Breaking: Bridge Collapse in Minneapolis

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

[ButThreeLeftsDo]

“One of two bodies pulled from the river today has been identified as Peter Hausmann, 47, of Rosemount, Minn. The second body has yet to be identified.”

[Abigail Adams]

No asphalt, it was concrete.

[ButThreeLeftsDo]

Latest update: http://www.startribune.com/10204/story/1353522.html

[Abigail Adams]

Thanks! I read that the second one found today was the baby. Very sad.

[ButThreeLeftsDo]

Morning Update.....http://www.startribune.com/10204/story/1353522.html

[Abigail Adams]

Wow, I wonder how they know that the man was trying to help others? It’s so sad that he survived the collapse, but then died later.

[ButThreeLeftsDo]

Evidently, from eyewitness reports. They’re saying he could have saved himself but chose to try to help. I’m hearing stories this morning that this is exactly the kind of person he was.

Very sad.

[ButThreeLeftsDo]

Afternoon Update...Another body has been recovered.

“Recovery workers pulled another body from the wreckage of the Interstate 35W bridge collapse just after noon today.
The body was not identified.

Pending notification of family members, the recovery would bring the number of dead in the Aug. 1 catastrophe to nine and reduce the number of people confirmed missing to four.”

[Palladin]

Thanks, AA. I have no idea what gusset plates are. I’ll have to read up on it.

[Abigail Adams]

Thanks for the update!

[Clark857]

I googled the MnDOT failure and found my way to this board. I was looking for closeup photos of the south pier. I believe the conclusions regarding the failure initiating over or near the south pier are correct. The speculation about the east truss going first is also interesting. There is a companion thread on this site that holds some very good, very high resolution photographs of that area. Forgive me not posting links, I’m a bit new to this.

The discussions of the “kingpost” and lower chord near the pier being an insignificant members are off track. As alluded to in some other posts, the truss is a three-span continuous structure. (See the general plan and elevation on the previously referenced MnDOT page) Tension in the top chord and compression in the bottom chord over the piers. Reverse that as you progress toward mid-span. The tension diagonals on either side of the "kingpost" are also significant members (as are almost all members in a truss). Those diagonals transmit the accumulated load to the “kingpost” which in turn transmits the reaction to the bearing.

The speculation regarding the SE bearing is interesting. I have yet to develop an opinion that it is either a cause or a consequence.

I do have a suggestion. Think in more than one plane. Most of this discussion has focused on the primary truss mechanics. Look again at the general plan and elevation from the MnDOT web site. Note the Section drawn near Pier 7 in the lower left corner. I have been wondering if there was a failure in a floor truss or the lateral bracing which allowed the trusses over the south pier to roll. The floor trusses are the smaller trusses which span between and cantilever over the main trusses supporting the deck stringers.

The term cantilever is being overused. The only cantilevers I have noticed are the single panel cantilevers at the north and south ends of the deck truss which catch the ends of the multi-girder approach spans and the cantilevers of the floor trusses outside the main trusses to support the deck.

Some have wondered about differences in the bearings over Piers 6 and 7. According to the drawings, the bearings over pier 7 (north) are fixed with expansion bearings over pier 6 (south). Hence the toothed rollers in the Pier 6 bearing assemblies.

Way over my 2 cents worth here. Thanks for tolerating. Yes, I am a bridge engineer.

[kwuntongchai]

Clark857

Here’s terminology question for you:

I find it curious that people are calling this a “truss” rather than a “cantilever” My understanding is that a cantilever is suspended out from the piers, tension on top, compression on the bottom. A truss, on the other hand, has the compression members on the top, the tension on the bottom.

Here’s a diagram, of the Quebec Bridge, a cantilever bridge with a suspended truss. The diagram shows the reversal of tension and compression members where the cantilever arms end and the truss section begins. http://www.brantacan.org.uk/QuebecSide2A.jpg

The transition between the truss segments and cantilever segments of cantilever bridges with suspended spans is pretty easy to see. In “through cantilevers” like the Quebec bridge or the first Carquinez Strait bridge in California, the truss sections are arched in typical truss fashion. With “deck cantilevers” the truss sections can be spotted where the thick compression members of the cantilever transition into the thin tension members of the truss, see http://www.cvrma.org/pictures/MISC/
dfrr5_170_so_young’s_high_bridge_tyrone_ky_1977.jpg (cut and paste link)

So, here’s a terminology question: What would you call
* The Quebec Bridge in Quebec? (I’d call it a “through cantilever with suspended truss”) http://www.hvq.com/fr_location.htm
* The Queensborough Bridge in New York? (I’d call it a “through cantilever without suspended truss”) http://www.brantacan.co.uk/QueensBoroughB.jpg
* The Aurora Bridge in Seattle? (I’d call it a “deck cantilever with suspended truss”) http://www.art.com/asp/display-asp/_/id—6725/Seattle.htm
* And I’d call the 35W bridge a “deck cantilever without suspended truss.”

