A Probabilistic Analysis of the Frequency of Bridge Collapses due to Vessel Impact

The collapse of the Queen Isabella Causeway in 2001, caused by a vessel collision, sent an alarming message to the state of Texas that vessel impact on bridges is a serious issue and that the possibility of such accidents needs to be considered in the design and evaluation of any bridge spanning a waterway. The Texas Department of Transportation (TxDOT) funded this research project at The University of Texas at Austin seeking to re-evaluate the current vessel collision calculations (both on the load and resistance side), create a database of vessel traffic in the state of Texas, and design a stand-alone computer program to perform the vessel collision risk calculations.

Currently the 2004 American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) design code regulates vessel collision analysis. Bridges are designed to meet a specified annual frequency of collapse based on a probabilistic model. While the basis for the computation of the probability of aberrancy and geometric probability are well justified, little research has been performed on barge to pier collisions to support the AASHTO LRFD method for probability of collapse. Using two models, one that determines the force imparted on a bridge pier by a vessel and another that determines what the ultimate lateral strength of a pier is, an enhanced method for determining probability of collapse was developed.