ABSTRACT
Currently there are approximately 605,000 bridges (over 20 ft. [6 m] in length) in all public highway systems in the United States. Approximately 23.8% of them are functionally obsolete and structurally deficient (FHWA 2012a). Bridges are classified as “Functionally Obsolete” (FO) when the deck geometry, clearance, or approach roadway alignment no longer meets current requirements and standards. FO can be classified as low load-carrying capacity, low waterway adequacy, insufficient deck roadway
16.1 Introduction ..................................................................................... 443 16.2 Historical Collapses and Bridge Conditions ............................... 445
16.6 Summary ........................................................................................... 484 References ......................................................................................................485
width (lane or shoulder width), insufficient horizontal and vertical clearances, and poor approach roadway alignment (FHWA 2011a) to serve traffic demands (vehicular size, traffic speed, traffic volume, etc.), or inadequate waterway to handle occasional flooding flows. Bridges are considered “Structurally Deficient” (SD) if significant load-carrying elements are found to be in poor condition due to deterioration and/or damage, or the adequacy of the waterway opening provided by the bridge is determined to be extremely insufficient to the point of causing overtopping with intolerable traffic interruptions (FHWA 2011a). In other words, the bridges have major deterioration, cracks, foundation scouring/undermining, or other damage and/or flaws that reduce their ability to safely support vehicle loads. SD bridges do not immediately imply that they will collapse or are unsafe, but need significant maintenance attention, such as repair, rehabilitation, retrofit, strengthening, or replacement (see Section 16.3.1 for definitions). The percentage of bridges falling into these two categories has gradually dropped down since 1992 as a result of new bridge constructions and, for the most part, replacement and rehabilitation (repair/ retrofit) of SD bridges over the years. Among all the bridges nationwide, 33.4% have steel superstructures (including plate girders, box girders, and trusses). On the basis of the year 2004 National Bridge Inventory (NBI), the average age of the bridges is 40 years and that for steel bridges is 48 years. Among steel bridges, 36.7% are SD and FO, the percentage is higher than that for all bridges.
