Project 3.1

Axial and transversal loaded joints and system connectors – analysis of influencing parameters on load-carrying & deformation behaviour and modelling

Pin-shaped joints, in particular screws and dowel-type fasteners, have reached an outstanding position in contemporary engineering constructions made of timber. Thereby, screws are primary stressed axially and dowel-type fasteners transversal in shear. If combined with steel plates both types show high performance in load-carrying behaviour. Beside these metal joints high performance can be also realised by bond joints.

Past and recent accomplished R & D on these jointing techniques showed partially missing or insufficient scientific background knowledge about some already in standards and / or approvals and guidelines given regulations. For example, topics like (1) the influence of variations in moisture content on the bearing behaviour of full-threaded self-tapping screws and full-threaded screw rods, (2) the stiffness characteristics (modulus of displacement), and in particular (3) the dynamical and fatigue behaviour in case of earthquake and wind actions, have to be mentioned. In particular part (3) is seen as topic which should get more relevance to serve the requirements defined by economy and society. Currently, the realisation of technically challenging constructions and buildings in timber, like wind towers or buildings in regions of high seismicity, is primary limited by insufficient knowledge about strength and stiffness behaviour of current available joining techniques. Therefore it is intended to stress mentioned topics more intensively.

Pin-shaped joints have in common that in a group not the total number of placed joints can be considered by their full resistance. This circumstance is currently regulated by the factor nef, defined by the share of effective acting to total number of placed joints. Comprehensive examinations on this factor are available for dowel-type fasteners but not for screws. Thus and in respect nowadays application of screws for joints carrying very high forces of up to a few MN there is definite need for action. First examinations and tests indicate that regulation by one single value of nef is not adequate. In fact it is indicated that system bearing behaviour can be modelled as a combination of serial and parallel system behaviour, in dependency of the type and region of fracture (achievement of withdrawal capacity or of steel strength) as well as by the construction itself and the position of steel plates in conjunction with the direction load is applied (if outer steel plates are pressed on (locked surface) or lifted from the surface (free surface)). During testing not only exceedance of withdrawal capacity but also failures in block shear were observed, even if the practical commonly used minimum distances were applied. Nevertheless, more detailed analysis is needed. Furthermore, relevant projects in the field of screw technology and timber engineering are given in conjunction with quality assurance in production and application, e.g. hydrogen embrittlement of the steel, tightening torque, as well as in regard the behaviour in case of moisture content variation in timber. It should not left unmention that the use of screws as joints has to be done in compliance with minimum distances, which defined to prevent splitting of timber. For the examination of these minimum distances, currently only a very time-consuming and cumbersome test configuration is given. It seems to be of high interest to define an improved test configuration, and further, to analyse the possibilities of screw application in hardwoods without pre-drilling, e.g. by compliance of boarder conditions in regard to density, thickness of timber components and diverse screw parameters.

A further and widely untouched or only in single projects discussed research field is given by use of bonding technology in high performance load bearing systems in timber engineering. Thereby insufficient knowledge about basic questions concerning the mechanical description, near reality and reliable design of bonded joints with timber / engineered timber products and steel joints can be attested. Furthermore, gluing-in of steel plates has up to now not reached the phase of practical applications.

It has to be stated that constructions made of timber in combination with state-of-the-art joining technique shows a remarkable performance. As a consequence it is meaningful to observe the behaviour of these contemporary constructions during their life-time. On one hand this gives further chance to prevent damages, and on the other hand this may also allow for adjustment of failure probabilities in design procedures if careful planning and adequate provisions can be secured. Last but not least it has to be clarified how defects, insufficiencies and / or damages indicated by monitoring can be corrected to again guarantee regular use.

To enable timber constructions competitive to other building materials, and to react on increasing lack of craftsman in view of appropriate accomplishment and design of joints, one main challenge in R & D is given by the development of system connectors, optimised for various fields, zones and actions of / on connections, but also in respect the connection to surfaces other than timber, e.g. to concrete, steel and masonry. It can be argued that future developments focus on multifunctional pin- or line-shaped joints, which, beside their ability to carry loads, also have to integrate building physic aspects, e.g. air tightness, noise protection and fire resistance.