When you drive over a bridge, you assume it's safe. After all, the truck right in front of you made it across okay, so your little Honda should do just fine. Don't be so sure. To find out how bridges in our state were doing, we turned to the National Bridge Inventory list, a database detailing all inspections of bridges in the United States with a length of more than 20 feet. In a story published in February, we found that one of every six bridges in the state needs major repairs. The bill for those repairs totals $4.8 billion - more than ten times what the state spends on bridge maintenance. The NBI database only told us part of the story. Once we located bridges in questionable shape, we took a look at them. What we found were bridges with fractured foundations, one that had been struck by a train (but remained in use), and one that had state officials so scared they inspected it daily.
We discovered that temporary repairs on bridges are usually done to the road surface so motorists won't feel the bumps. The real decay, which can cause a bridge to be shut down, occurs underneath.
One tell-tale sign of decay was wood slate on the underside of the bridge. Highway crews put the slate there to catch falling concrete so motorists below don't end up with part of the bridge in their laps.
At first glance, the NBI code guide can be intimidating. Its 100 pages provide mostly technical detail on 116 fields. But you only have to use a few of these fields to get a good idea of what shape a bridge is in. To understand how your area's bridges work, talk with a local engineer or county bridge inspection department. It doesn't make much sense to spend a lot of time looking at the "scour critical" field (dealing with water erosion under support piers) if most of your state's bridges are over desert. In New Jersey we used Item 67, structural evaluation, as our divining rod. A rating of 0, 2, or 3 (1 is not used) told use that a bridge was in pretty bad shape. Keep in mind that this database was designed by engineers, who tend to be pretty conservative creatures by nature. Nothing in this list will tell you if a bridge will fall down or how many more years of use it has. The latter used to be a field, but was discontinued.
Once we compiled a list of bridges, we examined the actual inspection reports at the state and county offices. These documents gave valuable insight into every problem with the bridge and up-to-date repair estimates.
Not every bridge is examined at the same time and the NBI data isn't always current. Bridges should be inspected every two years, but depending on how vigilant the inspection crews are, this can range from once a week to once a decade. Before printing a list of troubled bridges, double check with the inspection department to see if any repairs have been done.
An application that uses crosstabs can be helpful in grouping the ages of your state's bridges. We found that many of the bridges in New Jersey were built in the 1930s and 1950s, which means that most of them will have to be replaced in the next 10 to 20 years.
The NBI list also contained a lot of faulty locations for bridges, which can make mapping a nightmare. We found that most of the bridges in our areas were either in the wrong towns or used the wrong street names. Once again, we had to rely on the inspection department for proper locations. We later spent a lot of hours cleaning up the addresses.
To obtain a copy of the NBI list on 9-track tape for $250, contact the Federal Highway Administration at (202) 366-0660. NICAR also has the NBI list available for the entire U.S. ($160) or by state ($75). Also, ask for a copy of the "Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges."
Advice for working with NBI data
To get the most out of the National Bridge Inventory list, you have to know what to look for in the 116 fields. If you examine the data well in advance, you will probably end up using only about a dozen of those fields.
- In Item 5, use only bridges with a code of "1." This means it's an actual bridge. Any other code means a roadway passes under the bridge. - The NBI only deals with bridges more than 20 feet in length. If your favorite bridge isn't on the list, it may not meet the minimum requirement. - Don't get hung up on one rating number. There is no single data field that will tell you if a bridge is good or bad. There are separate ratings for the deck, superstructure, and sub-structure. The closest you will come to an overall safety rating is Item 67, Structural Evaluation. - Don't get too excited about the last item, Sufficiency Rating. This relies on a complex formula that takes into account how well traffic flows over a bridge, which has no bearing on the bridge's soundness. - Because the government is still paranoid about nuclear missile attacks, it makes mapping almost impossible. The latitude and longitude of each bridge will be deleted from your copy of the tape (for national security reasons). No amount of begging has convinced the government that journalists are not a security threat. - The database contains Federal Information Processing Standards (FIPS) codes rather than town names. You will have to join the FIPS with your own Census table to determine town names. - Beware of typos and miscoding in the location field. In our area, 20 percent of the bridges either had a wrong code or no code at all.
Editor's Note: The full text of the Asbury Park bridge article is available from the IRE Resource Center. Also available are a Cox Newspapers article and NBC Dateline transcript, both of which came from a joint NBI analysis by Elliott Jaspin and David Hinchman in December (#9718). You can contact Paul D'Ambrosio and Rick Linsk at this e-mail address: pmd@cscns.com.