In order to meet the increasing demands for sustainable & economic constructions, the European steel industry sees the increased use of more thin-walled sections and/or higher-strength steel grades as a main industrial goal. However, this leads to a number of scientific and engineering challenges, which stem from greatly increased relevance instability phenomena, as well as from the lack of appropriate design rules for slender, high-strength hollow sections. This project intends to address these points: 1) "Direct" design rules for the cross-sectional strength of hollow sections will be developed, on the basis of the "Overall Interaction Concept". The method will lead to a continuous strength function for the class 1 to 4 range and take advantage of beneficial effects (mutual restraint, real stress state, strain hardening, …). For CHS and EHS in particular, the new method will fill the current gap in design rules for class 3 and 4 sections. 2) The method will be expanded for the applications in beam-columns and interactive L-G buckling. 3) The elastic buckling behaviour of hollow sections will be studied in a systematic, (semi-)analytical way using the Generalized Beam Theory. 4) The safety level of the new design rules will be ascertained on the basis of the methodology of EN 1990, making use of the test data provided in the project (physical and numerical tests) as well as production data regarding material properties and geometric tolerances provided by the industrial partners. 5) The fields of application and of product improvement will be studied by R&D and engineering representatives of major steel industry stakeholders. Case-studies of structures built iusing traditional design rules will be re-assessed to determine the economic and technical advantages of the new design rules and developments in steel grades, shapes, and wall thicknesses. 6) Specific design guidelines and tools (software) will be developed and made available to the industry.