How To Stop Bridges From Falling Down

Using technology instead of manual inspections will detect structural issues in bridges before they bring on disasters

By Doug Thaler

The ravages of time leave their scars on everything, including highways and bridges. Most of America’s highways and bridges were built in the 1950s, and are now old and crumbling. Years of bearing the weight of relentless and growing traffic have taken their toll. Many of our bridges are in dire need of repair. Some require outright replacement.

In 2017, the updated National Bridge Inventory counted a total of 607,380 bridges in the US. Of these, 65,605 are “structurally deficient,” 20,808 are flagged as “fracture critical,” and 7,795 more bridges are flagged as both “structurally deficient” and “fracture critical,” placing them at a higher level of danger of collapsing. If a bridge is “fracture critical,” compromising of just one of its vital components is sufficient for the bridge to collapse without warning.

These assessments should have pushed highway authorities into action, but life goes on as usual, and 88 million trips on average are made across our bridges every day. It is our nature to wait till the last tree is standing before paying attention. Thankfully, our ingenuity also assures us that we will figure out a way to repopulate those trees overnight when there is dire need. When it comes to our infrastructure and bridges, the problem is that not attending to their maintenance can have dire public safety and economic concerns.

And major accidents have happened.

The nation’s attention was initially drawn to the condition of its older bridges following the collapse of the Silver Bridge between Virginia and Ohio in 1967. The bridge collapsed during rush hour and 46 people died. A crack in a single link had led to the collapse. An investigation found that apart from poor maintenance, the bridge was carrying much heavier loads daily than it had been designed to carry.

The collapse inspired the first legislation for regular inspection and maintenance of bridges. In 1971, the Federal Highway Administration, in the National Bridge Inspection Standards, laid out minimum requirements for inspecting highway bridges. The main instrument to be used was visual inspection.

On August 1st, 2007, Interstate 35W Bridge over Mississippi River suddenly caved in during rush hour traffic. Thirteen people died, 145 others were injured, and 111 vehicles were destroyed in this catastrophe. Following a National Transportation Board investigation, the cause of the collapse was traced to a design error involving steel connectors known as “gusset plates,” which should have been 1” thick, but were only ½” thick. This design flaw was overlooked because it was apparently not standard practice for bridge inspections to seek out design errors. If technology had been used instead of manual inspections, quantitative data might have indicated this abnormality.

The subjective nature and lack of quantitative data associated with visual inspections generally does not do enough to reflect ground reality. In addition, federal and state guidelines for manual inspection of bridges are about 50 years old, and due to budgetary concerns a significant number of bridges go beyond the stipulated two years without safety inspections.

If, instead of manual inspections, engineering firms and asset owners incorporated modern technology and robotics to conduct the biennial inspections, the asset managers and engineering firms would have substantially more data to monitor deterioration progression over time and address issues before it is too late. For example, Infrastructure Preservation Corporation’s BridgeScan™ can conduct a complete condition assessment of a bridge deck and approach roadway, looking for deterioration, delamination, voids, water intrusion, rebar placement, cover density and more versus the current visual inspection of walking a bridge deck and looking for cracks. What’s more, these inspections can be conducted within the current DOT’s maintenance budgets.

Systematic technological inspection processes provide specific data to categorize bridge conditions and to evaluate specific needs in terms of repair, retrofit, upgrades or replacements. For instance, the post tension tendons that help hold up many of our bridges and segmental structures are still inspected visually or by hitting the tendon with a hammer and listening for changes in the sound. IPC’s TendonScan® peers through the tendon locating both the precursors of deterioration (air, water, and bleeding grout) as well as loss of metallic area. It provides the action plan with what to repair and when.

Nondestructive testing and robotic bridge inspections will also yield quantitative data that will uncover issues at an early enough stage to avert disasters as well as prolonging the service life of these critical infrastructure assets. What is more, employing technology will actually trim costs and enable authorities to redirect more funds for repair and maintenance tasks.

Doug Thaler is President of Infrastructure Preservation Corporation, a nondestructive testing and robotic engineering firm that has developed patented robotic technology to provide asset managers with quantitative assessments for better allocation of assets and to preserve service life of critical infrastructure assets. To learn more please visit www.infrastructurepc.com

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