The concrete core structures consist of a number of connected shear walls that form a tubular section. The core section might be open or closed using a series of beams or floor slabs. Therefore, this type of structures has very high moment of inertia and strength and can withstand the applied lateral loads to mid and high-rise structures a lateral load carrying system. In some cases, due to damages caused by severe ground motions or change in the use of structure or addition of new stories, the initially designed and constructed concrete core may require strengthening or rehabilitation to withstand increased applied loads or to recover the loss of strength and stiffness due to damages. A practical method to strengthened this type of structural systems is to utilize steel plates or Fiber Reinforced Polymer (FRP) sheets. The performed evaluations showed that the main components of this system that required to be strengthened are spandrels which plays role of seismic fuse for the concrete core system. This paper is a case study that evaluates effects of application of steel plates and FRP sheets on spandrels of central core concrete shear wall system for a structure that required addition of two stories after initial design. A eight-story structure with central reinforced concrete shear wall system is designed and due to a decision to add two more stories in the construction phase, the designed structure required strengthening. Since the analysis showed that only spandrels of the core system do not comply with the new strength demand, two option of using steel plates and FRP sheets for strengthening of spandrels beams were considered. The evaluation and analysis of the system under seismic loading demonstrate acceptable performance of both methods, but for the sake of economic considerations, steel plates were used in the strengthening of the structure.

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