The Interstate 35W bridge over the Mississippi in Minneapolis collapsed last August after construction workers had put 99 tons of sand on the roadway directly over two of the bridge’s weakest points, according to a National Transportation Safety Board report.

In all, board researchers calculated a load of 1.26 million pounds, including 198,820 pounds of sand at the critical spots. However, the load would not have been excessive for a well-designed bridge, according to experts.

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