Steel circular hollow sections (tubes - pipes) can be applied for many applications, structural as well as in pipelines. Important failure modes are local buckling, severe ovalisation (flattening of the cross section) or rupture of the tube wall at local buckles or welds.

Within the framework of a European research project called Combitube sponsored by RFCS (Research Fund for Coal and Steel), the structural behaviour of spiral-welded tubes for application in combined walls has been investigated. Full scale tests were performed with FEA simulations and parameter studies.

In this paper the focus is on the analytical models that were developed to predict the elastic-plastic behaviour in bending. The analytical models are an extension of previous analytical models developed for pipelines, now including more parameters that influence the load deformation behaviour, in particular local buckling. They are validated with the results of the full-scale tests and parameter studies. The models lead to more safe and cost-effective design, because the effect of all relevant parameters that have an effect on the critical strain in local buckling is quantified, leading to less scatter in comparisons with test results.


Steel tubes are used for pipelines and also for structural applications. They may be seamless or longitudinally or spirally welded. An important application of spirally welded steel tubes is in combined wall systems for quay walls as indicated in Fig. 1. Spirally welded tubes are available in diameters up to about 3000 mm. The main loading is bending in combination with normal force, earth loads and tensile or compressive loads from infill sheeting. The governing failure mode is local buckling in the inelastic range of the steel.

Test results on tubes show big scatter in the local buckling strain. Several reasons for this scatter have been identified, but not yet well quantified. In a European research project (Research Fund for Coal and Steel – RFCS) the structural behaviour of spiral-welded tubes for application in combined walls has been investigated. Full-scale four-point bending tests and extensive numerical parametric studies have been performed, e.g., Van Es (2016), Vasilikis (2016) and the RFCS Combitube final report (2016).

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