The I-35W Mississippi River bridge (officially known as Bridge 9340 ) was an eight-lane, steel truss arch bridge that carried Interstate 35W across the Mississippi River in Minneapolis, Minnesota, United States. The bridge is noteworthy because of its catastrophic failure during the evening rush hour on August 1, 2007. It collapsed into the river and onto the riverbanks beneath, killing thirteen people and injuring 145. The bridge was Minnesota's fifth busiest, carrying 140,000 vehicles daily. The NTSB cited a design flaw as the likely cause of the collapse, and asserted that additional weight on the bridge at the time of the collapse contributed to the failure.
Immediately after the collapse, help came from mutual aid in the seven-county Minneapolis-Saint Paul metropolitan area and emergency response personnel, charities, and volunteers. City and county employees managed the rescue using post-9/11 techniques and technology that may have saved lives. Within a few days of the collapse, the Minnesota Department of Transportation planned a replacement bridge, the I-35W Saint Anthony Falls Bridge. Construction was completed rapidly and it opened on September 18, 2008.
Location and site history
Located in Minneapolis, Minnesota's largest city, the bridge connected the Minneapolis neighborhoods of Downtown East and Marcy-Holmes. The south abutment was northeast of the Metrodome, and the north abutment was northwest of the University of Minnesota East Bank campus. The bridge was the southeastern boundary of the "Mississippi Mile" downtown riverfront parkland.
Downstream is the 10th Avenue Bridge, once known as the Cedar Avenue Bridge. Immediately upstream is the lock and dam at Saint Anthony Falls, where Minneapolis began. The first bridge upstream is the historic Stone Arch Bridge, built for the Great Northern Railway and now used for bicycle and pedestrian traffic.
The north foundation pier of the bridge was near a hydroelectric plant, razed in 1988. The south abutment was in an area polluted by a coal-to-gas processing plant and a facility for storing and processing petroleum products. These uses effectively created a toxic waste site under the bridge, leading to a lawsuit and the removal of the contaminated soil. No relationship between these prior uses and the bridge failure has been claimed.
Design and construction
Southeast Steam Plant (left), Stone Arch Bridge, I-35W bridge (right) and behind it, the 10th Avenue Bridge and the University of Minnesota in 2005
I-35W bridge south end seen from West River Parkway
I-35W bridge (center left) west of the 10th Avenue Bridge (center right), over the Mississippi River, 2004
The bridge, officially designated ' Bridge 9340', was designed by Sverdrup & Parcel to 1961 AASHO (American Association of State Highway Officials, now AASHTO) standard specifications. The construction contracts, worth in total more than US$5.2 million at the time, went to Hurcon Inc. and Industrial Construction Company, which built the steel trusses and deck. Construction began in 1964 and the bridge opened to traffic in 1967.
The bridge's fourteen spans extended 1,907 feet (580 m) long. The three main spans were of deck truss construction while all but two of the eleven approach spans were steel multi-girder construction, the two exceptions being concrete slab construction. The piers were not built in the navigation channel; instead, the center span of the bridge consisted of a single 458-foot (140 m) steel arched truss over the 390-foot (119 m) channel. The two support piers for the main trusses, each with two load-bearing concrete pylons at either side of the center main span, were located on opposite banks of the river. The center span was connected to the north and south approaches by shorter spans formed by the same main trusses. Each was 266 feet (81 m) in length, and was connected to the approach spans by a 38 foot (11.6 m) cantilever. The two main trusses, one on either side, ranged in depth from 60 feet (18.3 m) above their pier and concrete pylon supports, to 36 feet (11 m) at midspan on the central span and 30 feet (9 m) deep at the outer ends of the adjoining spans. At the top of the main trusses were the deck trusses, 12 feet (3.6 m) in depth and integral with the main trusses. The transverse deck beams, part of the deck truss, rested on top of the main trusses. These deck beams supported longitudinal deck stringers 27 inches (69 cm) in depth, and reinforced-concrete pavement. The deck was 113 ft 4 in (34.5 m) in breadth and was split longitudinally. It had transverse expansion joints at the centers and ends of each of the three main spans. The roadway deck was approximately 115 feet (35 m) above the water level.
Construction on the bridge began in 1964 and the structure was completed in 1967 during an era of large-scale projects related to building the Twin Cities freeway system. When the bridge fell, it was still the most recent river crossing built on a new site in Minneapolis. After the building boom ebbed during the 1970s, infrastructure management shifted toward inspection and maintenance.
Black ice
In February and in December 1996, the bridge was identified as the single most treacherous cold-weather spot in the Twin Cities freeway system, because of the almost frictionless thin layer of black ice that regularly formed when temperatures dropped to 30s Fahrenheit (4 °C to -1 °C) and below. The bridge's proximity to Saint Anthony Falls contributed significantly to the icing problem and the site was noted for frequent spinouts and collisions. By January 1999, Mn/DOT began testing magnesium chloride solutions and a mixture of magnesium chloride and a corn-processing byproduct to see whether either would reduce the black ice that appeared on the bridge during the winter months. In October 1999, the state embedded temperature-activated nozzles in the bridge deck to spray the bridge with potassium acetate solution to keep the area free of winter black ice. The system came into operation in 2000. It has been raised as a possibility that the potassium acetate may have contributed to the collapse of the 35W bridge.
Maintenance and inspection
Since 1993, the bridge was inspected annually by Mn/DOT, although no inspection report was completed in 2007, due to the construction work. In the years prior to the collapse, several reports cited problems with the bridge structure. In 1990, the federal government gave the I-35W bridge a rating of "structurally deficient," citing significant corrosion in its bearings. Approximately 75,000 other U.S. bridges had this classification in 2007.
According to a 2001 study by the civil engineering department of the University of Minnesota, cracking had been previously discovered in the cross girders at the end of the approach spans. The main trusses connected to these cross girders and resistance to motion at the connection point bearings was leading to unanticipated out-of-plane distortion of the cross girders and subsequent stress cracking. The situation was addressed prior to the study by drilling the cracks to prevent further propagation and adding support struts to the cross girder to prevent further distortion. The report also noted a concern about lack of redundancy in the main truss system, which meant the bridge had a greater risk of collapse in the event of any single structural failure. Although the report concluded that the bridge should not have any problems with fatigue cracking in the foreseeable future, the bridge instrumentation by strain gages and continuous structural health monitoring had been suggested.
In 2005, the bridge was again rated as "structurally deficient" and in possible need of replacement, according to the U.S. Department of Transportation's National Bridge Inventory database. Problems were noted in two subsequent inspection reports. The inspection carried out June 15, 2006 found problems of cracking and fatigue. On August 2, 2007, Governor Pawlenty stated that the bridge was scheduled to be replaced in 2020.
The I-35W bridge ranked near the bottom of federal inspection ratings nationwide. The scale used was a "sufficiency rating" which ranges from the highest score, 100, to the lowest score,
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