Design Flaw Behind Minn. Bridge Collapse

NTSB Cites Undersized Steel Plates As "Critical Factor" In Collapse That Killed 13

(CBS/AP)  Undersized gusset plates in the Interstate 35-W bridge in Minneapolis were "the critical factor" in the bridge collapse last year that killed 13 people and injured 100, the National Transportation Safety Board said Tuesday.

Chairman Mark Rosenker said the plates, which connected steel beams, were roughly half the thickness they should have been because of a design error. Investigators found 16 fractured gusset plates from the bridge's center span, he said.

"It is the undersizing of the design which we believe is the critical factor here. It is the critical factor that began the process of this collapse. That's what failed," Rosenker said.

The Minneapolis bridge was a steel-deck truss bridge that opened in 1967. Rosenker said it wasn't clear how the design flaw made it into the bridge because investigators couldn't find the design calculations.

Once they made it into the completed bridge, he said, there was little chance they would be noticed by inspectors.

There are 465 similar bridges in the U.S. Investigators don't expect to find the same design flaw, but said today, those bridges should be thoroughly checked before weight or capacity is increased, reports CBS News' Ben Tracy.

The Minneapolis bridge was deemed "structurally deficient" by the federal government as far back as 1990, and the state's maintenance of the structure has been questioned. But Rosenker said the NTSB investigation has found no evidence that cracking, corrosion or other wear "played any role in the collapse of the bridge."

Investigators also found no flaws in the steel and concrete material used in the bridge.

In his update Tuesday, Rosenker also noted structural weight had been added to the bridge in two major renovations, as well as construction materials that were on the bridge the day it collapsed as part of a resurfacing project.

The investigation is now focusing on whether the heavy construction equipment on the bridge was the extra weight that snapped those already weak plates, Tracy reports.

The findings are consistent with what the NTSB said about a week after the Aug. 1 collapse, in which the bridge plunged into the Mississippi River. At the time, the NTSB said it had found issues with the collapsed bridge's gusset plates, but expected a full investigation to take more than a year.

Transportation Secretary Mary Peters was expected to issue an advisory urging states to check the gusset plates when modifications are made to a bridge - such as changes to the weight of the bridge or adding a guardrail, said a federal official with knowledge of the plans. The official spoke on condition of anonymity because Peters had not yet made the announcement.

Currently, such calculations are done for the entire bridge, but not down to the gusset plates, the official said.

Last August, Peters advised states to consider the additional stress placed on bridges during construction projects. An 18-person crew was working on the bridge when it collapsed.

Nearly three months later, she told a gathering in Washington of a "working theory" of a poorly designed gusset plate and a heavy load of construction materials.

Meanwhile, state lawmakers announced plans last month to spend up to $500,000 to hire legal counsel to aid in a legislative inquiry into the collapse.

Late last year, President Bush signed a massive spending bill which included $195 million to help replace the bridge. That came on top of the $178.5 million the federal government has already given Minnesota for the project.

The bridge was originally designed by Sverdrup & Parcel, a company acquired in 1999 by Jacobs Engineering Group Inc. of Pasadena, Calif. A message left by The Associated Press with Jacobs wasn't immediately returned.

[perrycbs1]

differnet:

Actually computers preceded WWII; but, what existed in those days were not capable of being used for most engineering.

In the mid 1970''s table top electronic calculators started replacing slide rules in office applications. New science and engineering students were still using slide rules as they were portable.

It was only in the very late 1970''s (1978 to 1979) that portable calculators started to replace slide rules. Design firms still did hand calculations and just got the numbers from the calculator instead of the slide rule.

The first programs for computers to analyze simple bridge trusses on mainframe computers did not really make much headway until the early 1980s (Programs were written in Fortran on keypunch cards).

Only the largest engineering firms could afford the mainframe computers in the 1980%u2019s that could run these programs.

The first desktop computers with simple engineering programs came about in the mid 1980%u2019s; and really usable programs were starting to be available by the early 1990%u2019s.

I would not expect any bridge designed before 1980 to have used a computer in our current sense of the word; and most bridges would still not have used modern computer design well into the 1980%u2019s. Almost all bridges designed after 1990 would have been computer designed.

I have not designed bridges; but, I have lived through all of these changes in the engineering field; and I still have a slide rule in my desk at home.

[differnet]

Perrycbs,

Computers have been around since WWII. They were large mainframes, but they did exist.

[perrycbs1]

Let%u2019s consider a few facts:

This bridge was designed in the 1960''s; with hand calculations and slide rules as computers did not yet exist.

Both the companies who designed and built the bridge are long gone.

The lead engineers should be dead from old age too...

Thanks to the conservative standards of the day, this bridge worked well for 40 years with key structural components that were half of the thickness they should have been.

This bridge was known to have problems - and the design of the bridge was known to not be single failure proof (i.e: would stand up if any single component failed). That was a problem with the design standards of the 1950''s and 1960''s as engineers did not know about the various failure modes of steel in those days. Now they do.

As mentioned in the article; even by today standards reanalysis of old bridges do not go all the way to the details of exact thicknesses of all the parts (such as the gusset plates). Of course, I suspect that will now become the standard for the future.
So, there really is no one to sue, no company to go after, and no way that you would expect the State DOT to have known of this problem. It is not possible to analyze and test everything to the nth degree because people are looking for reasonable priced items and reasonable priced bridges and reasonable priced government agencies.

Sometimes we just have to accept that mistakes happen - and stuff happens in life. Even when there is a tradedy.

[random_radar]

Who approved the bridge design? Government officials.

[consciousnes]

Why is the MNDOT Secretary still in office?
When are the people of Minnesota going to quit being nice and force the Governor to replace her with someone competent and honest?
This is concrete and steel proof that she does not know her job or care to learn it.
Get rid of her before we loose another 13 Minnesotans.

[Krazcarl]

Has anyone been fired or brought up on charges. The co. that built it ought to bankrupt now if not they will this idioucy will be expensive the state should sue a complete csae of incompetance and graft palms had to be greesed.