Issue 40

N. G. Pnevmatikos et alii, Frattura ed Integrità Strutturale, 40 (2017) 129-136; DOI: 10.3221/IGF-ESIS.40.11 130 design codes. Yang et al. 2003 [5] suggested the maximum control force to be a percentage of the building weight, while Cai et al., 1997, [6] give this force as a portion of the seismic force. Lee et al., 2004, [7] determined the upper limit of control force based on the response spectrum of the external earthquake. During the past years the design philosophy of new structure was to design stiff structures with high strength to resist the earthquake in the elastic range. After that the design philosophy moves one step further. Using the ductility of the material, structures were designed to resist lower level of earthquake forces within the elastic range but to have adequate ductility in order to face the attack of stronger earthquakes and prevent them from collapse. This drives to lighter structures compared to the previous structures and more economical. However, the capacity design and the reinforcement details increased the cost. Taking in account the cost of repair of the retrofitting of structure, after a strong earthquake, the design of ductile structures should be under consideration. The design philosophy proposed here is to use control devices installed in the structure and provide a reservoir of strength, stiffness or damping, necessary for preventing the structure from damage when an expected earthquake will occur. Thus, the control system will drive the structure to behave in the elastic range when it is attacked by the expected earthquake and no damages will occur. As far as the cost is concerned, it is possible to achieve substantial savings by avoiding retrofit of structure during the lifetime of structure and utilize these savings for installing a control system. A systematic procedure to achieve the above objective is proposed in this work. D ESIGN PROCEDURE FOR STRUCTURES EQUIPPED WITH A CONTROL SYSTEM he evolution of the design philosophy of structures passes through different stages. Fist the engineers design stiff and massive structures in order to behave elastically during the expected earthquake. As years passed and damages were observed after earthquakes the design philosophy was moved from the resistance of structure to energy dissipation capacity of the structural elements and design of structures with an overall ductile behavior. This drives engineers to perform capacity design for structures. This philosophy is nowadays included in all current design regulations. However, observing the damages to the structures that were designed with the latter philosophy and making calculations, the repair cost of the capacity design emerged and came into consideration. The answer to the previous consideration is the new and proposed design philosophy where the structure is oriented to capacity design equipped with control devices that will absorb a portion of seismic energy induced to the structure and as a result to keep the structure in the elastic range. The three design philosophies are depicted in Fig. 1. The proposed design procedure for the spectrum is calculated in such a way that one portion of earthquake forces is taken by the structure and the remaining ones are taken by the control devices. Figure 1 : The three design philosophies of design of structures. T