Optimum design of earthquake resistant modular structures.

Abstract

The primary interest in this dissertation is the optimum design of low to medium rise buildings subjected to intense earthquakes using modular structures. A modular structure can be thought of as a stack of rigid stories or modules separated by passive control elements. These elements, like current base isolators, require a structural mechanism which is vertically stiff, laterally flexible, and provides some means of energy dissipation. The only design variables of the optimization problems are the spring and damping coefficients of the control elements. The excitation and response relations are formulated in the frequency domain using probabilistic methods of random vibration. The excitation process used corresponds to strong earthquakes on rock deposits. Because the statistical characterizations of site-specific earthquakes are widely available in the form of response spectra, a method is developed to derive power spectral densities from response spectra. One and two modular models of a six-story building structure are used in the optimal design investigations. A one module case represents a base isolated building. In every case with two modules, the first level of control elements is placed at the base while the second level is placed at various higher locations. Within the limitations of the scope of this study, it is concluded that, for the same level of response, the base isolated building is less costly than the modular building. Since there are practical limits on the flexibility and lateral displacement capacity of a base isolation system, the modular scheme can be viewed as a method for response improvement without requiring impractical flexibility or displacement capacity at the base. The modular structure allows increasing the fundamental period beyond the limits established for base isolated structures. Therefore, it can be considered as a viable alternative for earthquake protection on soft soil deposits. The results show that a modular scheme is both effective and practical for the retrofitting of existing multistory structures with soft first stories. The example building required placement of one level of control elements under the roof or under the floor slab of the top story and supplemental braces and dampers in the first story. Overall elastic behavior was achieved for a severe earthquake on rock deposits.