Breaking: Bridge Collapse in Minneapolis

KSTP TV 5/ME | 8/1/07 | Me

[PapaBear3625]

While it’s true that anyone can rent a torch, cutting steel isn’t a low profile operation, and you don’t just whisk through three quarter or inch thick plates like a knife through butter either.

How about if somebody used thermite? (nice video here )

[spunkets]

Yes I noted those rust stains.

[wagglebee]

I agree, those are excellent pictures and great explanations.

As far as the conspiracy theorists go, it’s time that they acknowledge that this couldn’t be anything other than massive and catastrophic structural failure.

[spanalot]

Not much of a connection there - one would expect large gussett plates there - front and back.

[pillut48]

What if you were dressed to look as if you belonged to a construction/pot hole repair crew, and because of that, no one gave you a second thought?

I know TPTB *KNOW* terrorism wasn’t ‘involved’ in this (”Rigggggggggggghhhhhhhhhhhhhhttt!”/Dr. Evil off), but my tinfoil hat is still firmly in place. Too many ‘coincidences’ for my liking. :-(

May God watch over those involved in the recovery effort and keep them safe, and may God give comfort to those who lost loved ones or were injured in this. :*(

[spunkets]

Thanks.

The road bed is supported by trusses. Those trusses are used instead of plates to reduce weight. A truss is essentially a plate that contains no shear. The bridge has fixed points at both ends where it hits the earth, so it's not free at the ends. A cantilever is free to move up and down at one end. The top and bottom chords of the bridge can only move in expansion and contraction, so this is a mix of fixed ends and simple support. I included this fig to illustrate the forces just for the thread.

"The road deck shoreward of the pier is intact, while towards centerspan, it’s vaporized."

That came from the road bed flexing back and forth considerably when the SE side fell. The rebar from that is all drawn out and extends into the river. Concrete isn't any good in tention, so it just crumbled. What didn't go up in the air as dust slid down the embankment into the river.

Yes, both sides at the pad had notable rust stains.

[concretebob]

or powdered magnesium in a paste.
Wouldn't take long to fry a web or flange.
Sabotage or terrorism is way down the list of possible causes, given the real-time conditions.
There was enough going on on that bridge to create a cumlative effect.
Too much stuff in one spot, on a bridge that already had a less than 50% safety rating.

[SE Mom]

So, given what you’re saying- the combination of the train going under the bridge and the jackhammer we keep hearing about- could have an effect...

[freepertoo]

Wow.

[PennsylvaniaMom]

I have taken sometime to catch up reading this thread on the tragedy in Minneapolis. I want to thank the three of you (and others along the thread) for posting your explanations (hypothosis) of what you think happened...and by explaining (to the non-engineer like me) how a bridge 'works' and how a bridge can fail. Thank you for sharing your expertise...

As I drive over and under bridges around here, I will be looking at them in a whole new way.

[SilvieWaldorfMD]

Okay fine. So we’ve now established that Minn. - St. Paul is no Dearborn, Michigan.

Let me ask you: In the Muslim neighborhoods of Minn. - St. Paul, are the street signs in Arabic? Are they in Arabic with no English subtitles? (I’m not comparing the Chinatown street signs — here in DC and in NYC, they are in Chinese with smaller English subtitles in the bottom of the signs).

[spunkets]

"Not much of a connection there - one would expect large gussett plates there - front and back. "

Yes, I think that and the post does look small for all the weight above it. Considering that it's over 25 years old and they've already found fatigue and reinforced sections with added plates, I'd especially expect them there.

[concretebob]

River beds are by definition “bedrock”.

depends on where they are geograpically.
Ideally, yes, the steel piles or auger cast piles (A hole drilled in the ground and filled with concrete, also called caissons), are taken to rock or "driven to refusal".
However, not all bedrock is lying close enough to the surface to do this. Thats why we have geo-structural engineers to determine bearing capacity and how big the cassions (Auger cast piles) have to be and how deep they have to go to hit adequate bearing.
This was not a foundation issue.
This was a structural/mechanical failure. Either anchor bolts failed or the brige was point loaded beyond design capacity, which is frankly irelevant due to the construction/repair work.It's referred to as "moments". Time and distance travelled. It may only be a fraction of an inch of travel and milliseconds of time, but it's going on constantly, and if it's interuppted for any reason, it can be catastrophic.

[HereInTheHeartland]

Big picture question..
This crossing had 2 spans.
One is collapsed. The other is standing but looks to be the same construction, is it in any danger?

[bert]

The bridge structure is a truss with bolted connections. It is very likely that one bolted connection failed leading to progressive overloading of other connections.

Since the vertical truss components are still standing it is unlikely an anchor bolt failure.

[Sacajaweau]

Had a teacher of excellence for mechanics. Wish I had kept my books.

[spunkets]

You're welcome.

"As I drive over and under bridges around here, I will be looking at them in a whole new way."

Just hope the local bridge inspector, or engineers in charge does a good job.

[wagglebee]

Thanks for the info. Another thing that people need to remember is that Minneapolis routinely has temperatures below zero in the winter and highs in the nineties in the summer. These extremes lead to even more expansion and contraction than normal which will serve to weaken the steel.

This is an enormous tragedy, but it is not terrorism.

[jim_trent]

Actually, the bridge next to the collapsed one is of completely different construction. It looks like a concrete arch bridge from the 20’s or 30’s. It was probably the original bridge and was two way until the collapsed bridge was built and they were divided into one-way traffic for each bridge.

[Shelayne]

I pronounce it that way as well.

Years ago, I lived in both New York and just outside of Baltimore. I was teased ruthlessly for my pronunciation of roof and root, not to mention my use of “pop”. Though, they all did say that I do not have the Minnesoooota accent very much. It was just certain words.

I haven’t turned back. Well, lately, I have relaxed and now say “pop”.;^) However, I refuse—REFUSE—to pronounce Karaoke -Ka-rokey. That just irritates me. I don’t know why. :^D