The use of high-strength steels (HSS, 460 < f y ≤ 700 MPa) and particularly very high-strength steels (VHSS, f y > 700 MPa) is limited in steel structures in civil and offshore engineering. This is due to a lack of experience in structures made of these materials and a lack of understanding of the structural performance of HSS and VHSS. The main questions that are unanswered concern the topics of static strength, fracture toughness, fatigue, material (in) homogeneity and manufacturing. With regard to the static strength, it is unclear as to whether requirements currently stated in the Eurocodes for the yield-to-tensile strength ratio (f y /f u ) as well as the Von Mises yield criterion are applicable to structural designs in HSS and VHSS. With regard to fracture toughness, it is unclear as to whether the crack arrest capability of HSS and VHSS is currently sufficiently covered in Eurocode requirements. Furthermore, Eurocode requirements for maximum plate thickness are quite restrictive if they are extrapolated to HSS and VHSS. Other questions relate to the homogeneity of the material. Are HSS and VHSS sufficiently resistant to lamellar tearing and is material testing in different orientations necessary? This study gives an overview of the relevant questions, outlines the requirements in the current Eurocode system, presents answers to these questions where possible and provides a recommended research focus for the coming years.

Steel tubes can be applied for many applications, structural as well as in pipelines. They may be loaded by combinations of internal or external pressure, soil load, bending moment and normal force. Important failure modes are local buckling, severe ovalisation (flattening of the cross section) and rupture of the pipe wall at local buckles or welds. Within the framework of a European research project called Combitube sponsored by RFCS (Research Fund for Coals and Steel), the structural behaviour of spiral-welded tubes for application in combined walls has been investigated. The main loads are bending moment, normal force, soil loads and loads from the infill sheeting. Analytical models were developed to predict the elastic-plastic behaviour under these loads, i.e. the bending moment curvature diagram, the curvature ovalisation diagram and curvature strain diagrams. The analytical models as presented in this paper are an extension of previous analytical models developed for pipelines, now including more parameters that influence the load deformation behaviour, in particular local buckling. The new rules are intended for the new version of EN1993-5 on Piling. They are validated with the results of full scale tests and extensive parameter studies.