Uticaj promene krutosti na proračun i dimenzionisanje elemenata ramovskih konstrukcija od armiranog betona

Abstract

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.