Project 1.2

The overall design of constructions requires verification of ultimate and serviceability limit states, durability and robustness. It can be observed that building elements made of cross laminated timber (CLT), and in particular for ceiling elements are seldom controlled by ultimate, but more often by serviceability limit states design. In case of short spans up to 4.0 m normally limit values of deflection are the decisive criteria for design, but in larger spans limits in vibration become more and more design relevant. In all these cases degree of utilisation in ULS resistance seldom exceeds more than 50% and thus gets seldom design relevant. Consequently, optimisation of CLT ceiling elements can be achieved by (1) stiff outer layers, (2) by optimising the thickness ratios between longitudinal and transverse layers, and but more difficult (3) by controlling the mass of all (main) components of the ceiling construction.

Based on in-situ measurements of vibrations it was shown that current design procedures give too conservative values and show partly remarkable deviations to test data. Due to the fact that vibrations often govern the design of CLT ceiling elements it lies on hand that improvements in current verification procedure to a more realistically method have a direct influence on their profitability. Therefore in verification analysis of vibrations it is seen as relevant to consider (i) shear deformations, (ii) partial clamping action in nodes between walls and ceiling in conjunction with possible effects by elastomeric supports, (iii) a higher damping ratio in CLT due to more flexible in shear acting transverse layers (if compared to ordinary ceiling constructions of timber), (iv) the total ceiling construction and its composition, (vii) the influence of neighboured ceiling panels, and in particular (viii) the influence of non-load bearing partition walls. In view of current standards it can be argued that even the given limits and criteria are controversially discussed within the scientific community.

Generally and despite their low mass, ceiling constructions with timber have also the challenge to fulfil requirements in impact sound. This is in particular challenging in lower frequencies of 20 to 50 hearts because this range overlaps with the spectrum of (mechanical) vibrations. Because of the fact that in the meantime design of ceiling constructions in timber has to be done by means of the expanded frequency range, in case of impact sound with a lower value of 50 hearts, a gap in verification is obvious which can not be closed by calculations of the designing engineer nor by building physics considerations, but which shows to have a significant effect on experienced quality of the ceiling construction. Consequently, there is need for action for comprehensive and systematic analysis of both frequency ranges. This to increase the observable quality of ceiling constructions made of timber.

As already mentioned in the description of project 1.1 it can be observed that the solid timber construction technique in CLT is on way to get a building system technique in CLT. Therefore not only optimised building components, but also static-constructive, standardised and reliable leading details are required. It is intended to define construction details for the most important building components and joints (e.g. connection wall-ceiling-wall or connection wall- foundation) in diverse quality levels and for diverse classifications of structures (e.g. multi- storey solid construction or single family houses). The output should be made available to consortium members and public community. It has to be mentioned that similar leading details already exist. Nevertheless, the status quo of these details is judged as insufficient for the development of a standardised construction and in view of the development of a building system. Furthermore it has to be stated that a similar topic is also covered within project 3.2. Nevertheless, the scope within this project concentrates on the definition of leading details for the constructions within a building system and not on standardised joint details as focused in project 3.2. Even though the scopes are different synergies between both projects are expected and have to be utilised.

Having available definitions for main system components “building component”, “element”, “connection” and “detail” it is possible and meaningful to simplify and standardise the formulation of tenders and offers of building projects by providing standardised bills of quantities. This will also increase the competiveness of CLT constructions.