Minneapolis bridge collapse in 2007 caused by "Design errors" concludes NTSB

NTSB: Design errors caused 2007 bridge collapse

By FREDERIC J. FROMMER and JOAN LOWY – 21 hours ago

WASHINGTON (AP) — Undersized steel reinforcing plates were cited Thursday as the chief cause of last year's deadly collapse of a highway bridge in Minneapolis. Federal investigators also said the plates were overstressed by almost 300 tons of construction material piled on the bridge.

Investigators told the National Transportation Safety Board that the collapse of the Interstate 35W bridge on Aug. 1, 2007, was unavoidable once gusset plates in the center span failed.

When that happened, it dragged other sections of the bridge and rush-hour commuters into the Mississippi River, killing 13 and injuring 145. The plates are commonly fused to intersecting beams to reinforce the connection.

Investigators focused on the U-10 gusset plates, which were designed at only half the required thickness. But they also discussed the construction materials on the center span over the Mississippi River.

"Had the gusset plates been properly sized, this bridge would still be there," said Bruce Magladry, director of the NTSB's office of highway safety.

Board members criticized Minnesota officials for allowing 287 tons of construction materials to be stockpiled on the bridge's center on the day of the collapse. During rush hour that evening, the bridge shuddered and then dropped into the river.

Investigators told the board that Minnesota did not have any policy on weight added to bridges for construction projects.

Minnesota's transportation commissioner, Tom Sorel, attended the hearing and told reporters, "We've changed our specifications to make sure that doesn't happen again." Sorel became commissioner this year.

Investigators said the half-inch thick plates were inadequate to handle traffic and other stress factors and did not meet engineering guidelines when the bridge was built in 1967.

The board's final ruling was expected Friday.

In a statement, Minnesota Gov. Tim Pawlenty, who had come in for some criticism on bridge upkeep, noted that the board ruled out corrosion or cracking, adding that the design flaw "was unrelated to subsequent inspections or maintenance of the bridge."

But board member Debbie Hersman noted that a Minnesota transportation official had noticed bowing, or bending, of the U-10 gusset years before the collapse, and the state took no action.

NTSB officials said that was because gusset plates were always assumed to be the strongest link. Hersman suggested that assumption was unfounded. Other board members agreed.

"Where does this urban myth come from that gusset plates are so strong?" she asked.

The bridge was called "fracture critical." That meant a failure of any number of structural elements would bring down the entire bridge.

Safety board investigator Jim Wildey said there is "nothing inherently dangerous" about this type of bridge, as long as each structural element is designed to withstand the expected stress loads.

In St. Paul, Minn., a group of collapse survivors gathered at a National Guard armory to watch the NTSB presentation on the Internet. Michele McLane, 41, said the hearing was "the last door to close for me."

McLane, who drove her car safely off the northern end of the span, said the experience left her emotionally traumatized. "I finally get it now," she said. "I finally understand."

During Barack Obama's campaign for the White House, he cited the bridge collapse and called for spending more on crumbling highways, bridges and tunnels.

In July, the House passed legislation authorizing an additional $1 billion next year to rebuild structurally deficient bridges on the national highway system. The bill would require states to come up with repair plans for troubled bridges.

The Senate has yet to act on the bill. If no action is taken during a lame-duck session that starts next week, lawmakers would have to start anew on the legislation in January.

The Case for not rebuilding the Alaskan Way Viaduct

More evidence that Washington infrastructure collapse is over-hyped

By Knute Berger

Okay, classify this as a pet peeve, but it bugs me when politicians, including Christine Gregoire, wave the bloody shirt of the Minnesota bridge collapse as an all-purpose rationale to boost infrastructure spending. Gregoire has done this often. She raised the specter of the Minnesota disaster as an argument in favor of Proposition 1 last fall; she raised it again to argue for a new toll bridge across the Columbia River, and yet again at a national governor's meeting in February. I have no quarrel with repairing or inspecting roads and bridges — please, let's do that. But the fact is, we still don't have the final word on what happened in Minnesota, so the lesson there is unclear.

Even so, evidence is mounting that it wasn't simple neglect. The National Transportation Safety Board's interim report indicates the Minnesota bridge had a major design flaw: weak gusset plates at key points and no redundancy. Just a couple of weeks ago, the NTSP released the results of new simulation tests that indicate road work on the bridge may have caused the collapse. That work added massive amounts of weight over the weak sections, including 99 tons of sand. Stress at one of the bridge's weakest points was 83% more than it could hold. A second Minnesota bridge with bending gusset plates was shut down by the state just last week. (For more on the Minnesota bridge collapse and details on its faulty design, check here.)

