Doctor of Philosophy / Civil Engineering / Asadollah Esmaeily / Accurate and realistic assessment of the performance of columns in general, and those in critical locations that may cause progressive failure of the entire structure, in particular, is significantly important. This performance is affected by the load history, pattern, and intensity. Current design code does not consider the effect of load pattern on the load and displacement capacity of columns. A primary research sponsored by Kansas Department of Transportation (KDOT) was conducted as the initial step of the present study (No. K-TRAN: KSU-11-5). The main goals of the KDOT project were: (1) investigation of new KDOT requirements in terms of the column design procedure and detailing and their consistency with AASHTO provisions; (2) verification of the KDOT assumptions for the plastic hinge regions for columns and bridge piers, (3) provide assessment of the load capacity of the existing columns and bridge piers in the light of the new specifications and using the new load demand as in the new provisions; and finally recommendations for columns and bridge piers that do not meet the new requirements. A conclusion was drawn that there is a need for conducting more studies on the realistic performance of Reinforced Concrete (RC) sections and columns. The studies should have included performance of RC members under various loading scenarios, assessment of columns capacity considering confinement effect provided by lateral reinforcement, and investigation on performance of various monotonic and cyclic material models applied to simulate the realistic performance. In the study reported here, monotonic material models, cyclic rules, and plastic hinge models have been utilized in a fiber-based analytical procedure, and validated against experimental data to simulate behavior of RC section under various loading scenarios. Comparison of the analytical predictions and experimental data, through moment–curvature and force–deflection analyses, confirmed the accuracy and validity of the analytical algorithm and models. The performance of RC columns under various axial and lateral loading patterns was assessed in terms of flexural strength and energy dissipation. FRP application to enhance ductility, flexural strength, and shear capacity of existing deficient concrete structures has increased during the last two decades. Therefore, various aspects of FRP-confined concrete members, specifically monotonic and cyclic behavior of concrete members confined and reinforced by FRP, have been studied in many research programs, suggesting various monotonic models for concrete confined by only FRP. Exploration of existing model performances for predicting the behavior of several tested specimens shows a need for improvement of existing algorithms. The model proposed in the current study is a step in this direction. FRP wrapping is typically used to confine existing concrete members containing conventional lateral steel reinforcement (tie/spiral). The confining effect of lateral steel reinforcement in analytical studies has been uniquely considered in various models. Most models consider confinement due to FRP and ignore the effect of conventional lateral steel reinforcement. Exploration of existing model performances for predicting the behavior of several tested specimens confined by both FRP and lateral steel shows a need for improvement of existing algorithms. A model was proposed in this study which is a step in this direction. Performance of the proposed model and four other representative models from literature was compared to experimental data from four independent databases. In order to fulfill the need for a simple, yet accurate analytical tool for performance assessment of RC columns, a computer program was developed that uses relatively simple analytical methods and material models to accurately predict the performance of RC structures under various loading conditions, including cyclic lateral displacement under a non-proportionally variable axial load (Esmaeily and Xiao 2005, Esmaeily and Peterman 2007). However, it was limited to circular, rectangular, and hollow circular/rectangular sections and uniaxial lateral curvature or displacement. In this regards, a computer program was developed which is the next generation of the aforesaid program with additional functionality and options. Triangulation of the section allows opportunity for cross-sectional geometry. Biaxial lateral curvature/displacement/force combined with any sequence of axial load provides opportunity to analyze the performance of a reinforced concrete column under any load and displacement path. Use of unconventional reinforcement, such as FRP, in lateral as well as longitudinal direction is another feature of this application.

L’effet de l’espacement interlamellaire sur le comportement sous chargements monotone et cyclique de l’acier perlitique C70 a été étudié. Une démarche expérimentale conjuguant les essais de traction « in-situ » sous DRX et traction « in-situ » sous MEB couplées à une modélisation auto-cohérente a été adoptée. Les essais ont été effectués sur deux microstructures, à deux espacements interlamellaires différents. Il a été montré que l'écoulement plastique de la perlite est contrôlé par celui de la ferrite lequel est gouverné par l’espacement interlamellaire, il en est de même pour les distributions des contraintes résiduelles. La modélisation auto-cohérente met en évidence l'anisotropie élastique à l'échelle des phases et l'effet de l'espacement interlamellaire sur les rigidités et les limites d'écoulement directionnelles. L'analyse microstructurale des mécanismes de déformation montre que la déformation plastique de la perlite est caractérisée par le développement de bandes de cisaillement. Ces bandes sont le siège de l'amorçage des fissures par cisaillement des lamelles de cémentite pour les taux de déformation élevés. L’effet de l’espacement interlamellaire sur le comportement en fatigue giga-cyclique a été étudié par des essais de fatigue sous une fréquence de 20 KHz. Des différences de 30 MPa, pour la perlite à SP= 230 nm et de 50 MPa pour la perlite à SP= 170 nm entre les limites de fatigue conventionnelle (106cycles) et giga-cyclique (109cycles) ont été obtenues. L’effet de l’espacement interlamellaire sur le comportement en fatigue apparait peu significatif comparativement aux caractéristiques mécaniques de traction. Les limites de fatigue giga-cycliques obtenues intègrent l’effet bénéfique des contraintes résiduelles stabilisées ainsi que les effets d’écrouissage induit par l’usinage et par écrouissage cyclique. L’examen des sites d’amorçage, révèle essentiellement un amorçage en surface dans le domaine méga-cyclique et mixte en surface et/ou en sous-couches dans le domaine giga-cyclique. Ces résultats peuvent être interprétés sur la base des effets des propriétés de surface stabilisées et de la microstructure. Le recours à un critère de fatigue de type Goodman permet d’identifier d’une manière qualitative la contribution des différents facteurs d’influence, ci-dessus cités, dans l’amorçage des fissures de fatigue. / The effect of interlamellar spacing on the monotonic and cyclic behavior of C70 pearltic steel was investigated. Tensile tests under scanning electron microscope and under X-ray diffraction coupled with self-consistent model have been used. Tests have been carried out on two microstructures with different interlamellar spacing. It has been demonstrated that pearlite yielding is controlled by ferrite critical shear stress), which depend on the interlamellar spacing. The pearlite deforms inhomogeneously under tensile loading by localized shearing in intense shear bands. In the large shear bands, parallel cementite plate offsets before fracture and contributes to the occurrence of localized damage. The residual stress in ferrite is higher for the coarse pearlite. Giga-cycle fatigue tests were performed to identify the effect of interlamellar spacing on the fatigue behavior; tests were performed at a frequency of 20 KHz. The difference of the fatigue strength between 106 and 109 cycles is about 30 MPa for coarse pearlite and 50 MPa for fine pearlite. The effect of interlamellar spacing on the mega-cycle and giga-cycle fatigue limit appears not significant compared to the mechanical characteristics. Giga-cycle fatigue limit obtained includes the beneficial effect of stabilized residual stress. It also includes the effect of hardening induced by machining and cyclic hardening. However, any significant effect of interlamellar spacing on the C70 fatigue limits has been shown. SEM examinations shows that failures initiated on surface for the mega-cycle regime and on different sites for the giga-cycle regime. The experimental results can be interpreted based on the effects of stabilized surface properties and microstructure. The use of a fatigue Goodman criterion would identify qualitatively the contribution of different factors, cited above, in the fatigue life.