Bridge Demolition is Complicated

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.”