On August 1, 2007, the I-35W Bridge collapsed during rush hour, killing 13 people, injuring 145, and bringing massive economic disruption to the Twin Cities metropolitan area. The November 2008 National Transportation Safety Board (NTSB) report, which followed fifteen months of investigation, advised the public and transportation officials around the country that the bridge collapse was a one-time occurrence, caused by a simple design error that had gone undetected at the time of construction.
In short, the collapse was a one-off from which no lessons could be learned to avoid future failures. Barry LePatner says that just isn't true. He suggests the NTSB report whitewashed the true causes of the bridge collapse, and he stresses the need for reform in the way we fund and maintain the nation's infrastructure.
"NTSB's final report tellingly ignored a host of critical factors that were not disclosed to the public or other state bridge operators, which sadly delegitimized the efficacy of the report," says LePatner, author of the new book Too Big to Fall: America's Failing Infrastructure and the Way Forward (www.TooBigToFall.com). "A careful analysis of the NTSB report shows that it masked far more than it revealed about how the bridge was maintained, funded, and operated."
Design error or lack of action?
NTSB's final report said that the bridge's collapse was the result of a design error -- specifically, the under-design of certain gusset plates (metal plates used to connect structural members of a truss and hold them in position at a joint) at six nodes of the deck truss. The report revealed that these gusset plates should have been an inch thick, but instead were only half that thickness, contrary to the original design specification.
Adding to the stresses on the too-thin gusset plates, the report stated, were increases to the bridge's load. According to the NTSB report, had all the gusset plates met design standards at the time of construction, then -- even with the increased weight from the bridge's additions, the increase in traffic, and the weight of the construction materials and machinery -- the collapse would not have occurred.
"The reality is that the NTSB's findings virtually ignored sixteen years of inspections by the Minnesota Department of Transportation (MN/DOT) that reflected the steady decline of the bridge over a sixteen-year period," says LePatner, who coauthored Structural and Foundation Failures (McGraw-Hill, 1982) with engineer Sidney M. Johnson. "Widespread evidence of corrosion for critical steel members, frozen bearings that locked the bridge in place, and cracks throughout the bridge and approach spans rendered the condition of the bridge as 'poor,' which required some of the bridge's traffic lanes to be closed.
"All of these critical factors were highlighted in several outside engineering reports commissioned by MN/DOT," he adds. "These reports, which detailed the frailties of this fracture-critical bridge (meaning the failure of one structural member would trigger the collapse of the bridge), made a series of recommendations -- which went completely unheeded -- for addressing the problems resulting from neglected maintenance."
How did the NTSB fall short of really serving the people of Minnesota and the national public as a whole? LePatner explains:
The NTSB report wrongly exonerates MN/DOT. Based on the maintenance history of the I-35W Bridge's extensive wear and tear, corrosion, and signs of incipient failure for many years prior to its collapse, dismissing MN/DOT of any responsibility for its collapse is inexcusable. Inspections dating back to 2001 identified widespread corrosion and fatigue caused by weather and traffic volume.
Recognizing the fragility of the bridge's original fracture-critical design, engineering firm consultants stressed the need for added redundancy to strengthen the bridge. A June 2006 inspection -- the last one before the bridge's collapse -- found cracking and fatigue problems and gave the bridge a sufficiency rating of 50 percent. A rating of 50 percent or lower pursuant to federal standards is interpreted to mean that the bridge should be considered for replacement.