The tragic rush-hour collapse in Minneapolis of the Interstate 35W bridge over the Mississippi River is again forcing a re-examination of the nation's approach to maintaining and inspecting critical infrastructure.

According to engineers, the nation is spending only about two-thirds as much as it should be to keep dams, levees, highways and bridges safe. The situation is more urgent now because many such structures were designed 40 or 50 years ago, before Americans were driving weighty SUVs and truckers were lugging tandem loads.

It all adds up to a poor grade: The American Society of Civil Engineers gave the nation a D in 2005, the latest report available, after assessing 12 categories of infrastructure ranging from rails and roads to wastewater treatment and dams.

"One of America's great assets is its infrastructure, but if you don't invest it deteriorates," says Patrick Natale, executive director of ASCE.

Among scores of recent examples:

• Last month, a 100-year-old steam pipe erupted in midtown Manhattan, killing one man and causing millions of dollars in lost business.

• The inadequacies of levees in New Orleans became horrifyingly clear in the aftermath of hurricane Katrina. The city is still recovering.

• In 2003, the Silver Lake Dam in Michigan failed, causing $100 million in damage.

America's 577,000 bridges are of particular concern because they are subject to corrosion. According to the Web site of Nondestructive Testing, which advocates not damaging structures during testing, the average lifespan of a bridge is about 70 years. Bridges are inspected visually every two years. However, NDT notes, "it is not uncommon for a fisherman, canoeist and other passerby to alert officials to major damage that may have occurred between inspections."

In the federal government's rating system, any bridge that scores less than 80 — on a scale of 1 to 100 — is in need of rehabilitation. A bridge scoring below 50 should undergo reconstruction under federal guidelines. In 2004, 26.7 percent of U.S. bridges, urban and rural, were rated deficient, down from 27.5 percent in 2002, according to the U.S. Department of Transportation.

Minnesota's record is far better, with only 12.2 percent of its bridges falling into the deficient or obsolete categories.

Federal officials were quick to point out that those designations don't mean the bridges are unsafe.

"None of those ratings said there was any kind of danger," U.S. Transportation Secretary Mary Peters said in Minneapolis on Thursday. The ratings are used to point out deficiencies or overhauls that need to be conducted in the future.