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4.13 SCREWS
42-7-1 G Pirnbacher, R Brandner, G Schickhofer
Base Parameters of self-tapping Screws
Introduction
According to EN 1382:1999 the axial withdrawal strength of fasteners in
timber is determined by means of a standardised test under tightly regulated
loading conditions and exact rules for the moisture content of the specimens
(thereby also limiting the possible climatic conditions during preparation
and/or storage of the test pieces). The axial resistance is the primary
mechanism defining connections that employ axially loaded screws as
load carrying members – in general the dowel-type effect of the screw is
not taken into account for the design of the connection. Variations of the
moisture content, the temperature at “screw-in” and/or “pull-out” and
wether the screw is pre-drilled or not are not considered in the design rules
at all. Other parameters like the effective length and the angle between the
screw axis and the grain are taken into account in the different rules available
in the diverse codes like EN 1995-1-1:2004-1 and DIN 1052:2008
and technical approvals.
Conclusion
The parameters researched show significant effects on the withdrawal resistance
of screws. Most prominent is the effect of embedment with an increase
of at least 15% starting from only 15 mm embedment of the thread
into the wood.
Additionally the effect of embedment covers other effects – e.g. the effect
of the angle to the grain (earlier research without consideration of embedment
indicated no effect between 90° and even below 45°). The inclusion
of the embedment depth is proposed in the following form:
k � 1,15 if l � 2d
emb
emb
The moisture content shows an influence of 0.65% per percent of moisture
content. The
proposed correction takes the form of:
�1.00 8% mc . �12%
�
� � for
�
�
1.00 � 0.0065 ��u �% ��12
� 12% m. c � 20%
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.77
k
mc
The diameter has an effect of about -12.5% between the diameters of 8
mm and 12 mm. The relation included in the modelling of fax as function of
density is:
�0.428
� �
k � 2.44d d in mm
diam
The length can be considered by deducting the tip from the thread length,
where the correction is diretly related to the diameter and proposed in the
form of:
lkorr �lthread� klengthd k � 1.17
length
with
The temperature does not exert a quantifiable effect on the withdrawal resistance.
k �1.00 for � 20 to 50 C
temp
A modified Hankinson relation optimized to describe the 5% fractile can
be formulated by an adjustment of the exponents from the original value of
2.0 to 2.2 as follows:
1
khankinson,
modified �
2.2 2.2
sin � �1.30cos
�
� � � �
The dependency from density shows clearly in the regression analysis. An
example of the obtained mean value model shall be given here:
0.572
fax, mean,90 � 0.01353�test �0.28147 �2.4d � 2.18888
with
� �450kg/m and d � 8 mm
mean
3
0.572
fax, mean,90
� 0.01353� 450 �0.28147 �2.44 �8 � 2.18888
fax, mean,90
� 6.09 � 2.26 � 2.18 � 6.02 N/mm
The investigated effects show clear trends and can be normalized across
material and angle to the grain variations. Summing all single effects up
2