EXPERIMENTAL INVESTIGATION ON PARALLEL-TO-GRAIN
WOOD-TO-WOOD JOINTS WITH SELF-TAPPING SCREWS
Marco Ballerini 1
ABSTRACT: The paper presents the main outcomes of an experimental research performed on parallel-to-grain woodto-wood joints made with self-tapping screws in single shear. Screws have a nominal diameter of 12 mm and have been
investigated with and without washers. Joints with 8 mm dowels (obtained from the shank of the self-tapping screws)
have been also investigated for comparison purpose. Specimens have been designed to fail according to failure modes
of the European Yield Model that don’t correspond to a uniform stress distribution in wood elements. In detail the
failure modes taken into consideration are: rigid rotation of fasteners (mode c), development of a single plastic hinge in
fasteners in the point-side wood elements (mode d) or in the head-side wood elements (mode e), development of two
plastic hinges in fasteners (mode f). In designing, the strength increase due to the “rope effect” has been taken into
account. After testing, specimens have been processed to determine the actual failure modes of fasteners, their plastic
rotations and the maximum embedding deformations close to shear planes. The main results, in terms of strengths,
actual failure modes and plastic deformations, are compared for the four different failure modes and for the three
investigated fasteners: the 12 mm self-tapping screws with washers, without washers and the 8 mm dowels derived
from the shank of self-tapping screws.
KEYWORDS: Timber joints, Self-tapping screws, Rope effect
1 INTRODUCTION 1
Self-tapping screws are very interesting fasteners for
timber engineering since characterized by widespread
application potentials. Indeed, these screws have a lot of
properties that make them very suitable and profitable in
a wide number of situations.
First of all, these fasteners are particularly attractive by
the economic point of view. In fact, though characterized
by higher cost per unit compared to ones of conventional
screws, the lower processing needs on wooden elements
lead usually to significant savings in time and overall
costs. In addition, they have lower implementation costs
since require only simple and safe electric gears. Finally,
the lack of the requirement of predrilling allows larger
tolerances in fasteners positioning in the erection site.
Moreover, an important feature that makes these screws
particularly apt in the restoration of old/ancient timber
structures is the limited invasiveness of these fasteners
and their “reversibility”: the possibility of remove the
fasteners due to different reasons, like interim structural
needs or the definition of a different structural solution.
1
Marco Ballerini, Department of Mechanical and Structural
Engineering (DIMS), Faculty of Engineering, University of
Trento, Via Mesiano 77, 38123 Trento, Italy.
Email: marco.ballerini@ing.unitn.it
However, since self-tapping screws are quite recent in
production, the little availability of experimental data on
their mechanical properties and on their behaviour in
full-scale joints limits an efficient use of these screws for
structural purposes.
Due to this reason, an experimental research on the
effectiveness of self-tapping screws has been recently
carried out at the University of Trento [1].
The research concerns with wood-to-wood joints loaded
parallel-to-grain with different fasteners in single shear.
The aim of the research is the definition of the strength
of joints, with and without washers, varying some
parameters like: the fasteners, the failure modes, the
number of fasteners in force direction, the influence of
axial force in fasteners (the “rope effect”, by means of a
comparison with 8 mm dowel joints).
For a more accurate analysis of the structural behaviour,
specimens after testing have been sectioned in order to
obtain the actual failure modes of each fastener in the
joint, the location of plastic hinges, the amount of plastic
rotations of fasteners, the maximum values of embedding
deformations in wood elements (close to shear planes).
The paper presents the main results of the experimental
research. With respect to joints strength, the experimental
outcomes are analysed and compared between them and
with the design formulae embodied in the European
design code for timber structures (Eurocode 5 – EN 19951-1: 2009, [2]).
2 BACKGROUNDS
As it is well known, to evaluate the strength of wood-towood and wood-to-steel joints with cylindrical fasteners,
the European standard for timber structures takes the
plastic model proposed by Johansen in 1949 [3].
