Diagnostic Load Testing for Improved Accuracy of Bridge Load Rating

Bridge load rating provides a standardized procedure to determine the safe load-carrying capacity of a bridge, thereby allowing engineers to establish posting and permitting requirements. The structural condition of components, material properties, loads, and traffic conditions all contribute to the load rating, which describes the capacity of the controlling component of the structure. The primary purpose of bridge load rating is to ensure public safety. The bridge industry has permitted nondestructive load testing, at the discretion of the owner, to establish the safe load-carrying capacity of a given bridge when coupled with analysis and sound engineering judgment. The direct measure of structural response through diagnostic live load testing is viewed as a more accurate method of determining capacity and requires minimal assumptions regarding load distribution. Diagnostic live load testing often results in higher loading postings when compared to traditional analysis because of the elimination of conservative assumptions. A number of data acquisition (DAQ) systems are available for implementation in the load rating process. When correctly implemented, the systems provide an improved understanding of the live load response. Furthermore, deferring repair or replacement through diagnostic load testing can deliver immediate and long-term cost savings. As such, implementation of a diagnostic load testing program may have significant short- and long-term paybacks for both bridge owners and users. The current study employed three DAQ systems to demonstrate how diagnostic load testing can be used to augment traditional load rating procedures. The study revealed that diagnostic load testing of a rural, simple-span steel girder bridge resulted in an improved load rating of 273%, refining the test truck inventory rating from 1.5 to 4.1. Similarly, diagnostic load testing of a rural, simple-span steel pony truss bridge resulted in an improved load rating of 170%, going from a test truck inventory rating of 4.5 to 7.6. Analysis of the field-collected data demonstrated the improved ratings were attributable to a number of factors not considered during traditional load rating, such as site-specific factors and contribution of nonstructural components as well as a true measure of the dynamic amplification, load distribution, and composite action. The positive results showcase the possible improvement to load rating by using diagnostic load testing as compared to traditional load rating procedures, leading to a safer bridge inventory.