After more than 15 months of investigation, the National Transportation Safety Board began presenting their findings on the collapse of the Interstate 35W bridge at a two-day hearing that started Thursday. At the meeting in Washington, D.C., NTSB investigators said a major factor in the collapse of the bridge was the failure of gusset plates, which were not designed properly. Investigator Jim Wildey said the bridge was designed in such a way that if one portion failed, the rest of the bridge could potentially collapse. After the failure of the gusset plates at node U-10, the collapse of the bridge was unavoidable, he said. Wildey also ruled out other possibilities in the collapse, including corrosion damage, pre-existing cracking and the conditions of the bearings and piers. In his opening comments, Mark Rosenker, acting NTSB chairman, said every factor that may have played a role in the collapse has been examined. “We’ve done all we can to see that such a tragedy never, ever happens again,” he said. A number of the victims and family members were allowed at the hearing, while others affected by the collapse gathered at the St. Paul Armory downtown to watch a live webcast of the proceedings. A final ruling is expected near the end of Friday, when the NTSB will vote on final findings on the Aug. 1 bridge collapse that killed 13 people and injured 145. Check Friday’s Daily for more.