Reliving the nightmare

July 30, 2008
“Complacency must be fought. America‘s aging bridges need regular monitoring, preventive maintenance and, in some cases, intensive care.” -Ray McCabe, national director of bridges and tunnels, HNTB Corporation One can only imagine the instant and ...

Complacency must be fought. America‘s aging bridges need regular monitoring, preventive maintenance and, in some cases, intensive care.” -Ray McCabe, national director of bridges and tunnels, HNTB Corporation

One can only imagine the instant and overwhelming horror when the I-35W highway bridge crossing the Mississippi River north of Minneapolis began to give way back on Aug. 1 last year. Thirteen people died and some 145 were injured when it collapsed, crushing several cars and commercial vehicles under cascading tons of steel and concrete rubble, while pitching others like toys into the dark waters below.

(The wreckage of the I-35W collpase.)

What did those drivers think as the awful situation gripped them? For many, no doubt, the terror seized them after the collapse occurred, as they suddenly found themselves struggling to escape mangled vehicles tat just moments before were gliding unobtrusively to mundane ends - returning home from a busy day at the office, delivering freight, etc.

That a highway bridge of this size and scope could implode so suddenly, so viciously, caught everyone by surprise, though it shouldn‘t have. According to a report released this week by the American Association of State Highway and Transportation Officials (AASHTO) called “Bridging the Gap,” one out of every four of the 600,000 highway bridges in the U.S. needs to be modernized or repaired - despite the best efforts of state and local departments of transportation.

(An aerial view of the remains of the I-35W last August.)

AASHTO estimates the bill for such a repair order would top $140 billion if we were to start today - a staggering sum of money, but one that indicates a lot of problems have been ignored where our highways bridges are concerned for a long, long time. It‘s also important to recognize that highway bridges are a vital link within the U.S. transportation system, allowing it to operate seamlessly. Yet more than half of them were built prior to 1964 and of the 600,000 public road bridges in the country, some 74,000 - roughly 12% of them - are classified as structurally deficient.

“The I-35W bridge collapse in Minneapolis was a national tragedy and wake up call that America‘s older bridges, especially those on the National Highway System, require a systematic approach to ensure their safety,” said Ray McCabe, national director of bridges and tunnels for the HNTB Corporation, a construction company. “The probability of a bridge failure like this one is extremely low, but it isn‘t zero. It should be, except for failures due to an extreme event, such as a collision, an earthquake or a fire.”

(Divers got the worst possible duty after the bridge imploded -- canvassing the dark waters for bodies.)

OK, sure - McCabe and his company have some bias here as they build and repair bridges for a living. No doubt they see themselves getting a big slice out of that $140 billion pie should Congress and the President agree on a bridge repair plan of that magnitude. Yet McCabe‘s been in the “bridge business” for over three decades now and is a respected authority on the structural design, inspection and rehabilitation of long-span, movable, signature and complex bridges. He knows his stuff and knows just how deadly the consequences can be if highway bridges are taken for granted.

Take the old I-35W bridge for example - it carried more than 140,000 vehicles a day, and the loss of the bridge has been costing $400,000 per day in lost revenue, increased commuter expenses and burden on surrounding roads, according to Minnesota‘s Department of Transportation (MinnDOT). The collapse of the bridge affected not only road transportation, but also river, rail and air transit. MinnDOT noted its replacement, called the St. Anthony Falls (35W) Bridge, cost $234 million and should be ready for opening by next week.

While the National Transportation Safety Board (NTSB) has not officially released a final report on the collapse (it expects one to be completed by late fall this year at this point), its preliminary findings regarding the I-35W collapse released this January indicated that a design error led to the use of undersized gusset plates - some 16 in all - to hold the bridge together. Gusset plates are the large, flat steel connectors that hold load-bearing columns in place, tying together the angled steel beams of the bridge‘s frame - and the design error in using plates that were too small dates back to the original blueprints for this bridge, drawn up in the late 1960s.

(Few realize how much destruction can be caused when a highway bridge collapses.)

HNTB‘s McCabe says it is clear from this disaster that the U.S. must reassess its bridge inspection program and develop a risk-based approach to addressing the nation‘s deficient bridges.

“We do know that the bridge was inspected according to federal standards. Improving inspection procedures and techniques will allow us to better allocate available resources,” he notes. “It‘s equally important that the information gathered from inspections must be applied to a well funded and focused program of bridge repair and replacement to prevent future disasters.”

McCabe advocates a three-step plan to make that happen:

-- Make improvements to the bridge inspection and ratings system. Bridge inspections are generally visual, which lead to subjective conclusions. More rigorous training and certification programs must be enacted. Inspection frequency and inspector qualifications for any bridge should be based on the bridge‘s risk of potential collapse and the corresponding impacts.

-- Establish a dedicated method to allocate funding for structurally deficient bridges. More money is a necessary part of the solution, yet he stresses the money needs to be spent based on safety and prioritized using a risk-based approach. How new federal monies get distributed to states is critical, too, as states that have invested wisely in their infrastructure and have a small inventory of deficient bridges should not be penalized.

-- Apply advanced technologies, techniques and materials. New bridge designs and the rehabilitation of existing bridges must make full use of innovative technologies and more durable materials, such as high-performance concrete and steel.

(Ray McCabe, testifying on Capitol Hill about bridge safety.)

“Today‘s bridges need to diffuse loads and absorb stresses more effectively,” says McCabe. “New technologies, such as fiber optic sensors embedded in the bridge infrastructure, also provide more quantitative data and aid in prioritizing funding and repairs.”

An investment of more than $32 billion is needed to maintain the bridges on the National Highway System alone. McCabe emphasizes, though that this figure, while important, doesn‘t begin to address the growing 21st century demands of congestion relief and economic development.

“The way to ensure the safety of our nation‘s aging bridge infrastructure is not just additional funding or rigorous inspection or advanced technologies. It‘s putting all three to concerted use,” he stresses. “Let‘s not wait for the next failure.”

Amen to that.

About the Author

Sean Kilcarr 1 | Senior Editor

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