There are roughly 8,000 bridges across the country that have been designated as both "structurally deficient" and "fracture critical," and a website, saveourbridges.com, maps them all.

Structurally deficient, in short, means bridges that are in severe disrepair. 

Fracture critical, a term usually used for older bridges, means the structure has a steel component in tension, and if that component fails, some or part of the bridge could collapse.

(See more specific definitions of those terms here.)

Barry LePatner, creator of saveourbridges.com, was a guest on WBUR's Science Friday Aug. 31. He put the problem in less technical terms. When you have a bridge that is both structurally deficient and fracture critical, if any one component fails, the bridge could collapse.

In other words, he said on the radio show, no engineer can tell you when such bridges will collapse; it could be tomorrow, it could be a year from now, it could be some other time.

A look at the website's map shows a number of bridges in the Somerville area that fit this combination of problems.

Among them:

• Route 99/Alford Street bridge in Charlestown over the Mystic River
• Route 16 over the Malden River
• Route 28 over the Charles River (near the Museum of Science)

Absent from the map was McGrath Highway.

You can visit the site to see other structurally deficient and fracture critical bridges in the area.