Functionally absolute

May 11, 2009

The collapse of the I-35W Bridge in Minneapolis in August 2007 briefly diminished public confidence about the nation’s aging infrastructure.

Among engineers and lawmakers alike, questions emerged as to whether other pieces of America’s infrastructure would soon fail, whether it had the funding and other resources it deserved and, more directly, whether the more than 600,000 highway bridges in use across the nation were safe.

The collapse of the I-35W Bridge in Minneapolis in August 2007 briefly diminished public confidence about the nation’s aging infrastructure.

Among engineers and lawmakers alike, questions emerged as to whether other pieces of America’s infrastructure would soon fail, whether it had the funding and other resources it deserved and, more directly, whether the more than 600,000 highway bridges in use across the nation were safe.

Immediately, states assessed the reliability of their own bridges—chiefly, steel-deck truss highway bridges like the I-35W Bridge—and a picture began to emerge that surprised many.

Bridges across the country were—and are—safer than previously thought.

However, due to the attention given to bridge safety by the nation’s media, bridge engineering terms like “structurally deficient” and “functionally obsolete” were inaccurately reported and served to further complicate efforts to reassure a nervous motoring public. Neither term is a reflection of a bridge’s safety but, rather, are technical terms used to determine how best to address a bridge’s needs—through rehabilitation or replacement.

Swayed to inspect

On Dec. 15, 1967, the Silver Bridge—a 2,235-ft-long connection between Point Pleasant, W.Va., and Kanuga, Ohio, collapsed. The bridge opened in 1928 and was dubbed the “Gateway to the South.” The Dec. 22, 1967, edition of Time magazine said the bridge “swayed sickeningly to every vagrant breeze—so much so that Point Pleasant Mayor D. B. Morgan banned its use during parades.” At 4:50 p.m., all but six of the 37 vehicles on the bridge fell into the Ohio River or onto its banks. Forty-six people were killed.

Spurred by the disaster, federal officials created the National Bridge Inspection Standards (NBIS). As a result, bridges are now to be inspected at least every 24 months, and many are inspected more frequently than that.

Though there have been relatively few bridge collapses, some have expanded the knowledge base that enriched the NBIS and the inspection guidelines and training used today.

For instance, in April 1987 a 200-ft section of the 30-year-old Scoharie Creek Bridge collapsed on the New York State Thruway near Amsterdam, N.Y., killed 10 people. This incident identified the problem of scouring on underwater bridge supports and led to bridge inspections including scouring evaluations. Scouring is the erosion of streambed or embankment material by flowing water. It is often considered as being localized around piers and bridge abutments. When enough sediment has been removed, there is the risk of support failure and bridge collapse.

In May 1980 the 11-year-old Sunshine Skyway Bridge in Tampa Bay, Fla., collapsed after it was rammed by the S.S. Summit Venture. The ship, a 19,734-ton freighter on the last leg of its journey along the Gulf of Mexico, lost its radar in a tropical storm. While trying to make a difficult 13° turn between the bridge’s two main piers, the Summit Venture slammed into the bridge, sending 1,200 ft of roadway, including several cars and a Greyhound bus, into Tampa Bay. The incident claimed the lives of 35 people and heightened awareness of the threats to bridges posed by water traffic.

Two other collapses were key to NBIS enhancement over the years: Connecticut’s Mianus River Bridge in June 1983, which led to greater emphasis on fracture-critical bridges, and Alabama’s U.S. 43 Chickasabogue Bridge near Mobile in April 1985, which served to strengthen underwater bridge inspections.

As a result of such tragedies, bridge inspectors began—and have since refined—the world’s most rigorous bridge inspection program. The Federal Highway Administration (FHWA) made bridge inspection criteria among its first achievements and issued the first national bridge inspection regulations in April 1971.

