35W bridge in minneapolis collapses

[Mallignamius]

As we know from debunking 9/11 conspiracy theories, structural failure is generally an all-or nothing thing. When the structural members fail, the bridge would have essentially dropped in freefall. Even when the failure is buckling instead of breaking (this one probably had both), the ability of the members to hold up the weight/resist the fall drops by many orders of magnitude.

Isn't there any resistance from the connecting roads? I was just wondering if it would take slightly longer to begin a drop because of the steel beams (or stiffening girders?) running horizontally underneath, connected to the ends of the bridge and the anchorage blocks or abutments. If so, perhaps that would explain the "earthquake" descriptions, that the bridge has to pull away from the ends?

 

[BobG]

...Probably a bridge falling expert to comment on it. ...[/rant]

They did have a bridge expert on the news this morning to explain what might have caused the collapse. He was a reporter's worst nightmare. The reporter had to keep pushing him for an answer to why the bridge fell.

The short version of his answer (stripped of the technical info incomprehensible to the average viewer) was, "How could anyone possibly know until they get a look at the bridge?"

You could see it in her eyes: "You have to know because some stage technician led you up here to a chair in front of me! Don't you realize you're killing me right here on the air?!"

 

[Cyrus]

This is nothing but more of the same, media hype. A bridge fell. Report it correctly. Tell me what happened, how many people died and then move on. Then follow up a month later with a report explaining why it fell. But when you have all day news programs with nothing to talk about, they report the same crap for 12 straight hours on how the bridge fell. Yes, there was a school bus on the bridge......and? So what? Did everyone in the bus die? No. Is this good news reporting? No.


Quick, make an animation of the bridge falling, because were too stupid to think in our minds how a bridge falls. They even put all the cars on the bridge (bus included) as it fell in the animation. I wish one day a news reporter asked me a question live on the air. I'd call him a moron to his face.

 

[radou]

This is nothing but more of the same, media hype. A bridge fell. Report it correctly. Tell me what happened, how many people died and then move on. Then follow up a month later with a report explaining why it fell. But when you have all day news programs with nothing to talk about, they report the same crap for 12 straight hours on how the bridge fell. Yes, there was a school bus on the bridge......and? So what? Did everyone in the bus die? No. Is this good news reporting? No.

Welcome to the 21st century, bro. Sensationalism in its most pure form ever.

Quick, make an animation of the bridge falling, because were too stupid to think in our minds how a bridge falls. They even put all the cars on the bridge (bus included) as it fell in the animation.

 

[Cyrus]

Breaking news a timeline of bridge collapses!

http://www.cnn.com/video/#/video/us/2007/08/02/vosli.mn.i35w.bridge.collapse.side.view.cnn

Excuse me, I am going to go hide in my closet now with my duct tape.

 

[radou]

Excuse me, I am going to go hide in my closet now with my duct tape.

Short off-topic comment: at some point, you'll have to get out of the closet, though.

 

[Cyrus]

Here I found the animation.

http://video.wnbc.com/player/?id=138456

wow, did I really need this animation to see what happened.

 

[russ_watters]

Isn't there any resistance from the connecting roads? I was just wondering if it would take slightly longer to begin a drop because of the steel beams (or stiffening girders?) running horizontally underneath, connected to the ends of the bridge and the anchorage blocks or abutments. If so, perhaps that would explain the "earthquake" descriptions, that the bridge has to pull away from the ends?

Yeah, and of course, parts of the roads never detatched, so those sections only fell at one end. So perhaps the outer third on each side was slowed by that. But the center of the span would have fallen at near freefall acceleration. Also, though, supporting a falling object from one side makes half of it fall slower than g, but the other half faster than g because the center of gravity is trying to accelerate at g. That's why smokestacks that are demolished by toppling them break while they are falling - the different accelerations cause them to bend.

 

[Mallignamius]

(I only a few minutes ago saw the animation. I didn't know it broke up.)

So then could a connected end quickly snap upwards (following a detachment)? I suppose under such circumstances, cars could go flying if they're on those sections.

 

[Cyrus]

Not according to the animation! LOL

 

[russ_watters]

They have video of the actual collapse online now...

 

[ranger]

Yea, you should watch the video on cnn's website instead of those crappy animations.

As I was reading the morning papers, they were saying that the bring had an inspection in 1990. To sum it up, it was given "structurally deficient".
http://www.courant.com/news/nationworld/nation/wire/sns-ap-bridge-collapse,0,1159498.story?coll=hc_features_outdoors_util

 

[chief]

I like how the newspeople are asking, "well, they were jackhammering near the bridge, do you think that might have been a cause?"

Actually the jackhammering could have played a part in the collapse. If there were already corrosion problems and fatigue cracking in the welds, it is very possible that the vibration from the jackhammer amplified the problem and was the immediate cause of a truss connection failure... which in turn caused the rest of the bridge to come down.

 

[Cyrus]

A jackhammer is not really going to put a stead sinusoidal vibration on the bridge at exactly the resonant frequency of the structure. Could it play a role, possibly. The point is, it was a stupid question/statement for the reporter to make. I have seen a few clips of reportes saying stuff without knowing what there talking about, such as: "I know in a suspension bridge that if one section fails the whole bridge fails" and the expert told him, "well, that's not true". Quite sad.

Bumper to bumper traffic twice a day for 40 years of loading and unloading the bridge will fatigue crack it appart though.

 

[Alkatran]

The coverage makes me believe everyone should be forced to play armadillo run, or a similar game, in school at some point. It's amazing how small the difference between all-ok and near total collapse is.

 

[chief]

A jackhammer is not really going to put a stead sinusoidal vibration on the bridge at exactly the resonant frequency of the structure.

You don't have to have steady vibration at the resonant frequency of the truss to cause fatigue stress in the connections. I'm a licensed structural engineer by the way.

 

[Cyrus]

Interesting, you think jackhammers would have an effect on such a large structure? Personally, I find it rather hard to believe. I would expect the vibrations of the cars and tractors trailer going up and down the bridge to wash out the noise from a jack hammer.

 

[chief]

Interesting, you think jackhammers would have an effect on such a large structure? Personally, I find it rather hard to believe. I would expect the vibrations of the cars and tractors trailer going up and down the bridge to wash out the noise from a jack hammer.

I'm not saying the jackhammer caused the collapse, but it could have contributed. I think corrosion and fatigue were the main problems... but since only 2 of the 6 lanes were open at the time of collapse, something else must have played a part. Otherwise it would have collapsed on a day when it was more heavily loaded (all 6 lanes full).

 

[Cyrus]

Cheif, you should get a copy of the New Yorker, June 25, 2007, pg. 70. They have a nice article on Cecil Balmond.

He is one of the top structural engineers in the world right now.