The assessment of seismic performance of existing structures is becoming an important problem in earthquake engineering. Some important ructures are considerably old and, therefore, their strengths and ductilities are less than strength and ductility demands because of changes in codes and design methodologies. Such structures must be strengthened to resist future earthquakes. First, a structural model must be developed and then, based on seismic hazard and seismic risk analysis or code quantities, the design (or control) parameters can be determined. The next step is defining the damage indices in order to quantify the structural damage. Then the nonlinear dynamic analysis is carried out and damage indices are calculated. In the present paper, a power plant stack (located in Mashhad) is investigated for some levels of peak ground acceleration (PGA) considering return periods of 75, 500, 1000, and 2500 year. The height of stack is 100 m and the external diameter of the structure is 10 m. Several records are used for nonlinear dynamic analysis. It has been used from Park-Ang damage index that is a suitable model for concrete structures and has been considered in IDARC. It is clear that nonlinear dynamic analysis is necessary for the seismic vulnerability of existing structures. Special structures such stacks can be modeled with some 2-D or 3-D elements. However, the beam-column element is a proper model for special structures such as chimneys, considering calculation cost.


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