In the wake of the collapse, many people are pushing for major road building projects, but it seems like a tactic to use fear to scare up public support. We should design good roads and bridges and fix aging infrastructure, but let's not try and capitalize on disaster by conveying the idea that everything will crumble tomorrow if we don't cough up more billions for massive new projects.

And it's not as if Washington is falling behind the rest of the country. In fact, we're better than most. Washington gets one of the top grades in the nation for its state government in study released in early March. It was conducted by the Pew Center for the States and Governing magazine, and Washington got an A- for overall performance, with nice props for Gregoire. When it comes to infrastructure maintenance, Washington ranked in the top 11 with a B+. Sure, we could always go for A+, but the study hardly confirms a picture that we're falling to pieces here. (You can check the whole report here (pdf).

Citing the Minnesota fiasco is a way to get the public to support road expansion and controversial new funding mechanisms, like widespread tolling and road pricing. Simple repair and retrofitting are rarely on the real agenda. Look at the 520 bridge or the Alaskan Way Viaduct disputes. If safety were the primary issue, we could simply replace or retrofit the structures at lower cost, but the agenda for new infrastructure always unleashes the drive for bigger and more--tough to do when federal highway funds are on the wane. The thing that's really on the verge of collapse is our ability to pay for over-sized ambitions.

Minneapolis Bridge had design flaw

By P.J. Huffstutter, Los Angeles Times Staff Writer
January 16, 2008
CHICAGO -- Federal investigators announced Tuesday that a "serious design error" was a key factor behind last summer's deadly collapse of a Minnesota bridge, but also said that the mistake would not likely have been discovered during routine state inspections.

National Transportation Safety Board Chairman Mark V. Rosenker said the Interstate 35W bridge had been built with gusset plates -- the steel parts that connect the girders, which support the bridge -- that were too thin to hold up the bridge with increased traffic and additional weight of infrastructure improvements.


The Aug. 1 collapse killed 13 people and injured more than 100.

Investigators have found 16 fractured gusset plates from the center section of the steel-deck truss bridge, Rosenker said Tuesday at a news conference.

"Basically, those 16 gusset plates were too thin to provide the margin of safety expected in a properly designed bridge such as this," Rosenker said. "These gusset plates were roughly half the thickness that would be required -- half an inch thick rather than an inch thick."

Safety board officials said their investigation was far from over, and the exact cause of the collapse was still unknown. But investigators don't think that deficiencies in materials, maintenance or inspections played a significant role.

A final NTSB report on the rush-hour tragedy is expected by the end of the year.

In the days immediately after the bridge collapse, the NTSB had raised concerns about the plates.

The board's finding, which suggests that the 40-year-old bridge was structurally unsound from its debut, could alter national debate over maintaining the transportation infrastructure.

It is unknown how or why "there was a breakdown in the design review procedures that allowed a serious design error to be incorporated into the construction of the I-35W bridge," Rosenker said.

Investigators haven't located the original designer's calculations, he said, so "we cannot determine whether the error was a calculation error, a drafting error, or some other error in the design process."

Inspectors don't look for design problems that could date back decades, Rosenker said.

"The National Bridge Inspection Standards are aimed at detecting conditions such as cracks or corrosion that degrade the strength of the existing structure," he said. "They do not, and are not intended to, address errors in the original design."

The bridge was designed by Sverdrup & Parcel, a Missouri-based civil engineering firm that also designed and oversaw construction of the Louisiana Superdome in New Orleans and the Chesapeake Bay Bridge-Tunnel, which was once designated one of the Seven Engineering Wonders of the Modern World.

The company was later bought by Jacobs Engineering Group Inc. in Pasadena, which did not return calls for comment Tuesday.

Experts had warned for years that the I-35W bridge needed expensive repairs. In 1990, a federal inspection declared the bridge "deficient," noting a history of fractures along the plates that hold together its structural arches.

After opening to traffic in 1967, the I-35W bridge -- like many others -- was modified. Such modifications, which included thickening the driving deck, probably added strain to the bridge's weaker spots. And over the decades, traffic increased on the downtown bridge.

At the time that the bridge gave way, dropping traffic into the Mississippi River, maintenance crews were using tons of equipment and construction material on the bridge's deck.

