Performance Characteristics of In-Service Bridges for Enhancing Load Ratings: Leveraging Refined Analysis Methods

Report No: 21-R20

Published in 2021

About the report:

Bridge load rating assesses the safe live load carrying capacity of an existing or newly designed structure. In addition to load rating with previously defined standard load rating vehicles, the Federal Highway Administration issued additional guidance to states related to rating requirements for all the bridges with respect to specialized hauling vehicles and emergency vehicles that must be met by the end of 2022. It is recognized that the load effects (bending moment and shear) produced by these vehicle types on certain bridge types and spans might be greater than those caused by the previous rating vehicles. Therefore, a number of bridges within VDOT’s inventory may require posting when rated with these specialized vehicles. 

The goal of this study was to assess the likelihood of an increase in load rating factors through refined analysis methods for the bridge classes potentially vulnerable to load ratings under consideration of the new federal regulations and when using conventional, simplified equations for load distribution factors. In particular, the study focused on the evaluation of live load distribution factors for girder bridges and effective widths for distributing live loads in slab bridges through refined analysis. Three bridge classes (simple span steel girder bridges, reinforced concrete T-beam bridges, and concrete slab bridges)were selected for this refined analysis. Girder bridges were modeled using the plate-with-an-eccentric-beam analysis approach, while plate elements were used to model slab bridges within the LARSA 4D software package. The selected modeling approaches were validated through the simulation of the bridge structures with available field-testing results from the literature. A total of71 in-service bridges belonging to the three selected bridge classes were then modeled and analyzed to compute the load distribution factors for girder bridges or effective widths for slab bridges, and the results were compared with those obtained from the code-specified equations. Using the data obtained from these numerical simulations, a series of multi-parameter linear regression models were developed to predict the percent change in distribution factor and effective width, respectively, for girder and slab bridges with different geometrical characteristics if a refined method analysis is implemented. The regression models were limited to four parameters such that the results from regression models could be presented in table format.

The developed tables should be used as screening tools to provide guidance on the use of refined methods of analysis to improve the load ratings of bridges vulnerable to posting from previously existing load rating classifications as well as the recently introduced vehicles. Should VDOT use refined methods of analysis, there is good potential that the agency can avoid posting a substantial portion of its inventory, saving resources for more critical needs while safely keeping Virginia’s bridges open for commerce and the traveling public.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


Other Authors

Devin K. Harris, Ph.D., Osman Ozbulut, Ph.D., Abdou K. Ndong, Muhammad Sherif

Last updated: November 9, 2023

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