Many thanks Professor Wit Derkowski for the worthy explanation. At the moment my concentration is mostly on the foundation of Wind-Turbines, both those already built ones and those about to be installed, on different topographical sites indeed. As the wind-turbines are affected by cyclic loads and consequently their foundations are induced to fatigue phenomenon, I would like to know which measures could be fruitful.

Strengthening of reinforced concrete structures subjected to fatigue load should be done in such a way as to reduce the effort of individual materials.

First of all you need to recognize whether the problem of fatigue affects steel rebars or concrete. Well done strengthening may be done by means of prestressing which can reduce tensile stresses in reinforcing steel or compressive stresses in concrete. Sometimes, instead of strengthening of the structure you can change the static scheme of your structure to change the distribution of cross-sectional forces.

Many thanks Professor Wit Derkowski for the worthy explanation. At the moment my concentration is mostly on the foundation of Wind-Turbines, both those already built ones and those about to be installed, on different topographical sites indeed. As the wind-turbines are affected by cyclic loads and consequently their foundations are induced to fatigue phenomenon, I would like to know which measures could be fruitful.

Usually, fatigue is associated with a specific range of variation in stresses in the particular material. Therefore, during designing, it is necessary to limit the maximum value of stresses as well as their level of variation (for example, by changing the dimensions of the element or increasing the amount of reinforcement). It is much more difficult in the case of already existing structures - then the problem of effective strengthening method arises.

In my opinion the stress intentensity factor is a good measure.