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- Bond behaviour of self-tapping screws with continuous threads and long embedment length being used as reinforcement in glue-laminated timber elements
Ayoubi, Mazen; Trautz, Martin (Thesis advisor); Raupach, Michael (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2014
In regard of future challenges in modern architecture that demand slender and filigree constructions from environmentally, friendly and recyclable building materials, the significance of the timber structures continually increases. The use of self-tapping screws with continuous threads to reinforce and join timbers and glue-laminated elements represents an effective, simple and economic method. The high withdrawal resistance of the screws as well as its continuous bond with the wood enables an effect similar to steel reinforcement in concrete structures. The presented thesis aims at precisely describing the flexible bond behaviour and predicting the force transfer of the screws in glue-laminated elements as well as deriving design rules that enable the safe application of the screws as reinforcement in timber elements. Various parameters affect the bond behaviour of the screws and are described in own experimental and numerical investigations. The experimental investigations are comprised of over 160 pull-out tests with long embedment length and over 84 load distributions tests. The pull-out tests cover different angles to the grain and various screw diameters. Furthermore, they display the effect of the longitudinal cracks in the surrounding wood on the force transfer as well as the effect of the screw tip. The load distribution tests inform about the load transfer and the load distribution angles of the screw in timber in combination with different parameters. Continuative three-dimensional nonlinear FE-analyses provide deep insight into the load transfer mechanisms and the influence of the anisotropy of the material as well as the angle to the grain, and the anchorage length of the screws. Through various analysis of the bond behaviour between screws and wood, the experimental investigations form the basis for the calibration and evaluation of the numerical models. Herewith, a detailed knowledge of the force transfer mechanisms, in which the force within the screw changes along its length, as does the force in wood embedment, is essential to ensure an economic and safe design of the reinforced timber elements. This thesis allows an overall description of the bond behavior of self-tapping screws with long embedment length in different angles to the grain. With the proposed approaches, the anchorage length can be designed safely even if the screws drilled parallel to the grain. Furthermore, the influence of longitudinal cracks on the bond behavior can be predicted correctly.