Welded details in steel bridges are subjected to fatigue lives up to 200.10(6) cycles due to traffic loads. While the majority of these stress ranges are below the Constant Amplitude Fatigue Limit (CAFL), some few high stress cycles can trigger the start of fatigue damage and lead the remaining load spectra to become damaging. The behaviour under spectra loading is thus of major importance for the fatigue design of steel bridges. This paper focuses on the fatigue behaviour of welded joints under variable amplitude loads. Fatigue tests have been conducted under constant and variable amplitudes on a typical bridge detail. Experimental crack growth curves were obtained using the Alternative Current Potential Drop method (ACPD), which showed the detrimental effect of stress ranges below the conventional CAFL. A two-step model with initiation-propagation was established to estimate the experimental fatigue lives, using a local strain approach for the initiation life and fracture mechanics for crack propagation. The model was implemented in a probabilistic Monte Carlo framework to include variability on the main parameters and establish S-N curves for Constant and Variable amplitude. The results of the simulations show that load spectra shape can be correlated with the S-N curves, namely the 2nd slope value below the CAFL. (C) 2017 Elsevier Ltd. All rights reserved.