Live Load Test and Failure Analysis for the Steel Deck Truss Bridge over the New River in Virginia

About the report:

This report presents the methods used to model a steel deck truss bridge over the New River in Hillsville, Virginia.  These methods were evaluated by comparing analytical results with data recorded from 14 members during live load testing.  The research presented herein is part of a larger endeavor to understand the structural behavior and collapse mechanism of the erstwhile I-35W bridge in Minneapolis, Minnesota, that collapsed on August 1, 2007.  Objectives accomplished toward this end include investigation of lacing effects on built-up member strain measurement, live load testing of a steel truss bridge, and evaluation of modeling techniques in comparison to recorded data.  The most accurate model was used to conduct a failure analysis with the intent of then loading the steel truss bridge to failure.

Before any live load testing could be performed, it was necessary to confirm an acceptable strain gage layout for measuring member strains.  The effect of riveted lacing in built-up members was investigated by constructing a two-thirds mockup of a typical bridge member. The mockup was instrumented with strain gages and subjected to known loads to determine the most effective strain gage arrangement.  The results of the testing analysis showed that for a built-up member consisting of laced channels, one strain gage installed on the middle of the extreme fiber of each channel’s flanges was sufficient.  Thus, laced members on the bridge were mounted with four strain gages each.

Data from live loads were obtained by loading two trucks to 25 tons each.  Trucks were positioned at eight locations on the bridge in four different relative truck positions.  Data were recorded continuously and reduced to member forces for model validation comparisons.  Deflections at selected truss nodes were also recorded for model validation purposes.

The model validation process began by developing four simple truss models, each reflecting different expected restraint conditions, in the hopes of bracketing data from recorded results.  The models included a simple truss model, a frame model with only the truss members, and a frame model that included the stringers.  The final, most accurate model was selected and used for a failure analysis.  This model showed where the minimum amount of load could be applied to learn about the bridge’s failure behavior and was to be used for a test to be conducted at a later time.  Unfortunately, the project was terminated because of a lack of funding before the actual test to failure of the steel truss bridge was conducted. 

 Nevertheless, findings from the study led to two important recommendations:

1. When instrumenting a steel truss bridge for load testing by placing strain gages on built-up members, four gages, one placed on each flange of each channel, should be used.
2. When modeling deck truss bridges, the system should be considered to be a frame and should include the stringers in the model.

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