What "structurally deficient" means

Houston Chronicle:

Fourteen of the more than 2,000 state-maintained bridges in Harris and adjoining counties are classified as "structurally deficient," but all are open to traffic and safe to drive, Texas Department of Transportation officials said Thursday.

And Jon Holt, assistant bridge engineer for the department's Houston District, said all the bridges are scheduled for repair or replacement.

Rated least sufficient among them is the Brazos River bridge on FM 521, just outside Brazoria. That aged span, built in 1939, is a rarity in the Houston area — a steel truss design that still carries traffic.

Its design is similar to that of the Interstate 35W bridge over the Mississippi River in Minneapolis that collapsed at rush hour Wednesday, killing at least four motorists and injuring 79.

Federal officials alerted states Thursday to immediately inspect all bridges similar to the Minneapolis span. The Brazoria bridge is the only one fitting that description in the Houston District, according to a Federal Highway Administration database that rates bridges from zero to 100 for structural sufficiency.

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Holt said a deficiency classification can be "kind of misleading because if they were really bad, we would shut them down and there would be emergency work going on."

For instance, among the 14 deficient bridges in the Houston TxDOT district, which also includes Galveston, Fort Bend, Brazoria, Waller and Montgomery counties, two are overpasses near downtown that were hit recently by trucks and are undergoing repair.

One is on the East Freeway over Wayside and the other on Waco over the East Freeway.

A third bridge on the list is on the westbound Katy Freeway frontage road near Studewood. It has a hole near a joint in its deck and will be replaced due to deterioration, Holt said.

Most Houston-area bridges use steel-reinforced concrete beams or steel box girders, instead of trusses, to support their decks, said Todd Helwig, a University of Texas structural engineer who taught for 12 years at the University of Houston.

Helwig said truss bridges typically are found in spans more than 300 feet long and reduce weight through the use of arches and other forms of bracing in place of solid blocks or sheets of material.

Truss bridges are not inherently more dangerous, but may need more frequent inspection as they age, he said.

"The members that make them up are usually under tension or compression and if you lose one of the primary members, you can lose the bridge," Helwig said.

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Bridges are inspected at least every two years, Cross said, and those with critical issues are checked out more frequently.

Inspectors look for corrosion, deterioration and damage and check for cracks that signal fatigue, said Joseph Yura, a professor emeritus of engineering at the University of Texas at Austin who specializes in structural steel design.

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This is a pretty good none hysterical look at bridge maintenance and the classification system. As I expected the designation of the collapsed bridge did not imply that it was going to collapse under a normal lode. However, as the article suggest, the nature of its construction meant that any failure would like lead to a catastrophic failure. I suspect that the Brazos River bridge on FM 521 may be seeing less traffic for a while.

BTW, Brazos is Spanish for arms. Spanish explorers in the Gulf of Mexico had run out of fresh water and were dying of thirst when they came across the mouth of the river. Because the believed they had been saved by the "arms of God" they gave the river that name.