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• A new combination rule using virtually effective crack is proposed for coplanar surface cracks. • The shape development of twin surface cracks is simulated by numerical analysis based on Paris law. • The multiple-crack growth is sensitive to both the initial crack size and relative spacing. Multiple flaws are widely observed on the surface of corroded steel bridge wires, and the interaction between flaws can increase the stress intensity factor (SIF) to drive cracking. But the flaw combination criterion provided for plates in codes is not straight-ford to evaluate interacting surface cracks in steel wires. A model containing twin co-planar semi-elliptical surface cracks throughout a circular cross-section is thus established to investigate crack propagation of steel wires under axially tension in terms of initial crack shape, size and relative spacing. A new combination rule using a virtually effective crack is then proposed for interacting co-planar surface cracks based on the SIF amplification effect, which is quantified as a function of the initial crack aspect ratio, crack depth and relative spacing. The shape development of twin surface cracks is also simulated by numerical analysis based on the Paris law. The results show that the multiple-crack growth is sensitive to both the initial crack size and relative spacing, and the interaction should be considered for the neighbouring cracks when their relative spacing is within the crack depth range (s ≤ a). The comparison of the predicted fatigue lives between twin cracks and corresponding effective crack confirms the feasibility of the presented combination criteria. [ABSTRACT FROM AUTHOR]