---
title: 'Bridge Demolition is Complicated'
source: 'https://youtube.com/watch?v=7oi4yMr8Rjk'
video_id: '7oi4yMr8Rjk'
date: 2026-06-28
duration_sec: 0
---

# Bridge Demolition is Complicated

> Source: [Bridge Demolition is Complicated](https://youtube.com/watch?v=7oi4yMr8Rjk)

## Summary

This video explores the complex engineering behind demolishing the old I-74 bridges over the Mississippi River. It details the challenges of removing the structures safely without harming the environment or disrupting navigation, culminating in a controlled and spectacular explosion.

### Key Points

- **The need for replacement** [1:34] — The original I-74 bridges were built in 1935 and 1959, with narrow lanes and no shoulders, making them unsafe for modern interstate traffic.
- **Complications of demolition** [1:12] — Demolition engineering is often more complex than designing a new structure, especially due to water, shipping traffic, and environmental sensitivity.
- **Concrete deck removal** [4:37] — Concrete deck was sawcut into panels, and engineers had to calculate if the panels could support the weight of demolition equipment without collapsing.
- **Asymmetric loading challenge** [5:36] — Deck removal on suspension spans had to be symmetrical (staggered) to prevent towers from deflecting undesirably.
- **Temporary supports on barge** [6:53] — To avoid disturbing endangered mussels, demolition team floated temporary supports on a barge instead of placing them on the riverbed.
- **Modeling the bridge as built** [7:46] — A structural computer model was critical; it accounted for historical construction methods (like rivet order) to understand stress distribution.
- **Use of explosives** [11:05] — Shaped charges were used to cut cables and towers, but they are designed to make clean cuts rather than pulverize the structure.
- **Precutting before blasting** [12:43] — Before blasting, workers precut cables and towers so that the explosives only had to sever the remaining connections, ensuring controlled fall.