The safety board urged that all states and contractors take extra care with how much weight they place on steel-deck truss bridges -- of which about 465 remain nationally -- and reexamine the original designs before adding to or otherwise modifying them.

In Minnesota, 23 state bridges and 36 bridges under the jurisdiction of local governments have similar steel-deck truss designs.

State investigators had been reviewing seven of the largest.

"Now we're going to expand that to all of the state bridges and try to work with the local governments, to see if they're looking at the same things," said GOP Gov. Tim Pawlenty.

The $500,000 review is expected to be complete by June, he said.

Rebuilding of the I-35W bridge, estimated at $400 million, is underway.

The 10-lane bridge is to open this year.

That project has sparked federal and state political infighting, while weary commuters have been forced onto crowded surface streets.

And victims and their families have called for new state laws to compensate them

Governor Gregoire Hypes Disaster to sell bridges

Is Gregoire hyping disaster to sell bridges?

By Knute Berger 1/17/07

Gov. Christine Gregoire is eagerly sharing the happy news of her willingness to slap tolls on bridges and highways. She met with Oregon Gov.Ted Kulongoski in Clark County the other day to discuss moving ahead on a $4.2 billion new bridge over the Columbia River on I-5. The feds would pick up most of the tab, but Gregoire is set on imposing tolls. To push her agenda, she's using last summer's Minneapolis bridge collapse to make her case for urgency. Only problem is, it turns out the Minneapolis bridge disaster wasn't a case of aging infrastructure. The National Transportation Safety Board (NTSB) has concluded the problem was a design flaw.

Gregoire's statements on tolling the Columbia will ring bells for anyone listening to her rationales for exploring tolls the SR-520 bridge across Lake Washington. According to the Columbian, Gregoire is not only firm about making the new Columbia span--a key north-south link in the greater Vancouver-Portland area--a toll bridge, but she wants to look at imposing more tolls nearby:

Tolls, Gregoire said, will be a part of the finance package...

“The interesting questions there are: Do we start tolling before we even begin construction on the bridge to make sure the tolls can be lower when it’s ultimately opened? Do we do variable tolling to reduce congestion and give more choices to consumers? Do we have to toll someplace else because we’re going to push traffic in a big way” to other bridges? “In my mind, we are going to toll. How do we toll?”

The current Columbia bridge is considered a "bottleneck," but Gregoire is also raising the specter of safety and disaster. According to The Oregonian:

Gregoire said last summer's collapse of an interstate bridge in Minneapolis was "a wake-up call to this nation."

"The interstate bridges that connect Vancouver and Portland are heavily used and aging, and it is time to replace them in the name of safety as well as economic vitality," she said.

The Minneapolis bridge became the poster child for America's aging infrastructure, but Gregoire may have to look for another example. On Jan. 15, the New York Times reported:

Investigators said Monday that the Interstate 35W bridge in Minneapolis, which collapsed into the Mississippi River on Aug. 1, killing 13, came down because of a flaw in its design.

The designers had specified a metal plate that was too thin to serve as a junction of several girders, investigators say....

The I-35W bridge was of a type called “fracture critical,” meaning that the failure of any major member would cause a collapse, because it had no redundancy. The design is lighter and less expensive to build, but has gradually fallen out of favor with highway departments.

This doesn't mean that infrastructure isn't aging and that it doesn't need additional inspection, repair and possible replacement. But the situation in Minnesota isn't a case of neglected maintenance but likely a case of human error or poor judgement.

The case for bridge replacement can be made without hyping the Minnesota tragedy.

Minneapolis Bridge had been recently passed inspection

August 3, 2007
Minneapolis Bridge Had Passed Inspections
By MATTHEW L. WALD and KENNETH CHANG

The eight-lane bridge in Minneapolis that collapsed on Wednesday had been diligently inspected for years and had always passed, state officials said yesterday.

It did not, however, get stellar grades for its condition.

Additionally, officials said the bridge’s design had been considered outmoded for decades because a single failure of a structural part could bring down the whole bridge. About 11 percent of the nation’s steel bridges, mostly from the 1950s and 1960s, lack the redundant protection to reduce these failures, federal officials said.

Over all, the bridge was rated 4 on a scale of zero to 9, with 9 being perfect and zero requiring a shutdown. An inspection report last year said the supporting structure was in “poor condition,” far from the lowest category. Hundreds of other working bridges are in similar shape, but the report did indicate that the bridge had possible issues that needed to be regularly inspected.

