On September 26, 1996, the 1265-foot long, 18-year-old Koror-Babeldaob Bridge collapsed abruptly and catastrophically. The sudden failure occurred during a calm, mild evening under light traffic, that is, under very benign structural loading. The collapse shocked islanders because less than three years earlier, two independent teams of international bridge experts had evaluated the walkway and declared it safe.
Just prior to collapse, the bridge had been structurally renovated to correct a dip that had formed near midspan. The renovation involved adding high strength steel tendons inside the box girder that were strategically angled and anchored to provide lift at midspan (a technique known as exterior post-tensioning). Many engineers naturally assumed that the collapse was linked to the renovation, that is, the post-tensioning and related work must have somehow weakened the bridge. Exponent’s investigation of the cause of the failure included on-site inspections, material testing of the concrete and steel used to construct and renovate the bridge, and computer structural analyses of the bridge behavior. Exponent determined that the structural renovation did not contribute to the collapse in any way – it actually strengthened the bridge considerably. Surprisingly, it was a subsequent paving operation that caused the collapse. The repaving included aggressive removal and replacement of the pot-holed concrete surface. That aggressive removal, using large jackhammers mounted on trucks, that resulted in pervasive cracking of the concrete deck, which was heavily congested with steel reinforcing and post-tension bars of the original design. The cracking caused the original post-tension rods to lose anchorage, eventually triggering collapse.
