Project 3.2

Screw and gluing technology for the solid timber construction technique with cross laminated timber

At the beginning of development of the timber solid construction technique in cross laminated timber (CLT) the primary focus was on defining and answering of basic production requirements and on elaboration of basic design concepts. At this status the primary joining techniques for CLT-elements (in general self-tapping screws and ring-shank nails in combination with steel-angles and tension anchors) were taken from lightweight timber-frame constructions. Consequently, current available joining technique is in general not optimised and developed for CLT-elements. As more and more CLT is used in nowadays constructions, e.g. for multi-storey or even for numerous eight-storey office and residential buildings, current connection technique not seldom seems to reach its limit, in particular in earthquake design. Beside their origin of development, mentioned connection techniques if applied for CLT is often stressed eccentrically and thereby shows reduced efficiency.

CLT-elements can be joined in various configurations which can be classified into: joining (i) wall to wall, (ii) wall to ceiling to wall, (iii) ceiling to ceiling, and (iv) wall to base. Furthermore differentiation can be made in respect the positioning of fasteners, in (1) placement on narrow side or (2) on side surface of CLT-elements. As mentioned, currently primary pin-shaped fasteners like self-tapping screws, (ring-shank) nails or even dowel-type fasteners are common. For these types of fasteners it is of high importance to have verified and reliable knowledge about their bearing and deformation behaviour, in particular if they are applied on the narrow side of CLT-elements. Beside of a past project up to now only one more comprehensive international research project is available dealing with these subjects.

The main difference to joining technique used for solid or glued laminated timber is given by the cross- and multi-layered structure of CLT. Consequently, at first it is required to determine the bearing and deformation behaviour of self-tapping screws and dowel-type fasteners in CLT. For a comprehensive description of screw connections, knowledge concerning their withdrawal capacity for placement in narrow side or side surface is required. Thereby the main influencing parameters can be identified as (i) number and (ii) thickness of layers. For dowel- type fasteners in particular the characteristics of embedment capacity in dependency of force- fibre direction are of relevance. Thereby influence is given by possible gaps between the boards in each layer as well as by slots placed in single boards. Beside screws and dowel- type fasteners it is also possible to join CLT-elements by means of glued-in engineered wood products or metal plates. Thereby not only pin-shaped but also linear joining of elements can be achieved. It is intended to glue in these elements already in the assembly plant and joining them on-site by field connection, e.g. by simple screw or bolt jointing.

Furthermore it gets more and more in focus to join large-sized CLT-wall elements along the whole narrow face or to connect CLT-elements stressed out of plane rigidly in bending. Current practice uses therefore screw-press blocking joints. Nevertheless, as standardised rules are insufficient or even missing, the design of these joints is up to now more or less done on the basis of made experiences and by individual judgement. This circumstance is seen as absolutely unsatisfactorily.

From the side of building physics it can be stated that current used joining technique often initiates problems in regard to air tightness, which consequences in condensates, and acoustics. Nevertheless, currently available results are thereby insufficient to allow a comprehensive static-constructive solution. Beside of support and distribution of loads, in many cases it is also additionally required to insert e.g. sealing tapes between the CLT- elements. This to secure a safe and reliable function of the building over the whole life. In this respect it should be mentioned that with focus on the development of a system construction technique in CLT it lies on hand that joining of the two-dimensional product CLT by a one- dimensional, linear joining system (rail) would be very meaningful. Beside the functionality of these linear joining systems in respect to stress support and transfer and because of expected higher costs it would be also required to provide multi-functionality, e.g. by additional features like improvement of tightness, acoustics or the integration of elements relevant for building services (e.g. electricity, bus-systems).

Beside of so far discussed technique required for joining CLT-elements, already earlier a fast mountable & removable, reliable and safe joining technique for transportation and manipulation of CLT-elements in the production & assembly hall and in particular on-site is required. Thereby safety relevant and economical aspects have to be clarified for these only temporary stressed joining techniques. At present, company own developed joining techniques are commonly used. Nevertheless, as these techniques are in particular a safety problem, need for immediate action in regard to development of safe and reliable, standardised joining systems is given.