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Flexural Capacity of Fire-Damaged Prestressed Concrete Box Beams

CHAPTER 6. Summary

A fiery incident involving a gasoline tanker truck caused significant damage to an adjacent member box-beam bridge in southwestern Connecticut. Although ConnDOT decided to replace the superstructure of the bridge, there was a question as to whether the type of damage that these box beams experienced was sufficient to critically impair the structure's ability to serve its intended purpose. To answer this question, ConnDOT coordinated with the FHWA's Turner-Fairbank Highway Research Center to investigate the remaining flexural capacity of the beams in the bridge.

Four of the beams from the bridge were loaded in flexure to failure. These tests indicated that each of the beams retained sufficient flexural capacity to resist an 881 kN-m (650 kip-ft) moment while remaining elastic and to resist a 1,572 kN-m (1,160 kip-ft) moment prior to ultimate flexural failure. This ultimate value is greater than the rated ultimate flexural capacity of each beam. As such, it seems that these beams had sufficient remaining flexural capacity to serve their intended purpose in the immediate aftermath of the fire.

The long-term viability of these beams is more questionable. The visual and petrographic examinations indicated that the damage to the bottom flange concrete was sufficient to allow pathways through the concrete to the depth of the bottom strands. Thus, it is possible that the bottom flange concrete would have experienced accelerated deterioration, leading to accelerated deterioration of the bottom row of strands and a long-term decrease in the flexural capacity of each beam.