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The subject of this thesis is the numerical modeling of the behavior of plane reinforced concrete frame structures under long-term load with the purpose of qualitative and quantitative analysis of the influence of stiffness changes of these structures, as an effect of the occurrence of cracks, as well as creep and shrinkage of concrete, on the results of static and seismic analysis. The algorithm for the calculation of static influences in reinforced concrete frames, as well as dynamic characteristics and design seismic forces and influences by using multimodal spectral analysis, taking into account the real change of cross section stiffness after cracking of the concrete tensile zone, is formulated. This algorithm effectively solves the problem of determination of the stiffness of reinforced concrete structures under long-term load, and is significantly more precise and more detailed than the current recommendations in the literature. The new algorithm is implemented in the form of a package of computer programs, written in Fortran programming language, for the analysis of reinforced concrete frame systems taking into account the reduction of element stiffness. Application of these programs is simple, but effective, and among other, the real data on the geometrical characteristics of the elements of reinforced concrete structures with cracks is obtained, which can be used in other software for the calculation of reinforced concrete structures modeled by linear elements. In the static analysis of reinforced concrete frames two assumptions of linear theory are kept: the assumption of small deformations and the assumption of small displacements, while the material nonlinearity is taken into account by applying the numerical model with cracks, as well as using the Age - Adjusted Effective Modulus Method for the calculation of stress and strain changes during time. Using newly developed computer programs comparative analysis of results of classical static, as well as classical seismic calculation, in which the stiffness of columns and beams of reinforced concrete structures is calculated for gross concrete cross section, at one side, and the calculation that takes into account the changing of structure geometry due to the appearance of cracks, at the other side, is carried out. Conclusions are drawn that will be useful for the improvement of existing engineering practice.

This dissertation contains analysis of reinforced concrete (RC) walls performance under seismic loading. Limit states, ductility and energy indicators of RC walls behavior are examined, along with their failure mechanisms. Sources of theoretical and experimental research of RC walls behavior under static load and load that simulates seismic action are thoroughly and critically examined. Proposal of a nonlinear mathematical model of RC walls behavior under cyclic load was provided, as well as a simplified model suitable for practical analysis Results of analysis are verified by the available experimental research. Nonlinear numerical models of RC walls are used in this study for simulation of seismic action impact on the multi-storey buildings in dual structural systems. Using adequate (here developed or modified) numerical model provides information about the parameters required for proper behavior of RC walls in concrete structures. Based on the proposed model, building structures of various layouts, heights and arrangement of RC walls were analyzed. The analysis was performed using nonlinear methods: static nonlinear (push-over) method and dynamic method which integrates motion equation (time-history method). Storey drift, maximum inter-storey drift and acceleration of ceilings are comparatively analyzed, as criteria for assessment of seismic performances, due to their direct influence on the damage of structural and nonstructural elements.