This model states that the number of failure mechanisms
that a joint with cylindrical fasteners can shows is fixed
and governed by some parameters like number of shear
planes in fasteners, and thickness and position of steel
plates in case of wood-to-steel joints. In between this
family of possible failure mechanisms, the actual one is
the one that presents the minimum strength.
For instance, in case of wood-to-wood joints with
fasteners in single shear – the joints investigated in this
research – the possible failure mechanisms are the 6 ones
reported in Figure 1. To each failure mode the Eurocode
5, on the basis of the Johansen’s model, assigns the
following characteristic strengths:
f h,1,k t1 d
f h,1,k t1 d "
! + 2! 2 1+ " + " 2 + ! 3" 2 ! ! 1+ "
1+ ! $#
1,05
1,05
1,15
)
f h,1,k t1 d
(2 + ! )
f h,1,k t2 d
(1+ 2! )
2!
1+ !
(
)
(
)%'& +
Fax,R,k
4
t2
t1
t1
t2
t1
t2 t1
mode a
t1
t2
mode d
f h,2,k t2 d
(
t1
t1 t2
mode b
t1
t1
t2
mode e
mode c
t1
t1
t2
t1
mode f
Figure 1 – Failure modes of wood-to-wood joints with
cylindrical fasteners in single shear
"
% F
M y,Rk
$ 2! 1+ ! + 4 ! 2 + !
' + ax,R,k
!
!
2
$
'
4
f
t
d
h,1,k
1
#
&
"
%
M y,Rk
F
$ 2! 2 1+ ! + 4 ! 1+ 2!
! ! ' + ax,R,k
2
$
'
4
f h,1,k t2 d
#
&
F
2 M y,Rk f h,1,k d + ax,R,k
4
(
(
)
(
)
)
(
)
In above equations, β is the ratio between the embedding
strengths of timber elements (fh,2,k/fh,1,k), θ is the ratio
between the lengths of fasteners in the timber elements
(t2/t1), d is the fastener diameter, My,Rk is its plastic
moment.
The term Fax,Rk/4 represents the strength increase due to
the so called “rope effect”: namely the effect of the axial
force in fasteners in line with the load direction (this
effect can occur only in failure modes in which fasteners
rotate as rigid bodies or as a consequence of the
development of plastic hinges).
Previous parameters are well defined in case of traditional
cylindrical fasteners. However, in case of self-tapping
screws same fundamental parameters are not easy to
define (see Figure 2). This is the case of the diameter –
where three values are of relevance: the nominal
diameter (that corresponds to the outer diameter of the
threaded part), the diameter of the core of the threads,
the diameter of the shank – and of the yielding moment
of the screw – in this case at least two yielding moments
are of relevance: one for the shank and one for the
threaded part of the screw.
Moreover, also the embedding strength of self-tapping
screws needs to be newly investigated due to the
influence of different diameters in the same screw, to the
different shape of threads, to the different interface
situations along the screw (the threaded part and the
shank part of the screw in a wood hole generated by the
larger nominal diameter).
Figure 2 – A modern self-tapping screw, general view
and detailed view of cutter in point.
3 THE EXPERIMENTAL PROGRAMME
To supply to the lack of data on the mechanical behaviour
of joints with self-tapping screws, an experimental
investigation on wood-to-wood joints with fasteners in
single shear has been designed. The main aim of the
research is to exploit the influence on the strength of
some relevant parameters.
The parameters taken into account are:
1) the fasteners – self tapping screws with a nominal
diameter of 12 mm with and without washers are the
main object of the research; however, for comparison
purpose, joints with 8 mm dowels (derived from the
shank parts of screws) have been investigated;
2) failure modes – the 4 failure mechanisms associated
with the rotation of the fasteners as a rigid body or as a
consequence of the development of one or more
plastic hinges in each fastener (modes c to f, see
Figure 1) have been taken into account;
3) the “rope effect” – indirectly, by means of the comparison
the values of plastic moments experimentally determined
in [4]:
with the strength increase observed in joints with
fasteners characterised by different withdrawal strength;
4) the effective number “nef” – specimens with 2 and
with 4 fasteners on each side elements (respectively
with 1 and 2 fasteners aligned with the load direction)
have been investigated.
