Concrete structures for the hydropower industry
Non-linear analyses of cracks in aging concrete hydro power structures
The concrete structures at Swedish hydro power facilities were built during the early to mid-20th century and many of them are starting to exhibit age related wear and deterioration. It is important to ensure the integrity of these concrete structures from a dam safety perspective and also to secure a safe operation of the power facility in the future. With the latter in mind, this project study the concrete structures that house the power generating machinery of the facility, especially the parts close to the generator where the loads from the power unit are supported. Cracks observed in these structures will reduce its stiffness, which affects the operation of the rotating machinery. The project discusses some general considerations and loads that are of importance for this type of structures and highlights some typical cracks that have been observed in Swedish hydro power facilities. To complement this discussion, a case study is presented of a hydro power facility where cracks have been found in the concrete support structure of the power unit, especially at the interconnections between the unit and the concrete. The most likely cause of these cracks are investigated through nonlinear finite element analysis considering mechanical loads as well as physical loads such as drying shrinkage and temperature variations. It is concluded that the long-term physical loading is the most probable cause of the observed cracks. However, the operation of the power unit and changes in its operational pattern can cause further propagation of these cracks. Finally, suggestions on possible enhancement of the analysis methods used in the case study are proposed and discussed for further studies of this type of concrete structures.
Fluid structure interaction
The aim of this study is to investigate how Fluid-Structure interaction may be included in numerical earthquake analyses of dams. The basis for this project is an international ICOLD benchmark workshop on numerical analysis of dams. The focus of theme A was on how to account for fluid structure interaction in numerical earthquake analyses of dams. In this study, parametric numerical analyses have been performed where the purpose was to isolate some important parameters and investigate how these influence the results in seismic analyses of dams. These analyses were performed through the use of the finite element method are the choice of Rayleigh damping parameters, reservoir boundaries and wave absorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of a dam-reservoir-computation time, while allowing for more advanced features such as bottom absorption and non-be a challenging task, where it has a significant impact on the results. The method has a physical meaning in the sense that this method excites the same effective mass for the Rayleigh damped case as for the modal damped case. If a constant modal damping is desired or prescribed in a standard, this method provides a reasonable and sound method to choose the Rayleigh damping parameters fora complex structure. A more straightforward method is to choose the two frequencies in such a way that the span between the frequencies covers about 80% of the effective mass. The choice of reservoir boundary conditions parameter showed to be the one that least affected the results in the time-history analysis results and this coefficient should be used carefully.
Cracking of concrete buttress dam due to seasonal temperature variation
The largest and most important hydropower dams in Sweden are concrete buttress dams. These consist of a large number of concrete monoliths formed by a frontplate with a supporting buttress. Cracks have been observed in some of these dams, and these may affect their safety in the long term. As an important tool for the condition assessment of such dams, a finite element model based on nonlinear fracture mechanics and plasticity theory has been used to study crack development in a buttress dam. The combined effects of restrained thermal displacements and loads due to water were studied. The development of cracks due to seasonal temperature variations was simulated, especially the effect of an insulating wall installed some years after the completion of the dam. Thermal stresses, in combination with the load caused by water, were shown to be the reason for cracking. The addition of an insulating wall greatly contributed to the development of cracks in the buttress.