The tragic failure of the I35W bridge in Minneapolis is a wake up call to the nation that we have some pretty sick bridges in need of repair or replacement. Transportation authorities have known for some time that is the case, but efforts to fix the situation have lagged badly. Now, I suspect things will change, at least they should.

As a student in civil engineering, bridge design was part of my studies. I got away from civil engineering after college and got more into chemical engineering in the oil industry. Still, the principles have stuck with me probably because engineering, regardless of specific discipline, is really a thought process. With that as background, a few thoughts about the Minneapolis tragedy may be helpful.

I expect the I35W bridge was designed for less than the estimated 100,000+ vehicles that were moving over it daily prior to its collapse. Even more likely, it was not designed for the huge truck loads now experienced on road and bridge systems throughout the country. There are many different designs in existence but the principles involved are much the same. All of them take into account two basic types of loading, deadweight or static loads ( i.e., the weight of the bridge itself) and moving or dynamic loads imposed by traffic moving over the bridge. Other types of load may come into play such as wind conditions and harmonic vibration (read Tacoma Narrows) depending on the location environment. Many older bridges were built in steel while most modern ones employ concrete, many parts of which are prestressed.

Steel has one major problem...it corrodes. The corrosion process can be slowed with the use of certain protective coatings, but nothing lasts forever. Even concrete bridge spans rest on intermediate and abutment pivot points made of steel that are subject to corrosion. Corrosion ultimately weakens the steel and thus its ability to support loads. Constant flexing of structural members may eventually destroy the elasticity in a critical member causing the member to fail.

Concrete bridge construction has all but replaced steel today. It permits more flexibility in design and presents a more pleasing appearance. Except for intermediate tressels and abutments which require onsite concrete pours, stringers and decking quite often are poured offsite and hauled to the bridge site for installation. These members are prestressed to put them in an installed state of compression. This permits the menber to absorb tension creating loads without damage to the concrete.

Construction errors can result in ultimate structural failure. The possibilities are numerous and can only be eliminated by close observation during construction and objective inspection by experts before the structure is opened to traffic. The same holds for all inspections to follow once the structure is in service.


In the ideal world, bridge inspections would be made on a precise schedule by competent engineers with no connections with the designers and builders of the bridge. Defects noted would be reported along with recommendations for traffic restrictions to stay within the safety limits set by the inspectors until the next inspection and/or repairs. It would be incombent upon bridge authorities in state and local governments to post such restrictions on the bridge and see to their enforcement. In the real world, it is anyone's guess as to what is actually happening.