In light of the Minneapolis bridge disaster, let me veer off-topic for this blog to clear up some of the confusion regarding bridge inspections and safety. Terms like "structurally deficient", "functionally obsolete", "condition rating", and "sufficiency rating" are being discussed in the news, but it's hard to find good definitions for what these really mean.
I must warn you that I'm not a civil engineer, but I have photographed and studied hundreds of bridges.
The Silver Bridge disaster
In 1967, a suspension bridge over the Ohio River at Point Pleasant, West Virginia, suddenly collapsed, killing 46 people.
In the afternath, Congress authorized the National Bridge Inspection Program, requiring regular inspections for many highway bridges. Today, all bridges longer than 20 feet on a public road must be inspected, typically every 2 years.
The inspection data is collected into the so-called National Bridge Inventory. The nationalbridges.com website features a searchable database of bridges, but is currently offline. However, the raw data is available from the Federal Highway Administration.
The NBI data is complex, but the key elements are the condition ratings assigned to the bridge's deck, superstructure, and substructure. In short, the deck (what you drive across) is supported by the superstructure, which is supported by the substructure. These three areas are given a 0-9 rating. Here is what the scale means, as taken from a Federal handbook:
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.
If any of the three condition ratings are at 4 (Poor) or worse, the bridge is said to be structurally deficient. That was the case with the Minneapolis bridge: the superstructure was rated Poor.
The words "structurally deficient" sound scary, but this label does not mean that a bridge is necessarily unsafe. An Associated Press story about the Minneapolis bridge states:
Authorities said the "structurally deficient" tag simply means some portions of the bridge needed to be scheduled for repair or replacement. It wasn't a candidate for replacement until 2020.
The Federal Highway Administration website has a breakdown of structurally deficient bridges by state. In 2006, Missouri had 4,595 structurally deficient bridges out of 24,024 total. This is an improvement from 2000, when Missouri had 6,361 out of 23,388.
These numbers seem high, but remember that Missouri has a large number of bridges, period. We have more than our fair share of rivers and farm-to-market roads compared with most states.
Some bridges are classified as functionally obsolete. This means that the bridge does not meet modern standards for such things as guardrails, shoulders, lane widths, vertical clearance, and more. Back in the 1920s, bridges were commonly built that were only 20 feet wide -- downright tiny for handling today's tractor-trailers and SUVs. These bridges are obsolete, but they might be in perfectly good condition.
However, in news reports, functionally obsolete bridges sometimes get lumped together with structurally deficient spans. The two terms mean different things! When somebody says that X percent of bridges are "deficient", it's important to ask exactly which bridges they are counting.
In addition to the 0-9 scales described above, bridges are also rated on a single 0-100 scale called a sufficiency rating. This is a convenient way to summarize a bridge's condition for making decisions about repairs and replacements. The Minneapolis bridge was rated near 50 -- not great, but not terrible either.
If you're curious, this manual (PDF) provides the formula for calculating sufficiency ratings. It's not pretty.
The Minneapolis bridge was constructed with a deck truss design. Basically, a network of girders (called a truss) supported the bridge from below the deck. Just because this bridge collapsed does not mean that deck trusses are inherently unsafe.
Deck trusses are somewhat rare (as compared with more common designs where the trusses are located above the deck). Missouri has a handful of such bridges. The Hurricane Deck Bridge over Lake of the Ozarks is the closest match to the Minneapolis bridge:
The Pine Street Bridge at Poplar Bluff features a deck truss, but it consists of a single span without any kind of arch:
Many states have ordered inspections of their deck truss bridges, but this probably won't do much good until engineers understand what happened at Minneapolis, and know what to look for. Then, it will be important to apply that knowledge to all bridges.
Update: Aug. 6
The Minnesota Department of Transportation has posted a bunch of documents to their website, including inspection reports and construction drawings. The Fracture critical inspection report from 2006 is the most interesting, with lots of details and close-up photos of the doomed bridge.