In order to collect some relevant data, the samples have
been processed after tests execution. The main aims of
this part of the research are: collect the actual failure
mechanism achieved by fasteners in the investigated
joints; collect the magnitude of the plastic rotations in
fasteners and the location of plastic hinges; collect the
magnitude of maximum plastic deformation in the wood
(close to shear planes) due to embedding stresses.
M y,R,core =
90900 Nmm (cov = 3.78 %)
(2)
M y,R,shank = 123100 Nmm (cov = 0.17 %)
On the basis of previous results, the design of timber
elements to get the desired failure mechanisms has been
carried out. The design outcomes are summarized in
Table 1. For each joint configuration 3 samples have
been manufactured to have a minimum statistical base.
As a consequence, considering the 4 failure modes, the 3
type of fasteners, and the 2 fasteners patterns in joints, a
total number of 72 samples have been investigated.
3.1 SPECIMENS DESIGN
4 TEST RESULTS
Samples have been designed in symmetrical configuration
and for compressive loads in direction parallel to the
grain (see Figure 3).
Fasteners used in joints were 12 mm Würth Ecofast-Assy
self-tapping screws supplied by the manufacturer.
Timber elements were made of spruce (picea abies) with
a density ranging between 350 and 535 kg/m3.
The experimental research was carried out according to
provisions of EN 26891 [5]. Tests were considered ended
at a maximum joints displacement of 15 mm. The study
was performed by means of a servo-controlled hydraulic
actuator; the load and the joint slips were recorded
continuously. The test setup is shown in Figure 4.
With respect to the embedding strengths, a limited
investigation has been specifically designed to obtain the
strength values both for the threaded parts of screws
(inserted in wood without predrilling) and for the shank
parts (inserted in the holes produced by threaded parts).
With reference, as provided in Eurocode 5, to the core
diameter of screws threaded parts increased by 10% (dcore
= 7.1 mm) and to that of the shank (dshank = 8 mm), the
investigation has provided the following results:
f h,0,threads = 0,0733 ! !
The tests results of those of
measurements on processed
samples are collected in Figures
5 ÷ 9 and in Table 2.
Figure 5 shows the average
load-slip curves of different
joints. The reported loads are
normalized to the number of
fasteners. From Figure 5 it can
be observed that joints strength
generally decreases moving
from joints with screws and
washers, to ones with screws
only, and to those with dowels.
(1)
f h,0,shank = 0,0608 ! !
Concerning the plastic moments of screws My,R (for the
threaded part and for the shank), reference was made to
4 fasteners on each side
Figure 4 – Test set-up.
2 fasteners on each side
Figure 3 – Geometry of specimens with 4 and 2 fasteners on each side in single shear.
Table 1 –
Main geometrical parameters of joints and screws (mm).
Failure modes
c
d
e
f
b
t1 = s1
t2
s2
140
50
50
90
90
50
90
50
90
100
190
100
190
a1
120
a2
l screw
l thread
60
100
140
140
180
60
80
80
100
depends significantly by the activated failure modes
(modes c and d are the only ones that require the rotation
of screws heads) but also by the density of the wooden
elements (the same samples with 2 screws on each side,
characterised by a significantly lower density of wooden
elements, had not shown this tendency).
In samples designed to fail in mode c, the splitting of side
elements have been very moderate; this reflected in a very
limited drop in strength of these joints if compared with
the strength of same joints with 2 fasteners on each side.
On the contrary, the samples designed to fail in mode d
have shown an abrupt collapse consequently to the
Load on each fastener (kN)
Load on each fastener (kN)
Load on each fastener (kN)
Load on each fastener (kN)
However, some deviations in samples with 2 fasteners on
each side and designed to fail in mode e and with samples
with 4 fasteners in each side and designed to fail in modes
c and d have been noticed.
With respect to samples designed to fail in mode e, a
strength of joints with screws and washers very close to
that of joints with only screws is apparent. No supporting
elements have been found to explain this lower strength.
