After a 15-month probe, the National Transportation Safety Board has determined the probable cause of last year�s fatal I-35W bridge collapse in Minneapolis was a design error that caused the failure of gusset plates on the 41-year-old 1,907-ft-long steel-deck truss bridge. They could not carry loads that included deck upgrades, construction materials, equipment and rush-hour traffic when it fell, killing 13 people and in�juring 145. The eight-lane bridge was a non-redundant fracture-critical structure.

Along with the main finding, which the board approved on Nov. 14, NTSB also blamed the failure of the quality-assurance procedures of bridge designer Sverdrup & Parcel and Associates Inc., St. Louis, to ensure calculations for the gusset plates on the bridge’s main truss were performed, and federal and state transportation agencies’ “inadequate design review.” Sverdrup & Parcel was later acquired by Jacobs Engineering Inc., Pasadena, Calif., which did not respond to ENR’s calls seeking comment.