## Transcript

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.
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.
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.
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.”
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.
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.
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 reinforcement is cut too. And this is 
just a cool part of demolition engineering:  
using calculations and analysis not to design 
something new but to answer a tricky question  
like, “Can these concrete panels support 
the weight of a 35,000-pound excavator?”  
The answer was more complicated 
than just a simple “yes.” So the  
engineers imposed pretty strict positional 
requirements for the demolition equipment,  
in most cases making sure that the tracks of 
the excavator were always directly above the  
stringers instead of relying on the concrete deck 
panels to act like beams and transfer the weight.
Another challenge on the suspension span was 
asymmetric loading. It’s easiest to use the bridge  
to dismantle the bridge, systematically working 
your way toward either end. The problem is that  
if you take all the weight off one section of the 
bridge while it’s still remaining on other parts,  
the trusses are going to bow, the towers are 
going to deflect, and you could actually fail  
the bridge prematurely. It’s just like re-racking 
weights at the gym. If, instead of alternating,  
you take everything off one side of 
the barbell, you might have a bad time.  
So, on the suspension bridges alone, the 
deck removal was this multi-stage process.
Some slabs were removed with 
the excavator and loader.  
Others were popped up and left in place as 
counterweight to be removed by a crane later.  
It’s a lot more work with a crane 
(and slower), but it was the only  
way to get the deck off in a symmetrical way 
to avoid overstressing any part of the bridge.
Once the concrete deck was off, the contractor 
could start removing the steel trusses, beams,  
and stringers that make up the bridge structures. 
And this gets pretty tricky too. You can’t just  
go cutting up a bridge willy nilly. This is 
like jenga on hard mode with very high stakes,  
and that requires some structural engineering. 
On the continuous truss section, the demolition  
team wasn’t allowed to install temporary 
supports to avoid disturbing the mussels.  
So instead, they floated in the support on 
a barge. This allowed them to safely cut  
the trusses into pieces small enough for the 
crane to handle without causing a collapse.
The suspension spans were even more complicated. 
You can imagine how dangerous it is to cut a  
piece of steel that’s under significant 
stress. As soon as the member is severed,  
it could cause sudden movements and 
load redistributions within the bridge.  
Those cuts have to be carefully sequenced. When 
you’re actively weakening a structure, each step  
changes the stresses, shifting them around and 
altering their magnitudes and directions. You have  
to check each step before you do it to make sure 
it’s not going to endanger workers, ships below,  
or the environment. And there’s no way to know how 
much stress is in a member just by looking at it.
Instead, they had to create a structural computer 
model. But that’s not as simple as recreating the  
bridge in 3D. The order also matters.
One cool example of this: 
the rivets in the connections for  
the top chords of each truss weren’t installed 
until after the concrete deck was poured. So,  
most of the load was being carried in the bottom 
chords in tension. When they removed the deck,  
the whole truss responded by going into 
what engineers call “negative bending.”  
The top chords were in tension and the bottom 
chords in compression, the exact opposite of  
what you would expect. That’s something that 
could have derailed the demolition plan without  
having gone through the exercise of modeling the 
bridge exactly how it was originally constructed,  
modified, and retrofitted over the years. It was 
almost half engineering, half a history exercise.  
The engineer even used old magazine articles to 
understand exactly where those stresses would be.
Here’s another tricky part. To lift the 
truss sections off the suspension bridges,  
they had to put a crane on a barge. Anyone 
who’s been on a boat knows that they’re  
not the most stable platforms for high-stakes, 
high-center-of-gravity work, especially when you  
add in huge loads and the need for precision. 
They did it for the original construction of  
these bridges, but that doesn’t mean it’s easy. 
Operating a crane from a barge involves dynamic  
loads from lifting and swinging. Barges often 
use these spud legs to help keep them in place,  
but there’s still a lot of engineering that goes 
into checking the stability of a barge for the  
variety of loading conditions. Those calculations 
help you pick the right crane, its configuration,  
limitations on pick weights and movements, et 
cetera, to make sure the work can be done safely.  
And just like the concrete deck, these 
truss segments had to be removed in a  
staggered manner to keep the towers from 
deflecting too much in one direction.
Interestingly, sometimes to demolish a structure, 
you have to add parts first. The original lateral  
load system had to be removed because of 
how the bridge would flex during demolition,  
but the engineers didn’t want people working on 
a bridge with no way to withstand wind loads.  
So they had to design and build these steel 
bumpers that could transfer lateral loads from  
the superstructure during the demolition process. 
In another case, they had to install bearing  
restraints on the trusses of the continuous 
spans, again to manage wind loads during the  
time those trusses were partially demolished. And 
in another example, they had to build an entire  
stiffening truss, made from pieces of the bridge 
that had already been removed, so the last deck  
truss could be lifted and removed as a single 
piece. All this steel was brought to a location  
on the shore where it could be cut down using this 
hydraulic shear and then sent off for recycling.
At this point, basically all that was left of 
each bridge was the suspension towers and cables.  
Since those cables are essentially one long 
structural member, there’s really no way to  
safely cut them loose. Imagine getting snapped by 
an enormous rubber band. There’s a lot of stored  
up energy there, and you don’t want any humans 
nearby when they come down. This is where the  
explosives come in. And like every other part 
of the process, this is tricker than you think.
Explosives used for demolition aren’t 
really like the ones you see in the movies.  
You’re not trying to use them to 
blow everything into tiny pieces.  
On buildings, you get a lot of breakup anyway 
because of the kinetic energy of falling.  
But really, the explosives are just strategically 
severing columns and beams quickly to start the  
falling process in a more controlled way. 
On a bridge, usually what you want is big  
pieces that can easily be removed from the 
water. So the explosives are more like small,  
very exciting saws that can cut 
quickly, simultaneously, and remotely.
Demolition contractors use shaped 
charges that sever structural members  
in a relatively controlled manner, and 
more importantly, a specific location.  
It matters a lot what these pieces look like 
after the blast. They have to be small enough  
to be lifted and transported out of the water. And 
you want them to fall in specific locations where  
they’re accessible without blocking the navigation 
channel. So, before the explosives are placed,  
there’s an entire process of precutting. The goal 
is to reduce each location where explosives will  
be placed down to flat plates or smaller sections 
so the blasts are sure to completely cut through.  
A worst case scenario is an incomplete 
explosive demolition that doesn’t fully  
bring the structure down. When that happens, 
the whole process becomes much more dangerous  
and difficult because you have to finish 
the job using workers on a structure where  
it’s not entirely clear where the 
stresses are or where it’s safe to cut.
On the I-74 bridges, workers cut the outer 
strands of the main cable, leaving only 7 of  
the 37 strands holding. This was done in four 
locations on each cable to break it up into  
manageable pieces. The towers were also cut in 
strategic locations to allow the shaped charges  
to sever completely and control which direction 
they would fall into the water. And, by the way,  
this leaves the bridge extremely vulnerable. 
You’re basically marching right up to the line of  
stability so that the explosives can kind of carry 
you over the finish line to bring the structure  
down. But it means the clock is ticking. You’re 
checking the weather. Working long shifts. You  
don’t want a storm bringing the bridge down before 
you get to push the button. But once you do…
I’ve mentioned a lot of pros and cons of explosive 
demolition, but there’s one thing they do better  
than anything else: the spectacle.
You just can’t get around how 
cool it is to blow stuff up.
They got perfect shots for both bridges, bringing 
them down safely so that the barges could come out  
quickly to pick up the pieces. The road and 
navigation channel only had to be closed for  
a short period of time. Here’s a good look at how 
clean the cuts are from the shaped charges on the  
cables. And actually, they used explosives to demo 
the substructures later on.
The piers in the more  sensitive areas were taken out using conventional jackhammers as the final step. These sheet  
pile containment structures kept the debris from 
spreading out in the water. And of course they did  
a scan to make sure all the debris was picked up. 
It turns out one of those piers has become a great  
habitat for those endangered mussels, so you can 
see it still standing next to the new bridges.  
But other than that, there’s almost no sign 
that those old bridges were there at all.
I don’t just love bridges; I’m actually a 
licensed engineer, and part of that means  
every year I have to take a certain number 
of classes to stay current in the field.  
And that’s how I first heard of this job. 
Some of the engineers and contractors  
put the story together in a 
professional development class.  
So huge thanks to them for sharing so 
many interesting details of the project.  
And another thanks to Iowa DOT who answered 
our questions and shared photos and videos.  
I am so impressed with this bridge demolition. No 
shade at all to the new bridges, but honestly I  
think taking the old ones down was the coolest 
part of the entire replacement project. Bridge  
demolition is specialized, challenging work 
that takes a lot of engineering to get right.  
Hats off to the entire team on this project 
for getting it done so safely and efficiently.
You know, I was super interested in that  
sonar scan they did of the Mississippi River to 
make sure they got all the debris off the bottom.  
And really, I love any situation where you get 
to peer into the details of something that would  
otherwise be hidden from view. My friend Brian 
at the Real Engineering channel took that idea  
to a new level with his series, “The Anatomy 
of.” He’s putting everyday objects and devices  
into a CT scanner, so we can literally see 
inside them. This is such a cool exploration  
of what makes up our favorite gadgets, like 
the spin wheel in the original iPod or the  
RF shielding in the Nokia 3310. And if you want 
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