 1 and 1 failure of type 2
Figure 31 Load vs. time curves for bottom rail with pith up of Series 3 – Set 2. Washer size 60×60 mm, bolt position b/4 (b = 120 mm), 3 failure of type 2
Failure load
The failure load for the two brittle failure modes is defined as the load at which there is a first decrease in the load carrying capacity due to a propagating crack in the bottom rail. The results of the different tests of the two studies are summarized in Table 7 and 8. Since in other previous study [3, 4] the pith orientation turned out to be an important parameter at the evaluation of the test results, the failure load of the study is presented with respect to this parameter in Table 7 (pith downward) and Table 8 (pith upwards). Mean failure load and mean density are presented with respect to failure mode. The dry density, defined as the ratio
0 6 12 18 24
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
321 N 322 N 323 N
0 4 8 12 16 20 24 28
0 100 200 300 400 500 600
L oad [ k N ]
Time [s]
Pith Up
Load vs. Time
321 U
322 U
323 U
38
39
Series Se
of tests [kg/m ] value
Mean [%]
[kN]
Stddev [kN]
COV [%]
Mean [kN]
Stddev [kN]
COV [%]
Mean [kN]
Stddev [kN]
COV
[%] (1) (2) All (1) (2)
1
1 3 10.8 2.74 25.3 10.8 2.74 25.3 - - - 3 - 421 421 - 16.0
2 3 12.1 4.09 33.7 12.1 4.09 33.7 - - - 3 - 410 410 - 16.3
3 3 17.1 1.40 8.19 17.1 1.40 8.19 - - - 3 - 409 409 - 15.1
4 3 21.6 1.51 7.00 21.6 1.51 7.00 - - - 3 - 342 342 - 14.5
2
1 3 12.3 2.66 21.5 12.3 2.66 21.5 - - - 3 - 367 367 - 14.5
2 3 15.8 2.07 13.1 15.8 2.07 13.1 - - - 3 - 372 372 - 14.1
3 3 27.4 2.83 10.3 27.4 2.83 10.3 - - - 3 - 390 390 - 14.3
3
1 3 22.9 2.17 9.49 22.9 2.17 9.49 - - - 3 - 403 403 - 14.7
2 3 26.7 6.10 22.8 28.9 - - 22.5 - - 2 1 382 399 347 14.3
All Mean value 388 390 347 14.9
40
Series Se
