CROSS LAMINATED TIMBER: A MULTI-LAYER, SHEAR COMPLIANT PLATE AND ITS MECHANICAL BEHAVIOR Reinhard Stürzenbecher 1 , Karin Hofstetter 1 , Josef Eberhardsteiner 1 ABSTRACT: Cross Laminated Timber (CLT) is widely used for load bearing applications in timber engineering. Exhibiting a crosswise lay-up of layers of wooden boards, this engineered wood product constitutes a laminated composite with its distinctive mechanical behavior. The orthotropic material behavior of the individual layers and the extraordinarily high shear compliance of the resulting plate put high demands on an appropriate mechanical description of this wood product. Here, an advanced laminated plate theory is applied therefor, delivering accurate results for deformation as well as for stress components also for thick CLT plates under distributed and concentrated loadings, requiring only slightly higher computational costs than conventional plate theories, which are also included here. The actual, laminate-specific deformation behavior and stress state of CLT plates is presented due to an exact solution, delivering the peculiarities of the characteristic mechanical behavior and consequentially proposing advanced plate theories for their structural design. KEYWORDS: Laminated wood product, Deformation and stress state, Classical and advanced plate theories 1 INTRODUCTION 1 Cross Laminated Timber (CLT) constitutes an engineered wood product dedicated to structural applications. Made up of ordinary boards, glued together in a cross-layered fashion, the resulting wood product is employed as structural plate suitable for loading in-plane and out-of-plane. The orthotropic elastic material behavior of wood and the crosswise lay-up result in a quite distinctive deformation behavior of CLT. In mechanical terms, it represents a two-dimensional, multilayer, exceedingly shear compliant, laminated composite with thick and highly anisotropic layers. The high ratio of modulus of elasticity in fiber direction of lengthwise layers and the corresponding transverse shear modulus of the cross- layers (rolling shear modulus) provokes high shear deformations and a zig-zag shaped deformation pattern across the thickness of the plate. Hence, the courses of the transverse shear strains show considerable discontinuities at layer interfaces, while transverse shear stress distributions follow continuous, though strongly nonlinear courses across the plate thickness. In terms of structural design the continuous shear stress distribution is checked against shear strength values, which are 1 Reinhard Stürzenbecher, Karin Hofstetter, Josef Eberhardsteiner, Institute for Mechanics of Materials and Structures, Vienna University of Technology, Karlsplatz 13/202, 1040 Vienna, AUSTRIA Email: Reinhard.Stuerzenbecher@tuwien.ac.at ; Karin.Hofstetter@tuwien.ac.at ; Josef.Eberhardsteiner@tuwien.ac.at considerably different in adjacent layers, distinguishing the relevance of cross layers here. These CLT specific characteristics render an appropriate and accurate description of the mechanical behavior challenging, but for an intensive and safe use of CLT in heavily loaded structural elements this is nevertheless essential. The focus of this paper is to seek for a plate theory, which is able to reflect the laminate specific mechanical behavior of CLT and delivers accurate deformation and stress components for the structural design at reasonable computational costs. After a brief summary of the theoretical background of the considered plate theories, we first examine the deformation and stress state of surface loaded CLT plates based on an exact solution for the plate bending problem of laminated plates. This enables to depict the specific courses of displacements, strains, and stresses across the plate thickness, and serve as reference for later comparisons of the performance of various plate theories. In particular, we present an advanced plate theory based on the work of Ren [1], which combines accuracy of results with computational efficiency and shows only slightly higher computational costs than Reissner [2,3] or Mindlin [4] plate theory. Comparing the results obtained by Classical Plate Theory (CPT), First Order Shear Deformation Theory (FSDT) and Ren Plate Theory (RPT) with the exact solution delivers their suitable application ranges for structural design. Finally, the capability to capture the complex stress and deformation state of point loaded and of surface loaded and point supported CLT plates is examined.