Effective design concepts for mixed connections in steel structures

In order to guarantee and promote the economical, future-oriented and resource-saving use of high strength steels, it is necessary to develop modern design concepts for mixed connections of high strength and mild carbon steel in steel structures. The aim of the research project is the development of extended steel design rules of welded connections of high strength steels. This concerns butt welds which may fail in the weld because of the formation of a soft zone and thus do not reach the full load-carrying capacity and fillet welds whose design model is only valid for steel grades up to S700. For the verification of butt welds in steel construction, EC3 contains design concepts that are inadequate: Up to now, the design of butt welds is based on verification of the section of the member with the lower steel grade. The European discussion on the consideration of the possible soft zone leads to a high global reduction factor for the resistance. For butt welds on mixed connections, no consistent verification concept can be taken from the standard at present, although mixed connections are essential for the application of high strength steels. The meanwhile adapted concept for fillet welds of high strength steels is based on research for steel grade up to S700. Investigations on fillet welds of S960 are missing so far.

The aim of the research project is to develop an adapted design concept for mixed connections which takes into account the material properties of base and filler metal and the geometry if applicable. Recommendations should be developed for SME concerning weld preparation of butt welds at mixed connections as well as the energy input during welding in order to prevent the formation of a microstructure which negatively influences the load carrying capacity. On the basis of experimental and numerical investigations, design concepts will be developed for the standard EN 1993-1-8, which allow an efficient design of mixed connections of high strength steels.

University of Stuttgart, Institute of structural design
(Prof. Dr.-Ing. Ulrike Kuhlmann, Mareike von Arnim, M. Sc.)

Ilmenau University of Technology, Department of Mechanical Engineering, Production Engineering
(Prof. Dr.-Ing. habil. Jean Pierre Bergmann)

Fraunhofer Institute for Large Structures in Production Engineering IGP
(Prof. Dr.-Ing. Wilko Flügge)

FOSTA e.V. / AiF

March 1st, 2020 – August 31st, 2023