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The US has 578,000 highway bridges, which are the lifelines of US commerce. The average life span of highway bridges is about 70 years and the majority of bridges currently in use were built after 1945. However, significant environmental damage requiring repair typically occurs before the average bridge reaches mid-life. Corrosion, cracking and other damage can all affect a bridge's load carrying capacity. Therefore, all of the elements that directly affect performance of the bridge including the footing, substructure, deck, and superstructure must be periodically inspected or monitored. Visual inspection is the primary NDE method used to evaluate the condition of the majority of the nation's highway bridges. Inspectors periodically (about every two years) pay each bridge a visit to assess its condition. However, it is not uncommon for a fisherman, canoeist and other passerby to alert officials to major damage that may have occurred between inspections.

The potential penalties for ineffective inspection of bridges can be very severe. Instances of major bridge collapse are very rare, but the results are truly catastrophic. The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives. The bridge connected Point Pleasant, West Virginia and Gallipolis,Ohio over the Ohio River. The cost of this disaster was 175 million dollars but some experts estimate the same occurrence today would cost between 2.1 and 5.6 billion dollars. Furthermore, these cost figures do not take into account factors such as loss of business resulting from loss of access or detours, the cost resulting from blockage of a major river shipping channel, and potential environmental damage due to hazardous materials being transported over the bridge at the time of collapse.

The consequences of ineffective bridge inspection are usually not as severe as those at Silver Bridge. However, repair and retrofit costs on bridges represent a very significant portion of a state's transportation budget. In the future, replacement of a bridge will become an increasingly unattractive alternative. Growing construction costs, increased losses due to traffic disruption during repair or replacement, and continuing tight budgets will force life extension to be the only viable alternative for our aging bridges.

Fatigue cracking and corrosion will become increasingly important considerations as we go beyond the 75 year life expectancy and current visual inspection techniques will not suffice. The life extension approach will require increased use of NDE in a coordinated effort to obtain reliability assurance for these structures. NDE techniques such as magnetic particle inspection and ultrasonic inspection are being used with greater frequency. One of the newer NDE technologies being used is acoustic emission (AE) monitoring. Some bridges are being fitted with AE instruments that listen to the sounds that a bridge makes. These sophisticated systems can detect the sound energy produced when a crack grows and alert the inspector to the cracks presence. Sensors can be permanently fixed to the bridge and the data transmitted back to the lab so that continuous bridge condition monitoring is possible. The image provided here shows field engineers installing an AE monitoring system on the lift cables of the Ben Franklin Bridge in Philadelphia, PA