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001522
2018-06-06

FEM Modeling Approaches of Rigid Connections

To obtain significant results about the choice of the selected modeling approach, it may be helpful first to generate a structure that can be compared with the classical methods.

In a previous article, the design resistance of a rigid end plate connection was calculated with the formula according to EN 1993-1-8 [1]. In this example, the design resistance of the end plate should be loaded by means of an FEM modeling approach with the determined design resistance of 324.95 kNm.

Surface Model

The HEB 400 beam was entered with a length of 500 mm and divided into surfaces. Assuming that the end plate has a rigid behavior under the solid bolt heads, a rigid surface with a washer diameter of 44 mm was generated. To simulate the elasticity of the bolts under tensile stress, they were defined with a diameter of 24 mm and a length of 29 mm (= thickness of end plate + thickness of washer). To ensure that the load is introduced equally into the beam, a rigid end plate was used at the load application location.

Since a rigid support in the z-axis of the end plate can lead to convergence problems, a surface elastic foundation with 2 ∙ 108 kN/m³ was assumed, and at the locations where no deformation can take place due to the symmetry conditions of a beam joint or where the largest contact forces are expected, a line support was additionally designed. Both supports should fail in case of positive support reaction. Since EN 1993-1-8 [1] performs a plastic design, the "Isotropic Plastic 2D/3D" material model with the yielding criterion 23.5 kN/cm² was selected. As soon as the yield criterion is reached in the FE element, it cannot absorb further forces and a stress redistribution takes place.

Results

The failure criterion "Bolt failure with yielding of the flange" of the manual calculation can be validated.

When using the nonlinear material model "Isotropic Plastic 2D/3D", the area of the elements in the plastic state has to be evaluated. It has to be considered that the elements in plastic state correspond to the remaining damage. The FEM model shows plastic areas that are too large. This means that the connection is overloaded when considering it using the surface model. The determined bolt tension force with 279 and 288 kN also exceeds the allowable limit tension force.

Similarly sized plastic areas were determined with additional modeling, where the beam, end plate, bolts, and welds have been displayed with solids and the force introduction from the beam to the end plate takes place across the welds (=larger load surface). 


Author

Mr. Fröhlich provides technical support to our customers and is responsible for development in the area of reinforced concrete structures.

Links
References
  1. Eurocode 3: Bemessung und Konstruktion von Stahlbauten − Teil 1-1: Allgemeine Bemessungsregeln und Regeln für den Hochbau. Beuth Verlag GmbH, Berlin, 2010
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