Trucks carrying tens of thousands of pounds of crushed stone were parked on the Interstate 35W bridge, and more stone was sitting on the deck when the bridge collapsed, investigators said Sunday, raising suspicions that the added weight of materials intended for repairs may have played a role in the bridge failure.Meanwhile, new federal rules on bridge design and engineering are scheduled to take effect in October. The rules have been decades in the making and address issues like redundancy, longevity and current weight loads experienced by bridges across the country. The new rules will not be retroactive in nature, which means it will apply to new construction only.
The chairman of the National Transportation Safety Board, Mark V. Rosenker, said investigators had questioned employees of Progressive Contractors Inc., which was doing work on the bridge deck, regarding quantities of various materials, specific equipment they had put on the bridge, and where the materials and equipment were on the bridge. The weight and location will be entered into a computer program, Mr. Rosenker said, to calculate the stresses generated on each girder and other bridge components.
So far, investigators say they have ruled out nothing and will consider everything from the expansion and contraction of the bridge in the extreme weather conditions of Minneapolis to the possible corrosive role of bird droppings.
The stone was being used to add a two-inch layer to the bridge deck. Its weight, and the weight of the trucks carrying it, would most likely not be a threat to a bridge in good condition, engineers said, but could play a role in causing the failure of a structure that was already weakened.
Completion of the computer analysis is probably still weeks away, investigators said. Once the wreckage is removed from the Mississippi River, the plan is to lay out the debris on shore and identify each piece of metal and its role in the structure, in an effort to determine what failed first.
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 their federal financing.
The rules, highly complex, call for major improvements in the design of bridge steel, concrete and foundations. They are more conservative, for instance, in their assessment of the sturdiness of driven pilings, and in some cases will 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.
Among other things, the rules will 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,” Ms. 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. Today, we know a lot more about how loads are doing to affect the bridges, and why they fail.”
Surprisingly, experts say most bridge failures involve not structural failure of the span itself but of its foundations. For instance, churning water can undermine driven pilings, causing the whole bridge to collapse suddenly.