Conversely, the behaviour of samples designed to fail in
modes c and d can be easily explained by the fact that a
tendency of screws heads to induce longitudinal cracks in
side timber elements have been recorded. This tendency
Slip (mm)
Slip (mm)
Figure 5 – Average load-slip curves of specimens with 2 and 4 fasteners for each shear plane.
complete splitting of side elements well before the
maximum displacement.
Finally, Figure 5 shows clearly the different behaviour of
samples with dowels with respect to those with screws.
The behaviour of joints with dowel can be quite well
described by an elasto-plastic behaviour with no
hardening. On the other side, joints with screws are
almost always characterised by a significant hardening.
Failure mode c
Pictures in Figure 6 show the actual failure mechanisms
of samples. From pictures it is evident that all joints with
dowels fail according to respective design failure modes.
Conversely, it does not happen for all the joints with screws
or with screws and washers. In these joints, the design
failure modes e and f have been regularly observed, but the
design failure modes c and d have been found replaced by
modes e and f. This is due to screws heads and to washers
that prevent (to a different extent) the development of those
collapse mechanisms that require the free rotation of
fasteners ends.
The qualitative considerations exposed are presented in
Table 2. The data of Table 2 show clearly both the influence
of the density of wood elements and of the end-restraint
degree of different fasteners on the observed failure modes.
sample CAB-02
sample CAC-01
sample CBD-02
sample CBB-01
sample CBC-03
sample CCD-01
sample CCB-02
sample CCC-03
sample CDD-02
sample CDB-03
sample CDC-03
Failure mode f
Failure mode e
Failure mode d
sample CAD-03
Figure 6 – Observed failure modes in specimens with 4 fasteners for each shear.
Table 2 –
Design
failure
loads
c
d
e
f
3
Samples’ densities (kg/m ), failure loads (kN) – normalized to number of fasteners – with coefficients of
variation, observed failure modes in fasteners (if different to design ones).
Type of fasteners
Dowels
Screws
Screws with washers
Dowels
Screws
Screws with washers
Dowels
Screws
Screws with washers
Dowels
Screws
Screws with washers
average
density
376
395
487
386
384
386
394
401
436
377
410
473
2 fasteners on each side
observed
Fu (CV)
failure modes
4.76 (0.01)
8.30 (0.07)
8.91 (0.06)
5.57 (0.03)
8.29 (0.08)
10.30 (0.09)
6.21 (0.12)
7.40 (0.14)
7.20 (0.02)
7.02 (0.01)
10.79 (0.04)
13.42 (0.25)
It seems of relevance to stress out that the restraint
degree offered by washers appears more effective to that
offered by screws heads since it has not disadvantages.
On the contrary, screws with countersunk heads tend to
develop more easily brittle failures. The use of washers
appears therefore important in order to avoid the
premature splitting of side wooden elements in modes c
and d.
The normalized failure loads of joints are illustrated in
Figure 7 for the different fasteners and for the different
design and the observed failure modes. In the Figure,
boxes are associated to the different design failure
modes, symbols are associated to the different observed
failure modes, type of fasteners are ordered along the x
axis. Data related to joints with 2 fasteners on each side
are plotted on the left side of the vertical line associated
to each type of fastener while data of samples with 4
fasteners on each side are plotted on the right side.
From Figure 7 it can be noticed that normalized
strengths rise greatly from dowel joints to screw joints
and also from screw joints to those with screws and
washers. However, the latter increase is less pronounced.
It suggests that treads are more effective in developing
axial forces in screws, at the base of the “rope effect”,
than washers.
