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Characteristics of Sheathing-to-Timber Joints in Wood Shear Walls
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Characteristics of Sheathing-to-Timber Joints in Wood Shear Walls
Ulf Arne GIRHAMMAR
Umeå University
Sweden
UlfArne.Girhammar@tfe.umu.se
1980 Dr. Eng.; 1981 Assoc. Prof.;
1987 Eng. Consultant, wood and
building industry; 1989 Adjunct Prof.
1994 Senior Specialist, mechanics
industry. Presently at Umeå University.
Nils Ivar BOVIM
Assistant Professor
Agricultural University of
Norway
1971 Civ.Eng, 1973-1982 at
Norwegian Institute of Wood
Technology. Since 1982 Consultant
Eng. Since 1996 also at Agricultural
University of Norway
Bo KÄLLSNER
Adjunct Professor
Växjö University
Sweden
1977 Dr in Civ Eng. Since 1975 at
Trätek - Swedish Institute for Wood
Technology Research in Stockholm.
Since 2001 also at Växjö University.
Summary
The capacity of wood-framed shear walls is essentially governed by the characteristics of the
sheathing-to-timber joints. Their characteristics and failure modes vary, e.g. with respect to loadingto-grain directions and edge distances of the fasteners.
In this paper, results from an experimental study of sheathing-to-timber joints are presented and a
parametric equation to model the entire load-slip relationship is proposed. Load-slip curves for
different sheathing materials and load-to-grain directions are presented, and the influence of edge
distance of fasteners in the sheets is illustrated.
Key words: Sheathing-to-timber joint, load-to-grain direction, edge distance, analytical model.
1
Introduction
1.1 Background
The behaviour and capacity of sheathed wood shear walls are essentially governed by the
characteristics of the fasteners. In order to develop and apply accurate computerised design
methods for such shear walls it is important to know in detail the characteristics and failure modes
of the sheathing-to-timber joints, e.g. with respect to loading-to-grain directions and edge distances
for the fasteners in the sheets and wood members. In non-linear finite element modelling of shear
walls it is necessary to know the whole load-deformation curve of the joints including the different
joint failure modes and to be able to model the successive joint failures and, therefore, the gradual
failure of the shear wall as a whole.
Also, in order to apply plastic design methods of the type presented in the companion paper [1], it is
important to know the range of applicability with respect to the brittleness of the sheathing-totimber joints at failure. Due to economic reasons, it is desirable to use as few connectors as
possible. Hence, large fastener diameters are often used and edge distances to the sheet and framing
member become critical. This leads to brittle failure modes for the joints and the conventional
elastic and plastic methods that are based on ductile failure modes are not applicable.
1.2 Objectives
The purpose of this paper is to present the results of an experimental study and to give a
mathematical model for the behaviour and characteristics of sheathing-to-timber joints. The study
comprises of the following parts: (1) Tests of single joints with respect to different sheathing
materials, load-to-grain directions, and edge distances; and (2) Development and application of a
parameter equation modelling the entire load-slip behaviour of the joints.
1001
2
Testing Program
2.1 Test Specimens and Testing Procedure
All test specimens were designed as follows:
x Frame members: Pine (Pinus Sylvestris), C24, 45u120u300 mm.
x Sheathing: 150u150 mm.
x Nails: Annular ringed shank nails (Duofast), fy | 400-500 MPa, 50u2.1 mm. The joints were
hand-nailed and the nail holes were pre-drilled, 1.7 mm, in case of hardboard.
x Edge distances: For both frame member and sheet, 11.25 mm for parallel and 22.5 mm for
perpendicular load-to-grain directions in timber, respectively.
The test set-up is shown in Figure 1 for the single nailed sheathing-to-timber joints loaded parallel
and perpendicular to grain, respectively. The testing procedure was designed to conveniently study
the edge distance effect.
P
P
150
Free
edge
150
Free
edge
200
150
P
2
P
100+150+100
P
2
Figure 1. Test set-up for sheathing-to-timber joints subjected to tensile loads parallel and
perpendicular to grain, respectively.
All tests were performed under deformation control and with a deformation rate of 2 mm/min. For
each test, the density and moisture content was determined and a choice of fasteners was tested. No
adjustment of the test results was made with respect to these parameters.
