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

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dure requires the use of modification factors from a number of standards, of which EN 408 and EN 384 are the most important. Timber quality (in terms of bending strength, modulus of elasticity and density) influences the settings for the machine. Low quality timber usually leads to higher settings and vice versa. However, the settings are not defined on the basis of the weakest sample only, but on a mean value of all sub-samples used in the approval procedure. The origin of the trees has an important influence on the mechanical properties of the sawn timber. If the user of a grading machine uses timber from the same sources and in a similar proportion as used for the derivation of settings, the output material can be safe. On the contrary, if a sawmill only uses timber from a limited radius around the production site, the results can be far from safe when the original approval testing covered a much larger area. If the incoming material is of low quality only, the graded material will not meet the required characteristic values on a regular basis. Factors used in the derivation of settings include a depth factor kh and a kv - factor, which will be discussed in more detail. According to Eurocode 5, the depth effect may be applied on beams with a depth less than 150 mm. For depths above 150 mm, no correction for strength is allowed. However, during the derivation of characteristic values according to EN 384, the characteristic 5th-percentile value of sub-samples with a depth of more than 150 mm, shall be increased with the depth factor kh given as: �0,2 �150 � kh � � � (1) � h � It means that characteristic values following the dotted line in Figure 1, will be used by designers as following the continuous line. A discrepancy of around 10% can be observed for beam depths typically used for floors (> 200 mm). In addition to the depth effect, EN 384 allows a kv-factor to be applied on the characteristic value of the samples graded by a machine for strength classes up to C30 according to EN 338. The value of kv is set fixed at 1.12, independent of the quality of the machine or strength class combination. The idea behind this factor is that the variation of the characteristic values of sub-samples is smaller for machine graded timber as compared to visually graded timber. Other factors that influence the quality of visual and machine grading are the length of the pieces that were used in the analysis as compared to the lengths used in practice. Other than the influence of the length of the pieces during the bending tests according to EN 408 / EN 384, length effects have not been taken into account here. Figure 1: Discrepancy in the depth effect between EC 5 and EN 384. As no pan-European rules are used for visual grading of timber, EN 1912 regulates the application area of national grading rules. These grading areas are usually interpreted very wide. If visual grading is not sensible to the timber origin, this can be justified. The objectives and the analysis given in this paper can be summarized as: I) What is the influence of the origin and the grading principle on the characteristic value of the bending strength using the European standards EN 14081 and EN 384, after testing in accordance with EN 408? II) Does visual grading allow large growth areas like indicated in EN 1912? III) EN 384 and EN 14081 allow the use of some adjustment factors as mentioned previously: i. To what extend do these factors influence the grading output? ii. Is the kv-factor (kv =1.12) justified by a lower variation of machine graded timber compared to visual graded timber? iii. Is the variation in 5th-percentile values of sub-samples such, that different safety factors can be justified depending on the type of grading? Conclusions CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.102
Visual grading of wood into strength class C30 seems not to be justifiable, certainly not for the application range (growth areas) given in EN 1912. The coefficient of variation of the 5 th -percentile strength values is such that higher safety factors need to be applied than currently specified in Eurocode 5. In addition, in some sub-samples the yield is so low that no characteristic values can be determined. Visual grading into strength class C24 is possible on its own, leads however to a large scatter in 5 th -percentile strength values for the different sub-samples. This in return should require high γM values, much higher than currently specified. When only C24 is graded, also machine grading leads to a large variation in characteristic strength values within the grade (sub-sample level). When grading C24 by machine, a higher strength grade should be produced at the same time, increasing the threshold value for C24 and decreasing the variation within the grade. Grading of only one strength class by a machine should not be permitted, unless higher safety factors are prescribed for such material. Adjustment factors given in EN 384 and EN 14081 intensify the problems caused by deriving