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SCDOT Details 'Explosive' Bridge Cable Rupture

Wednesday, June 27, 2018

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Officials from the South Carolina Department of Transportation presented the findings of the investigation into last month's cable rupture on the James B. Edwards Bridge, in Charleston, to the SCDOT Commission at its June meeting last week, revealing that the incident left grout flying “a hundred feet away” from the actual break.

The bridge, actually a pair of twin spans, is the only post-tensioned concrete box-girder bridge in South Carolina; it was closed for more than two weeks after the May 14 discovery of a severed cable within the concrete structure. Less than 30 years old, the bridge has a long history of issues, including leaks and improper grouting. The twin spans carry Interstate 526 over the Wando River.

Cable rupture
Images: SCDOT

“It was a violent explosion; it actually burst out of its protective sleeve,” SCDOT's Leland Colvin told the agency's commissioners of the ruptured cable.

Deputy Secretary for Engineering Leland Colvin walked the commission through the break itself and the subsequent repairs and investigation, explaining that the location of the rupture, in pier 27 in the main span of the westbound bridge, and the originally unknown origin of the break led to the decision to shut the span down completely during repairs.

Corrosion due to water intrusion was later blamed for the problem.

“Basically, water intrusion and corrosion are an issue on this bridge, no doubt about it,” Colvin said during his presentation.

Colvin showed photos of the ruptured cable from both ends of the concrete diaphragm it was encased in, explaining the scale of the break. “It was a violent explosion; it actually burst out of its protective sleeve,” he told commissioners.

Daily Inspections

While the state normally inspects bridges every two years, SCDOT had increased inspections of the Edwards Bridge to weekly in October 2016 after another tendon ruptured at the same pier; it was one of those inspections that caught the newly ruptured cable in May. After the May 14 discovery, Colvin told the commission, the inspections on the bridge were increased yet again, to daily.

Edwards Bridge diagram

The location of the rupture, in pier 27 in the main span of the westbound bridge, and the originally unknown origin of the break led to the decision to shut the span down completely during repairs.

Colvin reviewed past incidents with the bridge, including a 2011 report that found water-intrusion issues in the bridge’s structure. That report led to further monitoring of the state of the cables, reapplication of grout and documentation of water intrusion.

Piers Inhibit Cable Inspection

The biggest maintenance issue on the bridge, Colvin explained, has been monitoring the status of the tendons within the concrete diaphragms, which he says can essentially only truly be accessed via destructive testing, which isn’t recommended because it could lead to even more moisture intrusion. Nondestructive testing methods are limited in their ability to assess the cables comprehensively.

Testing in the open areas of the concrete boxes is much easier but can’t account for the lengths of cable in the 7-foot diaphragms running through the piers.

Cable rupture

The cable ruptured where it runs through a 7-foot concrete diaphragm in the bridge pier.

“The tendon that failed, we tested it 2 feet from the location of the failure and found no signs of corrosion,” Colvin told the commission, underscoring the difficulty of inspecting the cables in the concrete box-girder span. “The effectiveness of the testing at the diaphragms is not very good.”

New Maintenance and Monitoring Plans

While some have called for the state to look into replacing the troubled bridge sooner than later, Colvin outlined SCDOT’s plans to keep the spans safe for the more than 20 years that remain in their projected service life.

One part of the plan involves redundancy: The number of external cables in both the westbound and eastbound spans will be increased to account for any future problems with the older cables. (These cables, Colvin stressed, are not necessary for holding up the dead load of the bridge, but exist to offset vibrations from traffic and other stresses. “There is a tremendous factor of safety in the bridge,” he told the commission.)

Additionally, SCDOT is in the process of installing equipment for real-time structural monitoring. The agency has already installed acoustical monitoring equipment including microphones in the westbound bridge and will expand that system into the eastbound span when a power supply is installed.

SCDOT also plans to take “a robust approach to sealing the decks and the pourbacks” on both the westbound and eastbound spans in an effort to stem moisture intrusion.

Colvin closed by noting the 50-year projected design life of the bridge, which opened in 1991, and without giving specific numbers, said the agency expects to get “many, many more years” out of both spans before they need to be replaced.

   

Tagged categories: Bridge cables; Bridges; concrete; Corrosion; Grout; Roads/Highways

Comment from Tom Schwerdt, (6/27/2018, 8:17 AM)

I have been hearing elsewhere that grouting bridge cables has been shown to frequently be an ineffective approach to corrosion protection.


Comment from Thomas Van Hooser, (6/27/2018, 12:47 PM)

Basic rule: Design for inspection and maintenance.


Comment from luiz de miranda, (6/27/2018, 4:40 PM)

We observed at Joa Viaduct, an important freeway built by prestressed grouting cables, the same failure aspect as the present case. However we observed a pH decreased at grout that envelopes the tendons to values less than 10.5.


Comment from Tom Schwerdt, (7/2/2018, 8:56 AM)

If the grout carbonates that fast, what's the point of using it?


Comment from chris atkins, (7/3/2018, 5:18 AM)

As with anything, if its done properly it works well, do it poorly and it doesn't (and sometimes designers need to recognise what they've designed sometimes can be drawn but not built). If you have inspectable unbounded tendons designed to be replaced someone still needs to inspect and decide to replace. We've had tendon failures on collapsed structures where quite a few had failed and no one had noticed.


Comment from Tom Schwerdt, (7/5/2018, 8:11 AM)

Chris, sure thing. That's one reason I'm in favor of a layered approach. Example: Galvanize the tendons, and also have them also protected with a DH system.


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