2.2 Joints with Different Sheathing Materials at Different Load-to-Grain Directions
Three types of materials were used:
x Hardboard (wet process fibre board, HB.HLA2, Masonite AB): C40, 8 mm.
x Particleboard (Byggelit AB): V100, 12 mm.
x Plywood (Spruce, Picea Abies, Schauman Wood Oy): P30, 9 mm (3.2+2.6+3.2).
The load directions were parallel (0q) and perpendicular (90q) to grain, respectively.
2.3 Joints with Hardboard at Different Load-to-Grain Directions and Edge Distances
Only hardboard was used as sheathing material. The dimensions of the timber member were
45u95u300 mm and the sheet 150u230 mm in these tests. The edge distance of the fastener in the
timber member was 22.5 mm in all tests.
For load directions parallel to grain (0q), the following edge distances in the sheet were tested:
x 1d and 2d, where d = 2.1 mm.
And for load directions perpendicular to grain (90q):
x 2d, 3d, 4d, and 5d, where d = 2.1 mm.
3
Test Results
3.1 Joints with Different Sheathing Materials at Different Load-to-Grain Directions
3.1.1 Hardboard
The test results for parallel and perpendicular load-to-grain directions, respectively, are shown in
Figure 2 (P = load, u = slip) [2]. In all parallel tests but one, a ductile type of failure took place (nail
yielding followed by withdrawal of nail). The brittle type of failure, which occurred after the
1002
1600
1600
1400
1400
1200
1200
1000
1000
800
P [N]
P [N]
ultimate load-bearing capacity was reached, had reference to bending failure of nail in the sheet. All
perpendicular tests exhibited a ductile type of failure (nail yielding followed by withdrawal of nail).
Mean values are given in Table 1.
Withdrawal
600
800
Withdrawal
600
400
400
Nail failure
200
200
0
0
0
5
10
15
20
(a)
25
30
35
40
45
50
0
5
10
15
u [mm]
20
25
30
35
40
45
50
u [mm]
(b)
Figure 2. Load-slip curves for hardboard: a) Parallel and b) perpendicular load-to-grain direction
Table 1. Mean values of test results for joints with different sheathing materials.
Sheathing
material
Hard0q
board
90q
Particle- 0q
board
90q
Ply0q
wood
90q
Displacement [mm]
Dry density
Moisture
3
[kg/m
]
Content
[%]
At max. At failure
load
(1)* (2)* Timber Sheet Timber Sheet
5
50
382
896
10.5
7.1
6
33
388
884
10.4
6.9
10
14 44
462
595
9.7
6.7
12
16 37
466
582
9.8
7.6
10
19
459
428
9.2
6.3
12
19 41
442
419
10.4
6.8
Maximum
load
[kN]
1.03
1.05
1.30
1.21
1.32
1.39
Number
of tests
10
10
10
12
10
12
* Displacement in case of (1) final brittle failure and (2) final ductile failure.
1600
1600
1400
1400
1200
1200
1000
Withdrawal
P [N]
P [N]
3.1.2 Particleboard
The test results are shown in Figure 3 [3]. Due to a mistake, the slip is somewhat overestimated.
Parallel tests: Three failures by withdrawal of nail, three by punching of nail head, and four by nail
failure in timber member. Perpendicular tests: Five failures by withdrawal of nail, four by punching
of nail head, and three by nail failure in timber member. Mean values are given in Table 1.
800
600
1000
Withdrawal
800
600
Punching
400
400
Nail failure
200
Nail failure
200
0
Punching
0
0
5
10
15
20
(a)
25
30
35
40
45
50
0
u [mm]
5
10
15
20
(b)
25
30
35
40
45
50
u [mm]
Figure 3. Load-slip curves for particleboard: (a) Parallel and (b) perpendicular load-to-grain
direction.
For particleboard and plywood, the maximum values seem to have been influenced by the higher
density of the timber members. Note also that the thickness of particleboard is 12 mm, about 50 %
thicker than the one of hardboard and plywood.
