An AP article leads with the headline, Thousands of U.S. Bridges at Risk of Freak Collapse. Apparently, almost a quarter of US bridges are now deemed structurally deficient. That means the bridge has been inspected and a portion is in bad repair or deteriorated. These 66,749 bridges are on the books for needed repairs.

The Freak Collapse idea, though, comes from another sort of design problem. Many bridges were designed to work only if each of the components works. In other words, they don’t have a redundancy in their design, allowing one component to fail but still keeping the bridge from falling.

For example, the Interstate 5 bridge over the Skagit River north of Seattle recently collapsed because an oversized truck clipped one of the trusses. The photo below shows the dropped span. A redundant design would prevent the complete failure if one component fails.

Of course, redundant design costs more to construct. So many efficient engineers designed and constructed the lowest cost structures that met the design code. They call these “Fracture Critical Bridges” and many states still allow this type of design.

The fascinating question, “How cautious do we want to be?” needs to be answered again and again. We need to address it as a society and as individuals.

I’m designing a 90′ high building right now and contemplating how much redundancy to put into the steel beam hanging grid just below the roof. I want to keep the cost low, but what if one of those welds fails in a few years. 90′ drops don’t end well.

We all need to consider how we live our lives. How much do we move to security and how much to efficiency/freedom? Where do you tend to land?