r - The Hong Kong Polytechnic University

eliability index of 3.5 for both moment and shear strength limit states for all the bridges considered here, (2) the
proposed values of IM for rating of existing bridges, and (3) the values of IM proposed by two of the authors in
a previous study (Deng and Cai 2010). From Table 2, it is observed that the IMs proposed in the present study
are close to those proposed in Deng and Cai (2010), except for the case of very poor RSC.
Table 2 Comparison between the proposed dynamic load allowance IMs in the present study and those by Deng
and Cai (2010)
Deng and Cai (2010)
Present study
Road Surface
Theoretical Minimum
Condition
Proposed IMs
Proposed IMs
IMs
Very Poor 1.99 2.50 2.40
Poor 0.99 1.00 0.90
Average 0.50 0.50 0.33
Good 0.33 0.15 0.20
Very Good 0.23 0.05 0.10
To verify whether the proposed IM can lead to a consistent reliability index of 3.5 for all RSCs, the reliability
indices are recalculated for the seven bridges designed following Eq. (23) while employing the proposed IM
values. These results confirm that using the proposed IM values produce more consistent reliability indices that
are very close to the target reliability index of 3.5, regardless of what the actual RSC is.
It is noteworthy that, when the RSC is below average, the proposed IM values are greater than the value of 0.33
suggested by the current LRFD code. In particular, the IM value proposed for very poor RCS is significantly
larger than 0.33 (i.e., more than seven times larger). Since a significant portion (i.e., more than 17%) of all
bridge decks in the United States belong to fair or worse RSCs, it is concluded that the use of different IMs,
which account for the different RSCs, is necessary for accurate performance evaluation of existing bridges.
SUMMARY AND CONCLUSIONS
This paper summarizes the research group’s work related to vehicle effects on existing bridges, using reliability
based performance assessment. The first part presents a simulation framework of fatigue reliability assessment
for existing bridges considering the effects of vehicle speed, road roughness condition, equivalent stress range
and the constant amplitude fatigue threshold. The second part develops a framework to estimate the extreme
structural response due to live load in a mean recurrence interval based on short-term monitoring of existing
bridges. The Gumbel distribution of the extreme values was derived from extreme value theory and Monte Carlo
Simulation. In the third part of this paper, the reliability indices of a selected group of prestressed concrete girder
bridges are calculated by modeling the IM explicitly as a random variable for different RSCs. Appropriate IM
values are suggested for different RSCs in order to achieve a consistent target reliability index and a reliable
load rating for existing bridges.
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