[0:01] In 2022, the world got a very cool new bridge. 
Two bridges, actually. The Iowa-Illinois Memorial  
[0:08] Bridges carries Interstate Highway 74 over the 
Mississippi River between Moline, Illinois and  
[0:14] Bettendorf, Iowa in the “Quad Cities” area. It’s a 
gorgeous pair of structures with the basket handle  
[0:21] arches carrying each deck over the main span of 
the river. But even after they were finished, Iowa  
[0:27] DOT had a problem. The two old bridges were still 
right there, also crossing the Mississippi River.  
[0:34] And even though they were kind of cool looking, 
they just couldn’t stay. The bridges were already  
[0:39] in poor condition, and without extensive ongoing 
maintenance, they would continue to deteriorate,  
[0:45] posing a danger to the public, affecting 
the sensitive environment along the river,  
[0:49] and even disrupting this critical shipping 
artery. The old bridges would have to come down.
[0:55] Demolition, on its face, seems kind of easy. 
For a billion-dollar-bridge replacement project,  
[1:00] the demo part feels almost like housekeeping. 
Smash the structure down or blow it up,  
[1:06] then just pick up the pieces. No engineering 
needed. The truth is that it’s anything but.  
[1:12] Demolition engineering, in many ways, is even 
more complicated than designing a new structure,  
[1:17] and the I-74 bridge is the 
perfect case study in why.  
[1:21] And don’t worry, there are explosions at the end. 
I’m Grady, and this is Practical Engineering.
[1:34] The original I-74 bridges, with just two lanes 
each, were way overdue for an upgrade in capacity.  
[1:41] This is actually a problem they faced and managed 
to overcome once before, many decades back.  
[1:47] Despite looking like twins, the original pair 
of bridges were built a generation apart. The  
[1:53] first span was completed in the mid 1930s, but car 
ownership and traffic exploded after World War II.  
[1:59] Engineers decided that the best way 
to increase capacity was to build a  
[2:03] nearly identical bridge right next to 
it. That new bridge was opened in 1959.
[2:09] Neither bridge was ever intended to meet 
interstate standards; they predate the  
[2:13] interstate system altogether. And yet, they found 
themselves carrying interstate-levels of traffic,  
[2:19] way beyond what the designers in the 1950s, 
and especially the designers back in the 1930s,  
[2:25] had considered. The lanes were narrow, there were 
no shoulders so they required a lower speed limit,  
[2:30] which bottlenecked traffic on I-74. 
Size isn’t everything, though; the bridges  
[2:36] were also just physically wearing out. Like an 
old car, it eventually got to the point where the  
[2:41] cost of replacing the bridges was outweighed by 
the constant maintenance and threat of disaster.  
[2:48] In 2012, Transportation Secretary 
Ray LaHood toured this structure,  
[2:52] reporting back that it was, quote, “one of 
the worst bridges I’ve seen in America.”
[2:58] You would think that already being close to 
falling down would be to their advantage when  
[3:03] it comes to demolition, but it’s quite 
a bit more complicated than that. These  
[3:07] were big bridges with 3 types of structural 
designs. There’s these three span continuous  
[3:13] truss units over the old non-navigable part 
of the river. There’s the deck trusses that  
[3:18] kind of act as connectors. And then you 
have the big 3 span suspension section.
[3:23] I’m sure you want to see the explosion, and I 
promise we’ll get there, but it’s basically the  
[3:28] last step. Of course, there are lots of cases 
where it makes sense to just blast a structure  
[3:33] down right away. You end up with a pile of rubble 
that you can manage with regular construction  
[3:39] equipment. It can be much quicker, easier, and 
safer than dismantling a structure piece by piece,  
[3:45] but it’s rarely true for bridges. Of course 
you’ve got the water that complicates things.  
[3:50] Removing debris from below 
the water line is challenging.  
[3:54] Long reach excavators can sometimes handle 
the smaller stuff, but you often need divers  
[3:59] to rig the big stuff to be lifted out by 
cranes. That’s dangerous and difficult work.  
