The effect of moment-rotation joint behaviour on the displacements of portal frames

[摘要] ENGLISH ABSTRACT: Higher grade steels are being rolled in South Africa by suppliers and results in structural members having anincreased axial and bending moment capacity due to an increased yield stress. Structural elements used indesigns 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. Thesevalues are not affected by the increased yield stress in higher steel grades and therefore have a negative effecton the displacements of the structure. The potential of these higher grade structural elements are not utilizedthrough serviceability limit state criteria, since the displacement determination does not account for the increasedcapacities 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 columnbases. It is assumed that connections and bases are either fully rigid or perfectly pinned. This assumptionis used in the analysis and design of the structure. Although it is assumed that connections and bases are eitherrigid or pinned, the real behaviour is in between these two extremes. Rigid connections exhibit a certain flexibilityunder loading whereas pinned bases provide a certain restraint under loading. The real behaviour of connectionsand bases are referred to as the moment-rotation behaviour of the connection. For a certain applied moment tothe 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 connectionsand bases in a structural analysis of a portal frame. A connection stiffness is determined from theconnection's moment-rotation behaviour, and is assigned to a rotational spring of zero length in a structuralanalysis. An experimental investigation was conducted to obtain the real displacement data of a portal framesubject to loads for two different support conditions, i.e. a perfect hinge and grouted-support. A perfect hingesupport was used to isolate the moment-rotation response of the ridge and eaves connection. The experimentalresults were used to compared to the results obtained from a structural analysis to determine the accuracy of thenumerical results.A real design case was investigated with load combinations imposed on the frame in accordance withSANS 10160:2011.Three methods of modelling connections and bases in an analysis were considered. Firstlymodelling connections as rigid and bases as pinned, secondly modelling connections as linear rotational springs andbases as pinned. Lastly was to model connections as linear rotational springs and bases as non-linear rotationalsprings. The outcome of the research was that more accurate displacements of a portal frame could be obtainedby modelling the real behaviour of rigid connections as rotational springs, but this is not the case with groutedcolumn bases. It is thus not feasible to model the real behaviour of connections and bases in a structural analysisas the current method of modelling connections as rigid and bases as pinned provides reliable and accuratedisplacement results.

[发布机构] Stellenbosch University