Cross laminated timber (CLT) in compression perpendicular to plane: Testing, properties, design and recommendations for harmonizing design provisions for structural timber products

Timber exposed to compression perpendicular to grain stresses is of concern in all timber constructions. The challenge is the high diversity of possible load configurations, which mandates for a generic approach with basic properties and case individual adaptations of resistance and stiffness. 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 serviceability limit state. This debate takes also its place in the currently ongoing revision of Eurocode 5 (EC 5), the European design code for timber structures. In the course of this revision regulations for structural timber products not currently anchored in EC 5, in particular for cross laminated timber (CLT), are now being sought. The product CLT is so far lacking in specifications for the testing, basic properties and design. In awareness of the necessity for generally applicable and reliable design approaches, I aim to present the state-of-the-art on compression perpendicular to grain/plane by focusing on the design, i.e. stress dispersion models, and cross laminated timber. In addition, I present the main data and findings from a comprehensive test campaign and adapt van der Put's stress dispersion model for stress dispersion in directions parallel and perpendicular to grain. This new approach allows an explanation of the influences caused by (i) contact area, (ii) load configurations, (iii) support conditions, (iv) layup and thickness of the CLT element, and (v) clear edge distances and clear spacing. The same approach is applicable for both strength and modulus of elasticity. The necessity to consider the orthogonal layup emerged in the case of CLT. I conclude with recommendations for testing and evaluation, propose basic properties and regulations for the main influencing parameters as also a generic design approach generally applicable for all linear and planar, unidirectional and orthogonal laminated structural timber products.