DOProC-based reliability assessment of steel structures exposed to fatigue

Abstract

Summary Attention to the fatigue cracks in steel structures and bridges has been paid for a long time. In spite of efforts to eliminate the creation and propagation of fatigue cracks throughout the designed service life, cracks are still revealed during inspections. Fatigue crack damage depends on a number of stress range cycles. This is a time factor in the course of reliability for the entire designed service life. The failure rate increases in the course of time and the reliability decreased. If possible propagation of the fatigue crack is included into the failure rate, it is necessary to investigate into the fatigue crack and define the maximum acceptable degradation. Three sizes are important for the characteristics of the propagation of fatigue cracks. These are the initial size, detectable size and acceptable size. The theoretical model of fatigue crack progression is based on a linear fracture mechanics. A tension flange has been chosen for applications of the theoretical solution suggested in the studies. Depending on location of an initial crack, the crack may propagate from the edge or from the surface. Regarding the frequency, weight and stress concentration, those locations rank among those with the major hazard of fatigue cracks appearing in the steel structures and bridges. When determining the required degree of reliability, it is possible to specify the time of the first inspection of the construction which will focus on the fatigue damage. Using a conditional probability, times for subsequent inspections can be determined. For probabilistic calculation of fatigue crack progression was used the original and new probabilistic methods – the D irect O ptimized Pro babilistic C alculation (“DOProC”), which uses a purely numerical approach without any simulation techniques. This provides more accurate solutions to probabilistic tasks, and, in some cases, to considerably faster completion of computations. FCProbCalc code has been developed using the aforementioned techniques. By means of FCProbCalc (which stands for F atigue C rack Prob ability Calc ulation), it is possible to carry out the probabilistic modelling of propagation of fatigue cracks in a user friendly environment and to propose a system of regular inspections which should reveal damage to the structure, using the described computational procedures.