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

More documents

Recommendations

Info

earing plates and shear panels itself. The application of CLT used as load-bearing plates requires information on strength properties, the design, however, is often governed by serviceability criterions like maximal deflection and vibration susceptibility. Hence, predicting the respective behaviour of such panels requires accurate information about their bending and shear strength as well as their elastic properties. Regulations regarding the derivation of performance characteristics, evaluation of conformity and (CE-) marking of wood-based panels for use in construction are given in EN 13986. According to this standard bending strength and stiffness of CLT have to be assessed following the procedure in EN 789. The latter requires 4-point bending tests of strip-shaped specimens with a width of 300 mm, cut off the CLT panels. The span has to be taken as 300 mm + 32 t, t being the nominal thickness of the CLT panel. By comparing results of bending tests on strip-shaped specimens and on full panels it is shown, that neither strength nor stiffness properties derived by testing strip-shaped panels are appropriate to assess the respective properties of the original panels. The analysis of the test data covers bending strength, bending MOE parallel and perpendicular to the grain direction of the face layers as well as shear moduli. Rolling shear failures which frequently occurred when testing strip-shaped specimens could not be observed in destructive tests of whole CLT panels. Additionally a verification by carrying out static bending tests (deflection measurements) under different loading situations showed, that the overall stiffness properties (elastic parameters of the stiffness matrix) can, alternatively to EN 789 tests or estimations with the compound theory, be derived directly by a modal analysis of full-size CLT panels. Summary and conclusions Strength properties can best be assigned to CLT by means of the compound theory. However, the mechanical properties (strength and stiffness) of the layers have to be known which means that the raw material has to be strength graded. Deriving stiffness properties of whole CLT panels with modal analysis is a good alternative to estimating them on base of the mechanical properties of the singly layers by means of the compound theory. Especially in cases where the raw material is not strength graded or its mechanical properties are not known with sufficient precision, the modal analysis can help in assigning correct stiffness properties to CLT. After having proven the correctness of the method by static proof loading, the panel properties were compared to bending MOE and shear moduli derived from tests on strip-shaped specimens cut off the CLT panels. One part of the tests additionally focused on bending strength and failure modes. The following conclusions could be drawn: – Bending strength and stiffness of CLT panels can vary quite strongly within one single panel. For both parameters differences between the strength and stiffness of stripshaped specimens cut off the panels of up to 100% have been found. Hence it is not possible to derive strength and stiffness properties of CLT panels from bending tests of few or single strip-shaped specimens. – The accuracy of the test results when performing bending tests of stripshaped specimens according to EN 789 is increased with increasing sample size. Mean values of at least 5 – 6 specimens better describe the actual bending stiffness of the panels. Average differences then amount to 10% (E11) and 6% (E22) but can still reach 20%. As asked for in EN 789, characteristic values of strength and stiffness properties of CLT have to be derived on samples which fulfil the criteria of EN 14358. – The variation of the stiffness properties depends on the degree of homogenisation of the actual CLT panel product. The smaller the components (lamellas) are and the less the variation in mechanical properties is (which can be reached by adequate strength grading of the raw material), the better it can be concluded from tests on strip-shaped specimens to the bending strength and stiffness properties of the whole CLT panel. – Compared to gross CLT panels, local non-homogeneities and faults (knots, pitch pockets, deviated grain, not adhesively bonded contacts, cuts, grooves, cracks) take more influence on the mechanical properties of the strip-shaped specimens. The smaller the width of such specimens is, the more their load-bearing behaviour is affected by local defects and non-homogeneities due to faults in the raw material or due to way of producing the panels. – The distances between middle layer parts not adhesively bonded at their lateral sides and the number of cuts and grooves, which are aimed at reducing the deformations of the CLT panel in case of changing moisture, take a big influence on the shear moduli. When deriving respective values on base of testing strip-shaped specimens this possible variation has to be taken into account by using empirical relationships. – When testing strip-shaped specimens, shear and rolling shear failures occur quite frequently, whereas such failure modes could not be ob- CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.20
served when testing whole CLT panels to failure. There bending failure was dominating. Punching, however, should be regarded, especially with thin panels and products with grooves and layers not adhesively bonded at their lateral sides. – Single tests on strip-shaped specimens may serve as an instrument of production control especially regarding the quality of bonding. They should, however not be used to derive mechanical properties of CLT panels. In scientific studies testing of strip-shaped cut offs of CLT should only be carried out on big samples. Geometrical dimensions should not be taken smaller than asked by the standards and generalization of conclusions in most cases is not possible (e.g. type of failure). CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.21
Page 1 and 2: CONTENT 2.1 COMPRESSION PERPENDICUL
Page 3 and 4: 23-12-4 H Riberholt, J Ehlbeck, A F
Page 5 and 6: 30-17-1 F Rouger A new statistical
Page 7 and 8: CIB-W18 Timber Structures - A revie
Page 9 and 10: 2.1 COMPRESSION PERPENDICULAR TO GR
Page 11 and 12: for solid sawn lumber and glulam ha
Page 13 and 14: The discussion is supplemented by t
Page 15 and 16: Figure 3. Load-deformation curves f
Page 17 and 18: The results indicate a clear affini
Page 19: which has been already mentioned ab
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 and 102:
- A higher COV of the raw material
Page 103 and 104:
Visual grading of wood into strengt
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
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?