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1Materials Testing Institute, University of Stuttgart, Timber Constructions, Pfaffenwaldring 4b, 70569 Stuttgart, Germany
©2016 Simon Aicher et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. (CC BY-NC-ND 3.0)
Previous research indicated that the rolling shear properties of European beech wood (Fagus sylvatica) are considerably higher than those of softwood. The aim of the presented investigation was to substantiate previous data on rolling shear modulus and strength of European beech wood and to further evaluate its substitution of softwoods in applications where shear properties are influential, namely as cross layers in cross-laminated timber (CLT). Further, the effect of the annual ring orientation within the boards on shear modulus and strength was of major interest. The beech specimens comprised four different sawing patterns, classified unambiguously with reference to the pith location. The shear properties were determined by 50, two-plate shear tests with specimen cross-section dimensions of 33 mm × 135 mm. A mean rolling shear modulus of 370 N mm-2 was obtained, whereby no significant detrimental effect for pith boards with cracks was observed. In agreement with continuum mechanics, the semi-quarter-sawn boards revealed the highest shear moduli whereas the quarter-sawn boards showed roughly 30% lower values. The mean rolling shear strength was 5.6 N mm-2 for all specimens, whereby pith specimens resulted in generally lower values. The 5% quantile, disregarding pith specimens, was 4.5 N mm-2. In conclusion, the rolling shear strength and modulus exceed the respective characteristic values for softwoods by roughly factors of 5 and 7, indicating great potential for beech wood cross-layers in CLT.
Rolling shear modulus and strength of the cross layers are decisive mechanical properties in cross-laminated timber (CLT) plates. The composite material CLT is built up in the slab thickness direction by orthogonally crossed layers of boards, hereby closely resembling solid wood-wise the classical plywood build-up of thin, cross-wise layered veneers. Currently, CLT, which is of ever increasing importance globally for medium and high-rise timber buildings, is almost exclusively manufactured from softwood (spruce/fir) boards. All existing European CLT approvals, which are based on the former European Common Understanding Procedure (OIB 2005) or the present European Assessment Document (EOTA 2015) as well as the recently approved European CLT standard EN 16351 (2015) address exclusively softwoods as lamination material. In recent years, the use of hardwoods in wooden construction products has become an ever increasingly important topic in central Europe. Lower grade beech wood (Fagus sylvatica, Linnaeus 1753) is utilized almost entirely thermally, but it is obvious that this material should be very apt for CLT buildups. This hypothesis is based on the assumption that the rolling shear properties of beech are probably very satisfactory (Stamer and Sieglerschmidt 1933; Hoefflin and Aicher 2000; Niemz et al. 2015). The intention is to gain a deeper insight into the mechanical properties of beech wood for homogeneous and hybrid CLTs within the framework of an ongoing European WoodWisdom research project on hardwoods. The present paper reports on the rolling shear modulus and strength investigations of single beech board slabs with emphasis on the boards’ sawing patterns and herewith associated growth and drying defects.
Theoretical background, materials, and methods
Results and discussion
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