The bridge has been inspected annually since 1993, but independent engineers acknowledged yesterday that there are well-known limits to how useful an inspection can be. Bridges, they said, are prone to a variety of problems, and some are hard to spot. At the Minnesota Department of Transportation, shaken engineers made it clear that they knew something crucial had somehow been overlooked.

“We thought we had done all we could,” said Daniel L. Dorgan, bridge engineer at the department’s bridges division. “Obviously something went terribly wrong.”

On Thursday, the United States Department of Transportation said it had told all states to inspect bridges similar in design and construction to the one that collapsed, or to review inspection reports. The department said there were 756 such bridges.

In 1982, a bridge inspector looked at the Mianus River Bridge in Greenwich, Conn., and did not see the metal-fatigued pin that would break nine months later, collapsing three lanes of Interstate 95 and killing three.

In 1987, a New York Thruway bridge near Amsterdam, N.Y., also had a clean bill of health, but inspectors had never gone underwater into the Schoharie Creek to look at the bridge’s footings, where flood waters had scoured the concrete base. When the footings slipped, the bridge fell. Ten people died.

Today, inspectors use ultrasound to check the pins in bridges similar to the Mianus one, and bridge footings receive much more attention.

"I think bridge inspections are the best they’ve ever been," said David Schulz, director of the Infrastructure Technology Institute at Northwestern University in Chicago.

The cause of the Interstate 35W bridge failure on Wednesday will probably become clear through metallurgical examinations of the wreckage, experts said, but recovering the metal parts will be delayed by the search for human remains and the need to keep investigators safe in the swirling waters of the Mississippi.

The bridge was undergoing repair work this summer, and Mr. Schulz said he would be stunned if the work did not play a major role in the collapse. “It’s too much of a coincidence,” he said.

But Mr. Dorgan said he saw no connection between the repair work, which was taking place mostly on the roadway, and the collapse of the steel support structure far below.

Mr. Dorgan said the bridge was believed to be in good enough shape until 2020, when it was due for either a major overhaul or replacement.

Parts of the bridge were considered structurally deficient because of corroded bearings and tiny metal cracks that had been spotted years ago but were considered stable. The rating of “deficient” is a common one that indicates the need for regular inspection and does not mean the bridge is dangerous, said Thomas D. Everett, a top bridge official with the Federal Highway Administration.

The most visible threat to a bridge is usually corrosion. But metal fatigue — the weakening of steel by the repeated weight of heavy trucks bouncing across the bridge surface — is harder to see. Bridges in northern climates are particularly vulnerable to metal fatigue because steel becomes more brittle and prone to cracking when it is cold.

“A crack is very difficult to observe visually,” said Steven J. Fenves, a guest researcher at the National Institute of Standards and Technology, part of the Commerce Department, and a professor emeritus of civil engineering at Carnegie Mellon University. “There may be paint over it, or maybe many layers of corrosion over it. It may be in an invisible place, in the second plate, not the outermost plate.”

The possibility that metal fatigue could cause a bridge to fail was not even considered by bridge engineers in the 1950s and 1960s, when the Minneapolis bridge was designed and built, Mr. Dorgan said. Research at Lehigh University in the 1970s showed that stresses could be much larger than had been thought. The I-35W bridge, which had been designed according to less rigid standards devised by the American Association of State Highway Officials in 1961, had components that would not be included in a bridge built today.

Fatigue cracks appeared in the approaches to the bridge, but no significant problems were detected in the center span. A study in 2001 by University of Minnesota researchers concluded: “The bridge should not have any problems with fatigue cracking in the foreseeable future.”

In a study completed in 2001 by the Federal Highway Administration, 49 working inspectors from around the country visually examined test bridges in Pennsylvania and West Virginia. The inspectors correctly identified fatigue cracks only 4 percent of the time.

Additional techniques like X-rays, ultrasound, magnetic particles or dye can help identify cracks.

Mark V. Rosenker, chairman of the National Transportation Safety Board, said yesterday that his agency would determine whether the criteria for inspections were adequate. “They may well not be enough,” he said. Or the procedures may be adequate but may not have been followed, he said.

Safety board investigators will use computer modeling to study the failure, Mr. Rosenker said, and will also reconstruct parts of the bridge from the wreckage to determine the cause. The board’s final report could be many months away, he said.