HIGH CYCLE FATIGUE BEHAVIOR OF REINFORCED CONCRETE

Towers and foundations that support wind turbines must be able to sustain high-cycle fatigue stress demands that are caused by waves, winds, and turbine operations. These demands often have high mean stress levels due to prestressing and/or mean wind speed, they can range into the millions of cycles of occurrence, and they often control design. In current practice, the fatigue design life of these structures is determined using “S-N curves” and Palmgren-Miner’s linear damage accumulation model. While this may be suitable for most steel structures, it is not suitable for most concrete structures. With funding from the U.S. Department of Energy, an experimental testing program is examining the impact of steel reinforcement, tensile stress, strain variation, and the order of stressing on compressive fatigue capacity. This is one of the first significant experimental testing programs to study the impact of steel reinforcement on the fatigue performance of concrete structures. The results from completed tests have identified shortcomings in existing fatigue models, and they are being used to suggest improved means to characterize damage and estimate fatigue life. This research project is also examining fatigue stress demands so to identify the primary sources of fatigue capacity utilization (demand/capacity for each stress state) and determine how they can be mitigated by turbine control settings. This includes the consideration of time dependent behavior including changes in the stiffness characteristics of structural concrete. This paper presents key observations and conclusions from the completed experiments, and a discussion of challenges in both fatigue testing and the development of suitable models and design requirements.