Project 2.2

Development of load bearing models and investigations of the 2D-load carrying behaviour of CLT elements for different kinds of actions and supports

Verified bearing models are of high importance for the design as well as the reliable and durable use of CLT-elements. Up to now the necessity of uniform and broad accepted definitions was neglected. Consequently, international accepted and verified bearing models are missing, which can be reproduced by comparison of technical approvals and judgement of current status of the European product standard. The design of CLT-elements base on agreed characteristic values which are normally derived by tests or comparative examinations, which themselves rely on general accepted test methods and configurations, as already discussed within the project 2.1.

In general, bearing models of the product CLT constitute all mechanical models necessary for ultimate limit state design done by engineers. Within this project focus is on analysis and development of bearing models for CLT-elements stressed in bending, shear, tension and compression (including also stability). In comparison with bearing models for GLT it becomes obvious that these models are not only relevant for the design but also for a consistent material and product description.

In particular system action and related effects (ksys) as consequence of common and parallel acting elements have to be mentioned. The accompanied homogenisation effect manifests in a reduced variability of characteristics which further leads to an increase of design relevant characteristics. Furthermore the influence of stressed volume and the distribution of stresses within this volume has to be considered, e.g. by kvol, ksize, kh. For example, an increase in the number of layers of CLT-elements stressed out of plane leads to a reduction of characteristic bending strength. Consequently, it is not only required to consider the arrangement and interactions of elements in CLT, but also the determination of a reference dimensions for CLT- elements as well as for the base element board.

A further main topic of project 2.2 deals with strength and stiffness behaviour of CLT-elements stressed in compression perpendicular to grain and out of plane. First results already exist. Within this project it is intended to determine finally up to now missing or discussed kc,clt,90- factors which are of high relevance for the design process, in particular for the design of multi- storey buildings.

Furthermore, it is seldom the case that CLT-elements are stressed in one direction and / or by a single load case only. In general combined stresses occur. Therefore, design of CLT- elements requires interaction formulas. As an example, the common interaction of normal stresses (tension / compression) and moments can be mentioned. Not of minor importance also the interaction of shear and tension / compression perpendicular to grain. Within the framework of this project these interactions should be also addressed.

CLT-elements are in many cases applied in architectural aesthetic constructions. In this regard topics like “pin-supported plates” and “effective width” and “local load introduction” are of high interest in general. “Pin-supported plates” require on one hand a two-dimensional consideration of load bearing behaviour of CLT-elements, and on the other hand the derivation of a solution for the static design and construction of this detail. Peak stresses in these zones often reflect localised overstress and the necessity for local reinforcements. Mechanics and empirical models for the description of “effective widths” are therefore relevant for the development of technical as well as economical more efficient and optimised CLT-elements with and without combination with linear elements, if they are stressed in bending out of plane. It has to be outlined that currently no scientific work on this topic is known.

A further important topic of practical relevance is given by local load introduction on CLT walls, for example by linear elements like beams or girders. Currently, design procedures for such cases are absolutely missing and require examinations concerning the load distribution angle. This is in particular of interest in case of local load introduction near wide openings. Current design relies on subjective judgement of the civil engineer.