P_1.2.2 screw_effective

Through novel manufacturing procedures, an optimisation of screw threads on the one hand and the hardening of steel on the other hand, it has been possible over the last few year to produce self-drilling woodscrews with full threads of lengths up to 600 mm and diameters of up to 12 mm. These screws can, with steelplate-wood connections, usually at an angle of 45°, be screwed into the wood without any predrilling, between screw angle and grain direction, and stressed axially. This method has enabled new connection techniques in timber engineering that can bear higher loads and are stiffer and more cost-efficient.

In the course of the k_ind-Project “P07_Standardisation“ by holz.bau forschungs gmbh, a series of questions were raised concerning the application of woodscrews. Among these are questions concerning the bearing strength – e.g. the efficient number of screws in a group of screws – but also questions concerning suitability of use, as is the case with the shift model, and ranging to principal questions of model (e.g. the static modelling of such connections).

Through the multitude of open questions it is indispensable to define the targets as tightly as possible without, however, ignoring significant influential parameters. Top priority must always be given to safety issues, and it is therefore only through proven models that engineers will be allowed to obtain seamless and reliable proof. And this, in turn, helps and promotes the discipline of timber engineering.

When dimensioning the connection techniques at the limits of bearing strength, it is common practice that the overall bearing strength of the connection when using several connection techniques in a series (in the grain direction) one after the other should be smaller than the sum of the individual bearing strengths of the connection techniques. This applies to shearing as well as to rod-like connection techniques and axially stressed screws. This circumstance is generally called the “group effect“ and represents the core theme of this project.

From this aspect of the so-called “group effect“, another question arises that is hoped to be able to be answered in this context, namely,

• "How far shall this group effect be considered with steelplate-wood connection techniques using a number of self-drilling screws, and are there any contrary effects that might – due to the compactness of the connection – increase the bearing strength?“ Clarifying this question will certainly contribute largely to the safety of timber engineering carried out by engineers, and for those commissioning and carrying out the works.