Project 2.1

Determination of characteristic strength and stiffness values of CLT-elements of various wood species under consideration of relevant production parameters and methods of testing and analysis

Within the last two decades numerous steps in research and development concerning the solid timber construction principle in cross laminated timber (CLT) have been made. In particular the topics “analysis of production relevant parameters” and “determination of characteristic values” got more and more in focus. The aim of project 2.1 is to examine up to know minor or even not treated actions and related resistances, primary by conducting and evaluation of practical test (results). Thereby need for action is given in regard to characteristics like bending and shear out of plane, shear and tension as well as compression of CLT stressed in plane direction. Normally, characteristic values are derived by testing full-sized specimen or by means of models which rely on properties of the base material “board” (in particular on tension properties). As well known, consequently derived characteristic values depend on the applied test configuration, the specimen’ dimension and the evaluation method. Nevertheless, only internationally agreed and accepted test and evaluation methods deliver in principle comparable test results, which are indispensable for standardisation as well as formulation of national and international technical approvals. Thus, the aim of project 2.1 is to elaborate and to propose relevant characteristics of CLT- elements together with associated and appropriate test and evaluation methods.

In this context characteristics of rolling shear are of particular importance, as they are remarkably relevant for ultimate as well as serviceability limit state design. For this reason one working package (WP) is dedicated to this subject. The aim is to isolate all parameters which have a relevant influence on rolling shear, and further to optimise the resistance of CLT- elements stressed out of plane by raising their resistance in rolling shear. Possible influencing parameters are e.g. density, the geometry of the boards (in particular their thickness) as well as the quantity of cross layers or the total volume under rolling shear. Furthermore it will be of relevance if single boards of cross layers are bonded side-by-side or placed by showing a constant gap in-between. Additionally, the aspect of slotted or not slotted face side is of relevance. As constructions in CLT gets more and more international important there is also the objective to not only concentrate on Norway spruce, but also on some relevant timber species common in further market places, e.g. Douglas fir, Radiata Pine, or hardwood species like birch and poplar.

A second main topic within this project 2.1 concentrates on face-bonding. It is intended to examine relevant parameters which affect the face-bonding characteristics of CLT-elements. It has to be assumed that these parameters deviate from parameters influencing bonding characteristics of glulam lamellas. For example, it can be expected that in comparison to glulam a reduced bonding pressure is sufficient to ensure an actuated and durable connection between CLT-layers. Besides bonding pressure further influencing parameters like surface quality and pressure time have also an affect on the bonding process. Additionally, parameters like the type of adhesive in conjunction with timber species, and applied quantity of adhesive are also of relevance. It is intended to analyse the effects of mentioned parameters also within the framework of this project.