What's in a color? Exploring the hues of Illinois' bridges

What's in a color? Exploring the hues of Illinois' bridges

Ever wonder why you don't see bridge painters with swatches, comparing hues and tints, looking for just the right shade or subtle tone?

When it comes to the appearance of Illinois Department of Transportation-owned bridges, there's a pretty limited palette, but there's a reason why you see only certain colors.

If it's not a stone-colored precast girder structure, the steel beams that support bridge decks are painted one of four colors: blue, green, gray or brown.

The official names sound like they were pulled from a box of Crayola crayons: Interstate Green, Munsell Gray and Reddish Brown, while blue is just "blue."

Engineer of Bridge Design Mark D. Shaffer in IDOT's Bureau of Bridges and Structures said there's a reason why there's four standard bridge colors: They are readily available and have been tested and approved.

There's also another, unofficial explanation.

"I have been told the reason why we have those specific colors is that they are supposed to stand for sky, grass, clouds, and earth," Shaffer said. "That always seemed kind of far-fetched to me, but that is what I have been told."

The decision as to what color a bridge would be painted is up to the district where that structure resides.

"Usually, the districts have a color that they like and that is what they use," Shaffer said.

Citing American Association of State Highway and Transportation Officials code, Shaffer said once bridges are built, they are anticipated to require repainting twice in their 75-year lifespan. Individual cases may vary.

"We expect them to be repainted every 25 years; however, we don't normally repaint them unless they need it," Shaffer said. "For some bridges this could be quicker, for others it's slower. Twenty-five years is probably a good average."

In many situations, the front-facing beams receive a fresh coat because of long-term UV exposure from the sun. Beam ends also can be repainted near joints because of salty liquid runoff from snow and ice efforts, which causes corrosion. Bridges are fully repainted when the corrosion extends further along the beams.

"When beam ends are repainted, the inspectors will note how much of the beam end is corroded. We will set up a painting contract to ensure that all the corrosion is removed," Shaffer said. "Usually this is the last 5 or 10 feet of beams."

River bridges go with the flow

There are some exceptions to the state's color palette when it comes to river crossings.

For example, the Interstate 255 Jefferson Barracks Bridge over the Mississippi River in the Metro East between Illinois and Missouri changed from reddish brown to gray in 2022. The change was more of a functional one, according to Joseph J. Molinaro, bridge engineer for the Missouri Department of Transportation's St. Louis District, which oversaw that bridge repair and painting project. The new color makes it easier to inspect the structure and identify any issues.

For some bridges, the color is a matter of preference. Peoria's I-74 Murray Baker Bridge over the Illinois River in District 4 changed from a salmon color to a "warm gray," a tone darker than IDOT's gray standard. Input from Peoria and surrounding communities led to that decision.

District 4 Bridge Maintenance Engineer Mark Eckhoff said the color choice draws from Peoria's massive "Upgrade 74" project in the early 2000s that rebuilt the interstate and truncated a section of the Murray Baker Bridge. Billed as the largest downstate project of its time at $460 million, he said designers weren't constrained by IDOT's standard color palette.

"They thought a project of this magnificence should have its own unique color for its bridges," he said.

The color isn't limited to the Murray Baker as the previously blue I-474 Shade-Lohmann bridges downstream were painted warm gray in 2017. Eckhoff said the Cedar Street Bridge also will be repainted warm gray in the future.

Another bridge, the I-39 Abraham Lincoln Memorial Bridge over the Illinois River in La Salle - the state's longest - is also up for painting in the future. Will it keep its signature color?

"We do not plan on changing the color," said District 3 Operations Engineer Junior Senat. "It will remain green"