Fatigue failures are often encountered in steel structures under heavy cyclic loadings. This paper presents metal fatigue problems in structural engineering using outcomes of recent advancements in numerical qualitative reasoning. Qualitative reasoning provides an effective and sound technique for solving complex and uncertain scenarios, regardless of the uncertainty or linearity of the design parameters and their constraints. This paper introduces the algorithms behind a software platform, built upon numerical qualitative reasoning for engineering applications. The software expresses the results of the analysis in variable ranges and diagrams showing a two-dimensional design space. The capability of representing design parameters and outcomes in solution spaces provides a practical way for engineers to leverage their existing knowledge and experience. Case studies in metal fatigue design are given to reflect on the capability of qualitative reasoning in engineering applications.