[4:04] You also have shipping traffic to consider. This 
stretch of the Mississippi is a busy part of the  
[4:09] inland waterway system, and closing it to clean 
debris out of the channel is a disruptive task.  
[4:15] The other thing making it tricky, in this case, 
is the environment. There are endangered mussels  
[4:20] living in the non-navigable channel below the 
continuous truss spans, so the demolition team  
[4:26] couldn’t use blasting or even temporary supports 
in that part of the river. The only option was  
[4:31] to dismantle the bridges more carefully and 
thoughtfully. Step one is to get the deck off.
[4:37] The strategy here was to sawcut all the concrete 
into pieces small enough to move with construction  
[4:43] equipment. An excavator with a slab crab 
attachment could lift each panel off the  
[4:48] steel structure, swing around, and pass to a wheel 
loader to carry it off the end. Sounds simple,  
[4:54] but it had to be done pretty carefully. Cutting 
the concrete into panels like this means that  
[4:58] the reinforcement is cut too. And this is 
just a cool part of demolition engineering:  
[5:04] using calculations and analysis not to design 
something new but to answer a tricky question  
[5:10] like, “Can these concrete panels support 
the weight of a 35,000-pound excavator?”  
[5:16] The answer was more complicated 
than just a simple “yes.” So the  
[5:20] engineers imposed pretty strict positional 
requirements for the demolition equipment,  
[5:25] in most cases making sure that the tracks of 
the excavator were always directly above the  
[5:30] stringers instead of relying on the concrete deck 
panels to act like beams and transfer the weight.
[5:36] Another challenge on the suspension span was 
asymmetric loading. It’s easiest to use the bridge  
[5:42] to dismantle the bridge, systematically working 
your way toward either end. The problem is that  
[5:48] if you take all the weight off one section of the 
bridge while it’s still remaining on other parts,  
[5:54] the trusses are going to bow, the towers are 
going to deflect, and you could actually fail  
[5:59] the bridge prematurely. It’s just like re-racking 
weights at the gym. If, instead of alternating,  
[6:04] you take everything off one side of 
the barbell, you might have a bad time.  
[6:09] So, on the suspension bridges alone, the 
deck removal was this multi-stage process.
[6:15] Some slabs were removed with 
the excavator and loader.  
[6:17] Others were popped up and left in place as 
counterweight to be removed by a crane later.  
[6:23] It’s a lot more work with a crane 
(and slower), but it was the only  
[6:26] way to get the deck off in a symmetrical way 
to avoid overstressing any part of the bridge.
[6:32] Once the concrete deck was off, the contractor 
could start removing the steel trusses, beams,  
[6:37] and stringers that make up the bridge structures. 
And this gets pretty tricky too. You can’t just  
[6:42] go cutting up a bridge willy nilly. This is 
like jenga on hard mode with very high stakes,  
[6:48] and that requires some structural engineering. 
On the continuous truss section, the demolition  
[6:53] team wasn’t allowed to install temporary 
supports to avoid disturbing the mussels.  
[6:58] So instead, they floated in the support on 
a barge. This allowed them to safely cut  
[7:02] the trusses into pieces small enough for the 
crane to handle without causing a collapse.
[7:07] The suspension spans were even more complicated. 
You can imagine how dangerous it is to cut a  
[7:13] piece of steel that’s under significant 
stress. As soon as the member is severed,  
[7:18] it could cause sudden movements and 
load redistributions within the bridge.  
[7:23] Those cuts have to be carefully sequenced. When 
you’re actively weakening a structure, each step  
[7:28] changes the stresses, shifting them around and 
altering their magnitudes and directions. You have  
[7:34] to check each step before you do it to make sure 
it’s not going to endanger workers, ships below,  
[7:40] or the environment. And there’s no way to know how 
much stress is in a member just by looking at it.
[7:46] Instead, they had to create a structural computer 
model. But that’s not as simple as recreating the  
[7:51] bridge in 3D. The order also matters.
