Bridge Demolition is Complicated

[00:01] In 2022, the world got a very cool new bridge. 

[00:08] Bridges carries Interstate Highway 74 over the 

[00:14] Bettendorf, Iowa in the “Quad Cities” area. It’s a 

[00:21] arches carrying each deck over the main span of 

[00:27] DOT had a problem. The two old bridges were still 

[00:34] And even though they were kind of cool looking, 

[00:39] in poor condition, and without extensive ongoing 

[00:45] posing a danger to the public, affecting 

[00:49] and even disrupting this critical shipping 

[00:55] Demolition, on its face, seems kind of easy. 

[01:00] the demo part feels almost like housekeeping. 

[01:06] then just pick up the pieces. No engineering 

[01:12] Demolition engineering, in many ways, is even 

[01:17] and the I-74 bridge is the 

[01:21] And don’t worry, there are explosions at the end. 

[01:34] The original I-74 bridges, with just two lanes 

[01:41] This is actually a problem they faced and managed 

[01:47] Despite looking like twins, the original pair 

[01:53] first span was completed in the mid 1930s, but car 

[01:59] Engineers decided that the best way 

[02:03] nearly identical bridge right next to 

[02:09] Neither bridge was ever intended to meet 

[02:13] interstate system altogether. And yet, they found 

[02:19] way beyond what the designers in the 1950s, 

[02:25] had considered. The lanes were narrow, there were 

[02:30] which bottlenecked traffic on I-74.

[02:36] were also just physically wearing out. Like an 

[02:41] cost of replacing the bridges was outweighed by 

[02:48] In 2012, Transportation Secretary 

[02:52] reporting back that it was, quote, “one of 

[02:58] You would think that already being close to 

[03:03] it comes to demolition, but it’s quite 

[03:07] were big bridges with 3 types of structural 

[03:13] truss units over the old non-navigable part 

[03:18] kind of act as connectors. And then you 

[03:23] I’m sure you want to see the explosion, and I 

[03:28] last step. Of course, there are lots of cases 

[03:33] down right away. You end up with a pile of rubble 

[03:39] equipment. It can be much quicker, easier, and 

[03:45] but it’s rarely true for bridges. Of course 

[03:50] Removing debris from below 

[03:54] Long reach excavators can sometimes handle 

[03:59] to rig the big stuff to be lifted out by 

[04:04] You also have shipping traffic to consider. This 

[04:09] inland waterway system, and closing it to clean 

[04:15] The other thing making it tricky, in this case, 

[04:20] living in the non-navigable channel below the 

[04:26] couldn’t use blasting or even temporary supports 

[04:31] to dismantle the bridges more carefully and 

[04:37] The strategy here was to sawcut all the concrete 

[04:43] equipment. An excavator with a slab crab 

[04:48] steel structure, swing around, and pass to a wheel 

[04:54] but it had to be done pretty carefully. Cutting 

[04:58] the reinforcement is cut too. And this is 

[05:04] using calculations and analysis not to design 

[05:10] like, “Can these concrete panels support 

[05:16] The answer was more complicated 

[05:20] engineers imposed pretty strict positional 

[05:25] in most cases making sure that the tracks of 

[05:30] stringers instead of relying on the concrete deck 

[05:36] Another challenge on the suspension span was 

[05:42] to dismantle the bridge, systematically working 

[05:48] if you take all the weight off one section of the 

[05:54] the trusses are going to bow, the towers are 

[05:59] the bridge prematurely. It’s just like re-racking 

[06:04] you take everything off one side of 

[06:09] So, on the suspension bridges alone, the 

[06:15] Some slabs were removed with 

[06:17] Others were popped up and left in place as 

[06:23] It’s a lot more work with a crane 

[06:26] way to get the deck off in a symmetrical way 

[06:32] Once the concrete deck was off, the contractor 

[06:37] and stringers that make up the bridge structures. 

[06:42] go cutting up a bridge willy nilly. This is 

[06:48] and that requires some structural engineering. 

[06:53] team wasn’t allowed to install temporary 

[06:58] So instead, they floated in the support on 

[07:02] the trusses into pieces small enough for the 

[07:07] The suspension spans were even more complicated. 

[07:13] piece of steel that’s under significant 

[07:18] it could cause sudden movements and 

[07:23] Those cuts have to be carefully sequenced. When 

[07:28] changes the stresses, shifting them around and 

[07:34] to check each step before you do it to make sure 

[07:40] or the environment. And there’s no way to know how 

[07:46] Instead, they had to create a structural computer 

[07:51] bridge in 3D. The order also matters.