So, back to your statement that it’s not a cantilever, seems that a truss would have the compression members on top, tension on the bottom, which is clearly not the case. So I’m curious as to the terminolgy used by bridge engineers—can you elaborate? If it was a “through” bridge rather than a “deck” bridge so it looked like the Queensborough Bridge in profile, would you still call it a “truss” or would you call it a cantilever?

Inquiring minds want to know
Kwuntongchai

BTW, posting links is easy—just paste in the location info of the page you’re steering us to.

[Clark857]

The terms cantilever and truss aren't mutually exclusive. Trusses are basically structures made up of triangles. A cantilever is a structure that extends beyond its last support. For instance, take a ruler, extend it over the edge of a desk and hold the part over the desk down with your hand. The part of the ruler extending over the edge of the desk is said to be cantilevered. A beam can be cantilevered as can a truss. Cantilever portions are most often used to balance truss design forces by moving the hinge location. They also serve as the location for expansion joints.

Through and deck trusses are so named by the location of the driving surface. If the driving surface sits on framing connected to the bottom chord, it’s a through truss. If, as in the case of the Minneapolis bridge, the driving surface is supported above the top chord, then it’s a deck truss. There are also trusses that make up an arch form which are called truss arches. The bowed form of the bottom chord of the MN bridge doesn't make it an arch. Truss arches can be deck structure, i.e. all below the deck. There are also truss arches that rise well above the deck and support the deck via cables.

The Quebec Bridge is a through truss composed of anchor, cantilever and suspended segments.

The Qeensborough Bridge is also a through truss. More particularly it is a continuous through truss. The continuity is evidenced by the additional depth at the towers. The MN bridge was a continuous deck truss.

The Aurora Bridge may be a deck truss arch. The depth of the lower chord arc may be such that the action is more arch than truss, i.e. the bottom chord remains in compression even at mid-span.

Continuous trusses are distinguished by depth at the towers as opposed to a series of simple span trusses. Simple trusses were often used for railroad bridges. Railroads cared less for structural efficiency than they did for maintenance. If there was a problem with one span, it didn't affect the others. See the Big Four Railroad Bridge (now abandoned) http://en.structurae.de/structures/data/index.cfm?ID=s0001373.

This site hosts a considerable database of bridges.

[ButThreeLeftsDo]

Sunday Update....more remains found.

http://www.startribune.com/10204/story/1358106.html

[ButThreeLeftsDo]

Latest recovered body identified..........

http://wcco.com/topstories/local_story_224141205.html

[kwuntongchai]

Clark857

Not to hyjack the thread, but it’s interesting that you consider all these to be “trusses.” Are there bridges in the world that you *would* call “Cantilever bridges?”

i.e. at http://en.wikipedia.org/wiki/Cantilever_bridge it lists the longest cantilever bridge spans in the world, but you refer to these as trusses. These all have compression members on the bottom, tension on the top (except in the suspended truss span). Compare to the examples at http://en.wikipedia.org/wiki/Truss_bridge where every bridge has the tension members on the bottom and the compression members on the top.

Not to belabor an issue, but can you steer me to a lexicon used by bridge engineers where these are defined as “continuous through trusses” rather than cantilevers? It’s always been something I’ve been curious about, how different groups use different terminology for the same bridge.

Or, what is a typical “basic text” used by bridge engineers that I could go dig up at the university library?

Thanks,
Kwuntongchai

[ButThreeLeftsDo]

Sorry, I’m not a bridge guy...

You need to be talking to jim_trent.

[ButThreeLeftsDo]

Sheriff Stanek told the boy mayor to close the footbridge that has been a gathering place for gawkers. And he did. It appears that there have been more remains pulled from the river today. Stanek told the mayor that the bridge should stay closed, out of respect, until ALL of the bodies have been recovered. And it will be so.

I’ve always liked Stanek.

[ButThreeLeftsDo]

“Hennepin County Sheriff Rich Stanek had opposed opening the pedestrian bridge. Victims’ families “do not want to see another picture of a body bag in a newspaper,” said Kathryn Janicek, the sheriff’s spokeswoman.”

http://www.startribune.com/10204/story/1364598.html

[Abigail Adams]

Thanks! Where did you hear about more remains?