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    Compressive behavior of large-scale square reinforced concrete columns confined with carbon fiber reinforced polymer jackets
    (Elsevier Sci Ltd, 2010) Turgay, Tahsin; Polat, Z.; Köksal, Hasan Orhun; Doran, Bilge; Karakoç, C.
    Several experimental and analytical studies on the confinement effect and failure mechanisms of fiber reinforced polymer (FRP) wrapped columns have been conducted over recent years. Although typical axial members are large-scale square/rectangular reinforced concrete (RC) columns in practice, the majority of such studies have concentrated on the behavior of small-scale circular concrete specimens. The data available for square/rectangular columns are still limited. This paper reports the results of an experimental research program on the performance of large-scale square RC columns wrapped with carbon fiber reinforced polymer (CFRP) sheets. Attention is focused on the investigation of the total effect of longitudinal and transverse reinforcement and FRP jackets on the behavior of concentrically loaded columns. A total of 20 large-scale RC columns were fabricated and tested to failure under axial loading in the structural laboratory. Three types of columns were primarily considered: unwrapped; fully wrapped; and partially wrapped. Based on the test results of RC columns, existing experimental data and procedures in the literature are also evaluated. Furthermore, stress-strain curves of the columns are successfully predicted by the analytical approach previously proposed for FRP-confined concrete. (C) 2009 Elsevier Ltd. All rights reserved.
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    Nonlinear finite element modeling of rectangular/square concrete columns confined with FRP
    (Elsevier Sci Ltd, 2009) Doran, Bilge; Köksal, Hasan Orhun; Turgay, Tahsin
    Numerous confinement models which can be categorized as design or analysis oriented have been proposed for predicting the behavior of fiber-reinforced polymer (FRP)-confined concrete columns. Analysis-oriented models generally use an incremental procedure for plotting the entire stress-strain response while design-oriented models predict the load-carrying and deformation capacities of the column with closed form expressions. As a comprehensive approach, this paper primarily deals with the nonlinear finite element modeling of rectangular/square concrete columns wrapped with FRP in order to simulate the compressive behavior under concentric loading. Adopting cohesion and internal friction values of Drucker-Prager criterion from a previous study of the authors, emphasis is placed on both the determination of confining stress and the lateral-to-axial strain relation. Thirty three small and large scale specimens. including slender columns, tested by four different researchers are numerically analyzed for this crucial relation between the behavior of concrete and composite jacket. The distribution of confining stresses at the mid-height plane of the columns is evaluated on the basis of analysis. Confining stresses obtained from nonlinear finite element analyses (NLFEA) are also compared with both uniform confining pressure for cylindrical specimens and effective stresses calculated by using a shape factor recommended by ACI 440.2R-02. Comparisons show that the confining pressure values obtained from the assumption of uniform stress distribution over the surface of concrete core are consistent with the maximum lateral pressure at the corners while effective lateral pressure can be considered as minimum confining stresses on flat sides. (C) 2008 Elsevier Ltd. All rights reserved.
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    A practical approach for modeling FRP wrapped concrete columns
    (Elsevier Sci Ltd, 2009) Köksal, Hasan Orhun; Doran, Bilge; Turğay, Tahsin
    Fiber reinforced polymers (FRP) have gained rapid popularity in recent years as one of the strengthening techniques of structural concrete elements. Particularly, increase in the use of FRP composite materials for strengthening and retrofitting of reinforced concrete columns has urged the development of several approaches to determine their compressive strength. Although substantial experimental and analytical researches have been conducted to model and simulate the response of concrete confined with FRP jackets under concentric loading. there is still an apparent need for the detail analyses and efficient numerical models to further understand the stress-strain behavior and failure mechanisms of the confined concrete. In order to predict the compressive behavior of concrete even under high confinement pressures, this paper introduces new relations for calculation of the cohesion parameter of Drucker-Prager criterion in terms of cylindrical compressive strength only. These relations are developed from a parametric study of a large number of nonlinear finite element analyses (NLFFEA) of FRP wrapped concrete columns to account for the axial load level and the shape of the stress-strain curve. Incorporating a realistic one-parameter failure criterion of concrete, the failure cone of Drucker-Prager model is enforced to approximate and coincide with the whole compressive meridian of the criterion up to the analytically predicted point of the ultimate hydrostatic pressure in the analyses. Based on this failure cone, mainly seven different relations corresponding to the various levels of lateral pressure are proposed for the compressive meridian and the cohesion while keeping the internal friction angle as a constant value of 33 degrees. The proposed approach is shown to fit quite well the experimental results of 42 specimens tested by eight different researchers, for various square and rectangular cross-sections under concentric loading. (C) 2008 Elsevier Ltd. All rights reserved.