Department of Civil Engineering

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Browsing Department of Civil Engineering by Author "Albertyn, Heindrich Louw"

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    The effect of moment-rotation joint behaviour on the displacements of portal frames
    (Stellenbosch : Stellenbosch University, 2011-12) Albertyn, Heindrich Louw; Dunaiski, P. E.; Haas, Trevor; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Higher grade steels are being rolled in South Africa by suppliers and results in structural members having an increased axial and bending moment capacity due to an increased yield stress. Structural elements used in designs are stronger and therefore lighter sections with sufficient axial and bending moment capacity are used. Displacements of structural elements are calculated using the stiffness and Young’s modulus of a profile. These values are not affected by the increased yield stress in higher steel grades and therefore have a negative effect on the displacements of the structure. The potential of these higher grade structural elements are not utilized through serviceability limit state criteria, since the displacement determination does not account for the increased capacities of higher grade steels, but only stiffness and elasticity of the members. Structural analysis of portal frames does not account for the real behaviour of steel connections and column bases. It is assumed that connections and bases are either fully rigid or perfectly pinned. This assumption is used in the analysis and design of the structure. Although it is assumed that connections and bases are either rigid or pinned, the real behaviour is in between these two extremes. Rigid connections exhibit a certain flexibility under loading whereas pinned bases provide a certain restraint under loading. The real behaviour of connections and bases are referred to as the moment-rotation behaviour of the connection. For a certain applied moment to the connection or base, the connection exhibits a certain rotation. The focus of this study is placed on the accuracy and feasibility of modelling the real behaviour of connections and bases in a structural analysis of a portal frame. A connection stiffness is determined from the connection’s moment-rotation behaviour, and is assigned to a rotational spring of zero length in a structural analysis. An experimental investigation was conducted to obtain the real displacement data of a portal frame subject to loads for two different support conditions, i.e. a perfect hinge and grouted-support. A perfect hinge support was used to isolate the moment-rotation response of the ridge and eaves connection. The experimental results were used to compared to the results obtained from a structural analysis to determine the accuracy of the numerical results. A real design case was investigated with load combinations imposed on the frame in accordance with SANS 10160:2011. Three methods of modelling connections and bases in an analysis were considered. Firstly modelling connections as rigid and bases as pinned, secondly modelling connections as linear rotational springs and bases as pinned. Lastly was to model connections as linear rotational springs and bases as non-linear rotational springs. The outcome of the research was that more accurate displacements of a portal frame could be obtained by modelling the real behaviour of rigid connections as rotational springs, but this is not the case with grouted column bases. It is thus not feasible to model the real behaviour of connections and bases in a structural analysis as the current method of modelling connections as rigid and bases as pinned provides reliable and accurate displacement results.