Bridge 11 Update

October 7, 2003
by Michael J. Chajes

Last week, Bridge 11 in Delaware was tested. The intent of the test was to load the bridge to "failure," or as close as we could get. The following is a summary of what happened. More detailed information will be put together and presented in papers and at upcoming conferences.

The bridge was a 3-span continuous slab-on-steel girder structure. The span lengths were 61 feet, 74 feet, and 61 feet. It also had a large skew. The bridge was in good condition.

Prior to last Monday's testing, the bridge underwent diagnostic load testing. The intended loading was to pile approximately 1 to 2 million pounds of asphalt millings on the main span of the bridge. The diagnostic test showed that the bridge would easily withstand the 1-2 million pounds without yielding. As a result, approximately 50% of each of the main-span tension flanges was removed prior to Monday's testing (a 10-foot region was reduced centered about the midspan of each girder). One girder had about 75% of the tension flange removed.

During the Monday testing, 2 of the 5 girders reached yield (after applying about 1.75 million pounds of load). The total displacement at midspan was on the order of 1.5 inches.

After Monday's testing, it was determined that further testing should be done. On Friday, both of the continuous concrete parapets were demolished at midspan, and then the deck over both interior piers was cut. Finally, the top flanges of all girders over one of the interior piers were cut.

At the end of the day Friday, the bridge remained standing. Four of the 5 girders at midspan had yielded, and the maximum midspan displacement was just under 4 inches. While we did not envision bringing down the bridge, it did prove to take much more load (and induced damage) than we would have expected to push it far into the inelastic region.

The good news is that this bridge (and likely other of our highway bridges) has a large amount of redundancy and reserve strength. Further analysis of the results should provide some interesting information about how bridges perform when they approach and reach inelastic conditions.