The missing haven’t been found and the bodies aren’t even cold yet but the finger pointing has already started. Of course, if you listen to the usual suspects, the collapse is entirely George W. Bush’s fault.

Disclaimer: I am not a structural engineer nor a civil engineer nor do I play one on television. I’m simply a user of bridges, just as you are, but I’m also a fan of old highways and old bridges. I’ve been snooping around old bridges for a long time and I like to know about the surfaces I’m driving upon.

The media are tossing around terms without knowing, let alone explaining, what they mean. What, exactly, does “structurally deficient” and “functionally obsolete” mean? I’ve heard those terms used interchangeably when, in fact, they mean something completely different.

A functionally obsolete bridge might be perfectly safe. The term is applied to bridges that were built decades ago. In the 1920’s, decks were often only 20 feet wide, acceptable in the days of Model T Fords but designers did not anticipate the huge tractor/trailers and SUV’s that we have today. Rail heights probably do not meet current specs along with other difficulties, but certainly not problems. Most functionally obsolete bridges have been bypassed by more modern roads and most functionally obsolete bridges are extant on secondary roads.

In 1967, the Silver Bridge collapsed between Point Pleasant, WV and Gallipolis, Ohio. The result was the National Bridge Inspection program. The NBI has a requirement that any bridge on a public highway, longer than 20 feet, be inspected periodically, typically every two years. In order to help determine which bridges needed attention, a scale is used called a condition rating. The NBI is a complex report, but for consumers, the key elements are the condition ratings assigned to the bridge’s deck, superstructure, and substructure. The “deck” is what you drive across, which is supported by the superstructure that, in turn, is supported by the substructure. These three areas are given a 0-9 rating determined by a federal guideline:

9 Excellent

8 Very Good

7 Good No problems noted.

6 Satisfactory Some minor problems.

5 Fair All primary structural elements are sound but may have minor section loss, cracking, spalling, or scour.

4 Poor Advanced section loss, deterioration, spalling, or scour.

3 Serious Loss of section, deterioration, spalling, or scour have seriously affected the primary structural components. Local failures are possible. Fatigue cracks in steel or shear cracks in concrete may be present.

2 Critical Advanced deterioration of primary structural elements. Fatigue cracks in steel or shear cracks in concrete may be present or scour may be removed substructure support. Unless closely monitored, it may be necessary to close the bridge until corrective action is taken.

1 Imminent Failure Major deterioration or section loss present in critical structural components, or obvious loss present in critical structural components, or obvious vertical or horizontal movement affecting structural stability. Bridge is closed to traffic, but corrective action may put back in light service.

0 Failed Out of service; beyond corrective action.

When a bridge has a rating of 4 or lower, it is labeled “Structurally Deficient.” This is not always bad, it typically means the bridge is in need of maintenance or repair. This was the case of the 35W bridge - it was not scheduled for replacement until 2020.

Bridges are also receive a sufficiency rating, which is a scale of 0-100. This is a convenient way to see what a bridge’s condition is to aid in making decisions about what bridge gets what attention and what bridge might be a candidate for replacement. The 35W bridge was rated near 50 - certainly not good but hardly a candidate for replacement. Many perfectly safe bridges seem to be in that range.

The 35W bridge was a deck truss, not commonly used in highway applications but is often seen on railroad spans. A deck truss has the truss superstructure below the deck surface. Older Wisconsin motorists are used to seeing what is called a “Through Truss” where the truss superstructure is above the deck. (Through trusses used to be very common but are being replaced all over Wisconsin.)

Until a failure investigation is completed on the 35W bridge, any frantic inspection of extant bridges is premature - inspectors need to know what failure points they are looking for.

(With a tip of the hat to James Baughn, the webmaster of http://www.bridgehunters.com and a fellow fan of old bridges and old highways.)