Imagine setting a fallen tree trunk across a crevasse. If the crevasse is only 4 ft deep, and the tree trunk breaks, you might sprain an ankle or break an arm, depending on how you fall, but the injuries will be minor.
If the crevasse is 1,000 ft deep, though, and the tree trunk breaks, you’re a goner. That’s an example of a failure with huge consequences.
If the tree trunk is 10 ft in diameter, there is little probability that it will break under the weight of an adult male. We might say it has low vulnerability. On the other hand, if the tree is a sapling and only 1 in. in diameter, it will have a hard time supporting the same weight. We might say it is highly vulnerable to breaking.
Consequences and vulnerability are the twin pillars of risk as set down in the Moving Ahead for Progress in the 21st Century Act (MAP-21), which was signed into law by President Barack Obama on July 6, 2012.
To comply with MAP-21, the Federal Highway Administration (FHWA) is now implementing policies that will incorporate risk- and performance-based management of U.S. bridges.
“You’ve got certain bridges that, if you look back in time, have performed very well,” Richard Walther, principal, Wiss, Janney, Elstner Associates Inc., Northbrook, Ill., told ROADS & BRIDGES. “As an example, concrete bridges located in Southern states, which see limited applications of deicing salts and freeze-thaw conditions, are likely to perform better than that same bridge in a Northern state, where there’s a lot of freeze-thaw cycles and frequent applications of salt on the roadway.”
As a result, a risk-based inspection program would allow bridges that are known to perform better to be inspected less frequently, and the money that would be spent on inspecting them can be used elsewhere.
“What we would expect as we move to a risk-based specification for bridge inspections,” said Walther, “is that we can apply our limited resources in the right places at the right time.”
MAP-21 does not stop at mandating risk-based, performance-based bridge inspection. The law also mandates that bridge inspections should collect element-level data on commonly recognized bridge elements.
For decades, bridge inspectors considered the bridge’s deck as a whole component and reported the condition of the deck overall. A single number on a 0-9 scale had to express the condition of the entire deck. The superstructure also received a single-number rating. The same applied to the substructure.
What MAP-21 requires for National Highway System (NHS) bridges is to quantify how much of the deck, for example, is in which one of four condition states.
“You take a bridge, and you break it into measureable quantities and pieces,” Tom Everett, team leader of the FHWA’s Bridge Safety, Preservation and Management Team, told ROADS & BRIDGES.
For example, a painted steel girder might be an element, and there might be 10,000 linear ft of painted steel girder on the bridge being inspected. Of the 10,000 linear ft of painted steel girder, the inspector may rate 8,000 ft of it in good condition, or condition state 1, 20 ft in really bad condition, or condition state 4, and the rest divided between condition states 2 and 3.
“What this does is really remove a lot of the subjectivity of the 0-9 scale,” said Everett, “and gives us much more specific, consistent, quantified data.”
Some states already collect element-level data on their bridges, even though it is not required by the FHWA. All states will be required to start collecting element-level data in October 2014 and report it to the FHWA next April.
Each state is responsible for inspecting its bridges and reporting the results to the FHWA. Some states perform their own inspections; some contract the work to a consultant. The frequency of inspection is typically 24 months. The interval can be extended up to 48 months, but only if the bridge meets very restrictive criteria. The bridge cannot have any main span greater than 100 ft long. The bridge cannot be scour-critical. No fracture-critical bridge can have its inspection interval extended to 48 months. There are other criteria as well.
Prior to MAP-21’s requirements for element-level data, bridge-inspection data was sometimes less than absolutely reliable. An extensive study of visual inspections—the type of inspection typically done, unless there is a need for something more—of bridges in the early 2000s found inspection ratings subjective and inconsistent.
FHWA’s response to the study was to increase inspector training and increase oversight of the program.
Subject of the object
“We brought every inspector out to the bridge and had them look at the same thing,” said Walther, who participated in the scientific study. “At the end of the inspection we asked them, How would you rate that? One inspector might say 6, and the other inspector might say 8. I would think that through the FHWA’s efforts at improving inspector qualifications and training that now, almost 15 years later, if the study were to be repeated that we would see that variability in rating would decrease, and the ability of inspectors to find defects would increase, particularly in regard to in-depth inspections.”
“We introduced into the regulation for the first time ever a refresher training requirement,” said Everett. “Now there is an expectation that inspectors would be refreshed on a periodic basis.”
They also added a requirement that inspectors who dive underwater to inspect elements below the water line would be trained to the same level as the inspection team leader. The other members of the teams are not required to have quite as much rigorous training.
