Reliability-Based Dynamic Load Allowance for Capacity Rating of Prestressed Concrete Girder Bridges

The current highway bridge design in the United States follows the AASHTO-LRFD specifications, which prescribe a dynamic load allowance, $IM$, of 0.33 for the dynamic effect of truck/tandem loading. Studies have shown that the $IM$ value prescribed by the LRFD code may underestimate this dynamic effect under poor road surface conditions (RSCs). One reason for this underestimation is that the $IM$ value employed in the AASHTO specifications was obtained from the statistical properties of the $IM$ relative to average RSC, as defined by the ISO 1995 standards. In addition, the $IM$, which is a random variable with certain statistical properties, was modeled as a deterministic constant in the code calibration process. In this paper, the reliability indexes of a selected group of prestressed concrete girder bridges, designed following the AASHTO-LRFD code, are calculated by modeling the $IM$ explicitly as a random variable for different RSCs. It is found that although the calculated bridge reliability indexes are usually above the target reliability index value of 3.5 under above-average RSCs, they can be significantly below the target value of 3.5 when the RSCs are below average. Following the load rating procedure proposed by the AASHTO load and resistance factor rating (LRFR) manual, it is also found that the code-employed $IM$ value may overestimate the rating factors when RSCs are below average. Based on these results, appropriate $IM$ values are suggested for different RSCs to achieve a consistent target reliability index and a reliable load rating. The results presented in this paper are particularly valuable for the rating of existing prestressed concrete girder bridges, for which the actual RSCs can be directly evaluated. The RSCs must be properly taken into account to accurately estimate the actual safety of the considered bridge.