Cross Laminated Timber under Compression Perpendicular to Plane


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.

Cross Laminated Timber under Compression Perpendicular to Plane - Advances in Engineering

About the author

Ass.Prof. Dipl.-Ing.(FH) Dr.techn. Reinhard Brandner
Graz University of Technology | Faculty of Civil Engineering Sciences
Institute of Timber Engineering and Wood Technology.

Bibliometric Parameters: 

Google Scholar ResearchGate  Pure ScopusID : 3988546200, ResearcherID : V-9084-2018, ORCID : 0000-0003-3359-2017

Career Summary:  After 4.5 years working as interior designer, Reinhard studied Forest Products Technology and Management at the Salzburg University of Applied Sciences / Kuchl (2002 – 2006) and received his diploma degree (Dipl.-Ing.(FH)) with distinction in 2006. From 2005 to 2014 he was a Research Associate at the Competence Centre Holz.Bau Forschungs GmbH (non-profit, private limited research company) and in charge of the positions (Sub-)Area Leader & Manager (2008 – 2014) and Assistant of Scientific Head (2009 – 2014). In 2009 Reinhard became a Teaching and Research Associate at the Graz University of Technology (TU Graz), Institute of Timber Engineering and Wood Technology, and in 2011 Deputy Head of the Institute.

Apart from his teaching assignments in timber engineering and wood technology, in research his focus lies on the statistical analysis and stochastic modelling of timber products, joints, structures and systems in general. In the first years achievements comprised optimization of existing and invention of new linear timber products as well as new and improved test and evaluation procedures for the determination of mechanical properties of structural timber products and joints which outcomes have also influenced European product, testing and design standards.

In 2012, Reinhard obtained his PhD (Dr.techn.) with distinction in Civil Engineering Sciences at TU Graz for his thesis “Stochastic System Actions and Effects in Engineered Timber Products and Structures” which was awarded in 2013 by the Leo-Schörghuber Prize by the Wood Research Munich (Holzforschung München) at the Technical University of Munich (TUM).

In 2014 Reinhard was appointed as Assistant Professor for Timber Engineering and Wood Technology at TU Graz (tenure track). Since that time, he extended his teaching and research portfolio by statistics, probability theory as well as basics of risk and reliability analysis. Since 2015 he is frequently invited as guest professor at the Technical University of Munich (TUM), Chair of Timber Structures and Building Construction.

He is an external expert in national and international standardization committees. From 2015 to 2018 he was (co)leader of WG 2 “Solid / Massive Timber” of the COST Action FP1402 “Basis of Structural Timber Design – from Research to Standards”, aiming on establishment and harmonization of properties, testing & evaluation procedures and design of CLT elements and structures within Europe. 


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.

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