Field stress/displacement monitoring and fatigue reliability assessment of retrofitted steel bridge details

This paper presents the field stress and displacement measurements in controlled load tests and long-term monitoring to investigate the performance of two retrofitted details, which were used to alleviate the cracking problems of an existing steel bridge. Effectiveness of the retrofit schemes is demonstrated by comparing the displacements of the retrofitted details with those of the non-retrofitted ones. Based on the field monitored data and the AASHTO Specifications, time-dependent fatigue reliability assessment is made, in which S–N curves are used to consider the influence of variable amplitude loading on the propagation of initial defects and fatigue damage. Effective stress ranges are derived from daily stress range histograms, and lognormal probability density functions (PDFs) are used to model the uncertainties in the effective stress range with the aid of regression analyses. It is observed that stress ranges in the instrumented details are all below the corresponding constant amplitude fatigue limits. Consequently, high fatigue reliabilities of these retrofitted details are expected. However, the reliability indices decrease significantly as time goes on, and this deterioration process may be accelerated with the increase in traffic volume. The analysis results provide references to bridges with similar fatigue cracking problems.