Sudath C. Siriwardane

The introduction of high-strength materials has made fatigue damage a critical limit state in modern structural design. Structures subjected to frequent repetitive or dynamic loading are particularly vulnerable to fatigue failure. In this study, an offshore structure subjected to cyclic wave loading during its service life is evaluated, focusing on the reliability of multiplanar KT-joints under wave loading uncertainty. Using the Hasofer–Lind approach, the reliability index (β) and probability of failure (Pf) were analysed across various operational scenarios. The results indicate that while the structure satisfies basic reliability criteria under low repetitive loading, these requirements are not fulfilled as the number of cycles increases. Specifically, the target reliability index of 3.8, defined by Eurocode 1990, was achieved in many low-cycle scenarios; however, this threshold was consistently violated under high-frequency loading. The study concludes that since the probability of failure increases significantly under high repetitive loads, rigorous maintenance, reinforcement, and structural strengthening protocols must be implemented – especially towards the end of the structure’s service life – to ensure the safety of both the asset and the personnel.

Purpose: This study aims to develop an analytical approach to accurately predict the lateral-torsional buckling (LTB) capacity of beams with varying cross-sections (VCS) such as beams with web openings (e.g castellated and cellular beams) and beams with patch corrosion. Existing expressions for elastic critical moments are limited to beams with uniform cross-sections and do not capture the influence of geometric variations. Design/methodology/approach: An explicit analytical formula for the elastic critical moment is derived using the energy (Rayleigh–Ritz) method, equating the strain energy during LTB to the external work done by loads. The derivation covers simply supported and fixed-ended beams subjected to end moments and extends to other loading conditions through moment modification factors. The analytical results are validated against existing experimental and finite element (FE) data for beams with web openings. Hence the corresponding design LTB moment capacities are established. Findings: The analytical predictions show very good agreement with experimental and FE results for beams with web openings. The method provides slightly conservative predictions when distortional buckling interactions are present. The results confirm that the proposed formula effectively captures the physical LTB behaviour in beams with VCS. Originality/value: This paper proposes, for the first time, an explicit energy-based analytical formulation for the elastic critical buckling moment of beams with VCS such as beams with web openings and beams with patch corrosion. The approach bridges theoretical understanding and design practice, providing a computationally efficient and physically consistent method suitable for assessing the structural integrity and reuse of existing steel structures.

Purpose: This study aims to develop a new multiaxial stress state analysis. Design/methodology/approach: The current two-part study proposes a new model to assess the interaction between shear-normal stresses in multiaxial fatigue life. The objective of these studies is to find a relationship between the fatigue behavior of shear and normal stresses, defined as the shear-normal stresses interaction and utilize it in fatigue life prediction of proportional and non-proportional loadings. In Part 1, a new multiaxial stress analysis is developed. Several proportional and non-proportional loadings are investigated in a parametric study. Through this investigation, five load parameters are identified. The proposed stress analysis is also extended to the high-strain loadings, including plasticity. The effects of the proposed load parameters on multiaxial fatigue are experimentally investigated and discussed. In Part 2, a new multiaxial fatigue model is proposed based on the load parameters introduced in Part 1. The validity of the proposed model is studied and compared with three well-known models. Findings: The discussion of the experimental program reveals that the introduced load parameters are essential in multiaxial fatigue assessment. Research limitations/implications: None known for this paper. Practical implications: The introduced stress analysis method is novel in the field of multiaxial fatigue. This analysis method shows its ability to identify the essential multiaxial load parameters required for life assessment. Originality/value: This paper fulfils an identified need to predict fatigue life under the proportional and non-proportional multiaxial loadings.

Purpose: The study aims to propose a new multiaxial fatigue damage parameter. Design/methodology/approach: Using the proposed stress analysis method proposed in part 1 and results of the design experimental program, the proposed formula is introduced. Findings: The performance of the proposed formula shows encouraging accuracy. This performance is evaluated via the comparison of the proposed formula with other well-known ones. Research limitations/implications: The proposed formula requires fatigue input data for non-proportional loadings in the low-cycle regime to predict the fatigue life in that regime. While accessing the available data in high-cycle fatigue is sufficient for life prediction of the non-proportional loadings in the high-cycle regime. Practical implications: The paper introduces the concept of the normal-shear stresses interaction in a multiaxial fatigue field. The importance of this phenomenon is evaluated via several available data. Originality/value: This paper fulfils an identified need to predict the fatigue life under the proportional and non-proportional multiaxial loadings.

Rib–deck (RD) welded joints in orthotropic steel bridge decks are prone to different fatigue crack mechanisms. Standard fatigue design methods are inadequate for some of these mechanisms under multiaxial non-proportional loading conditions. This study presents a framework to assess fatigue damage at RD welded joints, considering the different crack mechanisms based on the equivalent structural stress method and its extension to multiaxial non-proportional fatigue, which is the path-dependent maximum stress range (PDMR) cycle counting algorithm. The method is validated for uniaxial loading by using experimental data from the literature. Additionally, non-proportional fatigue damage at RD welded joints of a suspension bridge girder is investigated under simulated random traffic loading. The analyses reveal the limitations of the nominal stress approach to account for complex stress field variations. The PDMR method, more suited to capture the stress path dependency of non-proportional fatigue damage than the hot spot and critical plane-based methods, predicts higher fatigue damage. A comprehensive fatigue test campaign of full-scale RD welded joints is necessary to better understand their fatigue behaviour under multiaxial loading. Until more experimental data are available, the PDMR method is recommended for fatigue verifications of welded RD joints as it yields safer predictions.

Purpose: Incorporating pre-existing crack in service life prediction of reinforced concrete structures subjected to corrosion is crucial for accurate assessment, realistic modelling and effective decision-making in terms of maintenance and repair strategies.Design/methodology/approach: An accelerated corrosion test was conducted by using impressed current method on cylindrical specimens with varying cover thickness and crack width. Mechanical properties of the specimens were evaluated by tensile tests.Findings: The results show that, the pre-cracked samples exhibited shorter concrete cover cracking times, particularly with wider cracks when compared to the uncracked samples. Moreover, the load-bearing capacity of the reinforcement bars decreased owing to the pre-cracks, causing structural deflection and a shortened yield plateau. However, the ductility index remained consistent across all sample types, implying that the concrete had good overall ductility. Comparing the results of the non-corroded rebar and corroded rebar samples, the maximum reduction in the yield load was 25.22%, whereas the maximum reduction in the ultimate load was 26.23%. The simple mathematical model proposed in this study provides a reliable method for predicting the chloride ion diffusion coefficient in cracked concrete of existing reinforced concrete structures.Originality/value: A simple mathematical model was proposed for evaluation of the equivalent chloride ion diffusion coefficient considering crack width, average crack spacing and crack extending lengths for cracked reinforced concrete structures, which is used to incorporate existing crack in service life prediction models.