“We introduced a requirement to say the team leader must be on-site during the inspection,” added Everett, “because they are the people that we are dictating the training requirements for. It’s important that they be there when the inspection is conducted.”
The FHWA has not repeated the study, but they are confident that the increased training has improved the consistency of the inspection ratings, along with closer scrutiny of the data the states report.
“We have stepped up our oversight of the program, and part of that oversight is checking data quality and consistency,” Everett said. “On the data-quality side, we’ve seen fantastic improvements. The example I like to use is back in 2008, we estimated about 140,000 errors in our data, which sounds really bad, but if you think about 60 million pieces of data, even that’s not too bad. In 2012, it’s less than 4,000 errors. That’s a great story.”
FHWA field personnel also visit bridges that have been inspected to see whether they agree with the results that the state reported and whether the rating criteria are being applied consistently from one bridge to the next.
MAP-21 expires on Sept. 30, 2014, but the expectation is that the bridge-inspection requirements for a risk-based, performance-based approach and element-level data will continue.
Even before MAP-21, the National Bridge Inspection Standards (NBIS) required two levels of bridge inspection: routine inspection and in-depth inspection. A routine inspection is typically done visually, without touching the structure. In-depth inspections require close examinations, typically at arms length.
“We focus on teaching inspectors to recognize the seriousness or potential consequences of what they see,” said Everett. “A crack can be critical, depending on the location of it, so we teach inspectors where the key locations are on a bridge, where if they see a crack, they need to take immediate action. That’s the thrust of the training, first to recognize the defect and to understand the impact of that defect.”
There are a variety of types of deterioration of bridges, but by far the most common are corrosion and stress-related cracking.
An in-depth inspection typically is best suited to assess the effects of deterioration and makes use of tools that require the inspectors to put their hands on the bridge. There are various methods of investigating bridge deterioration. For a description of a handful of them, turn to “(Bridge) inspector gadget” on p 34.
Two of the most often-employed techniques for evaluating steel-bridge deterioration are the dye-penetrant test and the magnetic-particle test.
The dye-penetrant test is used for cracking in steel. A dye is applied to the site of a crack or a suspected crack. The dye spreads out, following the trail of the crack and making it easier for the inspector to see, measure and document.
In the magnetic-particle test, a steel piece suspected of being cracked is magnetized. Then iron filings are spread on the steel. The iron filings are drawn to the crack and make it stand out so it can be easily seen and documented.
Almost every bridge-inspection team routinely carries kits for the dye-penetrant and magnetic-particle tests when visiting a bridge site. These two tests are the most common and cost-effective nonvisual inspection methods.
Techniques for evaluating concrete bridges—such as using ground-penetrating radar, ultrasonic vibration, impact echo or acoustic emission—usually require the inspection team to leave the bridge site to fetch the equipment and then return to do the test.
On the bridge inspector’s wish list is a better, more cost-effective method for detecting what is going on inside of concrete, to know exactly when rebar starts corroding, how advanced the corrosion is or whether post-tensioning strands have started corroding inside their grouted ducts.
The perfect window into concrete eludes bridge inspectors for now, but they have plenty of excellent tools for detecting the elementary vulnerabilities of a bridge and eliminating risks to the traveling public. R&B
The world of bridge inspection was ready to take data on the road. When Bentley Systems released InspectTech Mobile, its iPad app for reviewing and collecting bridge-inspection data in the field, last fall the industry pounced and gobbled it up.
“The response of people downloading and trying it was the quickest uptake of any product that we’d ever had,” Ron Gant, global director of civil and transportation marketing for Bentley Civil, told ROADS & BRIDGES.
The mobile app makes it possible for bridge inspectors to take high-resolution pictures of the bridge and to view all previous photos of the bridge. The inspector can compare what the bridge looks like today with what it looked like last year and the year before and so on.
The inspector can carry an engineering model of the bridge to the bridge site and see how the deterioration of the bridge will affect the structure.
The inspector can enter data on the iPad and have it upload to the state’s inspection records and asset-management system. The data is immediately available for making decisions about maintenance.
InspectTech Mobile is currently available only for the iPad, but Bentley plans to expand it to Android and other mobile devices, but probably not smart phones.
“This is a market that the phone won’t quite work for,” according to Gant, because of the large forms that have to be manipulated and because the phone’s touch screen is awkward for a person wearing gloves. The tablets will have to be rugged, though, to withstand being hauled around in the field, paddling a boat underneath a bridge or hanging from a girder to get a good look at something.