[Assam] Potential Flaw Seen in Design of Fallen Bridge

[Manoj Das]

Our dear Saraight bridge is also nearing it's designed life. Someday it may
also collapse and snap the communications. There is talk of building another
4-lane bridge alongside. When this will come up is a matter of guess.

-manoj

On 8/9/07, Dilip/Dil Deka <dilipdeka at yahoo.com> wrote:
>
> If you have been following the Minnesota bridge collapse, you may find the
> following article interesting. These are speculations right now. But I
> suspect they are based on the shape of the steel debris. The extra weight
> due to maintenance equipment and material could have resulted in
> concentrated loading on some of the gusset plates. It is a standard practice
> in USA to keep some lanes open on roads and bridges during repair. The
> combined loading may not have been taken into consideration by those who let
> out the contract to repair.
>
> In the refinery and chemicals business, it is a standard practice to look
> at both operations and maintenace while designing the plants.
> Dilip Deka
> ================================================================
>
> *Potential Flaw Seen in Design of Fallen Bridge*
>   By MONICA DAVEY<http://topics.nytimes.com/top/reference/timestopics/people/d/monica_davey/index.html?inline=nyt-per>and MATTHEW
> L. WALD<http://topics.nytimes.com/top/reference/timestopics/people/w/matthew_l_wald/index.html?inline=nyt-per>
> Published: August 9, 2007
>  MINNEAPOLIS, Aug. 8 — Investigators have found what may be a design flaw
> in the bridge that collapsed here a week ago, in the steel parts that
> connect girders, raising safety concerns for other bridges around the
> country, federal officials said on Wednesday.
>  <http://topics.nytimes.com/top/reference/timestopics/subjects/b/bridges_and_tunnels/bridge_disasters/index.html>
>
>    Ruth Fremson/The New York Times
> Work continued on Wednesday in removing wreckage from the collapsed
> Minneapolis bridge.
> 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. Crews were doing work on
> the deck of the Interstate 35W bridge here when it gave way, hurling
> rush-hour traffic into the Mississippi River and killing at least five
> people.
> The National Transportation Safety Board<http://topics.nytimes.com/top/reference/timestopics/organizations/n/national_transportation_safety_board/index.html?inline=nyt-org>'s
> investigation is months from completion, and officials in Washington said
> they were still working to confirm the design flaw in the so-called gusset
> plates and what, if any, role they had in the collapse.
> Still, in making public their suspicion about a flaw, the investigators
> were signaling they considered it a potentially crucial discovery and also a
> safety concern for other bridges. Gusset plates are used in the construction
> of many bridges, not just those with a similar design to the one here.
> "Given the questions being raised by the N.T.S.B., it is vital that states
> remain mindful of the extra weight construction projects place on bridges,"
> Secretary of Transportation Mary E. Peters said in a statement issued late
> Wednesday.
> Since the collapse, the concern among investigators has focused on
> "fracture critical" bridges, which can collapse if even a single part fails.
> But neither the safety board nor the federal Department of Transportation on
> Wednesday singled out any particular design of bridge in raising its new
> concerns about gusset plates and the weight of construction equipment.
> Concerns about the plates emerged not from the waters of the Mississippi
> River here, where workers have only begun to remove cars and the wreckage
> with cranes, but from scrutiny of the vast design records related to the
> steel truss bridge.
> In Minneapolis, state transportation department officials seemed surprised
> by the sudden focus on the bridge's gusset plates, which are the steel
> connectors used to hold together the girders on the truss of a bridge. On
> this bridge, completed in 1967, there would have been hundreds of them,
> officials here said.
> Gary Peterson, the state's assistant bridge engineer, said he knew of no
> questions that had ever been raised about the gusset plates, no unique
> qualities to distinguish them from those on other bridges, no inkling of any
> problem during decades of inspections of the bridge.
> "I don't know what this could be," Mr. Peterson said. "I'm frankly
> surprised at this point. I can't even begin to speculate."
> 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 a week ago, 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.
> In an announcement, the safety board said its investigators were
> "verifying the loads and stresses" on the plates as well as checking what
> they were made of and how strong they were.
> 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<http://topics.nytimes.com/top/reference/timestopics/organizations/n/northwestern_university/index.html?inline=nyt-org>Center for Quality Engineering and Failure Prevention.
> A consultant hired by the State of Minnesota in the days after the
> collapse to conduct an investigation of what had gone wrong, even as the
> national safety board did its work, first discovered the potential flaw, the
> board said. Representatives at Wiss, Janney, Elstner Associates Inc., the
> consulting firm, could not be reached late Wednesday.
> *Federal authorities said one added stress on the gusset plates may have
> been the weight of construction equipment and nearly 100 tons of gravel on
> the bridge, where maintenance work was proceeding when the collapse
> occurred. A construction crew had removed part of the deck with 45-pound
> jackhammers, in preparation for replacing the two-inch top layer, and that
> may also have altered the stresses on the bridge, some experts said.*
> The chairman of the National Transportation Safety Board, Mark V.
> Rosenker, said on Sunday that investigators were calculating the stresses
> generated on each girder and other bridge components from the construction
> equipment and materials.
> While cautioning states on Wednesday about the weight of construction
> equipment and materials, the federal transportation department did not
> immediately issue any broader warnings about gusset plates. Brian Turmail, a
> spokesman for the Transportation Department, said on Wednesday evening that
> his agency was "conducting additional analysis to determine whether we need
> to ask the states to do checks of their designs."
> If there was a design error in the 1960s, failure to identify it before
> the bridge collapse<http://topics.nytimes.com/top/reference/timestopics/subjects/b/bridges_and_tunnels/bridge_disasters/index.html?inline=nyt-classifier>indicates a problem with the federal inspection program, said Thomas M.
> Downs, who was the associate administrator of the Federal Highway
> Administration from 1978 to 1980.
> Here, state officials were racing to respond to the new concerns about a
> design flaw, but said they had no details. "We're going to leave that to the
> N.T.S.B.," said Bob McFarlin, assistant to the commissioner of the
> Minnesota<http://topics.nytimes.com/top/news/national/usstatesterritoriesandpossessions/minnesota/index.html?inline=nyt-geo>Department of Transportation.
> Of a potential design flaw, Brian McClung, the spokesman for Gov. Tim
> Pawlenty, said the state's transportation department "will be looking into
> every single issue and possibility raised by the N.T.S.B. or the parallel
> investigation ordered by Governor Pawlenty, including this one."
> Mr. Peterson said that concerns about gusset plates might normally focus
> on questions of corrosion over time, but that he had never heard of a
> question over the original design or metal make up of a plate here. Had
> ultrasonic testing of the plates shown signs of corrosion or cracking, that
> would be a concern, he said. But in the case of the I-35W bridge, Mr.
> Peterson said he recalled "no gusset plate issues at all."
> When the bridge was built, in the 1960s, its hundreds of gusset plates
> were attached with rivets, though bridge designers here switched to bolts, a
> stronger option, in the 1970s.
> "Bolts are better," Mr. Peterson said, "but we wouldn't consider anything
> wrong with rivets."
>  Monica Davey reported from Minneapolis, and Matthew L. Wald from
> Washington.
>
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