Just How Fragile Are America’s Bridges?

Following last week's I-5 bridge collapse near Mount Vernon, Wash., the AP reported that the bridge was not structurally deficient but was functionally obsolete, and listed the bridge's ?sufficiency rating," at 47 out of 100. But what does any of that bridge jargon mean to anyone who isn't a civil engineer?

U.S. infrastructure is crumbling. But aside from the occasional report card issued by the American Society of Civil Engineers, which gives letter grades in categories such as bridges and drinking water based on the wide-ranging assessments of its members, there's no unified measurement for the status of different types of infrastructures. So when a bridge collapses, we're presented with terms fit for bridge inspectors.

Structural Deficiency

First, there's structural deficiency, a designation that can be based on any number of factors. America is awash in structurally deficient bridges. The most famous example in recent memory was the old I-35W bridge in Minneapolis that spanned the Mississippi River. It collapsed in 2007, killing 13 and injuring 145. But the Minnesota bridge was just one of 75,000 such structurally deficient bridges in the country at that time, and that I-35W bridge had carried that designation for 17 years before its failure. It had been slated for replacement in 2020, but the sheer number of structurally deficient bridges (the U.S. has 66,749, as of last year) meant it was simply part of a staggering nationwide backlog.

Functional Obsolescence

The I-5 bridge was not structurally deficient. But it was functionally obsolete, another relatively common designation that means it was effectively over the hill. The bridge was built in 1955, when traffic was lighter and so were the loads borne by trucks.

Functional obsolescence is about modern usage exceeding what the original design intended. It's not about specific cracks, wear, or other structural abuse. Bridges can be both functionally obsolete and structurally deficient. The two designations aren't mutually exclusive, but structural deficiency is more alarming.

Sufficiency Ratings

These are determined by a complex formula taking into account everything from clearance above and below the bridge deck to its ?essentiality" to the public. Sufficiency ratings tend to be based on some of the same factors as designations of structural deficiency and functional obsolescence. But because sufficiency ratings also include nonstructural aspects, the rating serves a different purpose?it attempts to prioritize the repair or replacement of that bridge.

With a sufficiency rating of 47 of 100, the I-5 bridge was a project that desperately needed addressing?not because it was doomed to fail, necessarily, but because it was grossly inadequate. By comparison, the I-35W bridge that collapsed in Minnesota was rated at 50. (Sufficiency ratings are public information, but, perhaps not surprinsingly, the information isn't easy to access. The best source we've see is Nationalbridges.com.)

The Rest of the Story

Still, these designations and ratings don't tell the full story of any given bridge. The I-35W bridge, for example, was a flawed design from the start. Its structure was considered fracture-critical, meaning that a large enough failure would lead not to a staged collapse, but to a systemic shattering. That seemed to be the case in 2007, when construction equipment overloaded its weakened deck during rush hour. When NTSB investigators analyzed the wreckage, however, they found that the I-35W bridge's design was even worse than originally thought, with its too-small gusset plates creating gaps and structural weaknesses far below the observable surface (which was already in a state of cracked disarray, though seemingly for other reasons).

So just because a bridge has a score in the range of these two bridges doesn't mean it's going to fail. Even landmark spans are scary if you look at the statistics: While portions of the Brooklyn Bridge are currently designated as either structurally deficient or functionally obsolete, local officials continue to assure the drivers of the 105,000 cars that cross it daily that it's nowhere near failure. The statistics mean what they mean, but aren't always harbingers of disaster.

All of this is why, when disaster strikes, we're left with the coldest comfort of all: structurally deficient and functionally obsolete bridges are everywhere. Our 84,748 functionally obsolete bridges are a problem and deserve attention. But there are also 66,749 structurally deficient bridges in active daily use in the U.S., representing 11 percent of all bridges, including some 7980 fracture-critical ones.

Those numbers are dropping, but slowly. But however perplexing or misleading the lexicon of civil engineering might be, make no mistake?American infrastructure remains in crisis.

Source: http://www.popularmechanics.com/technology/engineering/infrastructure/just-how-fragile-are-americas-bridges-15526104?src=rss

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