How to Demolish a Bridge — Practical Engineering

data-localized="{"invalidVideo":"Invalid video embed.","playButton":"Play"}"

[Note that this article is a transcript of the video embedded above.]<br>In 2022, the world got a very cool new bridge. Two bridges, actually. The Iowa-Illinois Memorial Bridges carries Interstate Highway 74 over the Mississippi River between Moline, Illinois and Bettendorf, Iowa in the “Quad Cities” area. It’s a gorgeous pair of structures with the basket handle arches carrying each deck over the main span of the river. But even after they were finished, Iowa DOT had a problem. The two old bridges were still right there, also crossing the Mississippi River. And even though they were kind of cool looking, they just couldn’t stay. The bridges were already in poor condition, and without extensive ongoing maintenance, they would continue to deteriorate, posing a danger to the public, affecting the sensitive environment along the river, and even disrupting this critical shipping artery. The old bridges would have to come down.<br>Demolition, on its face, seems kind of easy. For a billion-dollar-bridge replacement project, the demo part feels almost like housekeeping. Smash the structure down or blow it up, then just pick up the pieces. No engineering needed. The truth is that it’s anything but. Demolition engineering, in many ways, is even more complicated than designing a new structure, and the I-74 bridge is the perfect case study in why. And don’t worry, there are explosions at the end. I’m Grady, and this is Practical Engineering.<br>The original I-74 bridges, with just two lanes each, were way overdue for an upgrade in capacity. This is actually a problem they faced and managed to overcome once before, many decades back. Despite looking like twins, the original pair of bridges were built a generation apart. The first span was completed in the mid 1930s, but car ownership and traffic exploded after World War II. Engineers decided that the best way to increase capacity was to build a nearly identical bridge right next to it. That new bridge was opened in 1959.<br>Neither bridge was ever intended to meet interstate standards; they predate the interstate system altogether. And yet, they found themselves carrying interstate-levels of traffic, way beyond what the designers in the 1950s, and especially the designers back in the 1930s, had considered. The lanes were narrow, there were no shoulders so they required a lower speed limit, which bottlenecked traffic on I-74.

Size isn’t everything, though; the bridges were also just physically wearing out. Like an old car, it eventually got to the point where the cost of replacing the bridges was outweighed by the constant maintenance and threat of disaster. In 2012, Transportation Secretary Ray LaHood toured this structure, reporting back that it was, quote, “one of the worst bridges I’ve seen in America.”<br>You would think that already being close to falling down would be to their advantage when it comes to demolition, but it’s quite a bit more complicated than that. These were big bridges with 3 types of structural designs. There’s these three span continuous truss units over the old non-navigable part of the river. There’s the deck trusses that kind of act as connectors. And then you have the big 3 span suspension section.<br>I’m sure you want to see the explosion, and I promise we’ll get there, but it’s basically the last step. Of course, there are lots of cases where it makes sense to just blast a structure down right away. You end up with a pile of rubble that you can manage with regular construction equipment. It can be much quicker, easier, and safer than dismantling a structure piece by piece, but it’s rarely true for bridges. Of course you’ve got the water that complicates things. Removing debris from below the water line is challenging. Long reach excavators can sometimes handle the smaller stuff, but you often need divers to rig the big stuff to be lifted out by cranes. That’s dangerous and difficult work. You also have shipping traffic to consider. This stretch of the Mississippi is a busy part of the inland waterway system, and closing it to clean debris out of the channel is a disruptive task. The other thing making it tricky, in this case, is the environment. There are endangered mussels living in the non-navigable channel below the continuous truss spans, so the demolition team couldn’t use blasting or even temporary supports in that part of the river. The only option was to dismantle the bridges more carefully and thoughtfully. Step one is to get the deck off.<br>The strategy here was to sawcut all the concrete into pieces small enough to move with construction equipment. An excavator with a slab crab attachment could lift each panel off the steel structure, swing around, and pass to a wheel loader to carry it off the end. Sounds simple, but it had to be done pretty carefully. Cutting the concrete into panels like this means that the...