The National Bridge Inspection Program has grown more sophisticated since then, but continues to rely on time-tested techniques like chain-dragging and hammer-sounding. It also depends on newer technologies, like ground-penetrating radar and even dyes, which are central to noninvasive fracture tests. The federal program continued when the BPR was renamed the FHWA in 1969 and included in the U.S. Department of Transportation.

The one constant element, however, is the presence of a well-qualified bridge inspector. No matter how far technology may advance, visual inspections by experienced bridge inspectors can be supplemented but not replaced.

A study by Kumalasari Wardhana and Fabian Hadipriono of Ohio State University in 2005 looked at 503 bridge failures in the U.S. between 1989 and 2000. The bridges ran the gamut, from those designed to carry pedestrians over roadways to floating pontoon bridges across lakes. They defined “failure” as anything from collapse to damage so serious the bridge had to be closed.

They reported that 53% of bridge failures came during floods, when raging waters produced excessive scour or punished the structures with debris. Earthquakes accounted for another 3%. Collision with a car or boat was the second-leading cause of bridge failure, they wrote, accounting for about 12%. Car or truck collisions accounted for 14 bridge failures over that 11-year period. Ships or barges were responsible for 10, and trains for three.

Considering the volume of bridges in service at any one time, there are relatively few bridge tragedies. However, of the few that have happened, each offers new insights to bridge builders and bridge inspectors alike on how to further safeguard such structures.

The original Sunshine Skyway Bridge was replaced in 1987 with a 29,074-ft-long cable-stayed concrete bridge. Among its safety features, the bridge’s approaches are protected by “dolphins,” large concrete barriers that protect the bridge’s piers from collisions with freighters even larger than the Summit Venture.

When the replacement Scoharie Creek Bridge opened to traffic in December 1987, it featured numerous enhancements, such as pilings driven deeper, to protect the foundation from scour.

In 2003’s “Lessons Learned from the Scoharie Creek Bridge,” the American Society of Civil Engineers concluded, “Bridges across waterways must be designed structurally not only to carry their own weight and traffic loads, but also to resist the hydraulic forces imposed by rivers and other bodies of water. Moreover, the construction of the bridge abutments and piers alters the river’s flow, and may lead to new patterns of erosion and deposition. The collapse of the Schoharie Creek Bridge illustrates the importance of designing bridge piers to resist scour. The case also illustrates the importance of the inspection and maintenance of bridges.”

Clever crossings

By all accounts, the biggest challenge for bridges is not weather, scouring, seismic activity or any of the many factors bridge inspectors evaluate. Bridges, instead, struggle to adapt to changing traffic volume. Roads can be, and frequently are, easily widened to accommodate more lanes as traffic volume increases. Bridges are durable, but they are not easily adaptable. In such circumstances, bridge engineers are forced to either replace an existing bridge with a wider one or to simply build a second bridge near the original to provide the additional capacity needed.

To strengthen the service life and overall durability of bridges while improving adaptability, motorists of tomorrow will rely on new technologies, new materials and new design paradigms that result in multiple-hazard-resistant bridges. Such bridges will cleverly take advantage of new systems and better utilize common elements while keeping construction and maintenance costs low.

Though the National Bridge Inventory shows that approximately one in four bridges is rated as “structurally deficient” or “functionally obsolete,” it is important to note that none that is unsafe remains open to traffic. Moreover, TIFIA loans and new funding through the American Recovery and Reinvestment Act are helping states repair or replace such bridges to extend their useful life. By all accounts, more attention is being given to the state of America’s infrastructure now than at any other time since, perhaps, the creation of the Interstate Highway System in 1956. Work is under way, and the nation’s roads and bridges will be better for it.

American bridges are clearly still making the grade—and, thanks to the vigor of the inspection guidelines and the thousands of inspectors examining them, bridges long will remain a critical part of America’s future.

About The Author: Everett and Ernst, both professional engineers, are senior members of the Federal Highway Administration’s Office of Infrastructure, Washington, D.C.

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