Driving the attention to joints with same fasteners, the
following comments can be derived:
• dowel joints show a moderate and gradual
normalized strength increase with design/observed
failure modes: about 5, 6 and 7 kN on each dowel
(modes d and e have approximately the same
strength). No substantial difference can be detected
from the strength of joints with 2 and 4 dowels on
each side;
• screw joints show a higher normalized strength than
that of dowel joints. The normalized strength is about
of 8 kN for samples designed to fail in modes c, d
and e, while it is of about 10.5 kN for mode f. The
strength of joints designed to fail in mode d and with
4 screws on each side are significantly lower than
those of samples with only 2 screws on each side but
25 % e
100 % e
38 % f
average
density
489
475
461
460
471
446
436
451
477
444
445
445
4 fasteners on each side
observed
Fu (CV)
failure modes
5.62 (0.20)
7.22 (0.01)
7.51 (0.07)
6.58 (0.08)
6.73 (0.08)
10.44 (0.13)
5.46 (0.03)
7.58 (0.04)
8.52 (0.08)
7.30 (0.07)
9.57 (0.15)
10.98 (0.06)
25 %
70 %
100 %
17 %
Splitting
100 %
e
e
e
c
f
33 % c
this is due to the already mentioned premature brittle
collapse due to splitting;
• screw with washer joints are the ones with higher
strength. Samples designed to fail in modes c and d,
due to the rotation restriction offered by washers to
screws heads, failed respectively according to modes
e and f. Consequently, normalized strengths of joints
with design failure modes c and d are respectively
equals to those of modes e and f: 8 and 11 kN.
The data collected on processed samples are summarized
in Figures 8 and 9 with the same illustration technique of
Figure 7.
In Figure 8 the average values of plastic rotations in
fasteners of each sample are presented. For all kind of
fasteners, the average plastic rotations recorded for
failure modes d, e and f, range in between 4 to 16°. On
average, failure mode d is the one with the lowest values
while the e failure mode is the one with the highest
values.
With reference to failure mode c, some plastic hinges
have been recorded in some fasteners in spite of the fact
that the failure model does not require the development
of any plastic hinge. In case of joints with screws and
with dowels the average plastic rotations are very small:
less than 4°. In case of joints with screws and washers
the average plastic rotations are very large (between 8
and 16°) but, as already mentioned, these joints fail
according to failure mode e.
The recorded plastic rotations are in good agreement
with those predicted in [6] for a density of wood
elements ranging between 360 to 550 kg/m3. These
plastic rotations are correlated to yielding moments of
fasteners equal to 0.82÷0.86 the maximum bending
moment (achieved for plastic rotations of about 50°).
Figure 9 shows the values of average embedding
deformations recorded close to joints shear planes. These
plastic deformations range between 3 and 7 mm with a
prevalence of values around 5 mm. Such value is the
same set by EN 383 [7] for the experimental evaluation
of the embedding strength of wood loaded by cylindrical
fasteners.
Figure 7 – Values of failure loads (normalized to number of fasteners) of specimens with 2 fasteners on each side (on
the left of the reference line) and of 4 fasteners (on the right of the reference line) vs. design and observed failure modes.
Figure 8 – Recorded plastic rotations in fasteners vs. design and observed failure modes.
Figure 9 – Recorded maximum embedding deformations in wood elements vs. design and observed failure modes.
5 ANALYSIS AND COMPARISON
To assess the effects of the investigated parameters, the tests
results have been compared with the strengths predicted by
the Johansen’s model. Such strengths are provided by the
last 4 formulae of the set of equations reported in § 2. The
equations have been assumed without the numerical
coefficients and without the terms related to “rope effect”.
For the fasteners yielding moments, the 80% of the values
reported in (2) have been assumed to take into account of
the average plastic rotations of fasteners recorded (see
comments to Figure 8). Concerning the embedding strength
of timber elements, values reported in (1) were used.
The results of the comparison are illustrated in Figure 10
and in Figure 11 as a function of the average density of the
samples. In latter Figure, the results are presented according
to fastener type and to observed failure modes.
From both Figures it is immediately evident that
experimental strengths shown by dowelled joints are
correctly predicted by Johansen’s model. On the other side,
the screwed connections with or without washers are
obviously underestimated by the model since it does not
take into account the “rope effect”.
