The disastrous collapse of the I-35 bridge in Minneapolis on Aug. 1, 2007, has gripped the nation not only because of the loss of life, but because of the questions it raises as to the integrity of other highway structures. Claim personnel who deal with both large and small losses are familiar with the aftermath procedure of attempting to determine the cause of the loss, or in this case, the collapse of the bridge.
The National Transportation Safety Board (NTSB), the governmental agency tasked with transportation-related investigations, has indicated that the findings as to the cause of the collapse will probably not be available for about a year, which is typical of large losses. Data must be gathered, bridge sections in the Mississippi must be retrieved, and failure analyses must be performed in order to arrive at a probable scenario on what caused the collapse. Despite the long time required for a full reconstruction of the accident, the Internet is overflowing with failure analyses from a variety of sources ranging from non-scientific bloggers to college professors. In order to cut through the chatter, here is what is known at this time regarding the failure.
Bridge construction started in 1964, and was completed in 1967. It had a total length of 1,907 feet, with three lanes in each direction. The bridge was heavily traveled, carrying 140,000 vehicles daily. Due to its proximity to St. Anthony Falls to the west, the bridge receives constant moisture f, which causes considerable driving difficulty in the winter. An automatic de-icing system was installed in the bridge deck in 1999, using potassium acetate solution as the ice melting agent. At 6:05 p.m. on Aug. 1, 2007, the bridge collapsed without warning.
Large, unstable deflections toward the south end of the bridge suggest that some critical bridge structural element failed in that area, causing the collapse.
Apparently, at the time of failure, construction work was being performed on the bridge in the southbound lanes. The arrow points to concrete mixing equipment and other construction trucks. There were reports of more than 100 tons of gravel stored on the bridge in that area. Traffic had been diverted to the northbound lanes. Prior inspection reports indicate that the bridge was in need of repair, with several references to corrosion, fatigue cracks, and deformed members.
Soon after the failure, there were discussions by the NTSB of a possible design flaw and there was considerable interest in gusset plates, which are used to connect bridge structural members because of reported metal fatigue (arrow, Figure 5). Metal fatigue is a cyclic loading phenomena that tends to drive cracks through a steel structure over time.
So what is the likely cause of the collapse? There is insufficient information at this time to make a determination, but one can say with certainty that safety margin erosion played a significant role in the failure.
In 1964, the bridge was designed so that its strength far exceeded the critical stress or failure stress. The difference between the strength and critical strength is called the safety margin. The philosophy here is to build it stronger than necessary to account for unforeseen factors in the future. Over time, the bridge's strength had deteriorated because of a variety of factors, some unknown at this time. At the time of failure, there was corrosion evident on the bridge, there were fatigue cracks in structural members, and there were deformations to various structural elements. There was construction activity in the form of jack hammering operations, concrete mixing, and storing of building materials. Traffic was being routed to the northbound lanes and the failure occurred at, or near, rush hour. The construction activity and rerouting of traffic are examples of unsymmetrical loading, which can affect a structure as shown in Figure 7. The 200 lb symmetrical load is stable, while the 100 lb unsymmetrical load is unstable, even though it is a lesser load.
The center convention was being used for a flower show and sand was piled in one area on the floor, to be used in the flower displays. Sand, which can weigh up to 1.8 tons per cubic yard, overloaded the floor, causing the loss. Gravel can weigh up to 1.4 tons per cubic yard, with a mere 70 yards of gravel weighing 100 tons. The lesson here is that stored construction material may not be an insignificant load. Upon questioning by reporters, NTSB has remarked that construction loading did not cause the failure, which may be true. As in many failures, it is the accumulation of factors that lead up to the eventual failure of the structure.
Truck weight limits have increased in recent years. In the mid-1970s, truck weight limits were 50,000 lbs; now they are 80,000 lbs. (Some states allow even higher weights). Of course, these weight limits do not reflect overloaded trucks on the road, which could be as high as 30 percent of trucks. There are about four times as many heavy trucks on the road now as compared to the mid-1970s. This increase in vehicle weight obviously has reduced the safety margin, since load calculations were most likely based on earlier weight data.
In addition, design calculation methods of the 1960s lacked modern computer-based techniques such as finite element and finite difference analysis, which yield relatively accurate modeling of stress in structural members. Earlier, less accurate design methods could have led to a design flaw in the structure.
NTSB has alerted other states to be careful of construction-related loading on other bridges. Special emphasis is being placed on "fracture critical" bridges that can collapse if one of the truss members fail, as occurred in this case.
Charles C. Roberts, Jr., Ph.D., PE, is a consulting engineer based in Big Rock, Ill. He is primary author of the reference work "Technical Notebook: Forensic Aspects of Claims," released by ClaimsBooks.
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