McC BarsMcCormick School of Engineering HomeMcCormick School of Engineering HomeNorthwestern University Homepage

Infrastructure Technology Institute

New Data Gives Insight Into Bridge Collapse

The 790-foot main span of the Koror-Babeldaob (KB) Bridge, completed in 1977, connected the islands of Koror and Babeldaob in the Republic of Palau. The KB bridge set the world record for length of a prestressed concrete box girder bridge. While it is normal for bridges of this type to sag slightly as they age, the observed sag at the midpoint of the bridge after only 18 years in service was double what the designers predicted. A remedial repair was performed in 1996; however, the bridge suddenly collapsed three
months later.

In late 2008, ITI researcher Professor Zdenek Bažant, after taking part in a successful campaign to gain public release of the engineering data that had been sealed by litigation since the incident, completed the first of two analyses of the degradation and collapse of the KB Bridge. In this first analysis, Bažant addressed the cause of the sagging that eventually prompted remedial action that accelerated the collapse of the bridge.

Because it is impractical to build a full-sized bridge in a laboratory to test and measure how it will perform, engineers must combine small-scale experiments with mathematical models to determine to the safest ways to design a structure. Bažant’s analysis concluded that the excessive movement at the middle of the bridge was not caused by any error in fabrication or construction but by the design models that failed to predict the performance of the structure once it was built.

All models used in today’s engineering practice underestimate the degree of structural degradation that affected the KB Bridge over 18 years of service. By contrast, Bažant’s new model, which is not currently part of the standard by which structures of this type must be designed, accurately predicts the performance of the bridge. Bažant hopes to incorporate this new model into the standard design practices of concrete structures to prevent this type of incident in the future.