Out-of-plane bending properties of cross laminated timber (CLT)

Abstract

In Europe the structural timber building product cross laminated timber (CLT) is regulated via European Technical Assessments (ETAs), which require large and voluminous test campaigns to obtain. So far, there is no standardised CLT strength class system nor a set of harmonised properties. Such a strength class system and tabulated values could be accompanied by a set of so-called load-bearing models, analogous to the current regulations for glued laminated timber (GLT). The aims of this contribution are to comprehensively analyse the first known load-bearing model for out-of-plane bending of CLT from 2006 and to validate and benchmark this approach with possible further candidates. The aim is to initiate the establishment of a European strength class system for CLT based on its out-of-plane bending properties, for a more reliable, regulatory compliant structural building product and for enhancing its market acceptance. Therefore, a comprehensive analysis of various potential influencing parameters on out-of-plane CLT bending properties is effected by means of stochastic-numerical investigations as well as a large amount of test campaigns, the latter in the form of a retrospective study. The first approach from 2006, in particular in its adapted version, is successfully validated and confirmed as a valuable candidate for the future regulation of CLT due to its overall more accurate and more reliable strength predictions compared with the other candidates. In addition, models for the density and the modulus of elasticity parallel to the grain, as currently defined for glulam, are confirmed to be applicable also for CLT. An analysis of the variation in the properties of the base material and CLT motivates additional efforts towards grading and quality assurance in order to meet the expectations of the nominal strength class of boards and to fulfil minimum requirements for finger joint quality to better utilise the high performance of CLT and to increase its reliability.