The Mayor signed a large girder, which will form part of a new 1,400t weathering steel bridge that will cross the River Witham.
When complete, the five-span composite bridge will measure around 225m-long and will be six girders wide.
Mayor Ben Houchen said: “Cleveland Bridge is an iconic business that has helped build the world. I’m very proud it is a key part of Tees Valley’s economy, with a reputation that flies the flag for the area.
“I was delighted to put a Tees Valley seal of approval on the steel girder being produced at Cleveland Bridge’s factory in Darlington. It shows that this area has high quality manufacturing companies that can deliver on projects, whether they are on our doorstep or anywhere across the globe, as a result of growing, developing their expertise and investing in skilled workforces.”