From the NY Times:
"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.
The board, in the midst of a reconstruction of the circumstances of the collapse, released a diagram Monday showing the location of every car, truck and piece of construction equipment that was on the bridge at the time of the Aug. 1 collapse.
The diagram assigns a weight to everything on the bridge, using the car manufacturer’s weight for each vehicle and allowing 200 pounds for each adult, 50 or 100 pounds for each child depending on age, and 250 pounds for each of two portable toilets. It also puts four mounds of sand, each 12 feet wide and stretching about 55 feet, in a lane between the four operating traffic lanes.
Stress at one of the two weakest points was 83 percent more than it could have handled, according to an interim report released earlier by the Federal Highway Administration. "