The St. Anthony Falls Bridge carries Interstate 35W over the Mississippi River in Minneapolis. It is a classic example of a modern, post-tensioned, segmental design. The bridge opened September 18, 2008 and was built to last 100 years.
It replaced the old I-35W Mississippi River Bridge (officially known as Bridge 9340) which collapsed during the evening rush hour on August 1, 2007. That bridge was supposed to last 50 years, but it collapsed after 40, killing 13 people and injuring 145. As anyone living in Minnesota at the time can attest, the shock of the bridge disaster will never be forgotten.
The old bridge had opened in 1967. It was an eight-lane, steel truss bridge, built for two lanes in each direction and intended to carry 66,000 vehicles per day. In 1988, two more lanes were added in each direction by eliminating the shoulders. By 2004, an estimated 141,000 vehicles were using the bridge each day. In 2005, it was rated “structurally deficient” and in possible need of replacement, according to the U.S. Department of Transportations National Bridge Inventory database.
The National Transportation Safety Board cited the collapse was caused by inadequate load capacity due to a design flaw. Twenty-four of the gusset plates used to hold the bridge together were too thin, and in a fracture-critical design like the old I-35W Bridge, if one portion of the bridge fails, the entire bridge is subject to failure. Contributing to the design error was inadequate attention given to the gusset plates during inspections, the substantial increase in the weight of the bridge during previous modifications, and the additional load on the bridge the day of the collapse.
That day, road work was underway on the bridge, and four lanes were closed. Hundreds of thousands of pounds of construction equipment and a delivery of sand and gravel had been positioned on the south end of the center section that afternoon. Bumper-to-bumper traffic added to the load.
At 6:05 p.m. the bridge experienced catastrophic failure in the main span of the deck truss. There were 111 vehicles on the bridge when it fell. One was a school bus with 63 passengers (mostly children) and one driver. All were helped to safety due to the heroic efforts of ordinary citizens. Many say it’s a miracle that so many people survived the bridge disaster. Their chances certainly improved with the help of 100 passersby, construction workers, and a group of Red Cross medical trainees that assisted emergency workers from seven counties in the rescue effort.
When the bridge fell, it was the newest bridge in Minneapolis. Within hours, politicians pledged to rebuild it at an accelerated pace.
The new bridge is named the St. Anthony Falls Bridge, in recognition of its proximity to lower St. Anthony Falls. The Colorado-based firm, Flatiron Constructors, completed it three months of schedule, in what could be considered a textbook example of how a design-build project is supposed to work. That is, parts of the bridge were still being designed while other parts were being built.
The design includes multiple levels of structural redundancy, for a long-lasting bridge that will be economical to maintain. High-performance concrete provides superior durability, and state-of-the-art “smart bridge” technology monitors deck movement, stress, and temperature in real time. It is equipped with anti-icing sprayers which use a solution less likely to damage the bridge than ordinary road salt.
The minimalist bridge was designed with a theme of “Arches, Water, Reflection.” It is light rail ready, and it was the first bridge in America built to use low-energy, low-maintenance LED lighting. It has won 25 awards for excellence and, in 2009, was named one of America’s 10 best transportation projects.
Post-Tensioned Precast Concrete Box Girder
Ten Lanes of I-35W
The St. Anthony Falls Bridge is most often aglow in blue, but it can be lit with any color in the spectrum due to its innovative LED lighting system. For special events, organizations may apply to have their own specific colors displayed.