PLATES IN FOCUS PLATES IN FOCUS Federal investigators have concluded that steel plates on the interstate bridge that collapsed last summer in Minneapolis were inadequate to hold the structure together and appear to have been what allowed it to fail, killing 13 people, two officials familiar with the investigation said Monday. The National Transportation Safety Board (NTSB) plans to call Tuesday for states to perform safety assessments on the so-called gusset plates in steel girder bridges any time they add weight to a bridge, the sources said. Design changes in 1977 and 1998 added additional pavement and concrete barriers that increased the weight of the Interstate 35W bridge in downtown Minneapolis. The officials, and three others who discussed the findings, spoke on condition of anonymity because they were not authorized to discuss the investigators' conclusions before Tuesday's release. The Department of Transportation's Federal Highway Administration plans to act immediately to order states to conduct the safety assessments, said one of the officials. The bridge over the Mississippi River, one of Minneapolis' main commuting routes from the north, tumbled 115 feet into the river during afternoon rush hour on Aug. 1. In addition to the 13 deaths, about 100 others were injured. Gusset plates are flat steel structures used to bolt together the steel girders that carry the weight of a bridge. Bridge engineers typically design the plates to be far stronger than the girders because if one fails, the whole bridge will collapse. In the wreckage of the I-35W bridge, investigators found 16 gusset plates that were fractured, said one of the officials. Eight of the plates were in the location on the south side of the bridge where the collapse began, according to that official. The fractures prompted engineers to calculate whether the plates were adequate to hold the bridge together. What they found was that the half-inch thick plates should have been an inch thick — double the size. VIDEO: NTSB statements on bridge collapse The sources said the NTSB has many months of investigation before it can declare what caused the collapse, but the investigation is now focused on the plates. Several officials said they did not believe that a widespread problem exists with substandard bridges across the country. The I-35W bridge withstood 40 years of use before it collapsed. However, more attention should be paid to the gusset plates to ensure that bridges in the future don't collapse, they said. There are about 465 similarly designed steel truss bridges across the country. Such designs were popular during the middle of the last century, but have fallen out of favor. Today, engineers try to design bridges that have redundant designs that allow a single girder or plate to break without causing the structure to collapse. The findings suggest that bridge inspectors, who are required to regularly check for signs of wear or cracks, did not miss any problems on the Minneapolis structure, an official said. Enlarge By Morry Gash, AP The Interstate 35W bridge in Minneapolis, shortly after part of it collapsed on Aug. 1, 2007,. Conversation guidelines: USA TODAY welcomes your thoughts, stories and information related to this article. Please stay on topic and be respectful of others. Keep the conversation appropriate for interested readers across the map.