According to the American Society of Civil Engineers (ASCE) Committee for Fatigue and Fracture Reliability, 80–90 percent of failures in steel structures are related to fatigue and fracture. Therefore, fatigue and fracture reliability is one of the most important concern for the steel bridges.
For many of the steel bridges, an unsupported web gap (see Figure 1) exists between the end of the connection plate and the flange of the girder. Prior to the late 1970’s, it was a common practice to avoid transverse welds on the flanges. Under the live load-induced forces the two main girder of bridge often experiences a differential settlement, which results in tensile stresses and moment in the diaphragm. Due to this additional stress, the unsupported web gap is subject to out-of-plane distortion as depicted in Figure 1.
These stresses have caused horizontal cracks in the web or the web-to-flange welds and horizontal or vertical cracks at the end of the web-to-connection plate welds in many of the steel bridges. Moreover, the length of crack grows with repeated fatigue loading. Some of the typical cracks resulting from the web gap distortion in a welded plate girder is shown in Figure 2.
In this project, bridge reliability will be assessed probabilistically based on condition monitoring data. The main goals of this project are to:
• Propose a framework that uses the strain data for reliability assessment of steel bridges
• Develop a probabilistic model, which can be used to estimate the reliability index or remaining fatigue life of an existing steel bridge
The overall approach that will be undertaken in this project is illustrated in Figure 3, which consists of the following four key steps:
• Investigation of critical bridge component and strain-gauge placement
• Determination of stress range bin histograms using rain-flow counting method
• Estimation of stress range probability density function for binned histograms, and
• Probabilistic reliability assessment
