I-35W Mississippi River bridge
The I-35W Mississippi River bridge was an eight-lane, steel truss arch bridge that carried Interstate 35W across the Mississippi River one-half mile downstream from the Saint Anthony Falls in Minneapolis, Minnesota, United States. The bridge opened in 1967, and was Minnesota's third busiest, carrying 140,000 vehicles daily. After 39 years in service, it experienced a catastrophic failure during the evening rush hour on August 1, 2007, killing 13 people and injuring 145. The National Transportation Safety Board cited a design flaw as the likely cause of the collapse, noting that an excessively thin gusset plate ripped along a line of rivets. The amount of weight on the bridge at the time of failure was also cited by the NTSB as a contributing factor.
Help came immediately from mutual aid in the seven-county Minneapolis–Saint Paul metropolitan area and emergency response personnel, charities, and volunteers. Within a few days of the collapse, the Minnesota Department of Transportation planned its replacement with the I-35W Saint Anthony Falls Bridge. The construction of the replacement bridge was completed quickly, and the new bridge officially opened on September 18, 2008.
Location and site history
The bridge was located in Minneapolis, Minnesota's largest city, and connected the neighborhoods of Downtown East and Marcy-Holmes. The south abutment was northeast of the Hubert H. Humphrey 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 Saint Anthony Falls lower lock and dam. 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 that was razed in 1988. The south abutment was in an area polluted by a coal 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 has been claimed between these previous uses and the bridge failure.
Design and construction
The bridge, officially designated "Bridge 9340", was designed by Sverdrup & Parcel to 1961 AASHTO standard specifications. The construction contracts, worth in total more than $5.2 million at the time, were initially offered to HurCon Inc. and Industrial Construction Company. HurCon expressed concern about the project, reporting that one portion of the bridge, Pier 6, could not be built as planned. After failed discussions with MnDOT, HurCon backed out of the project altogether.Construction on the bridge began in 1964 and the structure was completed and opened to traffic in 1967 during an era of large-scale projects to build 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.
The bridge's fourteen spans extended 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 steel arched truss over the 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 in length, and was connected to the approach spans by a cantilever. The two main trusses, one on either side, ranged in depth from above their pier and concrete pylon supports, to at midspan on the central span and deep at the outer ends of the adjoining spans. At the top of the main trusses were the deck trusses, 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 in depth, and reinforced-concrete pavement. The deck was 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 above the water level.
Black ice prevention system
On December 19, 1985, the temperature reached. Vehicles coming across the bridge experienced black ice and there was a major pile-up on the bridge on the northbound side.In February and 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 below freezing. 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, Minnesota 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.
Although there were no additional major multi-vehicle collisions after the automated de-icing system was installed, it was raised as a possibility that the potassium acetate may have contributed to the collapse of the bridge by corroding the structural supports, though the NTSB's final report found that corrosion was not a contributing factor.
Maintenance and inspection
Since 1993, the bridge was inspected annually by MnDOT, with the exception of 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, no 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, regular inspection, structural health monitoring, and use of strain gauges 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 Tim Pawlenty stated that the bridge had been scheduled to be replaced in 2020.
The I-35W bridge ranked near the bottom of federal inspection ratings nationwide. Bridge inspectors use a sufficiency rating that ranges from the highest score, 100, to the lowest score, zero. In 2005, they rated the bridge at 50, indicating that replacement may have been in order. Out of over 100,000 heavily used bridges, only about 4% scored below 50. On a separate measure, the I-35W bridge was rated "structurally deficient", but was deemed to have met "minimum tolerable limits to be left in place as it is".
In December 2006, a steel reinforcement project was planned for the bridge. However, the project was canceled in January 2007 in favor of periodic safety inspections, after engineers realized that drilling for the retrofitting would, in fact, weaken the bridge. In internal Mn/DOT documents, bridge officials talked about the possibility of the bridge collapsing, and worried that they might have to condemn it.
The construction taking place in the weeks prior to the collapse included joint work and replacing lighting, concrete and guard rails. At the time of the collapse, four of the eight lanes were closed for resurfacing, and there were of construction supplies and equipment on the bridge.
Collapse
At 6:05 p.m. CDT on August 1, 2007, with rush hour bridge traffic moving slowly through the limited number of lanes, the central span of the bridge suddenly gave way, followed by the adjoining spans. The structure and deck collapsed into the river and onto the riverbanks below, the south part toppling eastward in the process. A total of 111 vehicles were involved, sending their occupants and 18 construction workers as far as down to the river or onto its banks. Northern sections fell into a rail yard, landing on three unoccupied and stationary freight cars.Sequential images of the collapse were taken by an outdoor security camera located at the parking lot entrance of the control facility for the Lower Saint Anthony Falls Lock and Dam. The immediate aftermath of the collapse was also captured by a Mn/DOT traffic camera that was facing away from the bridge during the collapse itself.
Mayor R. T. Rybak and Governor Tim Pawlenty declared a state of emergency for the city of Minneapolis and for the State of Minnesota on August 2. Rybak's declaration was approved and extended indefinitely by the Minneapolis City Council the next day. As of the morning following the collapse, according to White House Press Secretary Tony Snow, Minnesota had not requested a federal disaster declaration. President Bush pledged support during a visit to the site on August 4 with Minnesota elected officials and announced that United States Secretary of Transportation Mary Peters would lead the rebuilding effort. Rybak and Pawlenty gave the president detailed requests for aid during a closed-door meeting. Local authorities were assisted by the Federal Bureau of Investigation evidence team, and by United States Navy divers who began arriving on August 5.