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carrying capacity of steel bolts in end-grain under lateral loading can sufficiently
accurate be calculated by using the formula given in the paper.
25-7-2 J Ehlbeck, H Werner
Softwood and hardwood embedding strength for dowel-type fasteners,
Background of the formulae in Eurocode 5
Introduction
The embedding strength of timber and wood-based materials as well as the
yield moment of the fasteners are (besides the joint's geometry) governing
properties for determining the load-carrying capacities of joints with dowel-type
fasteners. The embedding strength depends on the type of fastener,
the joint configuration (such as member thicknesses, end and edge distances
as well as spacing of the fasteners), the manufacturing of the joint
(e.g. predrilled holes), and the wood species or the quality of the woodbased
materials. Thus, the embedding strength is not a special material
property, but a system property.
Many research works have been done to describe this property and to
collect sufficient test data. The test methods used differed, however, in
many cases significantly so that the results can not be compared without
any reservation. It was therefore an important task of the responsible European
standardization committee to produce a test standard EN 383
"Timber Structures - Test Methods - Determination of Embedding
Strength and Foundation Values for Dowel-type Fasteners".
The embedding strength of some hardwoods was tested according to
this European, harmonized test method. The tests were made under different
load-grain angles using bolts and dowels. Additional tests were performed
with European whitewood (picea abies) under 90° load-grain angle.
When evaluating all test data, the results published by [Whale and
Smith, 1986 a and b] were also used because these tests under loading parallel
to grain were in line with the main principles of EN 383.
Conclusions
The variance of the embedding strength of hardwoods (coefficient of variation
15%) is larger than the variance of the hardwood densities (coeffi-
cient of variation 8%). This taking into account, the characteristic embed-
f , can be derived assuming a Gaussian distribution:
ding strength, hk ,
Hardwood, parallel to grain
� �
f � 0,09 � 1� 0,01d
�
h,0, k k
Hardwood, perpendicular to grain
� �
f � 0,09 � 1� 0,016d
�
h,0, k k
f h,0
embedding strength parallel to grain, N/mm 2
f h,90
embedding strength perpendicular to grain, N/mm 2
� k characteristic density, kg/m 3
d fastener diameter, mm
Similar to the behaviour of wood under compressive stresses the embedding
strength decreases with increasing load-grain angle,� . This effect is
also depending on the fastener diameter, i.e. the decrease of embedding
strength is more distinct with large than with small diameters. As a good
fitting approximation can be used the Hankinson formula (see the figure):
Ratio fh, � / �kover load-grain angle � � . Hardwood, predrilled holes,
smooth round fasteners, diameter d = 30 mm.
For nailed joints without predrilled holes the embedding strengths are approximately
independent of the load-grain angle because incipient cracks
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.19