settings for large areas according the current standard. Therefore it is proposed to delete the kv-factor from EN 384/14081-2. The kv-factor allows more test results from the low strength range to become part of the sub-sample, having a higher scatter and consequently requires higher safety factors. The kh-factor from EN 384/EC 5 should also be deleted. Apart from the fact that no evidence of its existence is found in the test results, there is a clear inconsistency in the standards. When deriving settings according to EN 14081 the weakest subsample should be taken into account and limits on the allowable deviation from the target value should be specified. The advantage of machine grading (i.e. more reliable material with smaller scatter in strength properties) should lead to the specification of different safety factors for visual and machine graded timber. For grades C30, C35 and C40 produced by a machine, there is clearly a much lower scatter at characteristic strength level, justifying a difference in safety factor between visual grading and machine grading. This could be done by specifying different γM values, but another option is the specification of a modification factor, taking into account the difference in variability between visual and machine grading. CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.103
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CONTENT 2.1 COMPRESSION PERPENDICUL
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23-12-4 H Riberholt, J Ehlbeck, A F
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30-17-1 F Rouger A new statistical
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CIB-W18 Timber Structures - A revie
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2.1 COMPRESSION PERPENDICULAR TO GR
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for solid sawn lumber and glulam ha
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The discussion is supplemented by t
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Figure 3. Load-deformation curves f
Page 17 and 18:
The results indicate a clear affini
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which has been already mentioned ab
Page 21 and 22:
served when testing whole CLT panel
Page 23 and 24:
2) The mechano-sorptive effects in
Page 25 and 26:
and codified; optimised strategies
Page 27 and 28:
36-11-1 R H Leicester, C H Wang, M
Page 29 and 30:
For handling of moisture effects in
Page 31 and 32:
- The short-term tests had a mean m
Page 33 and 34:
criteria, they may offer decisively
Page 35 and 36:
Conclusion The relation between the
Page 37 and 38:
the laminations were determined and
Page 39 and 40:
27-12-3 E Serrano, H J Larsen Influ
Page 41 and 42:
shear stress offered by competing s
Page 43 and 44:
e alarmed when a significant check
Page 45 and 46:
Measurement of shear deflections du
Page 47 and 48:
41-12-2 M Frese, H J Blass Bending
Page 49 and 50:
tures". The Centre Technique du Boi
Page 51 and 52: By setting γG = γQ1 = γQ2 = …
Page 53 and 54: ment. For instance, extreme distrib
Page 55 and 56: method which was commonly used in c
Page 57 and 58: 2.7 LOAD DURATION 6-9-3 T Feldborg,
Page 59 and 60: son curve", derived from small clea
Page 61 and 62: 19-9-4 U Korin Non-linear creep sup
Page 63 and 64: 24-9-1 T Toratti Long term bending
Page 65 and 66: Summary Based on the analysis of th
Page 67 and 68: when a dry period is followed by a
Page 69 and 70: - Jansson and Leijten both demonstr
Page 71 and 72: - The used methods and technologies
Page 73 and 74: ISO, generally through the technica
Page 75 and 76: ed. The significance of the choice
Page 77 and 78: However, there is another equally i
Page 79 and 80: 25-17-3 D W Green, J W Evans Moistu
Page 81 and 82: tion of settings. Since this proced
Page 83 and 84: posed approach is that the reliabil
Page 85 and 86: Conclusion It was concluded that th
Page 87 and 88: - The difference between E in edge
Page 89 and 90: On the basis of long-term experienc
Page 91 and 92: Conclusions The possibilities of vi
Page 93 and 94: Finland and Norway). Consequences w
Page 95 and 96: Fig. 1. A wind-induced compression
Page 97 and 98: emphasis on the modelling of the ef
Page 99 and 100: some trends are apparent. Based on
Page 101: - A higher COV of the raw material
Page 105 and 106: 4 X(1 � X) Y �1� X � 3s�1
Page 107 and 108: F112 and F122. It is however shown
Page 109 and 110: strength tends to increase with ris
Page 111 and 112: y assuming a normal distribution. E
Page 113 and 114: 19-12-3 F Colling Influence of volu
Page 115 and 116: - WF/Ww: 2:1 The single span three
Page 117 and 118: 40-6-2 M Poussa, P Tukiainen, A Ran
Page 119 and 120: 2.12 SIZE AND CONFIGURATION FACTORS
Page 121 and 122: While the estimates of the size eff
Page 123 and 124: ters for all grades and species. Si
Page 125 and 126: Modelling of length-wise distributi
Page 127 and 128: different things that affect the si
Page 129 and 130: sponds to the statistical approach
Page 131 and 132: fects. As a summary we may, for the
Page 133: More important than the volume effe
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