During the past decades, extensive research on topics related to earthquakes and seismic loading has been conducted. This is partly due to the fact that earthquakes occur relatively frequent, and that the consequenses of earthquakes can be quite substantional. Many computational methods and programs have been developed in recent years, that makes it possible to analyse the earthquake response of structures to earthquake motions. In Norway, seismic events in a magnitude that involve a risk for structural damage or death rarely occur, but there is historical documentation and research that implies that strong earthquakes have occured in the past, and that they are likely to occur in the future.
A dam is a structure with quite special vibration properties. Standard designcodes for buildings are not directly transferable to dams. Dams subjected to earthquakes is a more complex problem than buildings, because of the dam-reservoir-interaction. This interaction makes the problem a combined structural and hydrodynamic problem. There are different ways to deal with the interaction, the most common is to increase the mass of the dam according to a formula developed by H. M. Westergaard in 1933. This thesis gives an overview of different theoretical approaches to this issue, and the developement that has taken place since the 1960's. It is also made an attempt to discuss the effects of the assumptions made in different theories. In this regard, it is referred to a lot of research done on the Koynadam in India, in the aftermath of the 1962 Koyna earthquake. In this thesis, a spesific dam geometry subjected to the 1976 Friuli earthquake in Italy, which had a magnitude around 6, has been analyzed. To fulfill this task, different methods for calculation has been used, both simple calculations by hand and finite element analysis in the frequency- and timedomain.
We learn that Westergaard's added mass has to be used carefully in more advanced analysis than simple hand calculations.The added mass increases the system's vibration periods, and this will in some cases lead to smaller response than if the dam was analyzed in its original state.