[07:56] One cool example of this:

[07:59] the top chords of each truss weren’t installed 

[08:05] most of the load was being carried in the bottom 

[08:11] the whole truss responded by going into 

[08:16] The top chords were in tension and the bottom 

[08:21] what you would expect. That’s something that 

[08:25] having gone through the exercise of modeling the 

[08:31] modified, and retrofitted over the years. It was 

[08:38] The engineer even used old magazine articles to 

[08:44] Here’s another tricky part. To lift the 

[08:49] they had to put a crane on a barge. Anyone 

[08:54] not the most stable platforms for high-stakes, 

[09:00] add in huge loads and the need for precision. 

[09:05] these bridges, but that doesn’t mean it’s easy. 

[09:11] loads from lifting and swinging. Barges often 

[09:16] but there’s still a lot of engineering that goes 

[09:21] variety of loading conditions. Those calculations 

[09:28] limitations on pick weights and movements, et 

[09:33] And just like the concrete deck, these 

[09:37] staggered manner to keep the towers from 

[09:42] Interestingly, sometimes to demolish a structure, 

[09:48] load system had to be removed because of 

[09:53] but the engineers didn’t want people working on 

[09:59] So they had to design and build these steel 

[10:04] the superstructure during the demolition process. 

[10:09] restraints on the trusses of the continuous 

[10:14] time those trusses were partially demolished. And 

[10:20] stiffening truss, made from pieces of the bridge 

[10:25] truss could be lifted and removed as a single 

[10:31] on the shore where it could be cut down using this 

[10:37] At this point, basically all that was left of 

[10:43] Since those cables are essentially one long 

[10:47] safely cut them loose. Imagine getting snapped by 

[10:54] up energy there, and you don’t want any humans 

[10:59] explosives come in. And like every other part 

[11:05] Explosives used for demolition aren’t 

[11:10] You’re not trying to use them to 

[11:15] On buildings, you get a lot of breakup anyway 

[11:20] But really, the explosives are just strategically 

[11:26] falling process in a more controlled way. 

[11:32] pieces that can easily be removed from the 

[11:37] very exciting saws that can cut 

[11:43] Demolition contractors use shaped 

[11:48] in a relatively controlled manner, and 

[11:53] It matters a lot what these pieces look like 

[11:57] to be lifted and transported out of the water. And 

[12:03] they’re accessible without blocking the navigation 

[12:09] there’s an entire process of precutting. The goal 

[12:15] be placed down to flat plates or smaller sections 

[12:22] A worst case scenario is an incomplete 

[12:27] bring the structure down. When that happens, 

[12:33] and difficult because you have to finish 

[12:38] it’s not entirely clear where the 

[12:43] On the I-74 bridges, workers cut the outer 

[12:49] the 37 strands holding. This was done in four 

[12:55] manageable pieces. The towers were also cut in 

[13:01] to sever completely and control which direction 

[13:07] this leaves the bridge extremely vulnerable. 

[13:12] stability so that the explosives can kind of carry 

[13:18] down. But it means the clock is ticking. You’re 

[13:23] don’t want a storm bringing the bridge down before 

[13:34] I’ve mentioned a lot of pros and cons of explosive 

[13:39] than anything else: the spectacle.

[13:42] You just can’t get around how

[13:46] They got perfect shots for both bridges, bringing 

[13:52] quickly to pick up the pieces. The road and 

[13:58] a short period of time. Here’s a good look at how 

[14:03] cables. And actually, they used explosives to demo 

[14:09] The piers in the more  sensitive areas were taken out using conventional jackhammers as the final step. These sheet  

[14:15] pile containment structures kept the debris from 

[14:20] a scan to make sure all the debris was picked up. 

[14:26] habitat for those endangered mussels, so you can 

[14:32] But other than that, there’s almost no sign 

[14:36] I don’t just love bridges; I’m actually a 

[14:41] every year I have to take a certain number 

[14:45] And that’s how I first heard of this job. 

[14:49] put the story together in a 

[14:52] So huge thanks to them for sharing so 

[14:57] And another thanks to Iowa DOT who answered 

[15:03] I am so impressed with this bridge demolition. No 

[15:09] think taking the old ones down was the coolest 

[15:15] demolition is specialized, challenging work 

[15:21] Hats off to the entire team on this project 

[15:27] You know, I was super interested in that  

[15:29] sonar scan they did of the Mississippi River to 

[15:34] And really, I love any situation where you get 

[15:39] otherwise be hidden from view. My friend Brian 

[15:45] to a new level with his series, “The Anatomy 

[15:51] into a CT scanner, so we can literally see 

[15:57] of what makes up our favorite gadgets, like 

[16:03] RF shielding in the Nokia 3310. And if you want 

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