Live-Load Response of a 65-Year-Old Pratt Truss Bridge

Abstract

The Kettle River Bridge is an arched, steel cantilevered deck, Pratt Truss bridge. To quantify the live-load performance of the bridge superstructure and provide a baseline for future comparisons, it was instrumented with 151 strain gauges on various beam, stringer, and truss members. In addition, 8 displacement gauges were attached along the truss. All gauges were simultaneously monitored while the bridge was subjected to a nondestructive, live-load test. The recorded bridge response was analyzed to quantify in situ behavior. The measured behavior was then used to validate a finite-element model using solid and frame elements, which was subsequently used to obtain an inventory- and operating-load rating of 2.03 and 2.64, respectively. Based on the results of the test, it was concluded that the large gusset plates added partial fixity to the truss members, as evident from the significant measured in-plane bending strains. However, the beam-and-stringer deck support system showed minimal rotational restraint between consecutive panels and behaved nearly simply supported. The validated finite-element model was used to determine distribution factors. The actual bridge distribution factors were found to be conservative in comparison to those calculated according the AASHTO LRFD specifications, by 17–44%.