The 40-year-old bridge that collapsed in Minnesota last night, causing the deaths of at least seven people, was graded "structurally deficient" two years ago but was not scheduled to be replaced until 2020.

Bridge 9340 was of a design known as a "non-redundant structure" by civil engineers, meaning that if a single part failed, the whole structure could collapse. It was completed before fundamental reform of bridge safety in America in the late 1960s.

In 1967, the year of its completion, another non-redundant bridge, the Silver Bridge in Ohio, fell into the river below because of the failure of a single chain link.

Forty-six people died and the disaster, like the Ronan Point tower block collapse in London the following year, prompted a period of greater caution in structural design.

The reforms came too late for I-35W bridge, which was finished in 1967. It had three spans but no supporting piers, to allow barge traffic to continue on the river below. It carried Interstate 35W and 140,000 cars and trucks a day over the Mississippi River just east of downtown of Minneapolis.

Professor Marios Chryssanthopoulos, an expert in structural systems at the University of Surrey, said there was "no similarity" between the structures of the Silver Bridge and the bridge that failed last night, but observed that a generation of bridges was built in the early 1960s according to safety rules that are now out-dated.

"Loads have been going up all the time and the US rules of the early 1960s have been found to be unconservative with regard to fatigue design," he said. "In other words, bridges designed in the early 1960s would have been designed differently 20 or 30 years later."

However, according to officials in Minnesota, regular recent inspections of the bridge had revealed no fundamental problems, and the engineering work being conducted on the structure when it collapsed was "overlay work" on the road surface, guardrails and lights.

Governor Tim Pawlenty said that he was told in 2006 that major rebuilding was not needed for at least ten years. “They notified us from an engineering standpoint the deck may have to be rehabilitated or replaced in 2020 or beyond,” he said.

Nonetheless, in 2005, the US Department of Transportation's National Bridge Inventory gave the bridge a rating of 50 per cent — the score at which replacement is considered. The rating described the bridge as "structurally deficient" and gave it a score of 4 out of 9 for its general physical condition.

Last night, the Minnesota Department of Transportation, said the department was aware of the 2005 assessment. "We've seen it, and we are very familiar with it," said a spokeswoman, who pointed out that many highway bridges shared the same description.

In 2001, a comprehensive technical study of the bridge concluded that the bridge did not need to be "prematurely replaced" and observed that "fatigue cracking of the deck truss is not likely", but identified three steel members carrying higher loads than any others.

The report, written by the Department of Civil Engineering at the University of Minnesota, said: "The bridge's deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system". It said the bridge did not need to be replaced but recommended thorough inspection of the three members that carried the highest loads.

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