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In October, the federal government will begin enforcing new rules for highway bridge design meant to make new structures more efficient, more reliable, safer and longer lived - and implicitly better than hundreds of thousands of existing spans across the nation.

The new rules have been in the works for two decades, and their enforcement by the Federal Highway Administration is unrelated to the Minneapolis bridge collapse, officials said. The rules will act like housing codes and not be retroactive. Many old bridges will suddenly be out of date but will be grandfathered in, as is the case with many old homes as community standards rise.

States have been slowly applying the new rules for years, some better than others. Minnesota is at the forefront, with 100 percent enforcement on new bridges as of last year. New York has been slower, and California slower still.

States that fail to follow the new rules after October could jeopardize federal financing.

The complex rules call for improvements in the design of bridge steel, concrete and foundations. They are more conservative, for instance, in assessment of the sturdiness of driven pilings, and may raise the cost of bridge foundations.

Officials called the rules the biggest change in federal regulations for bridge design since 1931. Behind the change, they said, are such factors as more realistic assessments of highway traffic as well as advances in the statistical analysis of past bridge failures and their causes.

The rules are expected to produce bridges better able to withstand peak traffic stresses and severe weather, as well as "extreme events" like ship collisions and earthquakes.

"It's going to increase reliability, which translates into bridges that are safer, more cost efficient, and have longer design lives," said Kelley C. Rehm, program manager for bridges and structures at the American Association of State Highway and Transportation Officials, which wrote the new rules.

"It doesn't mean existing bridges are going to fall down," Rehm added. "In the past, some were overdesigned, and some didn't have as much of a safety factor as we'd like to see."

Experts say most bridge failures involve not structural failure of the span but of its foundations. For instance, churning water can undermine driven pilings, causing the whole bridge to collapse suddenly.

The rules were written seven years ago, during the Clinton administration.