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American Society of Civil Engineers
Asset Management Publication
Structural Health Monitoring for Bridges
Peter J. Vanderzee
President and CEO
Frank B. Wingate, P.E.
Evaluating the health of large bridge assets has been an important component of bridge management for nearly 35 years. The Federal Highway Administration (FHWA) instituted mandatory visual inspections in 1972 as a result of the Silver Bridge collapse. Visual inspections are an expensive undertaking for owners, considering the U.S. has nearly 600,000 bridges in its transportation system. Over the past decade, questions have been raised about the efficacy of visual inspection, since that process drives billions of dollars in capital expenditures from State taxes and Federal funding. With the availability of rugged and highly accurate sensing devices, wireless communication and Internet connectivity, agencies now have the opportunity to obtain more accurate, objective, and timely performance information from bridges, supporting better financial decisions. When the objective information captured by a structural monitoring solution is used to evaluate a structure’s health, engineers are finding that structures exhibit quite complex behavior and could generally be in better health than visual inspection might suggest. That fact alone suggests there is value in the use of better diagnostic information before millions or billions of dollars are committed to repair or replacement. The focus of this paper is to discuss the value of using structural health monitoring to support decisions for repair or to replace perceived deficient assets, in particular, bridges.
Much effort has been expended over the past decade by the American Society of Civil Engineers (ASCE) to compile the biennial Report Card for America’s Infrastructure. While the total infrastructure financial ‘need’ now exceeds $1.5 trillion dollars over the next five (5) years, we can’t rest easy knowing that bridges ‘only’ need $200 billion over the next twenty years to eliminate perceived deficiencies. The ASCE should be commended for originating and promoting the concept of a national infrastructure report card; it has been useful for focusing the nation on this critical issue. But, to use an academic analogy, a C- at a program with substantially more rigorous requirements may be the grading equivalent of a B+ at a program with less rigorous requirements. Grades often depend upon criteria that are not readily apparent to the casual observer unless expressly articulated. The same principle applies to grading our nation’s infrastructure and the use of those grades to support funding requests for infrastructure renewal.
As consumers, we know that standards and best practices matter, especially when investing large sums. For example, the automobile buyer wants an objective understanding of price, quality, reliability, warranties and guarantees, and similar components of the transaction before making a buying decision. Those who have made decisions of this magnitude intuitively know the right questions to ask, websites to explore, rankings to study, and fine print to read. Sophisticated buyers don’t make major investment decisions using limited, subjective information. As asset management standards continue to evolve, infrastructure stewards have the opportunity to continue to improve their investment decision making processes by integrating explicit and objective criteria and standards for decision making.
For nearly 35 years, the Federal Highway Administration (FHWA) has required public owners to visually inspect highway bridges biennially or more often if conditions warrant, for structural sufficiency. This program is based on the National Bridge Inspection Standards (NBIS). The Federal Railroad Administration (FRA) also requires the railroads to inspect their bridges. At first glance, one might conclude that all this inspection effort is cost-effective, since bridges don’t fail, at least not very often. But, can that outcome be directly related to inspection expenditure or some other factor that may not even be considered by the responsible agency? Recent published articles, studies and papers; authored by experts, have characterized visual inspection protocols as subjective, highly variable and overly conservative. Perhaps perception is not always reality for bridge inspection.
Unfortunately, visual bridge inspection protocols do not effectively support repeatable, accurate, objective standards that can be used to assess the need for repair or replacement, and the programmatic expenditure of billions of dollars in limited funding. Here’s what others have been saying about this issue:
- “While the inspections associated with this (NBIS) effort provided useful information about the nation’s bridge inventory and their condition, it was insufficient to make reliable programmatic decisions in the face of dwindling funding dollars”. (1)
- “Simply looking at a bridge probably will not determine, for example, whether it has been overloaded. Frozen bearings, corrosion, and fatigue damage can exist without visible indications.” (2)
- “Load rating data for 40.5% of the structurally deficient National Highway System bridges did not match the bridge inventory. In these cases, important decisions on maintenance priorities are impaired.” (3)
Visual bridge inspection is inherently conservative by design. Consider a typical bridge inspector’s mission; they are not paid to take chances with public safety, so they are rightly trained to be conservative. In addition, Federal funding for bridge replacement is biased toward ever worse condition ratings, i.e. the worst bridges get top priority for funding. And, given abundant Federal funding in the past, there are few incentives to up-rate a bridge when lower ratings garner more Federal funds. The agency’s staff structural engineer compounds the visual inspection dichotomy by using the subjective, conservative inspection data as input for a highly conservative, prescriptive analytic protocol to determine bridge sufficiency. Finally, what are the implications of these statements for agencies with constrained budgets?
In short, bridge inspectors and DOT staff engineers are the de facto decision makers for maintenance, repair or replacement of bridges — not the Chief Bridge Engineer, CFO, or DOT Commissioner; despite their executive decision authority. When inspectors and staff engineers speak, DOT management must listen, because, without the use of more objective asset assessment technology, there is no basis to challenge visual inspection findings. In essence, the visual inspection protocol (NBIS), coupled with highly conservative analytics cannot peacefully co-exist given limited funding for transportation. One has to ask: Are there better asset assessment tools that can be used to justify project need and prioritization relative to the available funding? Structural monitoring is one such tool. It offers objective information to improve the effectiveness of investment decisions when the potential payoff of obtaining more objective information justifies the additional expenditures to conduct a structural monitoring program.
