Evaluation of seismic vulnerability of very flexible structures such as high-rise petrochemical and refinery stacks (chimney) and power plant chimneys is a challenging problem in earthquake engineering. Their equipments and structures are often considered as vital facilities and thus they must be fully functional after even very strong ground motions. From other point of view, numerical modeling of such mega-structures with numerous elements may not allow to consider all details of mechanical characteristics of consisting materials, particularly Nonlinear performance of elements during large deformations. Therefore, a simplified model corresponding to Dynamic characteristics of whole structure is substantially needed to investigate seismic performance and failure modes of these essential structures subjected to strong ground motions. The procedure for developing a 2-D simplified Nonlinear model based on moment-curvature in some plane-sections of a 3-D sophisticated model but linear system having almost identical Dynamic properties is discussed. If micro elements be used for structural modeling of the chimney like structures, static Nonlinear Analysis or Dynamic linear Analysis will be economically suitable for assessment investigation. But of the most important problems in earthquake behavior of the structures is hysteretic behavior of material and section properties. The significance of this study, if any, is mainly concentrated on model simplification that provides sufficient accuracy based on a Nonlinear discrete model. Tous power-plant chimney is investigated numerically as an example.