In the present thesis, the application of fracture mechanics in design and assessment of steel structures has been studied.The first case study concerns a ring-flange connection used in wind turbine towers. The flange is rolled from straight steel profile into a complete ring. Subsequently, at the ends of the ring are welded by way of electron beam welding. This weld providing the integrity of the ring-flange was designed against fracture. To avoid potential failure by fracture and/or fatigue, the fracture mechanics approach for engineering assessments (as described in the Swedish handbook for safety assessment of structures containing defects) was implemented to predict the maximum allowable crack size. Material properties along with the design stress and crack size are the three variables that must be taken in account in order to design a structure against fracture. To this end, toughness and mechanical properties of the weld metal were experimentally determined. The results confirmed that the material behaves in a ductile manner and the electron beam welding has a positive effect on material properties. The fatigue performance of the flange was examined using the fatigue load histogram experienced by the flange in its 20years lifetime. Detailed finite element analyses of the flange connection were conducted to determine stress distribution at the most critical location along a hypothetical crack plane. The stresses obtained from the finite element analysis were used to assess the fracture stability of the flange. The fracture and fatigue assessment results confirmed that the flange in question is susceptible to fatigue failure which turns out to be the governing criterion in order to define the maximum allowable crack size. Furthermore, a parametric study was carried out to investigate the effect of parameters such as crack eccentricity, material properties and crack geometry on the integrity of the flange connection.The second case concerned the maintenance problem of roller bearings of relatively old bridges. These tend to rupture and no satisfactory explanation exists in the literature. Two broken specimens were supplied by the Swedish Transportation Authority (Trafikverket). These were first subjected to fractographic examination to determine the crack initiation location. Afterward, Charpy, compact tension, tensile and chemical composition specimens were machined out of the broken roller pieces and the respective tests were conducted. The results showed a very brittle material but no deviations from the prescribed material properties that were set forth in the 1967 certificate of application (Zulassung) of the rollers in question. To explain the observed failures a mechanical over-stressing condition was sought. Two such over-stressing sources were investigated. First the Hertz contact solution traditionally used in the design of roller bearings (both DIN 4141 and EN 1337-4) is valid for an infinite length cylinder. At the edges of the roller contact zone, stress singularities of the sort appearing in the case of a rectangular rigid body indenting an elastic half-space are likely to appear: three-dimensional finite element analyses were conducted and a 30% edge stress increase was established. As a second source of over-stressing the imperfections of welded I-beams according to EN 1090-2 were introduced into the FE model including, as before, the girder end, the roller assembly and the abutment. A wedge-type imperfection between the lower flange of the bridge girder and the lower support plate of the roller can create detrimental stress concentrations even at values no higher than 50% of the EN 1090 limits. Finally, the effect of the contact stresses in the roller was examined with a linear elastic fracture mechanics approach. It was found that surface cracks at the roller edge may become unstable in Mode-II when traversing the limits of the contact zone as this is the area of maximum shear stress: daily thermal cycles cause such cracks to go through a full plastic cycle as they are forced to swing past both edges of the contact area of the roller. This causes plastic strain accumulation at the crack tip of even small stable cracks: this mechanism causes them to grow until they become critical. A fracture assessment diagram for one of the broken rollers was computed.