Spiral-welded steel tubes with diameter to wall thickness ratios between 60 and 140 are often employed in combined wall systems with local buckling as governing failure mode. In a European research project called Combitube, the structural behavior of spiral-welded steel pipes has been investigated because the current design rules in Eurocode 3 are not capable to obtain a good estimate of the real strength and deformation capacity. An analytical model has been developed that enables more economic designs with a better balanced safety level. Comparisons are made with full scale test results and numerical simulations. The results of this research project are also relevant for strain-based pipeline design, where more emphasis is on the critical curvature.
Spiral-welded steel tubes are often employed in combined wall systems, e.g. for quay wall systems as indicated in Fig. 1. In these applications, the diameter to wall thickness ratio (D/t) ranges between 60 and 140. The main loading is bending in combination with normal force, earth loads and tensile or compressive loads from the infill sheeting. The governing failure mode is local buckling in the inelastic range of the steel.
The current design rules in Eurocode 3 (EN 1993-5 and EN 1993-1-6) follow a design approach based on stress resultants, rather than strains and deformations. It has been shown that these current design rules are not well suited for tubes in combined wall systems. They are not capable to obtain a good estimate of the real strength and deformation capacity and lead to uneconomic designs.
In a European research project (Research Fund for Coal and Steel - RFCS), the structural behavior of spiral-welded pipes for application in combined walls has been investigated. Full-scale four-point bending tests and extensive numerical parametric studies have been performed Van Es (2014 and 2016) and Vasilikis (2016).
