Timber performs well when compressive forces are applied parallel to grain. However, when compressive forces are induced in a direction perpendicular to grain, the timber tends to yield under a comparatively lesser load. A similar trend has also been established for other properties. Consequently, structural designers tend to provide larger cross-sections in such situations; for instance, in single- or multi-span beams. Timber, under compressive loading perpendicular to grain, behaves as a ductile material, featuring hardening together with extensive deformation. Moreover, resistance of timber against compression perpendicular to grain is significantly dependent on the load configuration. The large variety in load configurations makes it impractical to regulate performance-based resistance and stiffness properties for all such configurations.
As the consequences of exceeding the allowed stresses are relatively small, there has been a lively debate on whether compression perpendicular to grain should be treated as an ultimate or better as a service ability limit state. Presently, the existing regulations neither encompass new structural products such as the cross laminated timber (CLT) nor resolve the aforementioned shortcomings.
To this note, Dr. Reinhard Brandner from the Institute of Timber Engineering and Wood Technology at Graz University of Technology in Austria looked thoroughly into compression perpendicular to grain/plane from a design perspective. In line with the ongoing revision of Eurocode 5, he developed reliable test specifications, encapsulating a design approach for CLT in compression perpendicular to plane as a candidate for a harmonized approach for the issue of structural timber products in general. His work is currently published in the research journal, Engineering Structures.
The author started by addressing the principle material behavior of timber as an anisotropic material. Next, he engaged in an in-depth review of the influencing parameters on the timber’s principle behavior. Afterwards, he carried out tests on industrially produced 5-layer CLT with a focus on the potential influence of contact area, load introduction versus load transmission with aligned load and support and lastly, the influence of moisture.
He also observed that the mechanical properties of the test specimens linearly decreased with increasing moisture content. In addition, he also found out that a minor approximately linear increase in strength and in elastic modulus with an increasing dimension of the contact area was recorded. Specifically, decreasing strength and modulus of elasticity were observed with increasing contact area. Lastly, he recorded that the variability in compression perpendicular to grain properties was a function of the contact area dimension, the member depth and the load configuration.
In summary, the study by Dr. Reinhard Brandner presented a novel approach which allowed for explanation of the influences caused by: load configurations, contact area, layup and thickness of the CLT element, support conditions, and clear edge distances and clear spacing. This approach was seen to be applicable for both strength and modulus of elasticity. Altogether, his research presented recommendations for testing and evaluating, proposed basic properties and regulations for the main influencing parameters and a generic design approach applicable for all planar and linear, unidirectional and orthogonal laminated structural timber products.
R. Brandner. Cross laminated timber (CLT) in compression perpendicular to plane Testing, properties, design and recommendations for harmonizing design provisions for structural timber products. Engineering Structures, volume 171 (2018) page 944–960.Go To Engineering Structures