Plate buckling in Movable Scaffolding Systems

Abstract

The main objective of the thesis is to determine the capacity against buckling of the main girder web at a launching stage. As the webs and the flanges of the main girder must defined as slender structural elements (class 4) instability will govern design resistance. The first part of the thesis will give the reader an understanding of the boundary conditions and phenomenon that will determine the loading conditions this steel girder is subjected to. All these observations will then be implemented to a nonlinear buckling analysis in ANSYS Workbench 12.1. This particular analysis will be executed in order to satisfy design guidelines given in \emph{NS-EN 1993 1-5: Plated structural elements}\cite{1} regarding material properties, imperfections and boundary conditions for this buckling phenomenon. With this type of analysis we can evaluate the post critical resistance of a plate. The geometry and loads which will be used as input in ANSYS will be retrieved from a reference project which is a MSS-system used to build a bridge in Portugal in 2004 , project identification: 25111 - VIZELA CALVOS. After obtaining results from this analysis the main girder`s web panel will collapse at loading from the launching wagon equal to 3879 kN. This particular result was satisfying as it demonstrated that the web panel had enough capacity and that the stiffening arrangement proved to be efficient as it provoked a local buckling mode in a subpanel. The utilization of the web against instability was rather satisfying at this project. A request from Strukturas regarding additional nonlinear analysis with ANSYS obtained with a new stiffening arrangement was as well executed. This will provide useful information regarding capacity at other MSS projects. The alternative stiffening arrangement included a transverse stiffener rigid for shear buckling. With this configuration the collapse load was found to be 5832 kN. Including a transverse stiffener will provide this web with additional stiffness against instability. The main reason for this is that the stiffener will provide the web panel with additional restraint and the critical buckling modes must be obtained with unsymmetrical loading. This is as well evident in design codes in NS-EN 1993 1-5 where plate panels can be treated separately if rigid transverse stiffeners are implemented. These rigid transverse stiffeners will decrease the buckling length and subsequently increase resistance. When these nonlinear analysis were executed the next objective of the thesis is to study the design formulas in NS-EN 1993 1-5. It soon became evident that there were some important issues regarding the calculation of resistance due to patch loading. Concentrated transverse loading applied perpendicular to the flange in the plane of the web is referred to as patch loading in Eurocode 3 and would in this case be the support reaction from the launching wagon. The inner web of the main girder has a special configuration with transverse stiffener c/c 750mm to add stiffness in these local regions at the flange and adjacent part of the web. As the ANSYS analysis state that the most critical mode is a local buckling mode between longitudinal stiffeners, the design formulas does not differ between failure modes. Bearing all this in mind that the transverse stiffeners configuration will contribute to spread the load at a larger area on the critical subpanel it`s safe to conclude that these design formulas must be discarded. The main girder`s web may as well suffer from instability due the influence of direct stresses and shear buckling. Their effect have been examined and it turns out that they will not govern the web resistance. These results were not unexpected because the main girder will experience a larger bending moment and support reactions at the concreting stage due to self weight of the concrete. As a consequence of these discoveries the nonlinear analysis in ANSYS will reflect that the patch loading will cause the most confusion regarding ULS capacity of the web panel.