In recent years, the development of structural design criteria for new structures and the renewed importance of the assessment of seismic vulnerability of existing under-designed buildings have broadened the objectives of seismic design. While safety against collapse is still the main goal, performance in terms of functionality and economy have assumed a central role in the design criteria. Hence, a great effort has been made to improve the current earthquake-resistant design methods in order not only to avoid collapse under a destructive earthquake, but also to limit the damage under moderate earthquakes. Furthermore, the new design philosophy is tending to multi-level probabilistic structural performance criteria, replacing completely the simple force strength approach. However, the implementation of all these new concepts requires the definition of a quantative damage index and measures. A key issue in the performance-based design is a reliable assessment of seismic damage potential. If the expected intensity of the earthquake is greatly over- estimated, the cost of new construction and seismic rehabilitation of existing structures could be excessive. On the other hand, if the intensity is seriously underestimated, the results may be heavy damage and loss of life. To this end, a reliable definition of seismic intensity has to relate to the effect of damage on structural behavior in order to assess the potential seismic hazard and to In this thesis, optimization of RC structures based on minimum damage index by using genetic algorithm was done. Different reports have been published recently on the use of a damage index and damage measures in earthquake engineering. They aim to clarify the different approach methodologies and to detail different proposed formulations. In this thesis after review in research, revised Park Ang damage index was selected for damage evaluation. The analysis of real buildings damaged by past earthquakes showed that when DI 1 structural collapse occurred, while for DI 0.4 structural damage was not irreparable. In the case of 0.4 DI 1.0, collapse did not occur, but the building could not be considered as repairable. On the other hand, the cases where damage is insignificant are defined by DI 0.2. The index can take into account both maximum plastic displacement and plastic dissipated energy and is supported by a wide correlation with observed. In this thesis, For dynamic analysis used from Opensees software that is an open source and power full .Then a program in Matlab was written and by summoning the output data from Opensees software, the revised Park Ang damage index was calculated for each elements and finally overall damage index for structures was evaluate. In this thesis by using Genetic Algorithm , two 3 and 6 story rc frame was optimized. Three earthquakes, Tabas, Manjil and Elcentro that scaled with code 2800 acceleration spectra was used for dynamic analysis. In this research, in addition to minimizing damage index, decrease in weight of optimized structure was second goal. Results shows that Gentic Algorithm is an effective tool for optimizing Damage index. Key words : optimization, damage index, park-ang, genetic algorithm, fitness function
