The 40-year-old bridge that collapsed in Minneapolis last week was built during an era when designers were confident they knew enough about bridge strength and weight loads that they could build bridges lighter and cheaper.

Sadly, a number of bridge collapses have taught engineers painful lessons about the frailty of bridges and the punishment they take from heavy trucks, strong tides and even the errant barge that slams into bridge supports, according to engineers, bridge builders and academics.

The challenge, they say, is that many of the nation's 594,709 bridges were built during the 1950s and 1960s, an era when designers didn't fully understand the effects of metal fatigue or other challenges. Now, many of those bridges are facing increased scrutiny.

"Maybe we out-thought ourselves for a little while," Mal Kerley, chief engineer of Virginia, told The Washington Post, referring to postwar bridge-building when the interstate-highway system was created. "What has happened over time is that we learned things."

In 1883, the Brooklyn Bridge opened to horse-and-buggy traffic. Now it carries more than 144,000 vehicles a day. Because engineers on the Brooklyn Bridge and other spans did not have sophisticated methods to accurately calculate loads, they made their best guess — and then multiplied.

The result was over-engineered, overbuilt metal building structures that will probably outlast their newer brethren.

"The smaller the amount of knowledge you have, the bigger the factor of safety you use," said Joseph Yura, emeritus professor of civil engineering at the University of Texas. "In those days, you would do the calculations and the chief engineer would just up everything by one size."

Today, bridge designers are again taking a more conservative approach. Spans are built with stronger materials, redundant support systems and super-strength steel building frames that can withstand the pounding of thousands of vehicles a day.

However, after World War II, with steel building prices as high as the nation's confidence, engineers thought they could "minimize materials and maximize stresses," Kerley said.

The result was structures such as the Frederick Douglass Bridge over the Anacostia River in Washington, D.C. Built in 1950, the Douglass Bridge sits on only two main horizontal steel beams, bolstered by vertical supports. The main beams were so thin that engineers at the time attached numerous vertical "stiffeners" to keep the beams from bending.

The stiffeners required a lot of manual welding, but "steel was more expensive and labor was less expensive" at the time, said Kathleen Penney, deputy chief engineer of D.C.'s Department of Transportation.

If the Douglass Bridge were built today, she said, the steel building material used would be 25 percent thicker, there would be more than two main beams in case one failed, and there would be far fewer welds and connections to minimize the number of corrosion and fatigue points.

"Collectively, there was more confidence in using less material," Penney said. "Since that time, we've had a lot of experiences that pushed us back to more conservative approaches."