Structural monitoring is an asset assessment tool that is used to gain an objective understanding of a structure’s health, monitor the progression of deterioration, or expand the safe operating envelope of a bridge or other structure. Structural monitoring is enabled by the innovative combination of unique sensing devices, wireless communication, and the Internet. While not intended to replace visual bridge inspection, structural monitoring should be considered best practice for objectively determining the actual condition of a bridge, after visual inspection rates a bridge as structurally deficient and/or determines that certain bridge elements are suspect (such as cracks in girders, out of plane bending, heavy corrosion, or fracture critical details).
From long-term monitoring data (more than one thermal cycle), structural consultants can use the captured information to build a calibrated finite element model (FEM). Using the mathematical model, DOT engineers can accurately and conclusively diagnose structural health, determine the location of hot-spots of concentrated stress, see the unanticipated effects of thermal loading, determine safe load carrying capacity, and objectively analyze other relevant bridge management issues. In essence, the DOT staff engineer has been transported to a whole new realm of understanding about how a bridge performs under actual service conditions. Better financial decisions follow.
Here are two examples highlighting the benefits of structural health monitoring:
Example #1: A tolled roadway received a third party (outsourced) visual inspection (NBIS) report that concluded observed corrosion of steel members was sufficiently severe to recommend a steel refurbishment project, strengthening the affected steel to safely carry anticipated service loading. When faced with a report from competent professional engineers, the agency had little choice but to follow the recommendation, or face increased liability exposure and possible failure, e.g. the visual inspection effect. The agency evaluated several contractors’ proposals for the steel repair; the lowest price was $875,000. Ready to award the contract, the agency decided to first install a structural monitoring solution to assess the effectiveness of the repair program by monitoring the bridge for six months prior to repair and six months after the repair had been completed.
After six months of monitoring, but before the repair started, the agency’s third party engineer had enough information to calibrate a finite element model of the bridge. Upon completion of the modeling, the owner’s third party engineer fully analyzed the structure and determined that the steel repair program was not necessary. However, the third party engineer concluded that the structure had another serious deficiency that had not been reported on the original visual inspection report. Faced with mounting pressure, the agency instead decided to take the safest course and implement the steel repair program. After six more months of monitoring at a total cost of approximately $125,000, including subsequent detailed structural assessment, the agency’s engineer concluded that his initial recommendation was correct; there had been no measurable improvement in bridge performance as a result of the $875,000 steel repair program. This case illustrates the potential for improving the cost-effectiveness of bridge maintenance and repair by selective identification of bridges that are potentially good candidates for structural health monitoring. The result: the agency could have saved over $800,000 had the structural health monitoring results been considered in the final action taken on the bridge, and these resources could have been invested in other areas of much greater need in the agency’s bridge inventory.
Example #2: A State DOT performed expensive repair actions to relieve perceived high stress and observed fatigue cracking in localized areas (crack stopper holes and steel retrofits) as a result of fracture critical structural construction details. Monitoring of key areas on the structure for seven months allowed the owner’s third party engineer to conclude that most of the retrofits were successful (good news), but that a severe hot-spot remained, which apparently was made worse from the retrofit, not better (bad news). In addition, thermal loading caused high stress excursions, indicative of bearing constraints that were not performing as designed. The owner asked the monitoring firm to reposition a number of sensing devices and continue the investigation with the third party engineer to resolve the hot spot and better understand structural behavior under service conditions. The objective for continued monitoring and analysis is to safely extend the life span of this bridge, which would seriously disrupt Interstate traffic and cost in excess of $100 million dollars to replace. The proven result: total expenses for this program will be less than one month’s interest on capital if the structure had to be replaced too early.
These examples demonstrate the value of using structural monitoring, a more precise, objective asset assessment tool available to transportation asset owners. Importantly, like most specialized tools, structural monitoring is not necessary for every structure, but rather for the structures that, when properly and objectively assessed, have a high probability of producing significant cost savings for the agency.
Some guidelines for best practice use of the structural monitoring asset assessment tool include the following:
- When a major repair program is being considered for superstructure corrosion (loss of section), typically exceeding $300,000 dollars.
- When a major replacement project is being considered for superstructure structural deficiencies that exceeds $2 million dollars.
- When an inspection-revealed structural member reveals cracking on concrete or steel that should be watched closely for increased public safety.
- When unexplained member movement (bending) is revealed by visual inspection and is apparently the result of service loading.
- On short span bridges having load postings that severely restrict commercial traffic.
- On bridges that are frequently permitted for heavy loads, to objectively determine if the heavy load movement has damaged the bridge (insurance recovery).
- On concrete bridges with embedded steel tensioning tendons that can be attacked by water and salt intrusion, resulting in loss of section from corrosion and subsequent loss of load carrying capacity.
- On bridges that are structurally deficient, but simply cannot be replaced due to lack of funding – Immediate warning alerts can be sent to responsible parties 24/7 in the event of high observed stresses or other deleterious events to protect public safety.
Improved asset management for transportation assets is driven by better information and use of best practice, situation-appropriate asset assessment tools. The visual inspection protocols mandated by the FHWA serve their purpose by providing early warning of certain structural deficiencies. However, given the new Federal funding paradigm, agencies must do more with less and vigorously adopt better transportation asset assessment tools. Asset management agencies must encourage and adopt better ways to safely extend asset life, drive life cycle costs lower, and safely defer repairs and replacements. It won’t be easy, but there is no other rational choice. The ultimate asset owners, the American taxpayers, deserve the financial benefits derived from safely extending asset life without resorting to unnecessary tax increases.
1 “Condition Assessment of Highway Structures – Past, Present, and Future”; Walther and Chase; Transportation Research Circular E-C104; September 2006.
2 “Bridging the Data Gaps”; Ghasemi; Public Roads; May/June 2007.
3 “Audit of Oversight of Load Ratings and Postings on Structurally Deficient Bridges on the National Highway System”; FHWA IG; Report Number MH-2006-043; March 2006.