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Before demolition, Penny Bridge undergoes stress test

Before demolition, Penny Bridge undergoes stress test

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Early in Lincoln’s history, the Penny Bridge connecting the Boulevards District with College View via Sheridan Boulevard crossed the Rock Island Railroad over a wooden trestle bridge.

That was replaced in the 1930s with a steel girder bridge topped with iconic cast-concrete railings near the boulevard’s intersection with 33rd Street.

For 80 years or so, the Penny Bridge -- named for the extra fare streetcar passengers would pay to cross it -- remained firmly in place for cars, pedestrians and some 38 annual Lincoln National Guard Marathons.

It will be torn down beginning Tuesday and replaced in a project scheduled through the end of the year.

But on Monday, the Penny Bridge became a research lab for engineers from the University of Nebraska-Lincoln and the Nebraska Department of Roads.

“We took an opportunity, since the bridge is going to be removed, to see how far we can go over the bridge’s posted limit,” said Foaud Jaber, an assistant bridge engineer at the Roads Department.

Using 32 sensors carefully placed at key structural points on the south bridge, engineers measured what would happen if the span rated for 17 tons was forced to endure nearly three times that. (The other bridge has a sign saying it is rated to hold 20 tons.)

Two dump trucks weighing 25 tons each crossed over the eastbound bridge one at a time, then side-by-side to measure how much stress the bridge could endure. The data was transmitted wirelessly from each sensor to a computer monitored by UNL engineers below the north bridge.

No collapse. No obvious cracking. Now, the engineers will analyze the data and see how the bridge designed to stand for 75 years did.

After a few hours of testing, civil engineers gained insight at how the weight affects the steel girder’s deflection, or how the weight on the bridge displaces the steel girders, and deformation, or how the weight causes each girder to stretch. A laser sensor placed under the bridge also measured the distance each steel girder moved under the force from the trucks.

“This is as exciting as it gets,” said state bridge engineer Mark Traynowicz, who said the test was designed to cause the bridge to yield, but not fail.

Typically, engineers avoid those kinds of tests because most bridges are still in use by counties.

“With this bridge, we can push the limits. It doesn’t matter if the trucks damage the bridge because (Hawkins Construction) is going to rip it out tomorrow,” he said.

UNL lab technician Peter Hilsabeck said the Penny Bridge is the third the team has tested this year. The other two were rural bridges still in operation.

“This is the first one I’ve ever done at the end of its lifespan -- it’s gone tomorrow with good reason,” Hilsabeck said. “This is a really good opportunity to find out just how bad it’s become.”

Or, as Josh Steelman, an assistant professor in civil engineering who specializes in evaluating structures for risk, put it: using science, technology, engineering and mathematics in a practical setting.

“This is sort of a real-world application of STEM,” said Steelman. “You’ve got to try to figure out if the bridge is safe or not. You’ve got testing and signal processing -- all that goes into structural analysis and modeling.”

Traynowicz said the project done through the Mid-American Transportation Center will help city and county officials learn more about the strength of local bridges rather than massive bridge structures administered by state and federal governments.

“There’s a lot of bridges out there that are old and corroded and by theory have low load postings,” Traynowicz said. “This will help us refine what we’re doing on individual bridges. It’s research like this that changes those types of formulas.”

Reach the writer at 402-473-7120 or cdunker@journalstar.com. On Twitter @ChrisDunkerLJS.

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