Modern bridge construction all over Europe in the last decades, has highlighted the importance of steel solutions for the design of bridges. Steel bridges have become slender and lighter with the generalisation of welded connections. These features increased the relevance of fatigue phenomena, and as a result, fatigue design has become a leading ultimate limit state verification. This research is focused on the fatigue behavior of welded joints in two domains: variable amplitude under bridge loadings and multiaxial interaction between normal and shear stress. For the assessment of weld details under variable amplitude loads, fatigue tests have been conducted under constant and variable amplitude in 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 Constant Amplitude Fatigue Limit (CAFL). A two-step model with initiation and propagation was used to estimate the experimental fatigue lives, using a local strain approach for the initiation life and fracture mechanics for long crack propagation. The model was then used in a probabilistic Monte Carlo framework to include variability on the main parameters and establish S-N curves for Constant and Variable amplitude fatigue. The results allowed to correlate the load spectra sequence with the shape of the S-N curves, namely the 2nd slope value below the Constant Amplitude Fatigue Limit. For the multiaxial interaction under shear and normal stress, a large scale setup was built that allowed tests on transversal attachments to be performed under proportional and non-proportional loads. Significant fatigue life reduction was observed under proportional multiaxial loads. The multiaxial experimental results were successfully described by a local notch approach with principal stress. A probabilistic traffic generation model was finally established for typical highway traffic loads from bridges in Europe. The model allowed to characterise the stress spectra shapes that influence the 2nd slope value of the variable amplitude S-N curves. The results of the traffic generation model were used to calibrate partial safety factors for the design code format safety checks, both for uniaxial and multiaxial fatigue.