I-74 Quad Cities Bridge Demolition: Complex Engineering Strategies

In 2022, the Iowa-Illinois Memorial Bridges, a pair of new structures carrying Interstate Highway 74 over the Mississippi River in the "Quad Cities" area, were completed. These bridges feature distinctive basket-handle arches. However, their completion left the Iowa DOT with the challenge of removing the two older I-74 bridges that previously occupied the same crossing. These old bridges, despite their historical significance, were in poor condition and posed risks to public safety, the environment, and shipping traffic if left in place.

The Complexity of Demolition Engineering

While demolition might seem straightforward—simply smashing or blowing up a structure—it is, in reality, a highly complex field of engineering. For the I-74 bridge replacement project, the demolition phase was as intricate, if not more so, than designing the new structures.

The original I-74 bridges, each with only two lanes, were built decades apart. The first span was completed in the mid-1930s, and a nearly identical second span was added in 1959 to accommodate the post-World War II surge in traffic. Neither bridge was designed to meet interstate standards, yet they ended up carrying interstate-level traffic, far exceeding their original design capacity. This resulted in narrow lanes, no shoulders, and lower speed limits, creating bottlenecks on I-74. Beyond capacity issues, the bridges were physically deteriorating, reaching a point where the cost of constant maintenance and the threat of disaster outweighed the cost of replacement. In 2012, then-Transportation Secretary Ray LaHood described one of them as "one of the worst bridges I’ve seen in America."

Demolition Challenges and Strategies

The poor condition of the bridges did not simplify their demolition. These were large structures with three distinct structural designs: * Three-span continuous truss units over the non-navigable part of the river. * Deck trusses acting as connectors. * A large three-span suspension section.

Avoiding Blasting and Environmental Concerns

While blasting is often used in demolition, it was not the primary method for the I-74 bridges due to several factors: * Water Complications: Removing debris from below the waterline is challenging and often requires divers, which is dangerous and difficult. * Shipping Traffic: The Mississippi River is a busy shipping artery, and closing it for debris removal would be highly disruptive. * Environmental Sensitivity: Endangered mussels lived in the non-navigable channel beneath the continuous truss spans. This prohibited the use of blasting or even temporary supports in that area.

Therefore, a more careful and thoughtful dismantling approach was necessary.

Deck Removal

The first step involved removing the concrete deck. The strategy was to saw-cut the concrete into pieces small enough for construction equipment to handle. An excavator with a slab crab attachment would lift each panel, transfer it to a wheel loader, and then move it off the bridge. This seemingly simple process required careful planning: * Structural Analysis: Engineers had to calculate whether the concrete panels could support the weight of a 35,000-pound excavator, especially after the reinforcement was cut. Strict positional requirements were imposed, ensuring excavator tracks were directly above stringers to avoid relying on the concrete deck panels as beams. * Asymmetric Loading on Suspension Spans: On the suspension spans, removing weight unevenly could cause the trusses to bow and towers to deflect, potentially leading to premature failure. To prevent this, deck removal was a multi-stage process. Some slabs were removed by excavator and loader, while others were left in place as counterweights and later removed by crane to maintain symmetrical loading and prevent overstressing any part of the bridge.

Steel Truss Removal

Once the concrete deck was off, the contractor began removing the steel trusses, beams, and stringers. This phase was akin to "Jenga on hard mode" due to the high stakes and the need for precise structural engineering. * Continuous Truss Section: Since temporary supports were not allowed due to the mussels, a support barge was floated in. This allowed the trusses to be safely cut into manageable pieces for crane removal without causing a collapse. * Suspension Spans: Cutting steel under significant stress is dangerous, as it can cause sudden movements and load redistributions. Therefore, cuts had to be carefully sequenced. Engineers used structural computer models to predict stress changes at each step, ensuring worker safety, protecting ships below, and safeguarding the environment. * Historical Modeling: A unique challenge was understanding the original construction and modifications. For example, the rivets in the top chords of the trusses were installed after the concrete deck was poured, meaning most of the load was initially carried by the bottom chords in tension. When the deck was removed, the truss responded with "negative bending," putting the top chords in tension and bottom chords in compression—the opposite of what might be expected. This required engineers to model the bridge exactly as it was built, modified, and retrofitted, even consulting old magazine articles to understand stress distribution. * Barge-Mounted Cranes: Lifting truss sections from the suspension bridges required barge-mounted cranes. This involved extensive engineering to ensure barge stability under dynamic loads, considering factors like spud legs, crane configuration, and weight limits. Truss segments were also removed in a staggered manner to prevent excessive tower deflection.

Adding Parts for Demolition

Counterintuitively, sometimes demolition requires adding new structural elements: * Lateral Load System: The original lateral load system had to be removed due to how the bridge would flex during demolition. To ensure safety against wind loads, steel bumpers were designed and built to transfer lateral loads from the superstructure during the demolition process. * Bearing Restraints: Bearing restraints were installed on the trusses of the continuous spans to manage wind loads during partial demolition. * Stiffening Truss: An entire stiffening truss, made from already removed bridge pieces, was constructed to allow the last deck truss to be lifted and removed as a single piece.

All removed steel was brought to shore, cut down with hydraulic shears, and sent for recycling.

Explosive Demolition

The final stage involved the suspension towers and cables. Since the cables are essentially one long structural member with immense stored energy, safely cutting them loose was impossible. This is where explosives came into play. * Controlled Severing: Demolition explosives are not meant to pulverize structures but to strategically and quickly sever columns and beams, initiating a controlled fall. For bridges, the goal is to create large, manageable pieces that can be easily removed from the water. * Shaped Charges: Shaped charges were used to sever structural members in specific locations. Precutting was crucial to reduce sections to flat plates or smaller pieces, ensuring the blasts completely cut through. An incomplete demolition, where the structure doesn't fully come down, is a worst-case scenario, making subsequent work much more dangerous. * Cable and Tower Preparation: On the I-74 bridges, workers cut the outer strands of the main cables, leaving only 7 of 37 strands holding in four locations on each cable to break them into manageable pieces. The towers were also strategically cut to control their fall direction into the water. This process left the bridge extremely vulnerable, requiring careful monitoring of weather and working long shifts to ensure the explosives could be detonated before a natural event brought the bridge down.

The explosive demolitions of both bridges were successful, allowing barges to quickly retrieve the pieces. Road and navigation channel closures were minimal. Shaped charges created clean cuts on the cables. Later, explosives were also used to demolish the substructures, while piers in sensitive areas were removed with conventional jackhammers, using sheet pile containment structures to prevent debris spread. A sonar scan confirmed all debris was removed, though one pier was intentionally left as a habitat for the endangered mussels.

The demolition of the old I-74 bridges was a testament to specialized and challenging engineering work, executed safely and efficiently.