یک اصل جدید برای طرح لرزه ای مبتنی بر انرژی قاب های خمشی فولادی A new methodology for energy-based seismic design of steel moment frames

 Introduction Like any earthquake spectrum, input and hysteretic energy spectra are derived from single degree-offreedom (SDOF) systems. Since most structures in reality are multi-degree-of-freedom (MDOF) systems, a methodology for estimating input and hysteretic energy demands for MDOF systems using information from single-degree-of-freedom (SDOF) systems is needed. Furthermore, for the purpose of design an energy distribution scheme is needed to apportion the total system hysteretic energy to each part of a MDOF structure. In this paper, a methodology to extend the energy demands from a SDOF system to a MDOF system is proposed. An energy distribution scheme that can be used for the design of multi-story steel moment frames is also proposed. In addition, a story-wise optimization design procedure for steel moment resisting frames is developed by utilizing their energy dissipating capacity from plastic hinge formation/rotation. Regardless of the size of the structure or whether it is a SDOF or MDOF system, the fi rst step in energy-based seismic design (EBSD) is to determine the seismic energy demand on the structure due to the design earthquake. Seismic energy demand or hysteretic energy is the inelastic component of the absorbed energy of the total seismic input energy imparted to a structure and it is a function of the hysteretic behavior of the structure. For a given design earthquake, structures of equal weight/mass but have different hysteretic behavior will experience different seismic energy demands. Expectedly, different seismic energy demands will require different energy dissipation capacities. In EBSD, the structure has to be designed so its energy dissipation capacity will exceed the energy demand. The fact that almost all practical structures are MDOF systems means one needs to go beyond SDOF systems and study the seismic input and hysteretic energy of MDOF systems. Compared to SDOF systems, the determination of seismic energy demand for MDOF systems is understandably more diffi cult. The number of dynamic equations involved along with the potential coupling effect of different responses makes the determination of seismic input energy and the accompanying hysteretic energy in MDOF systems rather diffi cult. However, using SDOF systems as the basis, research has shown that the input energy for MDOF structures can be obtained in an approximate manner. Akiyama (1985) used the S00E component of the 1940 El Centro record to compute the input energy using a Fourier Spectra for a fi ve-story building. He compared it with the input energy of an equivalent onestory building having the same fundamental period of vibration, total mass, and yield strength, and concluded that the input energy for the MDOF structure could be estimated from the input energy of the equivalent SDOF system. He also reported that the parameters that affected earthquake input energy were mainly the mass and period of the structure. Nakashima et al. (1996), in their study on the energy behavior of structures with hysteretic dampers, found that the total input energy and hysteretic energy for MDOF systems were approximately the same as those of the equivalent SDOF systems. They also found that this was true even for a large value of post-topre-yield stiffness ratio. The effect of post-to-pre-yield stiffness ratio was only important on the distribution of hysteretic energy at different levels of the structure.