NTSB bridge update looks at stress point, construction load

  • Article by: TONY KENNEDY , Star Tribune
  • Updated: March 17, 2008 - 10:41 PM

The agency released more findings about the I-35W bridge collapse, though a final report is still months away.

Federal investigators probing the collapse of the I-35W bridge have discovered precisely how individual rivets broke in a crucial gusset plate, but the final determination of what caused the bridge to fall is still months away, the National Transportation Safety Board (NTSB) said Monday.

In its fifth official update on the investigation, the NTSB released new findings on construction loads that were on the bridge. The agency also released results of a computer analysis of stress on the bridge's U-10 gusset plate, which is believed to be a point of initial failure.

Both areas of study have been well known since early in the investigation, when the NTSB said the bridge's original design may have been flawed by steel gusset plates that were only a half-inch thick. NTSB Chairman Mark Rosenker said Monday that investigators are making significant progress and he expects a final report by the end of the year.

One highlight from the newly released documents is a statement from forensic engineers at the State University of New York at Stony Brook. Their report said "it may not be likely'' that the bridge's steel superstructure had any sizable cracks. "However, a limited crack initiation and growth near rivet holes may be possible,'' the report said.

The Stony Brook team, hired by the NTSB to do computer modeling of stresses, said it will study the possibility of limited crack growth in the gusset connections.

So far, Stony Brook engineers have used computer modeling to pinpoint how rivets broke along a fracture line in the U-10 gusset plate on the west side of the bridge. The team said it will investigate the effects of friction between the gusset plate and edges of connecting truss members. Five beams connected at the juncture.

The NTSB said it is still examining the U-10 gusset plate connections on the east and west sides of the bridge. In addition, the agency said its materials laboratory is completing reports on corroded areas in the nearby pair of U-11 gusset plates. State bridge inspectors had noted corrosion in those joints before the bridge collapsed Aug. 1, killing 13 people and injuring 145 others.

Lucy Kender, a spokeswoman for the Minnesota Department of Transportation (MnDOT), had no comment about specific findings in the latest NTSB documents. "We're happy to see the investigation is continuing,'' she said.

Phil Sieff, a Minneapolis attorney who is helping to represent 96 victims of the bridge collapse on a no-compensation basis, said Monday that the latest information from NTSB will be closely scrutinized.

In January, the NTSB investigation into the bridge collapse prompted federal officials to advise states to verify load capacity calculations and stress levels in gusset plates and other parts of all steel truss bridges.

Construction load noted

Documents released by the NTSB on Monday noted that MnDOT's repaving contractor had loaded piles of sand and gravel on the bridge that weighed a total of 383,200 pounds. Photos take hours before the collapse show the piles of aggregate spread out across lanes that were closed to traffic. But construction workers told the NTSB that the piles were pushed closer to the bridge's center line before the collapse.

In all, seven construction vehicles, totaling 312,880 pounds, were on the bridge, according to the documents.

The agency also said it has interviewed MnDOT workers about the department's role in quality control of the bridge design process. "These interviews have focused on what types and level of review would have been conducted in the 1960s (when the bridge was designed and built) as well as how the process works today,'' the NTSB said.

Tony Kennedy • 612-673-4213

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