Earlier today, I noted that one focus of the collapse of the St. Anthony Bridge will be the methods used to inspect the bridges in Minnesota and across the nation. The latest inspection, less than three months before the collapse, never indicated that the bridge was in imminent danger of collapse, but the Star Tribune reports that the inspections mainly rely on eyeballs and ears as the highest-tech devices to find structural deficiencies. The technology of bridge inspections remains largely what it has been for the last several decades.
My father, Ed Morrissey (Sr), spent 29 years working on the space program, most specifically on the kind of non-destructive testing technology that would replace those eyes and ears on the bridges. He held a high certification level -- high enough to have a successful post-retirement career as a consultant at the same company from which he retired. I asked him to give us a primer on non-destructive testing techniques, their applications, and their limitations -- and he responded quickly.
There are problems with testing structures for cracks. Some of these problems are: surface rust, paint, build-up of grime and also the way the structure is designed. Assuming there are no inherent flaws, you would be testing for deep pockets of corrosion and cracking.
There are a number of methods available to search and locate flaws. First it would be necessary to identify the areas in which cracking or corrosion would cause catastrophic failure. Then you may use one or more of the non-destructive testing procedures such as:
penetrant inspection, magnetic particle inspection, ultrasonic inspection, radiographic inspection, eddy current inspection, acoustic emission inspection, and even thermal measuring techniques.
Each of the above methods has the ability to detect certain types of flaws:
Penetrant inspection will detect cracks that are open to the surface, but you must have a clean surface to apply it to get satisfactory results.
Magnetic particle inspection may be used to look for cracks which may be under a painted surface; however, the material which is inspected must be ferromagnetic (magnetizable steel).
Ultrasonic inspection will detect surface and subsurface flaws by sound reflecting off the surface of a crack and also detecting deep pockets of corrosion by sound absorption. Radiographic inspection (x-ray and gamma ray) will detect large pockets of corrosion but no small surface cracks. The cracks have to be large and parallel the path of the x-ray or gamma ray when passing through the tested material. Eddy current may be applied to test the metallic structure to detect small cracks. However, since bridges are constructed with magnetic steel for the most part, one has to eliminate the response of the magnetic field. Acoustic emission testing measures the noise created by stress on the structure parts under test to identify those specific sounds emanating from a crack. Thermal testing is performed by applying heat to the structural point and accurately mapping the heat dissipation for anomalies caused by crack or other flaws.
Each of the above methods has its own advantage in the detection of flaws, but they may only be of use under certain conditions. It requires a high degree of training and experience to apply these methods. Therefore most of this type of testing is performed by materials testing companies who specialize in these techniques.
The Admiral Emeritus offered a few more salient points in a follow-up call. In order to do this kind of testing, one has to already have identified the structural elements that would have the most likely fatigue or failure points. None of these tests could be easily conducted on an entire bridge structure; they're costly and not intended for 100% review. Inspectors would have to develop procedures and standards for the tests, and they'd likely have to do that for every bridge that they inspect.
Some of these tests do get performed for transportation system inspections. Railroad track inspections use NDT techniques, for instance, when they suspect a problem exists. The equipment could get positioned in place using cherry pickers and other lifts if used for bridges, but it would be slow and expensive to do.
Keep a link to this post handy. When more comes out about the limitations of the current inspection regimes and NDT techniques get discussed, it will make a good reference. Perhaps my father will expand on this technology in between rounds of golf as the story develops.