The Interstate 35W bridge over the Mississippi River in Minneapolis, Minnesota 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 on March 15th showing the location of every car, truck and piece of construction equipment that was on the bridge at the time of the collapse. This diagram assigns a weight to everything on the bridge and is very detailed. Stress at one of the two weakest points on the bridge was 83% more than it could have handled, according to an interim report released earlier by the Federal Highway Administration.

It should be noted that the Safety Board has not established the cause of the collapse, which killed 13 people and injured 145. It is expected to do so by the end of the year. Investigators have previously said that because of design flaws in the 40-year-old structure, several gusset plates, steel sheets that tie girders together, were too thin. The report, which can be found on the Board’s Web site, says investigators were looking into “what type of system of checks and balances would have been in place when the bridge was designed back in the 1960s.” In all, Board researchers calculated a load of 1.26 million pounds, or 630 tons, including 198,820 pounds of sand at the critical spots. However, some experts say the load would not have been excessive for a well-designed bridge.

Since the collapse, highway departments have begun reanalyzing bridges before bringing in large amounts of equipment and construction materials. The Board has hired the University of Minnesota’s Department of Civil Engineering to build a 1/200th scale model of the bridge to help investigators understand the bridge’s supporting structure. The bridge was “fracture critical,” which meant that it had numerous parts that had no back-up. Although the design was common when the Interstate highway system was built, what are referred to as “redundant” designs are more commonly used today. A final report on the cause of the collapse is expected by the end of the year.

Source: New York Times

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