Time-variant performance assessment and improvement of existing bridges

The serviceability and safety of buildings and bridges are expected to be maintained within a reasonable safety level throughout their lifetimes. However, the increase of the applied loads and degradation of structural performances reduce the safety of these structures over time. Therefore, the performance assessment of existing bridges with reliability theories is a worldwide problem in civil infrastructure systems. Theoretically, the bridge reliability, usually expressed by a reliability index, is quantified by comparing the structural capacity (R) with the load effects (Q), using the predefined limit state functions. A limit state function is a mathematical description of a boundary between the desired and undesired performance of a structure. The resistances of structures and live loads on the bridge are none stationary processes, where their statistic parameters, e.g., mean values and deviations, are time variant. Thus, traditional reliability analysis methods cannot be applied to the entire service life of bridges. In this research, the entire life cycle of bridges is treated as the sum of time series. During each time segment, both the load effect Q and the structural capacity R are assumed to be a stationary random process, and are expressed with a certain type of distribution. Thus, after obtaining the reliability probabilities for each time segments, the reliability probability for any length of mean recurrent intervals is obtained by the continued multiplication of the yearly reliability. The extreme structure response which reflects the extreme live load distribution for mean recurrence intervals is derived based on a short-term monitoring of a field bridge. The flexural capacity of bridge girders considering variation of concrete strength, corrosion of steel reinforcements in the concrete and steel components is discussed in details. The flexural capacity of bridge beams can be retrofitted with fiber reinforced polymers (FRP) materials. Finally, the flexural capacity of concrete bridge girders and steel girders strengthened with prestressed carbon fiber reinforced polymers (CFRP) are introduced. The time-variant reliability after the rehabilitation is calculated. The reliability of a bridge keeps decreasing all the time. There is a jump in the reliability when the bridge is strengthened. Rehabilitation of a bridge also slows down the rate of the performance degradation of the bridge