1003
3.1.3 Plywood
The test results are shown in Figure 4 [3]. Parallel tests: Nine failures by punching of nail head, and
one by nail failure in timber. Perpendicular tests: Three failures by withdrawal of nail and nine by
punching of nail head. The difference in failure characteristics between parallel and perpendicular
directions is due to the grain direction of the surface layers. Mean values are given in Table 1.
1600
1600
1400
1400
1000
P [N]
P [N]
1200
Punching
800
600
1200
1000
Withdrawal
800
600
400
400
200
0
0
5
10
Punching
200
0
Nail failure
15
20
(a)
25
30
35
40
45
50
0
5
10
15
20
u [mm]
25
30
35
40
45
50
(b) u [mm]
Figure 4. Load-slip curves for plywood: (a) Parallel and (b) perpendicular load-to-grain direction.
1500
1500
1200
1200
900
900
P [N]
P [N]
3.2 Joints with Hardboard at Different Load-to-Grain Directions and Edge Distances
The maximum load versus the edge distance in the case of parallel and perpendicular load-to-grain
direction, respectively, is shown in Figure 5 [4]. Parallel tests: 1d—One failure by withdrawal of
nail and one by edge failure; and 2d —All failures by edge failure. Perpendicular tests: 2d and 3d—
All failures by edge failure; and 4d and 5d—All failures by withdrawal of nail, except in one (for
4d) where nail yielding was followed by edge failure in a very late stage. Boundary for edge failure
is 2d for parallel and 4d for perpendicular cases, respectively. The number of tests is yet too small
to draw final conclusions. For details, see Table 2.
600
600
Edge failure
Edge failure
300
300
0
0
0
1
2
(a)
3
4
5
0
Edge distance [d ]
1
(b)
2
3
4
5
Edge distance [d ]
Figure 5. Max load vs edge distance: (a) Parallel and (b) perpendicular load-to-grain direction.
Table 2. Mean values of test results at different load-to-grain directions and edge distances.
Maximum
load
[kN]
Parallel
Perpendicular
1d
2d
2d
3d
4d
5d
971
1047
923
1212
1290
1162
Displacement [mm]
Dry density
Moisture
3
[kg/m
]
Content
[%]
At max. At failure
load
(1)* (2)* Timber Sheet Timber Sheet
8
18
34
414
922
11.7
7.1
5
38
413
930
11.5
7.3
3
5
433
912
12.4
8.0
7
9
431
907
11.8
7.6
7
13 >30
423
904
11.0
7.4
6
>30
415
904
11.3
7.7
* Displacement in case of (1) final brittle failure and (2) final ductile failure.
1004
Number
of tests
2
5
6
5
5
5
4
Analytical Evaluation
4.1 Analytical Description of Entire Load-Slip Relationship
Foschi proposed a three-parameter function to model the load-slip behaviour of timber joints [5]. To
include the softening behaviour of the joints, his equation is extended to a five-parameter function.
Thus, the entire load-slip curve (P, u) for sheathing-to-timber joints can be approximated as
P
( P0 K1u ) (1 e
K0 u
P0
D
u
E
)e
(1)
where P0, K0, K1 = Foschi parameters, and D, E = additional parameters, see Figure 6.
Evaluation of the five-parameter equation using
ordinary non-linear regression analysis and special
Load P
statistical software program is not a straightK1
K0
forward procedure in the present case. Here, an
evaluation procedure will be used as follows: (1)
Pm
The first on-loading part of the curve is forced to
pass through the maximum horizontal tangent
point, (um, Pm), where Pm is the mean failure load
P0
occurring at the corresponding mean deflection,
um, as found from the tests; and (2) The second
Ep
Pc
unloading part of the curve could be fitted to the
mean energy, Ep, consumed after the maximum
uc
um
load is reached as found from the tests, see the
Slip u
hatched area in Figure 6. As an alternative
procedure, other correlated parameters such as a
defined point of total collapse, (uc, Pc), could more
Figure 6. Load-slip curve modelled by a 5conveniently be used as the basis for the curve fit
parameter equation.
for this part of the curve.
Load-slip data from tests is a set of (u, P)-coordinates usually registered at constant time intervals.
The amount of data is dependent of time to failure and occasional variation in loading speed for
each test specimen. Using this raw data for the curve fit gives more weight to the test series with a
high number of curve points. The problem could be decreased by using a smoothed curve with
reduced number of points for each test. Each point represents the mean value of the test data at a
certain curvilinear distance in a normalized load-slip plot, where both the failure load and the slip at
failure are set equal to 1.
