CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...
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strength tends to increase with rising timber quality and decreasing dimensions.<br />
Regarding the safety (for low timber quality) and the economy (for<br />
high timber quality) <strong>of</strong> timber structures it would be desirable to take these<br />
effects into consideration when determining characteristic strength values.<br />
Discussion and conclusions<br />
The present investigations involving structural timber from Central Europe<br />
(mainly spruce) confirm that the ratio between tensile and bending<br />
strength depends on timber quality. This relationship can be derived using<br />
brittle fracture theory and expressed in terms <strong>of</strong> dependence on bending<br />
strength.<br />
According to the brittle fracture theory there is a constant relationship ft/fm<br />
which is independent <strong>of</strong> timber quality and dimensions. When taking into<br />
account the different material behaviour <strong>of</strong> timber loaded in tension and in<br />
bending by calculating different parameters for the strength distributions,<br />
the ratio <strong>of</strong> ft/fm increases with strength and can be said to be more realistic.<br />
For 2,642 tension and 1,739 bending tests in 28 and 27 samples respectively<br />
using test specimens in structural dimensions (constant dimensions<br />
per sample) ungraded timber resulted in the relationship<br />
1,20<br />
t � 0,36 m<br />
f f<br />
The bending tests were conducted with the weakest section located in the<br />
tension zone, so that a relationship in conformity with EN 384 based on<br />
bending tests involving random location <strong>of</strong> weak sections shows lower ratios.<br />
With consideration <strong>of</strong> test conditions involving non-random positioning<br />
<strong>of</strong> weak sections the 5 percentiles <strong>of</strong> tension strength were related to the<br />
7.5 percentiles <strong>of</strong> the bending strength. For strength graded timber there is<br />
a noticeable effect <strong>of</strong> both grading method and dimensions on the ft/fm ratio.<br />
There is no clear effect <strong>of</strong> timber quality on the ratio ft/fm in timber<br />
visually graded according to DIN 4074, while increase in ratio is remarkable<br />
for machine graded timber.<br />
No size effect was found for ungraded timber. A more pronounced size<br />
effect in strength graded timber loaded in tension lead to a size effect, on<br />
average, <strong>of</strong><br />
�lmrbmr� �<br />
l b<br />
�<br />
� m m�<br />
for visually graded specimens tested according to EN 408. Due to a constant<br />
ratio <strong>of</strong> b/l the effects <strong>of</strong> test length or span and specimen width or<br />
height could not be determined separately. There is considerable scattering<br />
regarding the ratio ft/fm. For safety reasons the lower ratios were more<br />
strongly weighted when determining the relationship for characteristic<br />
strength values ft,k/fm,k. In conjunction with the test data on visually and<br />
machine graded timber from literature the following equation was derived:<br />
1,28<br />
tk , 0,26 mk ,<br />
f � f<br />
(14)<br />
Table 4 compares characteristic tensile strength values for strength classes<br />
according to EN 338 derived from this equation with currently valid<br />
strength values. This reveals that tensile strength is being overrated as regards<br />
the lower strength classes while in the higher strength classes tension<br />
strength is being given too conservative. For economy and safety reasons<br />
proper consideration <strong>of</strong> the influence <strong>of</strong> timber quality would therefore<br />
be desirable, which would adequately highlight the particular advantages<br />
<strong>of</strong> machine graded timber.<br />
Tab. 4.- Comparison <strong>of</strong> characteristic strength values for strength classes<br />
according to EN 338 for load in bending and in tension with tension<br />
strength values calculated acc. to eq. (14).<br />
[N/mm 2 ] C 14 C 16 C 18 C 22 C 24 C 27 C 30 C35 C 40<br />
fm,k 14 16 18 22 24 27 30 35 40<br />
ft,k 8 10 11 13 14 16 18 21 24<br />
ft (14) 7.0 8.3 9.7 12.5 14.0 16.3 18.7 22.7 27.0<br />
36-6-1 P Glos, J Denzler<br />
Characteristic shear strength values based on tests according to EN<br />
1193<br />
Introduction<br />
Until today the effect <strong>of</strong> wood properties on shear strength <strong>of</strong> s<strong>of</strong>twood is<br />
not completely understood. The European standard EN 384 assumes a direct<br />
relationship between shear and bending strength: fv,k = 0,2∙(fm,k ) 0,8 .<br />
The strength properties proposed for the strength classes according to<br />
EN 338 are based on the same relationship. This means that the characteristic<br />
shear strength values are increasing with higher strength classes i.e.<br />
with increasing bending strength and density.<br />
<strong>CIB</strong>-<strong>W18</strong> <strong>Timber</strong> <strong>Structures</strong> <strong>–</strong> A <strong>review</strong> <strong>of</strong> <strong>meeting</strong> 1-<strong>43</strong> 2 <strong>MATERIAL</strong> PROPERTIES page 2.109