The tragic total and instant collapse of the Interstate 35 highway bridge in Minneapolis should force us as a nation to make a careful reassessment of the methods we use for inspecting and repairing our infrastructure.
Material deterioration, fatigue, vibrations, the effects of sustained loads and overloads, problems related to the foundation integrity, design weaknesses–all may be among the factors that contributed to the Minneapolis collapse. Sadly enough, this bridge may have been inspected in recent years, but the conventional technologies commonly used for such inspections generally are not up to the job. Many inspections are performed simply by naked eye or by very simple methods, such as pinging the bridge’s surface with a hammer.
We can achieve a more definitive assessment of the behavior and safety of bridge material and structure through the use of systematic and effective nondestructive testing methods that already exist.
Most of the nation’s highways and highway bridges were built in the 1950s; our railroad bridges are nearly a century old. Indeed, many of these structures may be in a critical state because we subject them to traffic loads and vibrations greater than their initial design capacity and life span. Today’s vehicles are often much heavier than their mid-century counterparts.
Concrete and steel, the most commonly used materials in our infrastructure, naturally deteriorate with age. Freeze-and-thaw mechanisms used in harsh winters are especially detrimental to concrete, as are hot and humid conditions and the application of salt to melt winter ice. The latter conditions can lead to the corrosion of the steel used to reinforce concrete, which expands the volume of the steel. The expanded volume results in internal stress on the structure and can lead to failure of the material or the entire structural system. The columns (piers), deck system, and joints in a bridge structure are all affected.
Research has been done on advanced concepts and laboratory techniques for nondestructive testing using, for example, radar, infrared thermography, and acoustic methods, but usually such developments have not been carried to the implementation phase to produce portable devices applicable in field conditions. The high-tech, nondestructive tools presently available for infrastructure inspection have not been embraced by the construction industry for widespread and systematic use.
We need to carefully evaluate and implement into our engineering and management practices the already existing high-tech methods for monitoring and testing bridges and other structures, and we must encourage the development of new technologies as needed.
We should develop field specifications for nondestructive testing and monitoring technologies and require the construction industry to adopt the use of these technologies. The public authorities responsible for our infrastructure should schedule systematic inspections, require predetermined levels of structural reliability, and demand rapid screening and detailed investigations. A systematic high-tech inspection process of this kind would allow us to rate the condition of bridges and other structures, and prioritize them for repair, retrofit, and upgrade. Decisions as to the options to repair, upgrade, implement a temporary solution, or build a new structure should be made by skilled engineers and based on scientific fact and sound engineering judgment, with consideration paid to the available monetary resources.
Finally, we should incorporate into our engineering curriculum the science of material deterioration, methods for developing advanced technologies for the assessment and repair of infrastructure, and the design of new and innovative materials for infrastructure applications. And we should insist that continuing education for practicing engineers and management staff in this field is essential.
It is possible to avoid another tragedy like that in Minneapolis, but only if we act quickly to make rapid and thorough inspections that utilize high-tech devices developed for the task.