[7:56] One cool example of this: 
the rivets in the connections for  
[7:59] the top chords of each truss weren’t installed 
until after the concrete deck was poured. So,  
[8:05] most of the load was being carried in the bottom 
chords in tension. When they removed the deck,  
[8:11] the whole truss responded by going into 
what engineers call “negative bending.”  
[8:16] The top chords were in tension and the bottom 
chords in compression, the exact opposite of  
[8:21] what you would expect. That’s something that 
could have derailed the demolition plan without  
[8:25] having gone through the exercise of modeling the 
bridge exactly how it was originally constructed,  
[8:31] modified, and retrofitted over the years. It was 
almost half engineering, half a history exercise.  
[8:38] The engineer even used old magazine articles to 
understand exactly where those stresses would be.
[8:44] Here’s another tricky part. To lift the 
truss sections off the suspension bridges,  
[8:49] they had to put a crane on a barge. Anyone 
who’s been on a boat knows that they’re  
[8:54] not the most stable platforms for high-stakes, 
high-center-of-gravity work, especially when you  
[9:00] add in huge loads and the need for precision. 
They did it for the original construction of  
[9:05] these bridges, but that doesn’t mean it’s easy. 
Operating a crane from a barge involves dynamic  
[9:11] loads from lifting and swinging. Barges often 
use these spud legs to help keep them in place,  
[9:16] but there’s still a lot of engineering that goes 
into checking the stability of a barge for the  
[9:21] variety of loading conditions. Those calculations 
help you pick the right crane, its configuration,  
[9:28] limitations on pick weights and movements, et 
cetera, to make sure the work can be done safely.  
[9:33] And just like the concrete deck, these 
truss segments had to be removed in a  
[9:37] staggered manner to keep the towers from 
deflecting too much in one direction.
[9:42] Interestingly, sometimes to demolish a structure, 
you have to add parts first. The original lateral  
[9:48] load system had to be removed because of 
how the bridge would flex during demolition,  
[9:53] but the engineers didn’t want people working on 
a bridge with no way to withstand wind loads.  
[9:59] So they had to design and build these steel 
bumpers that could transfer lateral loads from  
[10:04] the superstructure during the demolition process. 
In another case, they had to install bearing  
[10:09] restraints on the trusses of the continuous 
spans, again to manage wind loads during the  
[10:14] time those trusses were partially demolished. And 
in another example, they had to build an entire  
[10:20] stiffening truss, made from pieces of the bridge 
that had already been removed, so the last deck  
[10:25] truss could be lifted and removed as a single 
piece. All this steel was brought to a location  
[10:31] on the shore where it could be cut down using this 
hydraulic shear and then sent off for recycling.
[10:37] At this point, basically all that was left of 
each bridge was the suspension towers and cables.  
[10:43] Since those cables are essentially one long 
structural member, there’s really no way to  
[10:47] safely cut them loose. Imagine getting snapped by 
an enormous rubber band. There’s a lot of stored  
[10:54] up energy there, and you don’t want any humans 
nearby when they come down. This is where the  
[10:59] explosives come in. And like every other part 
of the process, this is tricker than you think.
[11:05] Explosives used for demolition aren’t 
really like the ones you see in the movies.  
[11:10] You’re not trying to use them to 
blow everything into tiny pieces.  
[11:15] On buildings, you get a lot of breakup anyway 
because of the kinetic energy of falling.  
[11:20] But really, the explosives are just strategically 
severing columns and beams quickly to start the  
[11:26] falling process in a more controlled way. 
On a bridge, usually what you want is big  
[11:32] pieces that can easily be removed from the 
water. So the explosives are more like small,  
[11:37] very exciting saws that can cut 
quickly, simultaneously, and remotely.
[11:43] Demolition contractors use shaped 
charges that sever structural members  
[11:48] in a relatively controlled manner, and 
more importantly, a specific location.  
