Significant corrosion found on Old Iron Bridge

The Elgin Courier: December 12, 2018

After the bridge was closed to all pedestrian access last month, a representative from Kimley-Horn presented the findings from their inspection of the Old Iron Bridge at the Bastrop City Council meeting on November 27.

The Old Iron Bridge, which spans the Colorado River near downtown Bastrop, was built in 1923 and served vehicle traffic until 1992, when vehicle traffic was redirected onto the newly-built Loop 150 bridge and the Old Iron Bridge was turned into a pedestrian bridge. During the most recent inspection in 2014, significant corrosion was found in parts of the bridge. In August, the Bastrop City Council awarded a contract for rehabilitating the Old Iron Bridge to Kimley-Horn, an engineering and design consulting firm, and they performed their inspections in November.

Brian LaFoy from Kimley-Horn began his presentation with an explanation of how the Old Iron Bridge is structured. The bridge is a Parker truss bridge; each of the steel members of the bridge are either in tension or compression. The Old Iron Bridge is considered a fracture critical bridge, which is defined by the Federal Highway Administration as a structure where if one of the members in tension were to fail, it could cause “significant or catastrophic failure of the structure,” causing the bridge to fall into the river, LaFoy said.

LaFoy said fixing the bridge will be a two step process: evaluation and rehabilitation. Kimley-Horn has performed a field evaluation, and now they are starting on a report looking at the deficiencies of the bridge and what can be done to improve it. Once the report is finished, the city will be able to make decisions about what to with the bridge and how it might possibly be restored.

LaFoy said Kimley-Horn found some minor issues with the bridge, including minorly-bent truss members, scouring on the underwater foundations and “flowering,” where layers of steel are beginning to flake away.

However, the biggest problem, and the reason why the bridge was closed down, is major corrosion in the gusset plates, which are sheets of steel where the diagonal and vertical members are riveted together.

LaFoy said the 2014 inspection found gusset plates with 50 percent section loss, while now some gusset plates have 100 percent section loss, meaning the corrosion has gone all the way through the plate.

“There’s significantly more corrosion than in the last report, or it wasn’t identified in the last report, that we were able to identify this time,” LaFoy said.

Another major issue is the lead in the coating of the bridge. Kimley-Horn received the initial lab results on the day of the meeting, confirming that coating on portions of the bridge is up to 13 percent lead, LaFoy said. The threshold for calling it a lead abatement project is one percent. The levels of lead means the bridge needs to be recoated and the lead fully contained and disposed of properly, LaFoy said.

“It’s not only just designing the repair to fix it structurally, it’s designing something to encapsulate it that the bridge can handle while we’re doing those repairs,” LaFoy said.

LaFoy clarified since the bridge is fracture critical, and because it would still require a design to contain the lead, demolishing the bridge would be a similar, and similarly expensive, project.

“Doing nothing and letting it fall down has its own worse consequences,” Bastrop Mayor Connie Schroeder said. “Because of the lead, now we’re in the Colorado River, and should the bridge fall, then it becomes a whole different project. It doesn’t just go away.”

LaFoy said sandblasting the bridge to get rid of the lead will be a tedious process, and a system to capture the lead during the process will be expensive to design and build. In addition, since the bridge is fracture critical, the pieces of the bridge can’t be taken apart and put back together, and a sequence would have to be created for removing bolts and rivets.

“We’re not out of the woods until the final contract is signed, executed, completed, and they walk away, and we’ve dodged a bullet,” city manager Lynda Humble said. “This is where we manage expectations up front on how precarious we are, because we’ve now waited until the point that we almost don’t have any runway, and this bridge is very close to not being savable.”

Humble said this is an 18- to 24-month process, and this is a project where the city will want to get a contractor for the best value, not the lowest bid, for the repairs.

“At any point during any of these processes, if they sandblast too much and they fracture something, we have critical failure,” Humble said. “If they are trying to repair it and don’t repair it in the right order, we have critical failure. This is art, this is science and a whole lot of luck.”

Humble said the city already has two million dollars allocated for the bridge from a certificate of obligation sale in September, but this project could cost as much as five million dollars. The next step for understanding how much it will cost and what funding options are available will be to get the report from Kimley-Horn at the beginning of next year.

Because of the severity of the gusset plate corrosion, and since Kimley-Horn has not yet been able to determine the true capacity of the bridge, the bridge is closed to pedestrian access for the foreseeable future.

“If you make the effort (to go over the barricades), you’re going to meet Bastrop’s finest,” Humble said. “This truly has eminent safety issues. You will be charged with criminal trespass, and we’re not going to play about it.

“I think it is safe for us to assume this bridge will be closed until we have a ribbon cutting ceremony on it, and we’ve lived through it and there’s still a bridge,” she added.

The next step is for Kimley-Horn to finish organizing their data and complete a load analysis to understand the remaining capacity of the bridge and present options of what to do with the structure and what the costs would be.

“Council, it feels like yet another one of those things we can’t just keep kicking down the road,” said Schroeder.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s