Design and load-bearing behaviour of joints with fully-threaded-screws pre-drilled with laser-technique - continuation of projekt

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Due to its important contribution to climate change, sustainable construction with wood is currently experiencing a significant boost, especially in multi-story buildings. Due to the increased loads, the joining points often require fully threaded self-tapping screws (STS) for reinforcement. The use of STS for joining and strengthening timber constructions is also represented in the current Eurocode 5, which will come into force in 2022. At the same time, the use of STS is being investigated in new fields of application, where screws with large engagement lengths (lE = 600 mm and more) are used. However, exact positioning of STS, especially with high lengths, remains challenging. The inhomogeneous wood structure and the high length cause the screws to stray from their designated axis. Pyrolytic pre-drilling utilizing laser radiation represents a suitable and innovative solution method for the exact insertion of STS. The advantages of pyrolytic pre-drilling compared to mechanical pre-drilling with regard to accurate insertion in glulam were worked out in the current DFG project. Within the project suitable process parameters for drilling deep holes in wood and the knowledge about the design of the laser drilling process were developed. Also, the effects on the wood matrix due to heat input were investigated and its influence to the bonding behaviour between the screw thread and the glulam matrix was determined in pull-out tests. Pre-drilled holes up to 300 mm were obtained with a reasonably constant drill cross-section with an average diameter of 6 mm. The gas evolution at greater depths causes more carbonizing burnup throughout the bore channel, adversely affecting the bonding strength of the inserted STS. This results from the stronger heating of the ablation products due to the longer interaction with the laser radiation in the drilling channel during the drilling process. Thus, it is necessary to improve the shielding gas supply to the laser drill point to prevent flames and waste gas disposal from ablating to the drill point. Therefore, for the proposed continuation project, it is planned to carry out the laser drilling process with the aid of a probe with at least one double cross-section for the supply and removal of shielding gas and exhaust gases. The new method improves the supply of inert gas to the ablation point and minimizes pyrolytic burn up due to the heating of the ablation products by the laser radiation. With the help of the newly developed probe, the targeted drilling depth is to be increased to at least 600 mm in this continuation project.