The Tollway's I-355 southern extension between I-55 and I-80 includes a bluff-to-bluff 1.3 mile long bridge across the Des Plaines River Valley. When the extension was opened on November 11, 2007, the entire I-355 tollway was renamed the Veterans Memorial Tollway and the bridge was named the Veterans Memorial Bridge.
On the way back home from a trip to Lockport, I stopped to take pictures from the south side. I've driven over the bridge many times since it was opened, but I had never noticed that it curved.
But I discovered in worldflicks that Mickey B. was able to find a nice view from the south. Note the red and green lights. The two red lights would be marking the edges of the shipping channel and helping to indicate bridge's clearance over the Chicago Sanitary and Ship Canal.
Months later I discovered a trail under the north side of the bridge.
I had noticed that the girders looked like concrete instead of steel. So I took a picture that had very little sky in it to get a good exposure of the girders.
Did they curve the road to go around this mound?
We see from a later picture from the south side of the trail that it is just that---a little mound.
My current theory is that it is a dolostone outcropping. This is why it resisted erosion by the historical Des Plaines river, which would have been much larger, and why the tollroad was willing to bend the bridge rather than chop dolostone. That is, it would not be cheap for the tollroad to remove a solid piece of rock so the cost of bending the bridge becomes a viable alternative. When I turn around from where I took the above picture, I get the rest of the bridge and some of the marine industry down by the Chicago Sanitary and Ship Canal
A video by the company that built the bridge, Walsh, saves me from a lot of typing. I describe the wetlands and endangered species in in my Towns and Nature blog. The video confirms that, instead of steel girders, they used concrete beams.
During the 10 years it took to do more traffic studies and to change the bridge's design to reduce the environmental impact, construction costs had skyrocketed due to the dramatic inflation in oil and raw material costs. The estimate for the original design had changed from $730 million to over 800. Rather than spend time doing another funding cycle, the engineers decided to get smarter. One change is that they repackaged the bids. Instead of seven large contracts, the work was broken down into 18 smaller contracts. This allowed many local contractors to get a piece of the action and saved $10 million. Since a lot of the large contractors would be out-of-state. The reduction of the large number of contractors needed reduced the transportation costs needed to get heavy equipment moved to the site. And it kept most of the money spent in the region, which helped bolster public support for the project. The original plans for the bridge included two designs---"a segmental precast concrete box design and a steel delta frame design," and a contractor could choose which design they wanted to build. They added a performance specification package and gave the contractors the third option of using their own design. [Rethinking] Walsh formed a team that developed a concrete beam design that was $8 million cheaper than the concrete box girder design and $50 million less than the steel plate girder design. The concrete beams are simple span prestressed bulb tee girders up to 170 ft and post-tensioned, segmental concrete girders spanning up to 270 ft. The beams were manufactured by Prestress Engineering Corporation in Blackstone, IL, [ConcreteProducts], and by DSI (WIDAG-Systems International) in Bollingbrook, IL. DSI also provided the strand tendon bundles. [DSI] A third bid change is that the Tollway Authority removed boilerplate specifications that had become obsolete. For example, they used to require a maximum strand diameter was 0.5". By allowing the designers to use 0.6" strands, they were able to save money. So the 10-year delay was a blessing in disguise---it forced the tollway to figure out a better way to contract the construction of roads and bridges.