Good afternoon ladies and gentlemen and thank you for coming.
As we all remember, on August 1, 2007, the city of Minneapolis suffered a tragedy when a bridge carrying Interstate 35W over the Mississippi River collapsed, killing 13 people and injuring more than 100 others.
The NTSB dispatched investigators that evening and we continue to investigate the accident. We released the accident site to the Minnesota Department of Transportation on October 12, and completed our on-scene investigation on November 12, although we have retained control of important portions of the bridge and might need to revisit the scene. Since the early days of the investigation, we have been working with the Federal Highway Administration and the Minnesota Department of Transportation to develop finite element analyses of the bridge to help identify the stresses in the bridge components resulting from the loads on the bridge at the time of the collapse.
I want to make it clear that we have not yet determined the probable cause of the accident, but there has been a development that I want to share with you today. We are issuing a safety recommendation to the Federal Highway Administration, which will be available at the conclusion of this conference and will be placed on the Board’s website shortly.
First, a little history. This bridge was built during the mid-1960s and opened to traffic in 1967. The collapsed portion of the bridge was a steel deck truss, which was considered “fracture critical” because the load paths in the structure were non-redundant, meaning that a failure of any one of a number of structural elements in the bridge would cause a collapse of the entire bridge. There are approximately 465 steel deck truss bridges in the National Bridge Inventory, according to the Federal Highway Administration. In the years since it opened, the I-35W bridge experienced two major renovations, in 1977 and 1998. As part of these renovations, the average thickness of the concrete deck was increased from 6.5 inches to 8.5 inches, and the center median barrier and outside barrier walls were increased in size. These changes added significantly to the weight on the structure. On the day of the collapse, the bridge was undergoing repaving operations and there was construction equipment and material on the bridge.
The deck truss portion of the bridge consisted of steel beams that were connected to each other at 112 nodes, or joints, by gusset plates. There were two gusset plates at every node, and consequently there were 224 gusset plates on this bridge. During the wreckage recovery, we encountered fractured gusset plates from eight different nodes located in the main center span; all 16 gusset plates from those eight nodes were fractured. The other major gusset plates in the main trusses were generally intact. Gusset plates are generally designed to be stronger than the beams they connect and one would not expect to find them fractured. The damage patterns and fracture features uncovered in the investigation to date suggest that the collapse of the deck truss portion of the bridge was related to the fractured gusset plates, and in particular may have originated with the failure of the gusset plates at one of those eight nodes. As you might imagine, this created questions about the materials used to construct the bridge. However, materials testing performed to date has found no deficiencies in the quality of the steel or concrete used in the bridge.
Subsequently, the Safety Board and FHWA conducted a thorough review of the design of the bridge, with an emphasis on the design of the gusset plates. The investigation has determined that the design process led to a serious error in sizing some of the gusset plates in the main trusses; specifically, the gusset plates at the eight nodes I mentioned earlier. Basically, those 16 gusset plates were too thin to provide the margin of safety expected in a properly designed bridge such as this. These gusset plates were roughly half the thickness that would be required – half an inch thick rather than an inch thick.
In an effort to determine why those gusset plates are undersized, we wanted to examine the bridge’s design methodology used in the 1960s to verify that it was sound. Unfortunately, the calculations used for the main truss gusset plates could not be found, so we cannot determine whether the error was a calculation error, a drafting error, or some other error in the design process.
We also examined the bridge inspection records and, although those inspections identified and tracked some areas of cracking and corrosion, at this point in the investigation, there is no indication that any of those areas played a significant role in the collapse of the bridge.
It is important to understand that bridge inspections would not have identified the error in the design of the gusset plates. The National Bridge Inspection Standards (NBIS) are aimed at detecting conditions such as cracks or corrosion that degrade the strength of the existing structure; they do not, and are not intended to, address errors in the original design.
During the course of our investigation, we have determined that errors of this nature have little chance of being discovered after construction. Routine design calculations that are performed for bridge modifications and special use permits do not typically include the level of detail that would be required to discover an original design error of this nature.
The Safety Board is concerned that, for at least this bridge, there was a breakdown in the design review procedures that allowed a serious design error to be incorporated into the construction of the I-35W bridge. The bridge was designed with gusset plates that were undersized, and the design firm did not detect the design error when the plans were created. Because of this design error, the riveted gusset plates became the weakest members of this fracture-critical bridge, whereas normally gusset plates are expected to be stronger than the beams they connect. Further, there are few, if any, recalculations after the design stage that would detect design errors in gusset plates. Finally, other programs to ensure the safety of our nation’s bridges, such as the methods used in calculating load ratings and the inspections conducted through the NBIS program, are not designed or expected to uncover original mistakes in gusset plate designs or calculations.
It is important to note that the Safety Board has no evidence to suggest that the deficiencies in the various design review procedures associated with this bridge are widespread or even go beyond this particular bridge. In fact, this is the only bridge failure of this type of which the Safety Board is aware. However, because of this accident, the Safety Board cannot dismiss the possibility that other steel truss bridges with non-redundant load paths may have similar undetected design errors. Consequently, the Safety Board believes that bridge owners should ensure that the original design calculations for this type of bridge have been made correctly before any future major modifications or operational changes are contemplated.
I want to repeat that we do not yet know what caused the I-35W bridge to collapse that day, nor do we have indications that other similar bridges in this country have the kind of design flaw we found in this one, but we think that in the future, before major work is performed on a “fracture critical” bridge, all structural elements should undergo load capacity calculations.
I’ll take some questions.
Speeches & Testimony