For these fasteners, form Figure 10 a different strength
increase can be detected for failure modes c and e and
for failure modes d and f. Indeed, while samples related
to modes c and e show an average strength greater of
about 40-50% the ones predicted by Johansen’s model,
samples related to failure modes d and f have a larger
average strength: about 50-100% the ones of Johansen’s
model. There are some exceptions to the described
behaviour. The first one is related to the large resistance
shown by the screwed joints designed to fail in mode c if
compared to analogous samples with 4 screws. The
second exception concerns the screwed joints designed
to fail in mode d and with 4 screws on each side, which
look very weak due to the splitting failure of side
elements.
Figure 11 does not show a marked influence of the
average density of samples on the strength values with
Figure 10 – Comparison between recorded failure loads and the original Johansen formulae that don’t consider the
reduction in strength due to multiple fasteners in line (neff) and the increase in strength due to the “rope effect”.
Figure 11 – Comparison between recorded failure loads and the Johansen formulae vs. density of wood elements
(lighter markers refer to specimens with 2 fasteners).
the only exception of the data of screwed joints. To the
author, this influence seems only apparent since these
data encompass the wake strength of samples that fail
due to splitting (characterized by a density of 460-480
kg/m3) and the exceptional resistant samples with 2
screws designed to fail in mode c (characterized by a
density of 380-420 kg/m3).
As regards the efficiency of fasteners aligned in load
direction (neff/n), from both Figures can be seen that
samples with 2 fasteners on each side (in lighter marks in
Figure 11) usually have a resistance greater than those
with 4 fasteners (in darker marks). The higher strength is
contained unless the already mentioned exceptions.
For a numerical appraisal, the average strength of all test
configurations are reported in Table 3. The data seem to
confirm the remark previously advanced about the
strength of joints failed in modes c and e and those of
joints failed in modes d and f.
With respect to the efficiency of the number of fasteners,
ratios neff/n computed for samples failed in modes c and
e generally show a reduction in strength of about 20%
for dowelled joints and of 10% for screwed joints. The
same ratios for joints failed in modes d and f show a
reduction of about 7% with a lesser influence of
fasteners type.
These values are quite different from those proposed by
Eurocode 5 that are more dependent by fastener
diameters (they give a reduction of only 3% for the 8
mm dowels and of about 13% for the 12 mm screws).
However, a more important matter is that the efficiency
seems to be influenced by the failure mode.
Table 3 –
Dowels
Screws
Screws with washers
(2)
(3)
Finally, the joints strengths have been compared with
those proposed by Eurocode 5 taking into account the
opposite effects of the “rope effect” and of the number of
fasteners aligned in the force direction. The comparison
is shown in Figure 12. From the graph it can be noticed a
quite good overall estimation of the joints strengths: the
maximum overestimation is of about 15-20% and the
maximum underestimation of about 55%.
Looking in more detail the results it can be observed:
• concerning the dowelled joints, strengths are
correctly predicted for samples failed in mode c and
generally underestimated (of about 10%) for the
samples failed according to the other modes;
• with reference to joints with screws, strengths are
generally underestimated. The underestimation is due
to the limited value of the “rope effect” that is
credited to screws head. The maximum underestimation is for failure mode f;
• for joints with screws and washers, strengths are well
estimated for samples failed in modes d and f while
they seems slightly overestimated in case of failure
modes c and e.
Ratios of average Fu / FJ for all joint configurations.
Type of fastener
(1)
Table 4 reports the strength increases of screwed joints
with respect to those of dowelled joints once the effect of
the number of fasteners in line with the force is taken into
account (neff/n). The strength increase can be associated to
the “rope effect”. From the Table an evident influence of
failure modes on strength increase is detectable. The “rope
effect” can be evaluated of about 45% for failure modes c
and e, and of about 85% for failure modes d and f.
