High capacity saw tooth connections under cyclic loadsResearch of high-capacity saw-tooth-connections in cyclic loads for the transfer of extremely high and concentrated tangential forces between slender steel and concrete elements

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In a current DFG research project the applicant of this research proposal demonstrated, that saw-tooth connections are exceptionally well suited to transfer very high and concentrated tangential forces from steel into concrete elements, for example in bridge structures. However, the analytical analyses and practical tests were only investigating static loads. In contrast, the most important applications of saw-tooth connections are in buildings and structures, which are predominantly exposed to dynamic loads. The goal of this research proposal is a first and fundamental investigation of saw tooth connections under cyclic loading. The theoretical pre-analysis, conducted so far, indicates, that saw tooth connections will show a very high load bearing capacity in cyclic loading. The main reasons are the large areas of concrete and steel, which are available for the bearing of the compressive forces. As a result, the stresses in the concrete and the steel are quite low and uniform, without high peaks of stresses. It is supposed, that the stresses in the steel are lower than its fatigue resistance. Until today there is no scientific research, theoretical or practical, available on this subject. The first preliminary tests of concrete prisms with inclined saw tooth connectors, were carried out by the applicant. They showed no reduction in the load capacity after 2 Mio load cycles, with a Pmax/Pu = 0,67 and a deltaP/Pu = 0,48. However, in reality and in contrast to the prism-tests, the load distribution along the saw tooth connector is not constant, but shows a peak of forces at the tip of the saw tooth connector. That is why the performance of saw-tooth connections in a more realistic component test cannot be predicted by today. In addition, the connections in the mid-span of continuous beams may be subjected to changing loading directions. The effect of changing loading directions will be examined, for the first time ever, in this research proposal. In the preliminary tests, small cracks parallel to the compression struts were observed in the concrete. In the case of changing loading directions, this would result in perpendicular cracks, with the compression struts crossing the cracks. It is absolutely not clear, how these cracks will influence the load bearing capacity of saw tooth connections under dynamic loads. For the first time, this effect will be investigated in the proposed research project. A continuous ultrasonic measurement will be used during the tests, to detect the areas of fractured concrete. The aim of this research project is the first fundamental research into the basic principles of the load bearing behavior of saw tooth connections under pulsating and alternating loads. Special emphasis is laid on the research into the effects of crossing cracks in the area of saw tooth connections.