**1. Introduction**

One of the challenges for infrastructure owners of large national networks of highway and railroad bridges is maintaining the performance of the bridges over their designed service life without the risk of failure. But bridges do fail for a number of reasons. A bridge failure is most often the result of some failure in the life cycle of a bridge from concept through design, construction and maintenance during its service life. The direct failure of bridges is the result a number of factors to include structural over loading, material failures, poor designs, faulty construction, inadequate oversite and control of bridge construction, inadequate inspections, delayed maintenance, earthquakes, and flooding.

In research work on the frequency and causes of bridge failures K. Wardhana, et al., studied over 500 failures of bridge structures in the United States between 1989 and 2000 with the age of the failed bridges ranging from 1 year (during construction) to 157 years, having an average age of 52 years. They state "The most frequent causes of bridge failures were attributed to floods and collisions. Flood and scour, with the major flood disaster in 1993, contributed to the frequency peak of bridge failures

(almost 53% of all failures). Bridge overload and lateral impact forces from trucks, barges/ships, and trains constitute 20% of the total bridge failures. Other frequent principal causes are design, detailing, construction, material, and maintenance. Comparison made among three periods of similar studies (1977–1981, 1982–1988, and 1989–2000) revealed almost similar trends, with most failures occurring during the bridge's service life. Also, human-induced external events occurred frequently in all three periods, but were most dominant in the first and third periods [1]. While Wardhana studied three periods of bridge failures in the United States the statistics are representative of bridge failures around the world in all years.

This chapter examines bridge failures and structural deterioration for the purpose of understanding the elements that cause deterioration in bridge structures and the factors that lead to the structural failures. There are a number of elements that cause the material in bridge structural components to degrade and deteriorate over time, in both steel and RC (reinforced concrete) bridges. The two dominate elements causing deterioration are deicing chemicals (chlorides) and vehicle over loading. These two elements affect all types of bridges and their components. In extreme cases extensive deterioration potentially leads to structural failure. Of the bridge failures presented in this chapter each failure has one or more of these factors which contributed to the bridge collapsing.
