Field testing and fatigue evaluation of a Pennsylvania bridge

By: Carl Bowman and Ian C. Hodgson, P.E., S.E.

The Shippingport Bridge in Beaver County, Pa., spans the Ohio River, connecting the boroughs of Shippingport to the south and Midland to the north. Constructed in 1961, the main river crossing consists of a three-span, combined deck/through truss, with span lengths of 341 ft, 620 ft, and 341 ft. The southern approach to the bridge consists of a two-girder, two-span unit, while the northern approach is a single-span, multigirder unit.

Lehigh University’s Advanced Technology for Large Structural Systems (ATLSS) Research Center performed field-testing of this bridge as part of a larger evaluation and rehabilitation project. A detailed fatigue evaluation was conducted by measuring the in-situ behavior of the bridge. The scope of the work included controlled-load testing using a test truck of known geometry and weight, and long-term monitoring of the bridge. Estimation of remaining fatigue life at critical details was also performed using the collected data.

Instrumentation Program

Strain gages and displacement sensors were installed throughout selected spans and were placed at locations known to be fatigue sensitive or in locations that would provide insight into the global load distribution characteristics and general behavior of the bridge. The locations included floor-beam/truss connection, stringer/diaphragm connections, longitudinal stiffener terminations, floor-beam web copes, gusset plate terminations, floor-beam knee braces and web gaps at diaphragm connection plates.

The data were collected using a Campbell Scientific CR9000X datalogger. The logger is a high-speed, multichannel, digital data-acquisition system that was installed in a weather-tight box located on the bridge inspection walkway.

Controlled-Load Testing

A fully loaded, triaxle dump truck was used for the controlled-load testing phase of this project. The individual axle weights and the overall geometry of the test truck were measured prior to the testing. The test truck was driven across the bridge multiple times in each lane at both crawl speed and normal traveling speed. The crawl tests were used to determine the static response of the bridge while the dynamic amplification was evaluated using the results from the tests at normal traveling speed.

Remote Monitoring

The bridge was monitored remotely between November 2005 and January 2006. During the monitoring period, the CR9000X was triggered to record high-speed, time-history data when high-stress events were detected. In this way, data records were obtained for crossings of the heaviest vehicles.

At the same time, stress-range histograms were recorded continuously for all channels throughout the monitoring period. Every ten minutes, histograms were updated using the rain-flow cycle-counting algorithm. For the fatigue evaluation, the stress-range histograms were truncated at a level equal to approximately one fourth of the constant-amplitude fatigue limit (CAFL) of the detail specified in AASHTO.

The CR9000X operated reliably throughout the monitoring period, even in the cold winter months. Remote communication with the datalogger was established using wireless modems. Data download was performed automatically via a server located in the ATLSS Center in Bethlehem, Pa. This link was also used to upload new programs as needed.

Findings

Field instrumentation and testing provide data that allows engineers to understand the actual behavior of the structure and, from there, make informed decisions regarding bridge rehabilitation. This project provided an opportunity for ATLSS researchers to use controlled-load testing and long-term monitoring to characterize the global and local response of the bridge and to optimize the rehabilitation scheme. Improved confidence in the fatigue life estimates is also achieved.