Would FEA Have Saved The I-35 Bridge?

March 6, 2008
Federal investigators recently released a few preliminary findings about the infamous Minnesota I-35W bridge failure.

By chance, the announcement came as FEA and CAD vendors were converging on SolidWorks World, a major trade show. FEA experts at the show had plenty to say about the federal findings. Their conclusion: Had FEA been around when the bridge was built, it would have caught the errors that seem to have lead to the bridge’s collapse.

The I-35 bridge, a truss design, consisted of steel beams connected to each other at nodes or joints by so-called gussets, basically steel plates bolted or riveted to hold the beams together. Federal officials say several gussets fractured, most likely because they were too thin, only 0.5-in. thick when they needed to be 1 in. The deck, though probably designed with a certain safety factor, carried tons of construction equipment on the day of the collapse. So loads were higher than normal. Also, the structure had a nonredundant design, which means a single component failure could bring down the entire structure.

The original bridge calculations were evidently lost, making it hard to tell whether the undersized gussets resulted from a drafting mistake or a calculation error. Experts claim newer FEA programs help eliminate such mistakes. Structural and multiphysics analyses take place before any concrete is poured or beams get bolted, thus catching design flaws early.

“The first goal of structural analysis is to size beams correctly,” says Suchit Jain, vice president of strategy at SolidWorks Corp., Concord, Mass. “FEA software lets engineers account for various loads affecting the whole structure. For example, the steel itself has weight, and so it is a load. And there are direct mechanical loads such as vehicles, side loads from wind, and seismic loads from earthquakes. And on a smaller scale, ‘microseismic loads’ go into vibration and dynamic analysis.”

After sizing the beams, engineers analyze individual joints to predict if they will withstand operating stresses, says Jain. “Modern FEA software lets users see how structures weaken over time. Engineers can also ensure resonances will not come into play, as happened with the Tacoma Narrows bridge failure.”

Bridges today are much more flexible than those of the past, says Comsol Branch Manager John Dunec. “Better bridge materials let designers make bridges longer and lighter, with thinner cross sections, but they require large deformation analysis,” he says. “Fortunately, current software performs nonlinear single and multiphysics analyses, the kind of analyses this task requires.”

For example, in this age of terrorist attacks, it’s necessary to predict what would happen should a large truck or ship hit a bridge and spill corrosive chemicals. “So, variables now include static loading, impact loading, flow, chemical diffusion, and reaction processes weakening the bridge, among many others. This is a problem multiphysics software solves using partial differential equations and matrix math,” he says.

It’s possible to model an entire bridge. And for some analyses, that might be necessary, says Dunec. “Needless to say, such models have millions of degrees of freedom and require lots of computational power. But it lets analysts see where all the loads are distributed, then hone in to the component level to perform detailed studies on parts of the bridge. If the 1-35W designers had used modern advanced FEA, they probably would have found the gusset problem before the bridge was built.”

Check out this Flickr link to see an image of a real gusset plate: http://www.flickr.com/photos/80651083@N00/984752381/in/pool-35w-bridgedisaster

The I-35W Mississippi River Bridge, an eight-lane, 1,907-ft-long structure in downtown Minneapolis, collapsed in August of last year killing13 people and injuring at least 100 more. Analysts are using modern FEA and other techniques to determine why the 40-year-old bridge failed.

A CAD rendering shows an example of a gusset plate.

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