 It is the evening rush hour on the 1st of August 2007 and traffic crawls its way along the I-35 West Mississippi River Bridge. The crossing has been here for over 50 years and is an essential part for many journeys around Minneapolis. Bridge 9340 as it is officially known is a steel girder structure stretching for 14 spans roughly 1907 feet long. The bridge has become a bit of a bottleneck as four of its eight lanes were closed for resurfacing as such the evening rush traffic files through the limited space but little did the commuters know that they were traveling on borrowed time. The bridge failed in the blink of an eye plunging the vehicles down into the Mississippi River below. Today we're covering the Mississippi Bridge collapse and as such I'm going to rate it here 6 on my disaster scale and again here 6 on my historical legacy scale. In the early 1960s plans were drawn up to cross the Mississippi River as part of the colossal construction of the I-35 a monumental 1569 mile north south cross country interstate highway project. Parts of what would become the I-35 had been constructed pre 1956 the date on which the interstate system was formed in the US. The route would eventually be completed in 1982 and it ran from Minnesota in the north all the way down to Texas in the south and part of the route would have to travel through Minneapolis necessitating the crossing of the Mississippi River. As a side the I-35 in the region actually splits into two the I-35 west and the I-35 east. This allowed the highway to be brought into Minneapolis and St Paul respectively. Both branches rejoined in Columbus. In 1962 plans were begun to be put on paper on the I-35 west river crossing. As part of the design process engineering firm Sverdup and Parcel were hired on to produce a preliminary engineering report. Great final design plans checked by a registered professional engineer and check design computations. On the 12th of April 1963 two options were presented to the Minnesota Department of Transport a four and a two trust design Sverdup and Parcel recommended the latter for economic reasons which was the option selected by the department. The accompanying report from Sverdup and Parcel stated welding is planned throughout for the makeup of girders and trust members and in this connection high yield strength still conforming to T1 will be used extensively. They also commented that the use of welding compared to riveting would save the department money. The reality of the use of T1 steel had an overall cost implication. By September both Minnesota Department of Transport and the Federal Highways Administration began to express reservations about using such large quantities of T1 steel in the bridge. You see T1 is a very strong construction material but this comes with a drawback the way it is heat treated and as such members made of T1 are limited by the size of the heat treatment facilities where they are produced. This meant that there would be a need for more joints which incurred extra cost during construction. Not only that but the material was more expensive compared to other steel types at the time. T1 was 38 cents per pound compared to the say weaker A441 steel that was around 31 cents per pound and could be made in longer sections due to the different heat treatment processes. During the March 1964 conference, Minnesota Department of Transport and the Federal Highways Administration decided that sphered up and parcel eliminate T1 steel from all main trust members and use A441 and A242 steels instead. This required a redesign as the A441 and A242 steel had a capacity of 27,000 pounds per square inch versus 45,000 pounds per square inch for T1. Due to the weaker steel the design had to be replanned and required thicker steel which in turn added more dead weight to the structure. However, some of the original 0.5 inch T1 steel gussets weren't thickened when the change to weaker steel was employed. The ones that were improved in the final design were up to 1 inch thick and they might be worth while remembering this for later on. The final design was settled upon by March 1965 however initial works to build the piers had already started in 1964. Let's first look at the actual design accepted. The structure used welded built up steel beams for girders and trust members with riveted and bolted connections. The bridge was 1907 feet long and carried 8 lanes of traffic, 4 northbound and 4 southbound. The bridge had 13 reinforced concrete piers and 14 spans numbered south to north. 11 of the 14 spans were approach spans to the deck trust portion which was only 3 spans long but was over 1000 feet in length. The bridge deck in the approach spans was supported by continuous welding steel plate girders or by continuous voided slab construction. The three main spans used deck trust construction consisting of two reinforced concrete deck slabs separated by about 6 inches with a total width of the deck slabs of about 113 feet 4 inches. This was enough to accommodate all 8 lanes of traffic. The main trusses comprised of two parallel main sections. This was made up of an upper and lower cords that extended the length of the deck trust portion of the bridge. For added strength the cords were connected by straight vertical and diagonal members. To connect these together at each junction gusset plates were installed and riveted together. These were made of steel. To connect the east and west trusses bracing members were utilised. The bridge deck was completed in 1967 and opened to traffic and for 10 years or so all seemed well until it reached its first renovation in 1977. Back in 1967 when the bridge was initially constructed it had 1.5 inches of concrete over the unhoted top reinforcing bars of the bridge deck. This was pretty standard for the time but by the 1970s a number of similar structures were experiencing rebar corrosion. In order to combat this a thicker layer of concrete was poured on top of the bridge bringing the total thickness to 3 inches. But this came with an issue and that was the increase in weight on the truss structure in the region of roughly 3 million pounds or 13.4%. All throughout the bridge's life the structure received an annual inspection starting in 1971. Part of these inspections were a general condition and evaluation report. But after 1991 the results took a nosedive. As part of the annual survey the bridge is given a status and at the