number of fasteners
on each side
4
2
4/2
neff /n
4
2
4/2
neff /n
4
2
4/2
neff /n
Fu / FJ ratios vs. the design failure modes and the observed ones (in parenthesis)
c
e
d
f
0.82
0.86
0.93
0.98
1.03
1.08
0.99
1.05
0.80
0.80
0.94
0.93
0.80
0.93
1.25
1.30
1.15
1.71
1.75
1.43
1.66
1.85
0.71 (1)
0.91
0.69 (2)
0.93
0.91
0.93
1.31 (e)
1.44
1.71 (f)
1.80
1.49 (e)
1.31
1.83 (f)
1.85
0.88
1.10 (3)
0.93
0.97
0.88
0.95
value don’t taken into account in computing neff/n due to the too much higher failure loads of specimens with 2 fasteners;
value don’t taken into account in computing neff/n due to splitting occurred in specimens with 4 fasteners;
value don’t taken into account in computing neff/n due to the too much lower failure loads of specimens with 2 fasteners.
Table 4 – Experimental strength increase in joints with screws and screws with washers.
Fu / [(neff /n) FJ ]
Screws
Screws with washers
(1)
c
1.56
1.45 (e)
Design failure modes and the observed ones (in parenthesis)
e
d
1.42
1.46 (1)
1.43
1.84 (f)
value afflicted by the splitting of specimens with 4 fasteners.
f
1.86
1.90
Figure 12 – Comparison between recorded failure loads and the ones predicted by Eurocode 5 taking into account both
neff and the “rope effect”.
6 CONCLUSIONS
The research has focused on the experimental
determination of the strength of wood-to-wood joints
with self-tapping screws in single shear. To this end, a
parametric study that takes into account different
parameters has been carried out.
The research allows deriving the following conclusions:
• joints with screws and washers have a higher strength
of those with screws without washers which in turn
are more resistant than those with dowels;
• the strength increase is essentially due to the “rope
effect” which can take place only if fasteners are
effectively bounded;
• washers, providing a rotational constraint to fasteners
ends, prevent the origin of failure modes that requires
this degree of freedom (modes c and d);
• screws heads tend to behave as washers, however are
less efficient and consequently cause higher local
stresses in wood. Screws with countersunk heads
used in joints designed to fail in modes c and d can
lead to premature collapses for splitting of timber
elements. For these failure modes, the use of washers
is strongly recommended;
• test results, although with some conflicting data, have
allowed the estimation of the strength increase to the
“rope effect” and the strength reduction due to the
number of fasteners aligned in the direction of the
force;
• the values derived are not excessively far from those
established by the European standard for timber
structures which are generally more conservative.
• a better appraisal of the actual strength of joints with
self-tapping screws can be based on the classical set
of equations assuming more appropriate values of
basic parameters (yielding moment and embedding
strength).
ACKNOWLEDGEMENT
The author wishes to thank the company Würth Srl of
Egna (BZ), the company Heco Italia Srl of Romano
d'Ezzelino (VI) and the company Rotho Blaas Srl of
Cortaccia (BZ) that kindly provided the fasteners used in
the research.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
F. Asson: Experimental investigation on wood-towood joints with self-tapping screws (in Italian).
Thesis, University of Trento, Italy, 2009.
EN 1995-1-1 – Eurocode 5: Design of timber
structures. Part 1-1: General – Common rules and
rules for buildings. CEN, European Committee for
Standardization, 2009.
Johansen K. W.: Theory of timber connections.
International Association of Bridge and Structural
Engineering, Publication No. 9:249-262, 1949.
C. Montresor: Determination of plastic moments of
self-tapping screws for joints in timber structures
(in Italian). Thesis, University of Trento, Italy,
2008.
EN 26891 – Timber structures. Joints made with
mechanical fasteners. General principles for the
determination of strength and deformation
characteristics. CEN, European Committee for
Standardization, 1991.
M. Ballerini, and C. Montresor: Mechanical characterisation of joints with self-tapping screws for
reversible structural refurbishment works (in
Italian). In Consolidamento delle strutture in legno,
pages 129–163, Hevelius Edizioni, Naples, Italy,
2009.
EN 383 – Timber structures. Test methods.
Determination of embedding strength and
foundation values for dowel type fasteners. CEN,
European Committee for Standardization, 2007.