Andreas Taras

Local and global buckling of box columns made of high strength steel

The international increased competition of the steel industry requires modern construction solutions to save material and thus labour costs. Combined with architectural trends to lightweight structures, the development of high strength steels, with steel grades exceeding a yield strength of 460 MPa, is a logical solution. These materials are not fully covered yet in the European Design Standards, especially in regard to stability issues. As the difference of high strength steel and mild steels lies mainly in the deviating plastic behaviour, the focus of the study at hand lies firstly on local buckling, where local plastifications might influence the ultimate load of a structural element. In the respective Eurocode 3-1-5, the resistance curve used to represent the reduction factor of plated elements due to local failure is based on the so-called Winter-curve, which was derived by George Winter using a semi-empirical approach in 1947. This design curve reproduces the mean reduction values achieved in the experiments conducted by Winter and other researchers at that time. More recent tests on welded, squared box sections from steel grades S275 up to S960, and also the 34 experiments conducted within this study, showed the un-conservativeness of the Winter-curve with increasing local slenderness, independently of steel grade. The evaluation of test results revealed a considerable scatter, which is partly attributed to the stability failure mode, but could be also found to be originated from lack of information concerning unintended eccentricities in the experimental test setup. Therefore, the data set was divided into two sets, one consisting of all data, and one including only tests where these information were available and thus had reduced scatter. A new reduction curve was derived, which represents the mean function for all test data available in the whole slenderness range. To ascertain a defined level of failure probability, the mandatory safety concept of EN 1990 was applied on the existing and the newly proposed resistance definition, to derive the corresponding safety factor γ M . The procedure is thereby based on assumptions regarding the material and geometric property-specific Coefficients of Variation (CoVs). Standard values often given in literature, although correct for their specific applications, cannot be simply adjusted when including high strength steel material. In the study at hand, it was thus focused on weighting the impact of different properties like e.g. yield strength, and show where the safety concept has to be adapted. Different approaches and variations are presented and discussed in this work, leading to scientifically justified γ M -values. After clarification of the local buckling resistance, the interaction with global buckling was focused on. While there are precise analytical models available to assess the calculation of critical and ultimate load for global and local buckling separately, the interaction of both modes prove to be difficult as membrane effects and imperfections are of major impact. In an experimental programme, 13 tests on columns with high b/t-ratio were carried out on squared, welded box sections made of S500 and S960 steel material, varying the global slenderness. The experiments were re-calculated with the Finite-Element-programme ANSYS. The calibrated numerical model was subsequently used for parametric studies. iii In these studies it was aimed to cover a range of both local and global slenderness up to 2 and load patterns including bending. The interaction design check in Eurocode 3-1-1 is based on the Ayrton-Perry format using linear-elastic assumptions, which allow for separate assessment of local effects, axial forces and bending. Several additional correction factors are thereby to calculate. The study at hand provides an analytic approach to determine a slenderness depending reduction factor under an early combination of axial forces and bending such that additional factors can be neglected. This approach, subsequently denoted as “generalised slenderness approach (gs)” is still orientated on the Ayrton-Perry format. Local effects are not included by omitting parts of the cross-section in the gs-approach, but by adding an additional equivalent global imperfection. The magnitude of this imperfection is based on the earlier derived new reduction curve. Design charts for box sections were developed to ease the application.

Numerical methods for the fatigue assessment of welded joints: influence of misalignment and geometric weld imperfections

AbstractThe fatigue design life of welded joints in steel structures is increasingly assessed by using numerical models and methods, such as the structural (hot-spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S-N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically “imperfect” welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S-N curves are known to cover imperfections up to the common tolerance classes for fatigue-prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented...