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

41-12-2 M Frese, H J Blass
Bending strength of spruce glulam: new models for the characteristic
bending strength
Abstract
A comprehensive research project regarding the bending strength of beech
glulam showed: Combined visual and mechanical strength grading of
boards is very competitive and a strength model, in which the glulam
bending strength depends both, on the board tensile strength and the finger
joint tensile strength, is a completely transparent model being particularly
suitable to determine requirements for the board and for the finger joint
tensile strength.
This paper describes the application of these principles to an alternative
and new strength model for spruce glulam. For that the effect of the board
and finger joint strength on the glulam bending strength is numerically determined
by means of simulated glulam beams. According to the findings,
described in this paper, current requirements for boards and finger joints
are insufficient to ensure the nominal strength values of GL24 to GL36 according
to EN 1194.
Conclusions
38 bending tests on full size spruce glulam beams were performed. Since
the strength values were obviously too low compared with EN 1194, this
investigation was motivated. It was the aim to explain the low bending
strength values by developing new strength models for the characteristic
glulam bending strength.
By means of a computer model, suitable to simulate different strength
grading methods, the mechanical properties of glulam beams were calculated
and bending tests on those beams were numerically performed. The
simulation results and the board tensile strength, belonging to the grading
methods, forma database which was used to perform a regression analysis
in order to derive new strength models. Two models, being of importance
to calculate the characteristic glulam bending strength, were determined.
They are particularly suitable to predict the bending strength of glulam
manufactured from visually and mechanically graded boards, respectively.
There is good agreement between the strength models and Colling's results
from the 1990s. They can also be verified by bending tests published
in the literature. The all-over ratio of predicted values to test results in the
literature on average amounts to 1.02. In this ratio 451 bending strength
values of glulam are considered.
One can infer from the strength models, that the current requirements
for board tensile and/or finger joint strength are not sufficient to ensure the
characteristic glulam bending strength values assigned to the strength classes
GL24 to GL36. This finding is to be explained for homogeneous
GL32: Whereas the current standard EN 1194 stipulates 22 N/mm 2 and
38.8 N/mm 2 for characteristic values of board tensile and finger joint
bending strength, respectively, the new strength models lead to demands
of about 27 N/mm 2 and 43 N/mm 2 . For combined beams even higher values
are required. Against the background of knowledge the poor characteristic
bending strength values of the 38 test beams were caused by too low
requirements for boards and finger joints.
For further standardization, the paper contains a basic proposal for the
characteristic glulam bending strength in the well known format of EN
1194. In addition, it contains an elaborated proposal for prEN 14080 for
the chapter "Strength and stiffness properties of glued laminated timber".
43-12-3 M Frese, H J Blass
System effects in glued laminated timber in tension and bending
See 3.14 System effects
43-12-4 C Faye, F Rouger, P Garcia
Experimental investigations on mechanical behaviour of glued solid
timber
See 3.14 System effects
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.47