[11:53] It matters a lot what these pieces look like 
after the blast. They have to be small enough  
[11:57] to be lifted and transported out of the water. And 
you want them to fall in specific locations where  
[12:03] they’re accessible without blocking the navigation 
channel. So, before the explosives are placed,  
[12:09] there’s an entire process of precutting. The goal 
is to reduce each location where explosives will  
[12:15] be placed down to flat plates or smaller sections 
so the blasts are sure to completely cut through.  
[12:22] A worst case scenario is an incomplete 
explosive demolition that doesn’t fully  
[12:27] bring the structure down. When that happens, 
the whole process becomes much more dangerous  
[12:33] and difficult because you have to finish 
the job using workers on a structure where  
[12:38] it’s not entirely clear where the 
stresses are or where it’s safe to cut.
[12:43] On the I-74 bridges, workers cut the outer 
strands of the main cable, leaving only 7 of  
[12:49] the 37 strands holding. This was done in four 
locations on each cable to break it up into  
[12:55] manageable pieces. The towers were also cut in 
strategic locations to allow the shaped charges  
[13:01] to sever completely and control which direction 
they would fall into the water. And, by the way,  
[13:07] this leaves the bridge extremely vulnerable. 
You’re basically marching right up to the line of  
[13:12] stability so that the explosives can kind of carry 
you over the finish line to bring the structure  
[13:18] down. But it means the clock is ticking. You’re 
checking the weather. Working long shifts. You  
[13:23] don’t want a storm bringing the bridge down before 
you get to push the button. But once you do…
[13:34] I’ve mentioned a lot of pros and cons of explosive 
demolition, but there’s one thing they do better  
[13:39] than anything else: the spectacle.
[13:42] You just can’t get around how 
cool it is to blow stuff up.
[13:46] They got perfect shots for both bridges, bringing 
them down safely so that the barges could come out  
[13:52] quickly to pick up the pieces. The road and 
navigation channel only had to be closed for  
[13:58] a short period of time. Here’s a good look at how 
clean the cuts are from the shaped charges on the  
[14:03] cables. And actually, they used explosives to demo 
the substructures later on.
[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 
spreading out in the water. And of course they did  
[14:20] a scan to make sure all the debris was picked up. 
It turns out one of those piers has become a great  
[14:26] habitat for those endangered mussels, so you can 
see it still standing next to the new bridges.  
[14:32] But other than that, there’s almost no sign 
that those old bridges were there at all.
[14:36] I don’t just love bridges; I’m actually a 
licensed engineer, and part of that means  
[14:41] every year I have to take a certain number 
of classes to stay current in the field.  
[14:45] And that’s how I first heard of this job. 
Some of the engineers and contractors  
[14:49] put the story together in a 
professional development class.  
[14:52] So huge thanks to them for sharing so 
many interesting details of the project.  
[14:57] And another thanks to Iowa DOT who answered 
our questions and shared photos and videos.  
[15:03] I am so impressed with this bridge demolition. No 
shade at all to the new bridges, but honestly I  
[15:09] think taking the old ones down was the coolest 
part of the entire replacement project. Bridge  
[15:15] demolition is specialized, challenging work 
that takes a lot of engineering to get right.  
[15:21] Hats off to the entire team on this project 
for getting it done so safely and efficiently.
[15:27] You know, I was super interested in that  
[15:29] sonar scan they did of the Mississippi River to 
make sure they got all the debris off the bottom.  
[15:34] And really, I love any situation where you get 
to peer into the details of something that would  
[15:39] otherwise be hidden from view. My friend Brian 
at the Real Engineering channel took that idea  
[15:45] to a new level with his series, “The Anatomy 
of.” He’s putting everyday objects and devices  
[15:51] into a CT scanner, so we can literally see 
inside them. This is such a cool exploration  
[15:57] of what makes up our favorite gadgets, like 
the spin wheel in the original iPod or the  
[16:03] RF shielding in the Nokia 3310. And if you want 
to check it out, it’s only available on Nebula.
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