Forcing the five-parameter load-slip curve through the points, (um, Pm) and (uc, Pc), the non-linear
curve fit is reduced to finding the best value of the three Foschi parameters K0, K1, and P0 from
load-slip data of the on-loading phase. An Excel spread-sheet and the Excel Solver have proved to
be excellent tools for finding the set of Foschi parameters giving the best value of the curve fit. In
the solving process the parameters D and E are found by iteration for each set of Foschi parameters.
4.2 Example of Curve Fits to Test Results
As examples of curve fits using this analytical equation, the load-slip relationships for particleboard
in Figure 3 will be used. Three curves are presented in Figure 7 showing curve fits based on, (a) all
curves, (b) curves with ductile type of failure mode, and (c) curves having brittle failure
characteristics. The different parameters are given in Table 3.
5
Comments and Conclusions
Load-slip curves for single sheathing-to-timber joints with different sheathing materials and loaded
parallel and perpendicular to grain are presented. The type of fastener used gives the desired ductile
failure mode for hardboard. In case of particleboard and plywood, the optimum type of fastener is
still to be found. The head of the present type of fastener is too small for soft materials as
particleboard and, especially, plywood.
The influence of the edge distance of the fastener in sheets of hardboard is illustrated. Preliminary
1005
0
5
10
15
20
25
(a)
u [mm]
30
35
40
45
1600
1400
1200
1000
800
600
400
200
0
50
P [N ]
1600
1400
1200
1000
800
600
400
200
0
P [N ]
P [N ]
results indicate that the minimum edge distance is 2d for parallel and 4d for perpendicular load-tograin direction, respectively.
A 5-parameter equation to model the entire load-slip relationship is proposed and its application to
joints with particleboard is illustrated.
0
5
10
15
20
25
(a)
u [mm]
30
35
40
45
1600
1400
1200
1000
800
600
400
200
0
50
0
5
10
15
20
25
(c)
u [mm]
30
35
40
45
50
Figure 7. Load-slip curves for sheathing-to-timber joints with particleboard: Curve fits based on
(a) all curves, (b) curves with ductile failure, and (c) curves with brittle failure.
Table 3. Parameter values in the analytical equation.
P0
[N]
648
151
861
All curves
Curves with ductile failure
Curves with brittle failure
* A measure of the accuracy of the estimation.
6
K0
[N/mm]
888
1277
731
K1
[N/mm]
102
462
55.3
D
3.08
0.860
10.3
E
D
[mm ]
5208
5.20
71.7 ˜ 1010
R2 *
[%]
92.3
91.5
88.5
Acknowledgement
We gratefully acknowledge the assistance of Jonas Eltoft, B.Sc. and Samuel Palm, B.Sc., who
performed the tests at Umeå University, Department of Applied Physics, Civil Engineering. This
work is part of a Nordic Wood project on panel structures.
7
References
[1]
Källsner B. and Girhammar U.A., “A plastic lower bound method for design of wood-framed
shear walls,” 8th World Conference on Timber Engineering, Lahti, Finland, 2004.
Eltoft, J. and Palm, S., “Tests of Joints in Sheathing-to-Timber Walls with Masonite as Sheet
Material” (in Swedish), Umeå University, Department of Applied Physics & Electronics,
Civil Engineering, Report 2003:2, Umeå, Sweden, 2003.
Eltoft, J. and Palm, S., “Tests of Joints in Sheathing-to-Timber Walls with Different Sheet
Materials” (in Swedish), Umeå University, Department of Applied Physics & Electronics,
Civil Engineering, Report 2002:4, Umeå, Sweden, 2002.
Eltoft, J., “Tests of Sheathing-to-Timber Joints with Masonite at Different Edge Distances”
(in Swedish), Umeå University, Department of Applied Physics & Electronics, Civil
Engineering, Report 2003:4, Umeå, Sweden 2003.
Foschi, R. O., “Load-slip characteristic of nails,” Wood Science, Vol. 7, No. 1, pp. 69 – 76,
1974.
[2]
[3]
[4]
[5]
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