start of the 1990s it received an unsatisfactory structural deficiency rating. A status does not indicate that a bridge is unsafe but only that the structure is in need of maintenance, repair or eventual rehabilitation and as such a weight restriction is given. However in 1993 some gusset plates had experienced corrosion. In 1998 another refurbishment project was underway at the bridge. This involved improving several features including the central barrier and outside traffic railings which did not meet the at the time safety standards. And like in 1977 the permanent changes increased the dead load on the bridge by about 1.13 million pounds or 6.1%. In the early 2000s cracking was discovered and was drilled out and reinforced and a process of six monthly inspections was enacted and this brings us to the bridge's final renovation leading to four of the eight lanes being closed on the I-35 bridge. The 2007 renovation involved removing some on the concrete and adding a two inch thick concrete overlay on the deck. Part of the construction works involved staging materials on the bridge ready for use. This increased the static load by roughly £578,000 right over one of the 0.5 inch thin gusset plates. During the works throughout July the staging of materials was not raised as a concern and as such for each surface pour tons of sand, rock and water was stored right over some of the most vulnerable portions of the bridge. On the afternoon of the August 1st 2007 the contractors working on the bridge were preparing to pour a 530 foot overlay in the southbound inside lanes. The pour was set to be undertaken at 7pm due to the hot summer sunshine and in lead up to this large numbers of materials were stored on the bridge and two cement tankers were fully loaded at £80,000 and were parked over pier 6 and a weakened gusset plate. To add more weight to the bridge from the concrete pour the busy evening rush hour traffic crawled its way over the open lanes further concentrating the weight on the structure. The inevitable would happen at 6.05pm. The central span of the bridge suddenly gave way followed by the neck spans. The structure and deck collapsed into the river and onto the riverbanks below. There was a motion control security camera and this captured the dramatic and fatal moment of the structures failure. There were 111 vehicles on the bridge when it collapsed and this included public as well as personal vehicles. Many were plunged into the river, the banks and the rail yard below the span of the bridge. The first call from 911 dispatch went out at 6.07pm and at 6.08pm Minneapolis 911 dispatch made a distress call requesting that all available emergency assistance personnel attend the scene. Many of the first people involved on the scene were locals and others stuck in traffic. Just five minutes after the collapse police were on scene and began to coordinate the rescue efforts. Within an hour of the collapse 12 public safety agencies responded with 28 watercraft to assist with the river rescue operations helping to recover many victims from the Mississippi River. Some sections of the roadway were resting on top of the river leading to the occupants of some of the vehicles being stranded necessitating 93 being rescued from various precarious situations. In the summer evening many lined the banks of the river to watch the unfolding dramatic rescue efforts launched by the first responders. By 7pm the rescue efforts started to move towards search and recovery as the submerged vehicles needed to be identified. By the next morning many were still unaccounted for and as such the river was lowered by two feet by the four dam downstream. It would take three weeks for the final body to be removed from the wreckage site. The collapse had a total of 190 casualties 34 of whom were seriously injured and 13 would be killed. On the 6th of August the United States House of Representatives and the United States Senate each voted unanimously to secure $250 million in emergency funding for the state of Minnesota. With traffic now rerouted away from the bridge site many dead and millions of dollars were for damage. The NTSB set out to find the reason for such a dramatic and southern structural failure. Every piece of the structure that was recovered underwent scrutiny. Investigators found that gusset plates at the upper node 10 had fractured. The fractured gusset plates coupled with fractures in the lower cord members between L9 and L10 nodes resulted in completely separating the main trusses in this area thereby allowing the centre span to drop. Analysis of the security camera footage showed that failure had begun towards the southern end of the centre span centred on the gusset plates and their material thickness. The NTSB concluded that some had inadequate capacity for the expected loads on the structure which was further compounded by the increased weight during the 1977, 1998 and 2007 refurbishments. Ultimately the NTSB report released in 2008 concluded due to a design ever by spared up and parcel and associates of the gusset plates at the U10 nodes which failed under a combination of substantial increases in the weight of the bridge which resulted from previous bridge modifications. It's further up and parcel received a great deal of blame along with the state transport officials for giving inadequate attention to the gusset plates during inspections. Needless to say the financial implications went beyond the recovery and cleanup for the state. On the 2nd of May 2008 a $38 million agreement to compensate victims of the bridge collapse was settled. Another $52.8 million was received by the victims from URS Corporation and Jacobs Engineering Group, the successor of Spared Up and Parcel. They also agreed to pay the state $8.9 million in an out of court settlement. The bridge would be replaced and opened on the 18th of September 2008. This video is a plain default production. All videos on the channel are creative commons attribution share alike licensed. Plain default videos were produced by me, John, in a currently sunny south eastern corner of London, UK. Help the channel grow by liking, commenting and subscribing. Check out my Twitter for all sorts of odds and sods, as well as hints on future videos. I've also got Patreon and YouTube membership as well, so check them out if you fancy supporting the channel financially. And all that's left to say is thank you for watching.