Michael S. Lenett

Subjective and Objective Evaluations of Bridge Damage

A decommissioned, 40+-year-old reinforced concrete deck on a steel girder bridge was subjected to a series of induced damages, nondestructive field tests, and visual evaluations to compare objective and subjective methods of bridge-condition assessment. Prior subjective evaluations of bridge condition often produced highly variable results. For example, inspectors with different backgrounds and field experience disagreed on how severely certain forms of deterioration and damage influenced bridge behavior and safety so, consequently, different assessments of bridge condition were generated. Furthermore, a load rating of the “as-is” state of the bridge (e.g., the state before any induced damages), according to current Ohio Department of Transportation procedures, indicated that the bridge could only support truckloads of 227 804 N (51,192 lbf). However, the objective data acquired during nondestructive field testing of the bridge, which was subjected to truckloads of 282 130 N (63,400 lbf), revealed maximum superstructure deflections and live-load stresses of 0.190 cm (0.075 in.) and 15 985 kPa (2,320 psi)—values well within AASHTO limits. These values also imply that the bridge can support loads much greater than those indicated in the load rating. Comparing subjective and objective assessments for the induced damage scenarios yielded similar results. Essentially, data revealed that subjective methods of bridge evaluation and assessment were unable to properly characterize intrinsic bridge mechanisms and the influence that such mechanisms have on bridge behavior. Condition assessment of a typical reinforced concrete deck on a steel girder bridge should therefore include objective evaluations of bridge condition and behavior.

Structural identification for condition assessment of civil infrastructure

The experimental aspects of structural identification for global and local condition assessment of constructed facilities are summarized. Experimental techniques suitable for meaningful field testing have been explored on an operating reinforced concrete deck-on-steel stringer highway bridge which served as a test bed. Modal analysis and instrumented monitoring applications conducted on this bridge serve to exemplify the experimental steps of a possible global structural identification methodology for health monitoring of civil infrastructure. The results given indicate that this is a feasible approach to for condition assessment and structural health monitoring.

HEALTH MONITORING OF IRONTON-RUSSELL BRIDGE FOR RATING PURPOSES

The Ironton-Russell truss bridge spanning the Ohio River between the cities of Ironton, Ohio, and Russell, Kentucky, consists of a three-span cantilever through truss with a suspended truss in the center span and a fourth span that is essentially a simple-span through truss. Visual inspections of this bridge revealed that recent welded repairs and modifications to various truss members reduced their fatigue classification as well as modified original design and construction details. When the visual inspection team identified these details along with section loss at certain truss verticals and diagonals, it was their recommendation to reduce the allowable load on the bridge from 208 260 to 26 700 N (23.4 to 3 tons). However, a 26 700-N (3-ton) load limit would hinder the transportation of heavy materials across the bridge and therefore have a severe impact on the local economy. Consequently, a series of nondestructive field tests were organized and performed on the bridge to provide better assessments of load capacity and in situ structural health. Controlled truck load tests involving resistance-based strain gauges were conducted, and an instrumented monitor utilizing low-speed (vibrating-wire) strain gauges was installed at the site. The respective data recorded during these field efforts were used not only to evaluate environmental effects on the bridge but also to compute the load rating for each of the instrumented members. The resulting objective-based load ratings and their corresponding allowable live loads greatly exceeded the recommended 26 700-N (3-ton) load limit. Essentially, objective data identified or characterized in situ structural mechanisms and therefore provided the basis for more sound and realistic load ratings.

Structural identification-based condition assessment: a demonstration of methods applied to a laboratory scale model testbed

This paper reports on results from an ongoing project to study structural identification-based approaches to the condition assessment of constructed facilities. Under such an approach baseline and sequentially identified system models would be used comparatively to track changes in system parameters and to infer the onset, location, and extent of structural damage, deterioration, and/or defects. As part of the initial phases of this research a scale model grid representing a steel girder highway bridge was designed constructed, instrumented and tested at the University of Cincinnati. This paper presents the development, testing, and analysis of this scale model structure and offers the data generated as a benchmark problem on structural identification for the members of the research community.