Collpase of Silver Bridge in West Virginia spurred initial federal inspection program.
Just as West Virginia's Silver Bridge collapse in 1967 marked a new era for bridge inspections and awareness of U.S. infrastructure issues, so will Minnesota's Interstate 35W bridge collapse be another ante-upping chapter. The chapter is still being written. U.S. Dept. of Transportation Secretary Mary Peters has vowed a “top-to-bottom review” of federal bridge inspection guidelines. The specific structural issues that may be reshaped depend largely on what the National Transportation Safety Board will determine from its investigation.
Fatigue cracks, lack of redundancy, bearings corrosion, welding codes—a variety of possible factors have been thrust on the national stage. But engineers caution against premature theories regarding why the 40-year-old steel truss bridge collapsed Aug. 1. What does seem clear is that this will lead to updates in inspection guidelines, increased use of monitoring technologies and renewed attention to the complex issue of funding.
“One of the things that I would anticipate…is recommendations to change what we are doing in the way of inspection and monitoring bridges,” says Gene Corley, senior vice president of CTLGroup, Skokie, Ill.,
The Federal Highway Administration's National Bridge Inspection Standards require 2-year inspections of all public highway bridges with at least a 20-ft span. Federal regulations for inspections and load ratings started in 1971 after the 1967 collapse of the eyebar chain suspension bridge in West Virginia. In 1983 they were amended to include fracture-critical inspections after the I-95 Mianus River Bridge collapse in Connecticut. In 1987 came an amendment to include underwater inspections following the collapse of a New York State Thruway bridge due to scour.
In the Works
Some updates are coming, though not due to the collapse and not specifically for the bridge inspection program. Kelley Rehm, program manager for the American Association of State Highway and Transportation Officials' bridges and structures subcommittee, says new design load resistance standards are to be fully implemented for all new, federally funded bridges by Oct. 1. They are based on the statistical likelihood of “extreme events” such as hurricane-level winds, barge impact and seismic forces, and on newer types of truck loads. The new code should lengthen design life to 75 years, compared with 50 years, she says.
In the 1960s, she notes, designers focused on cost-efficient, lighter-weight bridges with fewer members to take expected loads. But that changed more than 30 years ago, Rehm adds. Since the early 70s, “they quit building the non-redundant structures."
Engineers search for clues to collapse of I-35W bridge (shown here in 2005), which could spur changes.
Many point to the fact that the I-35W bridge was non-redundant. Sena Kumarasena, associate vice president with HNTB Corp.'s Boston office, suspects failure of a fracture-critical member on I-35. If so, collapse could have been avoided by “making sure that there is sufficient redundancy within the structure by deliberately providing alternate load paths,” he says.
He notes that a 217-ft span of the I-794 Daniel Webster Hoan Memorial Bridge suffered a partial collapse Dec. 13, 2000 (ENR 1/15/01 p. 17). Two of three support beams failed with fracture-critical members, and the deck sagged 4 ft. But the third beam kept the bridge up until the span was demolished two weeks later in a controlled demolition.
Nicholas Altebrando, bridge practice leader for STV Inc., New York City, recalls seeing a steel truss during an inspection 28 years ago that held up despite a missing bottom chord member. “It would not be fair to say it's automatic” that a non-redundant truss should fail, he cautions. Robert J. Healy, deputy director with the Maryland State Highway Administration's office of bridge development, agrees: “Just because it was a deck truss bridge doesn't mean that was a reason why it collapsed.” Still, he sees merit in FHWA's request that all states now re-inspect their steel arch truss bridges. “The public needs to know that states can give them some comfort level that the bridges are safe.”
1940: Tacoma Narrows Bridge, Washington state
1967: Silver Bridge, Point Pleasant, W.Va
1983: Mianus River Bridge, Connecticut
1987: Schoharie Creek Bridge, New York state
1989: Hatchie River Bridge, Tennessee
2006: Autoroute 19 Bridge, Laval, Quebec
But what of funding for future maintenance of the 160,570 bridges rated structurally deficient or functionally obsolete by FHWA's National Bridge Inventory? “With the strain on transportation revenue, a debate on the national and local level needs to happen on how to sustain transportation funding,” says Ananth Prasad, chief engineer for Florida Dept. of Transportation.
With renewed attention from Congress, there may be some extra federal funding coming, along with higher levels of inspection, says Rehm. House transportation committee members have been asking how Congress might better identify and prioritize what federal highway funds are paying for, she notes.
House Transportation and Infrastructure Committee Chairman James Oberstar (D-Minn.) was scheduled to announce Aug. 8 in Minneapolis a proposed new trust fund to help rehabilitate the 6,175 structurally deficient bridges on the National Highway System. Details on how much funding would be involved, and where it would come from were not available at press time. Oberstar's proposal also calls for updated federal bridge inspection standards, immediate inspections and recalculated load ratings for all deficient NHS bridges. The NHS comprises about 160,000 miles of the country's major roads, including the Interstate system.
State DOTs seem confident of their current inspection programs. However, a county closed a 90-year-old, 700-ft-long steel truss span over the Meramec River in Missouri immediately, rather than later this summer. “The horrors of that disaster obviously weren't something we wanted to experience on a first-hand basis,” says Garry Earls, St. Louis County's highway and traffic director. St. Louis County had lowered the weight limit on Old Gravois Road Bridge from 5 tons to 3 tons, which motorists kept violating.
Missouri DOT, which is inspecting 11 of 14 deck truss bridges, notes that pressing needs motivated its Safe & Sound Bridge Improvement Program, which will repair or replace 800 of its worst bridges via design-build.
More than 60 of FDOT's 11,000 bridges have steel arch trusses, Prasad says. Some were inspected recently. He is debating whether to reinspect. “It's hard to inspect unless you know what you're looking for,” he adds. Only 273 Florida bridges are rated structurally deficient.
FDOT, with the University of Florida, is developing an electronic data collection technology embedded into piles to monitor pile capacity loads, Prasad says. Prevention of future catastrophes will include a robust inspection program, funding, continuous improvement in specifications and “a very good monitoring system,” he concludes.
Jean-Louis Briaud, manager of Texas Transportation Institute's Geotechnical and Geoenvironmental Program, says TTI is developing a methodology for better predicting erosion around bridge pilings. TTI estimates 600 of Texas DOT's 50,000+ bridges are at risk of failure due to scour.
Some 2,125 Texas bridges are rated “structurally deficient.” TxDOT is inspecting 16 bridges similar to the I-35W bridge. Randy Cox, state bridge engineer for TxDOT, says that problem bridges are shut down or given weight restrictions. A 2006 audit by the U.S. DOT said Texas failed to post maximum weight limit signs on some aging bridges and had unclear weight maximum calculations for certain bridges, but TxDOT says the audit was based on a small sample gathered more than five years ago.
Massachusetts state inspectors have reviewed inspection reports on its 24 steel arch truss or steel deck truss bridges. No problems were found, but 10 structures will receive hands-on inspections.
New York State's DOT is working with the state's Thruway Authority and Bridge Authority to re-inspect 49 bridges with truss designs similar to I-35W. These include major I-87 structures such as the Major Deegan Expressway and the Tappan Zee Bridge, I-84's Newburgh-Beacon Bridge, I-90's Patroon Island Bridge in...