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A new semi-analytical procedure is derived for the determination of buckling of the reinforced concrete column exposed to fire. The fire analysis is performed in three separate steps, of which the time development of temperatures in the fire compartment is performed first, followed by the coupled heat and moisture transfer analysis and, finally, by the mechanical analysis. A particular emphasis has been given to the critical buckling time and the remaining critical buckling load at a selected time. For this purpose, a parametric study has been performed by which the influence of different geometric parameters on the buckling load capacity of reinforced concrete columns has been assessed. The results of this study show that the load-carrying capacity of the column reduces significantly with the increasing time of fire exposure and the column slenderness. Moreover, the initial mechanical load has a small, although not negligible effect on the buckling load capacity. [ABSTRACT FROM AUTHOR]

A three-step model for the performance-based numerical simulations of the fire response of steel–RC two-layered beam-like composite structures is presented and validated. The first step consists of the determination of the evolution of temperatures in the structure׳s surroundings. Moisture and the heat transfer through the RC layer and the conduction of heat over the steel layer are obtained in the second step. In concrete, the transfer of water vapour, dry air, and free water is discussed as well as the evaporation and liquefaction phenomena and the dehydration of concrete and its thermal and mechanical degradation. Within the framework of the third step, a geometrically and materially non-linear mechanical response of the structure is proposed accounting for interlayer slips and uplifts as well as for various material-related phenomena such as the material hardening/softening and creep. The governing equations are solved numerically. An efficient, novel strain-based finite element formulation is introduced for the mechanical analysis. Due to its generality and consideration of several different possible non-linear material, geometrical, and interlayer contact phenomena and their couplings the model can be of a use to a broader fire science community for exploring the impact of different physical parameters on the results of the addressed numerical simulations, thereby providing directions for further research. In the paper a case of such a study is also demonstrated exploring the contribution of the steel sheet and the flexibility of the interlayer connection of a standard trapezoidal steel–RC slab to its ultimate fire resistance. A reasonable contribution of the sheet is proved if the stiffness ratio between the integrated and the external tensile reinforcement of the RC plate is low provided that the contact connection is sufficiently stiff. [ABSTRACT FROM AUTHOR]

Abstract: Failure of concrete under compression results in a localized region of softened material subject to high strain. The paper attempts to find out the error of the numerical method for the mechanical analysis of planar reinforced-concrete beams subject to temperature increase simulating typical fire conditions at high temperatures, if the strain softening in concrete is considered, while employing continuous finite elements. The results are confronted with the formulation using constant-strain crack band elements in the localized regions combined with the continuous elements everywhere else. The width of the crack band is, in principle, temperature and stress–state dependent. In the paper we assume the crack band elements of constant width throughout the analysis, which may be a valid assumption at high temperatures, as indicated by some data given in literature. Results of the analyses of a two-story, two-bay reinforced concrete frame, designed strictly in accordance with the European standards, and subject to self weight, dead and service loads and, additionally, to the increasing fire-like thermal load are presented. The use of solely continuous finite elements, yet accounting for strain softening of material wherever it occurs, yields spatially oscillating results for strains around the localized region. This sometimes yields a totally false result or loss of convergence of the global Newton method. The width of the localized band tends to zero with the growing density of the finite-element mesh, with the results being highly mesh sensitive. The finite-element meshes where constant-strain crack band elements in the localized regions are combined with the continuous elements everywhere else yield, on the other hand, a computationally stable and accurate post-instability response for a wide range of crack-band element widths. Its critical point is, however, obtaining a reliable estimate of the width of the crack band element at high temperatures. This requires extensive experimental testing with hot concrete in compression. [Copyright &y& Elsevier]

Abstract: The way the non-linear constitutive equations in the spatial beam formulations are solved, influences the rate of convergence and the computational cost. Three different approaches are studied: (i) the direct global approach, where the constitutive equations are taken to be the iterative part of the global governing equations, (ii) the local (or indirect global) approach, where the constitutive equations are solved separately in each step of the global iteration, and (iii) the partly reduced approach, which is the combination of (i) and (ii). The approaches are compared with regard to the number of global iterations and the total number of floating point operations. The direct global approach is found to be the best choice. [Copyright &y& Elsevier]

Abstract: A new finite element formulation is presented for the non-linear analysis of two-layer composite planar frames with an interlayer slip. For each layer the geometrically non-linear Reissner’s beam theory is assumed. A small slip between the layers is considered. Non-linear, time-dependent constitutive models for the layers, and a non-linear relationship between the slip and the shear stress at the interface are accounted for. As only a few finite elements are needed to describe a beam or column of a frame with great precision, the new finite element formulation is prefectly suited for practical calculations. [Copyright &y& Elsevier]

Abstract: This paper presents a mathematical model for investigating the buckling of geometrically perfect, materially and kinematically non-linear, locally weakened reinforced concrete columns. The present analytical model is based on the linearized stability theory and is capable of predicting exact buckling loads. Based on a systematic parametric analysis, the effects of the weakness location, its magnitude, the number of weaknesses and the material non-linearity of concrete and reinforcing steel on buckling loads have been investigated. The results are given in terms of graphs showing in detail that the material non-linearity of concrete and reinforcing steel and the local weaknesses have a significant influence on the buckling load. [Copyright &y& Elsevier]