Why did the I-35W Mississippi River bridge collapse?

Question

I remember that after it collapsed and all the debris was cleaned up, there was never anything on the news about why it collapsed. The whole story just kind of disappeared. Does anybody know why the bridge collapsed?

Answer 1

The short answer is:  Age caught up with it.

The longer answer is from the day anything gets built it ages and weakens.  The question is,"How old can something get before it gets repaired or replaced?"  Remember it takes time and money to repair or replace anything.  With limited resources, what do you fix this year?  You can't fix everything every year. What can be let go another year?  With bridges, they get inspected and the inspector makes a judgment as to "Will this bridge be OK another year?"  "Are there other bridges in worse shape that needs repair this year?"  In the case of the I-35W bridge, the inspector make a bad call.  On the other hand, some bridge somewhere else may NOT have failed as a result of getting maintenance.  

Assume you are married.  Your car needs tires soon.  Your wife's car needs a brake job, soon.  Either job costs $400 and you can afford to do only one this month.  Which one do you do?  Which one do you put off for next month?  If the car that you don't fix is in an accident and someone gets killed, were you negligent or stupid?

Answer 2

there were some stress issues on some of the connectors and pillars and stuff and the DOT doesnt really care about that kind of thing until people die. they were negligent. the old age the weight of years of traffic came to an unfortunate head. and it was in the news, you proly watch to much fox news.

Answer 3

Yeah, it died as a news story in the national news.  No, people didn't completely forget about it.  The civil engineering world and MSP area were still paying attention.  Why did the bridge fall down?

*  Gusset plates that were supposed to be 1" steel were 1/2" steel.

*  The deck was loaded with construction equipment and rush hour traffic.

*  Corrosion.  Too many Minneapolis winters and salt trucks.

Answer 4

The plates holding the girders at the joints failed.  These plates were the weak link.  The steel girders would probably have lasted a thousand years.  

The interesting question that cannot be answered is what was it about the plates that failed?  Was it the steel in plate?  Did the steel bend, or buckle under the stress and strain of the bridge?  Did the plates become brittle due to cold and then hot temperatures?  What other possible reasons might there be that would cause a plate to fail?

Perhaps it was the rivets that held the plates to the girder.  What is it about a rivet that could have failed?  Did the rivet sheer?  Did the rivet crack due to environmental conditions or old age?  What other possible reasons might there be that would cause a rivet to fail?

Given that the bridge consisted of X number of steel girders and Y number of steel plates and Z number of steel rivets - what do you suppose the likelihood of failure is?  What if there were two joints that had already failed?  What if there were three?  How many joint failures does it take to make a bridge like that fail?

I am not a civil engineer, but I think a bridge like that would remain standing unless four or more joints had failed.  If more than four joints failed, then I think the entire bridge would go.  

The civil engineers that I know are very good at what they do.  A bridge like that is pretty solid, and very unlikely to fail due to the failure of just one girder-joint-plate-rivet system.