Controlled damage based design of composite structures under seismic loading
Authors: Σέρρας Διονύσιος 
Published: September 2021

The main purpose of this Ph.D. Disseration is to investigate in-depth the seismic behavior of steel/concrete composite structures consisting of circular concrete – filled steel tube columns and composite beams (steel I – beams connected with concrete floor slabs). After an extensive study of the literature review, it is found that there is a lack of a simple simulation of the inelastic hysteretic response for circular concrete – filled steel tube (CFT) columns under seismic loading. More specifically, there is a lack of a hysteretic model which is based on the theory of concentrated plasticity so as to be easily applied to analytical simulation models and to reduce drastically the computing time of the inelastic analysis, providing reliable and accurate results with other corresponding simulations which are complex and computationally time consuming. The limited knowledge of the inelastic response of composite structures subjected to monotonic loading is a further aspect which is missing from the investigation of this type of structures. It is evident from the fact that the pertinent literature is constrained only to steel structures. At the same time, the research study for the seismic behavior of composite structures under repeated earthquakes as well as the impact of the seismic incident angle is considered a further investigation in this research. Therefore, the main research areas of this doctorate dissertation can be briefly described as follows:i.Investigation of the inelastic behavior of circular concrete – filled steel tube columns under monotonic and cyclic loading using detailed simulations with finite elements. Their accuracy is verified by available experiments of the pertinent literature. Also, an additional study is developed where monotonic and cyclic loadings are compared, exporting to useful findings.ii.Development of a simple hysteretic model of concentrated plasticity for the seismic behavior of circular concrete – filled steel tube columns, whose parameters are functions that derived from parametric studies on the finite element simulations.iii.Evaluation of the analytical simulations and analysis procedures which are proposed in the pertinent literature for the seismic behavior of structural members via comparisons with experimental data.iv.Extensive parametric studies with dynamic inelastic analyses of steel/concrete composite frames in order to create a responses database aiming at developing new and more rational seismic design methods of steel/concrete composite frames as well as the improvement of the existing provisions.v.Assessment of the seismic behavior of steel/concrete composite irregular frames under repeated earthquakes and damages quantification into the structural members taking into account the incident angle of each seismic event.vi.Case study: Adoption of proposed empirical equations related to soil which are verified via comparisons between the analytical results from the proposed equations and experimental results from various published works. Additionally, a series of comparisons between a composite steel/concrete onshore structure with a one consisting of different material (i.e. reinforced concrete) is conducted under monotonic, cyclic and seismic loadings, providing useful findings.According to the above research areas, the present dissertation is divided into five sections:The first section refers to the methodology which is developed in order to create proposed empirical equations of circular concrete – filled steel tube columns under monotonic loading. Simulations for circular concrete – filled steel tube columns as well as various structural members of circular cross – section are adopted from the relevant literature.The second section refers to the methodology which is followed in order to develop a proposed simulation for the hysteretic behavior of circular concrete – filled steel tube columns under cyclic loading.The third section is related to the investigation of the seismic inelastic behavior of steel/concrete composite frames aiming at developing new and more rational seismic design methods of composite steel/concrete frames as well as the improvement of the existing provisions. Thus, various empirical relations are proposed which related to the dimensioning of the steel/concrete composite buildings and to the direct identification of the controlled damage of the structural members, demonstrating the effectiveness and usefulness of this investigation.The fourth section refers to the damages assessment in 2 – D and 3 – D steel/concrete composite irregular frames under repeated earthquakes. These frames are also investigated depending on the incident angle of each seismic event, providing useful findings.Finally, the fifth section presents a proposed analytical approach related to soil and verified by comparing it with experimental and computational results obtained by the pertinent literature, leading to useful conclusions. In this section, a comparison between composite materials (steel and concrete) and other specific materials (steel or concrete) is also taken place. More specifically, two onshore structures consisting of circular concrete filled – steel tube piles and piles from reinforced concrete are investigated and compared under lateral loadings (monotonic, cyclic and seismic loadings) concluding to useful findings.