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Bridges

Bridges are essential engineered and constructed structures that support elevated roadways for transporting vehicles, trains, and foot traffic over waterways, roads, valleys, and transportation obstacles such as cities and railway yards. The safety of bridges is often taken for granted by the public, and bridge failures that result in mass casualties have been rare until the last decade. Bridges in many parts of the world are aging and have outlived their engineered life spans and need to be replaced or retrofitted. Thousands of obsolete bridges remain in use. Obsolete bridges are often structures that were constructed over a century ago; they may not be unstable but they are not capable of sustaining the weight loads of today's vehicles, nor can they provide the multiple lanes needed to support the increasing volume of traffic. Bridges also provide terrorists with targets of opportunity.

Recent Bridge Engineering Research Findings

In 2009, the American Society of Civil Engineers (ASCE) issued findings on the safety of critical infrastructure in the United States. The study assigned letter grades on the safety of dams, highways, levees, sewers, and bridges. On a graded scale of “A “to “F,” bridges received a grade of “C” due to the high number of unsafe bridge findings. The report found that 26 percent of U.S. bridges were structurally deficient. For a bridge to designated as structurally deficient, it must have a defect significant enough to require posted weight load limits, speed restrictions, and signage warning of flood dangers in locations where flooding regularly occurs. Internationally, bridge safety was evaluated in a 2009 technology study that identified bridge strikes, where bridges are struck by objects such as barges, boats, and locomotives. Large-object strikes accounted for a high number of bridge failures in the United States and Europe but accounted for few collapses in other parts of the world, often due to underreporting. Bridges carrying overloaded vehicles and trains were identified as the most frequent factor contributing to bridge failures. Although the 2007 Minneapolis bridge collapse involved multiple causes, overload was determined to be a contributing factor. Increased weight loads and increased numbers of vehicles on the road have put unmanageable strains on aging bridges.

Bridges in the United States and Canada are engineered for safety and serviceability for a period of approximately 50 years. A large number of them are at least 43 years old and approaching the need for replacement. Of 600,000-plus U.S. bridges, 12 percent were found to be structurally deficient in 2008, and almost 15 percent were described as functionally obsolete. In rural areas, 25 percent of bridges were deemed to be structurally inadequate, as well as 30 percent of urban bridges. Motor freight traffic has increased by nearly 50 percent in the last 20 years, and urban areas account for the largest share of traffic in the United States, indicating that urban area bridge safety is a critical public safety concern. Bridges that are in the process of being constructed account for the second most frequent cause of collapses worldwide.

Bridge Protection against Terrorist Attack

A commission comprising governmental, engineering, scientific, and security experts convened in the United States reported that nearly 1,000 of the 600,000-plus bridges in the United States were vulnerable to terrorist attack. The commission called for risk assessment evaluations of bridges, enhanced security measures, and structural retrofitting to diminish the potential for terrorist activities. As recently as May 1, 2012, five men describing themselves as anarchists were arrested planning to blow up an urban, highly trafficked bridge in Cleveland, Ohio. Worldwide, in May 2012 three completed terrorist attacks on bridges resulted in explosives detonated in Myanmar and India, and one bomb scare occurred in Kenya.

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