By: Seth Medwick, P.E., and Paul Kaufman, P.E.
On Aug. 2, just one day after the I-35W Bridge collapse in Minneapolis, the Federal Highway Administration issued a technical advisory urging departments of transportation and other bridge owners to conduct immediate inspections of all steel-deck truss bridges with fracture-critical members, even if they had been inspected just last year.
“At a minimum, state transportation agencies and other bridge owners should review inspection reports, including those for routine, in-depth, fracture-critical and underwater, to determine whether more detailed inspections are warranted,” the advisory stated.
Following the advisory, requests for emergency re-inspections surged. Ironically, these reinspections relied on the same inspection techniques used previously. DOTs did not ask engineers to use the latest technologies to aid their inspections.
Advancements in bridge inspection technology can provide bridge owners with clearer, more accurate pictures of their structures’ conditions, helping them make more informed decisions about how best to allocate resources and safeguard motorists.
Agencies know these technologies exist but have been slow to adopt them for several reasons. First, some agencies and engineers are reluctant to adopt new technologies because of the risk inherent in using what may be perceived as unproven technology. Others believe the current tools of the trade have served the industry well and are reluctant to change for change’s sake. Second, many newer technologies are designed for specific applications, not routine inspections, making them irrelevant in that application. Third, new technology traditionally is expensive. Cost, combined with the newness and limited application, lead most cash-strapped DOTs to view new bridge inspection technology as a luxury they simply cannot afford.
However, all that could be changing.
Galluping support
A convergence of industry developments may increase acceptance of new bridge inspection technology. Decreasing costs and wider availability of new technology are contributors to wider acceptance. Equally influential is an increased awareness of the diminished state of our nation’s bridges due to recent structural failures.
More than half of the respondents of a national Gallup Poll, which surveyed 1,012 Americans soon after the I-35W collapse, said they thought the disaster was indicative of broader problems rather than an isolated incident. More than seven in 10 Americans said they favored Congressional legislation to spend more than $100 billion to repair and rebuild the nation’s bridges.
If public opinion about our bridges is changing, political agendas and appropriations may change, too. Just one day after the Minneapolis tragedy, the Senate unanimously passed the National Infrastructure Improvement Act of 2007—legislation to address the deteriorating condition of America’s roads and bridges, among other infrastructure.
Pulsating results
As the number of proponents grows and purse strings loosen, DOTs and other bridge owners can seriously consider the new bridge inspection technologies now available to them, such as magnetorestrictive testing.
Until recently, a structural engineer often was forced to make judgment calls while inspecting certain aspects of a bridge. When examining a suspender rope, for example, the inspector might count the number of broken wires around the perimeter of the rope and approximate the overall section loss. He then would compare those values to predetermined cutoff values, which are developed for the inspection and are based solely on the condition of the visible outside wires. Since the condition of the interior wires cannot be visually determined, the engineer must make a judgment call as to the overall condition of the rope. Perhaps it is the right call. Perhaps it is not.
Magnetorestrictive testing takes the guesswork out of inspecting hidden details, such as the condition of interior suspender rope wires. Magnetorestrictive testing is a nondestructive testing technique used on suspender ropes to detect section losses or broken wires on either the interior or exterior of a rope. A transducer is attached to the rope and sends a guided acoustic pulse into the rope. Where there is a defect in the rope resulting from corrosion or fatigue damage, a portion of the pulse is reflected back to the source. These reflections are collected via a laptop computer and can be measured and quantified to determine the relative size and location of the corrosion or fatigue damage in the rope. Decisions to replace the suspender rope then can be based on hard evidence and not expert conjecture.
There are others
Magnetorestrictive testing, a decidedly high-tech method, already has become a proven practice. The following are other techniques that may or may not be considered high-tech but still are highly effective.
Strain gauges are useful in assessing the load-carrying capacity of a structure when existing conditions are unknown. For example, during one inspection, a consultant was asked to load-rate a bridge composed of prestressed concrete beams; however, no design drawings or as-built drawings were available to reference. Without that information and because he could not see the reinforcement and the tendons inside the beams, a load rating performed conventionally was not possible. However, a load rating was possible with the aid of strain gauges. During that particular inspection, strain gauges were attached to all of the beams and then trucks of known weight were run over the bridge. The data collected was used to “back into” the structure’s carrying capacity.
Additional inspection best-practice advancements include ground-penetrating radar, which determines delamination of bridge decks hidden by a wearing surface, and magnetic particle testing, which evaluates the extent of cracks in steel members.
Other techniques that are not quite high-tech but still deserve to be considered as best practices are:
- D-meter testing for steel thickness: A digital meter with a transducer attached to it, a D-meter essentially is an ultrasound machine. The transducer on the D-meter sends sound waves into the known metal. The D-meter measures the time it takes for the sound wave to bounce back. This time is then converted into a thickness;
- Fiber-optic borescopes with photographic and video capabilities, which help engineers visually inspect hard-to-reach areas; and
- Digital moisture meters, which help identify rotting timber.
Built for looks
In addition to new technologies, changes in bridge design are aiding inspections. Today’s bridges are being built with self-monitoring devices that alert agencies of deterioration or excessive load levels. Sensors embedded in concrete decks report chloride ion content, saving owners the time and expense of extracting core samples for testing. Today’s bridges are being designed with alternate load paths, making them structurally redundant to significantly reduce the risk of collapse. Engineers are designing in 3-D, allowing them, for example, to create trusses where the top and bottom diagonals carry load. The Cooper River Bridge in Charleston, S.C., was designed this way.
In addition to design advancements, today’s bridges have inspector-friendly features, such as:
- Box girders with access holes or hatches, so engineers can get inside. In fact, designers are moving away from covered or encased details in general. Some engineers even create foam board mock-ups of their designs to test accessibility;
- Traveler systems, or permanent motorized scaffolding systems, on long-span structures, such as suspension bridges and large through trusses;
- Piers with catwalks and ladders to make accessing bearings and pedestals easier and cheaper, as lift trucks are no longer needed; and
- In New York, deep girders are designed with handrails, so engineers can inspect by walking along the girder’s bottom flange.
Finally, there also have been advances in access technology, or how engineers access a structure for inspection. One technique growing in popularity is rope access, an extension of rock climbing. It has been used extensively in oil rig inspections in the United Kingdom and now is gaining popularity among bridge inspection engineers. With rope access methods, structural engineers climb ropes, allowing them to gain hands-on inspection of areas previously not easily accessible by more commonly used means such as bucket trucks or scaffolding. The Metropolitan Transit Authority Bridges and Tunnels in New York first used this method to inspect bridges in New York City in 2003. Since then, it has been gaining popularity with inspection consultants and agencies alike.
Technology is like water. It has found its way into our industry through small openings, such as design and access, despite our general hesitation. Fortunately for the safety of our nation’s bridges, those openings may be widening, allowing new technologies to flow into bridge inspections. It is a natural progression and a needed practice as bridge owners strive to ensure the safety of their structures.
About The Author: Medwick is manager of bridge inspection services and Kaufman is a senior project engineer. Both are employed by HNTB Corp.
