This project focuses on new design methods, modelling, and assessments of safe and reliable steel structures subjected to degradation processes attack; especially by very quick and dangerous brittle fracture, fatigue appearing at stress alternating in time, or large plastic deformation caused by creep. Since to predict the moment of the final fracture instability of steels structure, e.g. its lifetime, is very complicated and bears a high degree of risk, the structure must be monitored during service. Then new special testing methods, namely those for which only a small amount of material is needed to reveal the state of structure and properties, must be developed for use immediately at the instance of testing. Using these small-scale test results and models of microstructure damage, the time at which the reliability of the structure is becoming diminished can be successfully estimated. The outlined new assessment in the development of the methods of quantitative fractography will help to formulate microstructural conditions of materials with high fracture resistance. Thereby the strength and structural potential of the material could be completely exploited. Application of this method can also provide suggestions on how to control microstructure and local processes of damage to gain high resistance of material against failure. Besides structural design and related calculations of load capacities, the project also is also concerned with the influences of the lack of structural homogeneity, and effects of individual mechanical properties that have been used for design of engineering constructions. All of this implies the possibility of better prediction of the construction boundary conditions, utilisation of modern probability methods for designing purposes, and also employment of new numerical methods for operational simulations of the construction behaviour.