system
Given the following post, respond as if you're commenting on Reddit:
human
Why did the I-5 bridge in Washington collapse?

Can someone also describe "functionally obsolete?"
gpt
It was designed in the 1950s and was an obsolete and dangerous design. Structural Engineering back then was downright primitive compared to today. That bridge design was "fracture critical" meaning failure in a single area could cause the entire bridge to collapse. Modern bridges are designed to sustain damage to multiple critical areas before failure. 

Think Jenga - if you build a tower without any middle blocks, removal of a single block anywhere will cause the tower to fall. Modern bridges have all 3 bricks and can survive lots and lots of bricks being removed. 

An oversized truck struck and damaged a crucial part of the bridge which caused it to collapse. This was a series of human errors. The truck had a lead car to look for obstructions and didn't spot the low clearance, the trucking company was issued permits for the route saying it was safe, and the bridge wasn't marked as a low obstruction. Only overhangs lower than 14' were required to be marked, this bridge was 14'7", and the truck was 15'9".