At 6:05 PM on August 1st, 2007, the I-35W bridge over the Mississippi River in Minneapolis was packed with rush-hour traffic. Making the roadway even more congested was summer road construction on the bridge and a Minnesota Twins game scheduled to start in an hour. Cars were bumper to bumper.

Disconcertingly, the bridge started swaying, and then a rumbling was heard. Suddenly, the bridge deck plummeted into the river below. There were 111 vehicles and 190 people on the bridge during the collapse, 13 of whom died and 145 sustaining injuries serious enough to require hospitalization. Seventeen cars had to be salvaged from the Mississippi River.

What could have caused such a disastrous failure?

The I-35W Bridge in Minneapolis

The I-35W Bridge was designed by the engineering firm Sverdrup & Parcel, with the blueprints receiving final certification in 1965. The structure complied with the 1961 standards of the American Association of State Highway Officials (AASHO) and the 1964 standards of the Minnesota Highway Department.

It was a steel truss bridge with no supports (truss or suspension) above the roadway. The 1,064-foot deck truss portion was supported by concrete piers anchored in the ground, with a web-like assembly of steel trusses providing structural strength and load-bearing capacity. The truss nodes, where more than one truss member connected, were connected by two gusset plates, heavy steel plates riveted to each of the trusses they connected, establishing a firmly supported node.

One advantage of truss structures is that they are naturally load-path redundant; no single truss is critical to the integrity of the structure. The I-35W Bridge was undoubtedly expected to have a similar redundancy of critical members, but at the time it was built, the technical understanding of metal fatigue was unsophisticated. Later advancements in metal fatigue would show that weakening gusset plates did, in fact, make the bridge non-load-path redundant, meaning a single truss failure would cause the entire structure to collapse.

In 1977, renovations increased the amount of concrete covering the steel rebar from 1.5 inches to 3 inches. This was in response to new guidelines for protecting the rebar from the extreme climate of Minnesota (with its hot summers and very cold winters). This additional concrete added more than 3 million pounds of weight to the bridge. Then, in 1998, the median barrier was replaced, improved concrete traffic railings were installed, and repairs were made both to the concrete piers and to the steel cross girders under the deck slab. These improvements further added over a million pounds to the bridge weight.

In 2007, further repairs were underway to replace 2 inches of concrete on the bridge deck. Because concrete sets rapidly, contractors staged almost all of their equipment on the bridge itself, adding over half a million more pounds. Most of this extra weight was concentrated in a small area, vastly increasing the load borne by several truss nodes–one of which would fail and cause the collapse.

Causal Factors

Insufficient quality control at the engineering firm for the bridge design: Investigators found that design verification procedures of the engineering firm that designed the bridge “contain[ed] no explicit procedure for ensuring that all necessary calculations are performed.” Notably, shear calculations, which should have been performed on three of the nodes, including the one that collapsed, were omitted.

Inadequate gusset plates: The gusset plates on the bridge were only 0.5 inches thick, instead of 1 inch thick as required by building codes in 1967. It is unknown why this happened, but design and finished construction review procedures should have identified and corrected this at multiple phases of the bridge’s life.

Exclusion of gusset plates in guidance for bridge load ratings: When establishing load-bearing capability, calculations (and later, computer modeling) evaluated loads on the truss members but not the gusset plates. Gusset plates were considered to be much stronger than the truss members they connected and, therefore, generally weren’t evaluated.

Lack of inspection criteria for gusset plates, and guidance for gusset plate distortion: Inspections in 1999 and 2003 did note that gusset plates on the bridge were distorted, a common indicator of fatigue. Yet because gusset plates were believed to be the strongest element of a steel truss, inspectors attributed the distortion to installation factors such as the riveting process and did not consider it a concern.

Insufficient regulations for bridge design review and approval: All of the approved methods for evaluating the load capability of a bridge structure, including 17 approved computer-modeling programs, omitted connections such as gusset plates.

Lack of guidance for placement of construction loads on bridges during maintenance and repair: The “final straw” causing the collapse was likely the added, concentrated weight of the construction material.

Investigation and Aftermath

The National Transportation Safety Board (NTSB) investigated the incident and found that all four gusset plates at one of the nodes had fractured due to fatigue and excess loading, which caused the separation of the main trusses and the collapse of the bridge span.. In their report, the NTSB wrote,

The initiating event in the collapse of the I-35W Bridge was a lateral shifting instability of the upper end of …[a] diagonal member and the subsequent failure of … node gusset plates on the center portion of the deck truss.

Essentially, the placement of construction crews, with their extremely heavy equipment, induced a “lateral shifting instability,” and the added stress and strain caused the mechanical failure of two gusset plates. The NTSB also called out the excessively thin gusset plates installed, concluding, “if the gusset plates had been designed in accordance with … [required] specifications, they would have been able to safely sustain these loads, and the accident would not have occurred.”

On August 8th, 2007, the Federal Highway Administration issued a Technical Advisory directing that during bridge construction, regulators and contractors “ensure that any construction loading and stockpiled raw materials placed on a structure do not overload its members.” Mn/DOT later revised its Standard Specifications for Construction to limit the weight of vehicles, equipment, and materials that may be staged on bridges during construction.

The NTSB also concluded that,

Had [official] guidance included gusset plates in load ratings, there would have been multiple opportunities to detect the inadequate capacity of the …gusset plates of the I-35W bridge deck truss …[But because engineers] generally consider gusset plates to be designed more conservatively than the other members of a truss, [they] typically ignore gusset plates when performing load ratings, and the resulting load ratings might not accurately reflect the actual capacity of the structure.

In the years after the collapse, the engineering firm responsible for inspecting the bridge agreed to a $52m settlement with victims’ families and survivors and a $5m settlement with the State of Minnesota. The construction firm working on the bridge at the time of the collapse settled with victims’ families and survivors for just over $10m and with the state for $1m. It is shaky whether States can be sued for negligence, but the Minnesota Legislature set aside some $37m for victims’ families and survivors. The owner of the original engineering firm settled with the state for almost $9m in damages.

Have you previously studied the collapse of the Minnesota I-35W Bridge? Feel free to share your thoughts with us below.