As with most infrastructure catastrophes, there has been plenty of finger-pointing taking place in the days after a Minneapolis bridge collapsed early this month and killed what is now determined to be nine people. (Many others were injured, and divers are still searching for more victims nearly two weeks after the collapse.)

Neither the federal Department of Transportation nor the National Transportation Safety Board (NTSB), whose full investigation is set to take months, has indicated any particular design or definitive cause for the tragic collapse of the eight-lane bridge.

Yet safety officials say they have found a “design issue” with gusset plates — the steel connectors that tie together the angled steel beams of the bridge’s frame — at particular locations on the bridge, raising new concerns about the gusset plates and the weight of construction equipment.

Vehicles are strewn among the wreckage of Interstate 35W bridge, which collapsed over the Mississippi River Aug. 1 in Minneapolis.
Credit: AP / M. Spencer Green

Since the collapse, investigators’ concerns have focused on “fracture critical” bridges, which can collapse if even a single part fails. Concerns about the plates emerged from scrutiny of the vast design records related to the steel truss bridge.

In an announcement, the safety board said its investigators were “verifying the loads and stresses” on the plates as well as checking the materials used to make them.

The New York Times reports from Minneapolis:

State authorities here said the plates were made of steel, and were, in most such bridges, shaped like squares, five feet by five feet, and a half inch thick. Such plates are common in bridges as a way to attach several girders together, said Jan Achenbach, an expert in testing metals at the Northwestern University Center for Quality Engineering and Failure Prevention.

There have been hundreds of gusset plates, which are the steel connectors used to hold together the girders on the truss of a bridge, on the bridge since its 1967 completion.

The Times reports:

If those who designed the bridge in 1964 miscalculated the loads and used metal parts that were too weak for the job, it would recast the national debate that has emerged since the collapse […] about whether enough attention has been paid to maintenance, and raises the possibility that the bridge was structurally deficient from the day it opened. It does not explain, however, why the bridge stood for 40 years before collapsing.

When the bridge was built, its hundreds of gusset plates were attached with rivets, though bridge designers switched to bolts — a stronger option — in the 1970s.

One possible stress may have been the weight of construction equipment and materials on the bridge at the time of the collapse, the NTSB reported on its Web site. A construction crew had removed part of the deck with 45 lb. jackhammers, in preparation for replacing the 2 in. top layer, and that may also have altered the stresses on the bridge, some experts said.

The Federal Highway Administration swiftly responded by urging all states to take extra care with how much weight they place on bridges of any design when sending construction crews to work on them.

“The collapse of [the] interstate highway bridge over the Mississippi River in Minneapolis took everyone, including engineers, by surprise,” Henry Petroski, a professor of civil engineering and history at Duke University, wrote in a Los Angeles Times op-ed soon after the devastating collapse. “We do not expect our bridges to drop out from beneath us, and their designers take great pains to ensure that they do not.”

Petroski, author of Success Through Failure: The Paradox of Design, Pushing the Limits: New Adventures in Engineering and To Engineer is Human: The Role of Failure in Successful Design, wrote:

Among the surest ways to obviate the failure of any structure is to anticipate all the different ways in which it can fail. Thus, in designing the Minneapolis bridge, engineers had to consider the consequence of a single steel member breaking, buckling or otherwise failing to carry its intended load.

In a decade, from 1995 to 2005, the weight load on urban highways increased by half. Since 1970, the weight carried on rural highways has gone up nearly 7-½ times, according to Federal Highway Administration statistics.

Petroski’s conclusion:

No matter how carefully bridge designers anticipate failure on the drawing board (or computer screen), their structures will only be as reliable as how carefully built, maintained and inspected they are. Just because a bridge has given decades of successful service under adverse conditions of increasingly heavy traffic and neglect does not mean that it will continue to do so. It is the function of regular and careful inspections to catch what designers might not have anticipated.

"Movement toward inspection of all inventory is a mix of valid precaution and knee jerk," writes Richard M. Gutkowksi, a professor of civil and environmental engineering at CSU and past chairman of the American Society of Civil Engineers Administrative Committee on Bridges, at The Denver Post. "Until the causes of the I-35W demise are determined, complete examination of like bridges (in type and age) is a responsible reaction. Once found, the causes should lead to more targeted checking of other bridges, and those concerned should feel some relief."

Meanwhile, a plan to replace the bridge “is on the fastest of fast tracks,” The Associated Press reports.

State officials, who want the span open by the end of next year, have an ambitious schedule to award contracts to replace the bridge next month. Three days after the bridge collapsed, the state had already begun looking for companies interested in erecting a new bridge in just 16 months.

According to AP:

Erecting a new bridge like Minneapolis’ would ordinarily take about three years, even if the design and building phases were overlapped to save time, said Bill Cox, owner of Corman Construction Inc. in Annapolis Junction, Md., a road and bridge construction firm.

The goal of awarding contracts in mid-September is highly ambitious given the array of questions to be answered, including whether to mimic the former bridge’s alignment, how much traffic to accommodate, how much to spend and what it will look like.

Recommended: Bridge Engineering a Constantly Evolving Science

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