Fatigue capacity of partially loaded areas in concrete structures
MetadataVis full innførsel
Because of growing energy demands, wind turbine suppliers are required to create larger turbines, which again require higher towers. Higher towers imply larger bending moments at the base, which causes stresses under the shaft to increase. The specimens constructed for this investigation are derived from a scaled model of a wind turbine with a circular foundation, with special focus on the connection between the steel shaft of the turbine and the concrete foundation. There are three main limitations in this investigation. The reduced size of the specimens tested will have an effect on the outcome of the results. The time available only allowed for testing of 12 specimens. The number of specimens should have been higher in order to get better accuracy in the results. In order to observe the specimens during testing, the boundary conditions from the real structure could not be simulated completely. In spite of these limitations, valuable data has been produced. Six of the 12 specimens were tested statically, three with splitting reinforcement, and three without, in order to assess the effect of the splitting reinforcement. The remaining six specimens were tested dynamically at different load levels in order to establish the relation between stress levels and life-time. The results from the dynamic tests were plotted in a Wöhler curve. The confinement-effect of the partially loaded area, and the effect from the inclusion of splitting reinforcement, gives a static compression capacity factor increase from 1.73 (NS-EN 1992-1-1) to 1.9. The specimens with splitting reinforcement also experienced a much more ductile failure mode, compared to the specimens without reinforcement. Fatigue strength results were compared to formulation given in the old Norwegian standard, NS3473, but with a reference concrete strength higher than the uniaxial concrete strength. The reference strength used was calculated using factors from the NS-EN-1992-1-1 for partially loaded areas. Based on the fatigue strength results, expressions between the stress levels and the lifetime, expressed as a number of cycles until failure at the different stress levels, are presented in a Wöhler curve. Compared to fatigue tests done on specimens without reinforcement, the multiplication factor C1 increases from 16.8(±0.1), to 19.48(±5.67). This indicates that the factor C1=12 used in the NS3473 is adequate for design in the high strength range. One important factor is the location in height of the splitting reinforcement, in relation to the partially loaded area, and the distributed load area. It was observed that the reinforcement closest to the location of the distributed load area carried the highest amount of stress. Further testing with a higher number of specimens, as well as a larger amount of cycles, is required in order to fully determine the effect of splitting reinforcement, and if the increased contact pressure capacity for static loading is also valid in fatigue. Another important aspect is the impact of humidity on the specimens, and covering the specimens in water during testing could produce more reliable results. Dynamic testing of specimens without reinforcement should also be performed, in order to figure out the contribution of the confinement effect from concrete, and the confinement effect provided by the splitting reinforcement.