| Willamette River Bridge demolition |
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| Willamette River Bridge demolition: what goes up must come down |
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An excavator demolishes the I-5 Willamette River Bridge
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Tearing down a 2,000-foot long, 50-foot high freeway bridge requires massive machines crushing, smashing, bashing, cutting and crunching cement, asphalt and steel until all that is left is rubble and debris. The demolition, which may appear chaotic to an untrained eye, is actually a carefully orchestrated performance that can occur only after months of careful planning and preparation intended to leave nothing to chance.
During a 2002 inspection, Oregon Department of Transportation engineers identified shear cracks in the Interstate 5 Willamette River Bridge’s structure severe enough to require posting weight limits for the bridge. Heavy-haul trucks were rerouted 200 miles around the bridge, punching a hole in the vital north-south corridor for the West Coast, until 2004 when the agency erected a temporary detour bridge to keep freight and other vehicles moving.
In 2009, planning, engineering and preparation came to a head and it was time to start construction of the permanent replacement bridge—a graceful deck-arch bridge designed to last at least 75 years.
Before the agency could start building the new structure, ODOT needed to demolish the old one. Teams spent months planning the process and preparing the area, working diligently to account for every aspect of tearing down the four lanes of freeway standing high above the river. They devised a demolition plan that called for excavators to pull the bridge apart from both above and below simultaneously with little or no impact on surrounding parks, roads or natural areas. Predemolition engineering determined that the bridge would be strong enough to support the machines working from the bridge's surface, munching and crunching the bridge away from beneath them as they worked.
As the first girders were cut away, however, engineers received an unwelcome surprise: The bridge's actual capacity and strength did not match original estimates outlined in the plans. Although the design was strong enough for daily highway traffic, it would not support the excavators' weight as they weakened the structure during demolition.
To keep the crew safe, all demolition had to occur from below, requiring specialized machines able to reach 50 feet or more into the air to disassemble the bridge overhead. For the challenging task, ODOT's team brought in a pair of high-reach excavators with custom-designed booms capable of reaching the former interstate bridge 40 feet above and reducing it to a pile of rubble and rebar.
The machines worked from a custom-designed work platform beneath the bridge. The temporary platform, which stands about 10 feet above the water, not only protects the river from debris, it also provides a stable surface for the workers and machinery during the demolition and reconstruction. At approximately 120,000 square feet, this structure—composed mainly of steel and massive timbers—has a footprint as large as some Costco stores.
The work bridge is the last line of defense for the river; all machines on the work platform were carefully wrapped with tarps to protect the river below from any leaks or drips. This preparation, affectionately known as diapering, was the last step before the machinery, operated by skilled demolition experts, broke the bridge above into pieces and pulled it onto the deck. Operators then crushed large pieces into smaller rubble and separated materials for reuse and recycling.
Contractors stripped the steel of almost all concrete and grouped it into long bundles or compact balls, which were then hauled away to scrap recyclers. The machines sorted all other material for easy removal. Much of the concrete went to recyclers to be crushed and made into gravel, and some was used as fill by local gravel mines. All told, ODOT removed about 30 million pounds of demolished material from the project site.
Despite some surprises along the way, ODOT’s team found creative engineering solutions to safely and effectively deconstruct the project ahead of schedule. In just a couple of months, the team reduced a nearly 2,000-foot-long interstate freeway bridge to rubble and left a blank slate for the construction team to build the new I-5 Willamette River Bridge.
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