CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

Figure 2: Test arrangement
Summary
The tensile strength perpendicular to grain of 187 wood specimens was determined
using the test arrangement of EN 1193 (see figures 1 and 2). 53
% of these specimens failed right at the bond line at the intermediate
wood. By Finite Element calculations it was shown that stress intensities
are evoked at the bond line and the stress distribution in the specimen is
non-uniform.
An important aim of the project was to determine the correlation of tensile
strength perpendicular to the grain with other properties, especially
with the density. By separating the largest group of the tangential specimens
by the location of rupture, the result was a comparatively high coefficient
of correlation for specimens which failed at the joint and a very
small coefficient of correlation for those with failure in the wood. One
possible hypothesis for this result is based on Weibull's probabilistic fracture
theory.
It has to be emphasised that the tests were done with nearly clear wood
specimens without visible cracks or splits, which often occur in structural
size timber. Especially because of cracks, the tensile strength perpendicular
to grain in structural timber will be lower than the values given in table
5.
32-6-4 H J Blass, M Schmid
Tensile strength perpendicular to grain of glued laminated timber
Introduction
The tensile strength perpendicular to grain of glued laminated timber according
to the European standard EN 1194 is based on the following equation:
(1)
ft,90,g,k = 0,2 + 0,015 ft,0,l,,k
where ft,90,g,k is the glulam strength perpendicular to grain and ft,0,l,k is the
tensile strength parallel to grain of the planks. According to Eurcode 5 this
characteristic strength ft,90,g,k is related to a reference Volume of V0 = 0,01
m 3 . A test method for determining the tensile strength perpendicular to
grain of glulam using this reference volume V0 is given in EN 1193.
Test series according to this test procedure are reported by Aicher und
Dill-Langer. The material they used was especially produced for their research
project. The planed planks of strength class C40 had a constant
thickness of 33 mm. Planks including pith were excluded (so called 3x-log
sawing, centre boards with pith were sorted out).
The aim of the project presented here was to determine the tensile
strength of glued laminated timber of different strength classes commonly
used in Germany. The material of the test specimens should forma representative
sample taken out of numerous beams manufactured by different
glulam producers.
Conclusions
The tensile strength perpendicular to the grain shows no significant correlation
with wood density and does not differ for different glulam strength
classes. This contradicts EN 1194. The lowest values determined in tests
are close to zero. Structural details with tensile stresses perpendicular to
the grain should be avoided or the timber should be reinforced perpendicular
to the grain.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.132