As earthquake ground shaking affects all elements and components of a hydropower project, i.e. the dam body, appurtenant structures, underground works, and all hydromechanical and electromechanical components etc. at the same time, the earthquake load case is probably the most challenging one for dam engineers. Thus all these elements and components must be able to resist some degree of earthquake action. This also applies to temporary structures like cofferdams, which, depending on the duration of dam construction may have to function for several years, retaining walls, slope protections, etc. In addition, the earthquake safety of the dam and other structures of hydropower plants must also be guaranteed during the whole construction phase. It is important to note that there is no structure, which is inherently safe against earthquake such as, e.g. underground works. Also to declare the earthquake risk as an accepted risk is no longer consistent with the current state of practice. The seismic design criteria depend on the safety class of the differen
t elements and components. Safety class1 includes elements and components, which must remain in an operable state after the ground motion of the safety evaluation earthquake (SEE), i.e. bottom outlet and spillway gates. Safety class 2 includes the dam body and other water retaining elements, which must be able to retain the water in the reservoir after the SEE until the water in the reservoir can be lowered if the dam experiences structural damage. Safety class 3 includes mainly appurtenant structures such as the powerhouse, switchyard, transmission towers etc., which have to be able to withstand a design ground motion that is less severe than that of the SEE. Finally, safety class 4 includes non-critical structures like office buildings, storage facilities etc., which may be designed according to building code regulations. Furthermore, special criteria have to be established for temporary structures and for special construction phases. A systematic and detailed approach is presented for the case of large hydropower project.