Bridge Collapse in MN 29

[jike]

Sway trusses are in the vertical plane between the main trusses.

I was curious whether there were any truss members in the horizontal plane directly beneath the deck. It looks like there might be but then again my eyes might be fooling me.

 

[darkwing88]

Off the main topic - The structural engineer used GTSTRUDL to model the bridge for the 2006 URS report. Here are some of the things he did that I think is silly in this day and age: 1) he neglected the vertical slope of the bridge 2) neglected the horizontal curve of the North and South ends 3) cross slope of the structure was neglected. Is GTSTRUDL that difficult to use to not model the bridge correctly? I use STAAD and modeling those features would not be difficult at all.

 

[darkwing88]

jike: yes the sway frames and portals above the piers are in the vertical plane. I need to get a handle on the bridge terminology.

 

[archeng59]

Would the concrete bridge deck be the bracing element for the top flanges of the bridge framing? If the cross section of the concrete deck was reduced, would that cause lateral buckling to occur? From the photos, there does not appear to be a metal deck. Seems like the sway trusses would prevent that, but since I'm not a bridge designer I don't know.

 

[archeng59]

in my previous post, I should have written... "If the cross section of the concrete deck was reduced, would that ALLOW lateral buckling of the trusses to occur?"

 

[jike]

The concrete deck is most probably the bracing element of the stringers (beams).

Whether it is the bracing element of the truss top chord, I am not sure. This why I ask the question whether there was any truss members in the horizontal plane just below the deck or stringers. For a bridge of this size, I would guess there is just so it could be erected safely and to maintain truss stability for deck replacement.

 

[civilperson]

My political rant: As a Libertarian, the government and all its departments are incompetent in many if not all areas of construction and engineering. Political entities should not be building roads, bridges, maintaining roads or bridges but perhaps may be a source of funds for roads and bridges. The government should do what only governments can do: police, defense, courts, etc. (a very short list for the etc.) Early American bridge engineers had obtaining private funding as a first step in the design process. The heads of DOTs answer to politicians.

 

[bridgebuster]

Go to Page 1-3 of the URS report it shows an elevation of the truss as well as the plan of the upper and lower laterals. The upper and lower laterals create the wind bracing system.

The roadway stringers sit on transverse trusses rather than floorbeams. The trusses were probably used for economy considering the large cantilever (in the transverse direction) of the roadway.

Page 3-25 provides some description of the foundations.

I don't think neglecting the grade of the bridge in the analysis is a big deal; it's less than 1% over the length of the bridge (Page 4-13). The cross-slope of the deck doesn't need to be considered; both trusses are at the same elevation.

Is it OK to neglect the horizontal curvature? It looks as though it's within the AASHTO parameters. Should they have done so, hard to say? Particularly if the collapse was the result of a bearing failure at Pier 5. Skews always produce a racking force; without doing our own analysis we can't say what the effect would be in this case.

 

[bimr]

 

[humvee01]

Land to the East of the bridge pulled East.

SOZT 8/2/2007
We have predicted that bridges crossing the Mississippi will be affected when the New Madrid and related fault lines adjust, going into the pole shift. Was this bridge collapse which crossed the Mississippi in Minnesota caused by such an adjustment, the footings on one side of the bridge moving in an opposite direction from the footings on the other side, or perhaps the bridge being pulled apart? The Mississippi River is born in Minnesota, tumbling out of the headwaters in the highlands of Minnesota over a series of natural falls. This is a clue that adjustments in the rock strata could be involved. The highlands of Minnesota come to a point at Minneapolis, with lower land lying to the East along this point. What caused the land to the East to drop, unless this land was stretched in the past? We have stated that the ripping apart of the St. Lawrence Seaway ends in the rumpled Black Hills of SD. Run a line from Montreal, at the mouth of the seaway, to Rapid City, SD and the line runs through Minneapolis. Why would an adjustment be made in the MIDDLE of this stretch zone while the seaway itself did not part? When we described the diagonal pull the N American continent is enduring, and just how this will snap when adjustments are made, we did not intend that this process would occur smoothly, all at once as described. Weak points along the rip lines give way one by one, each such adjustment placing stress on other points in a domino manner. The I35W bridge, being the larger of the bridges crossing the Mississippi at this point, was less able to adapt to a change in position vs-a-vs its footings on either side of the river, as it was an interstate bridge supporting several lanes, and thus had massive and thus rigid supports. Smaller bridges have more flexibility as they are built to withstand uneven loads on either end, thus are more springy by design. Will there be more such disasters along the Mississippi and in the cities that will be affected by the New Madrid and seaway rip? This is just the start, and when the pace picks up, there will be no question that something OTHER than Global Warming is the cause.
EOZT

Bridge collapse probe focuses on unexplained shift
Aug 3, 2007

http://www.cnn.com/2007/US/08/03/bridge.structure/

Investigators trying to figure out what caused Wednesday's massive bridge collapse are focusing on the southern end of the span, which "behaved differently" as it fell, the National Transportation Safety Board said Friday. The NTSB says one part of the bridge shifted 50 feet as it fell, while other sections collapsed in place. What's getting investigators' attention is the way the southern part of the bridge fell in a video they've already examined -- recorded by a security camera near the bridge's north end -- and the way the section settled after the collapse. It appears that it has shifted approximately 50 feet to the east and when we compare that to what we've seen in the rest of the bridge -- the rest of the bridge appears to have collapsed in place.

---------- Original Message ----------------------------------
From: "chriscon"<humvee@myway.com>
Reply-To: humvee@myway.com
Date: Fri, 3 Aug 2007 16:06:07 -0400 (EDT)

>

>Nancy: Was the collapse of the I-35 bridge across the Mississippi River in Minneapolis due to the land on the west end of the bridge moving NW in relation to the eastern end moving to the SE?
>

 

[bimr]

humvee01 (Electrical), you are reading incomplete news stories:

NTSB says bridge shift wasn't cause of collapse

http://www.ydr.com/newsfull/ci_6546111

There was no "unexplained shift". After the collapse of the main span, the south approach deck tipped over in a secondary collapse like a house of cards, falling towards the west.

You would have to have an "unexplained shift" in the north south direction to pull the bridge off the supports and that did not happen.

 

[unclesyd]

Not that there all bad but do you haveone in your backyard.

http://www.fhwa.dot.gov/pressroom/sdtbinv.htm

 

[bimr]

NTSB evaluating resurfacing project on I-35W bridge

When the bridge fell, a construction crew was replacing part of the bridge deck, transportation officials said.

Workers were shaving about 2 inches of concrete off 9-inch-thick slabs and replacing them with 2 fresh inches of concrete.

Workers were replacing all 9 inches in about eight parts of the bridge.

"In some parts, the 7 inches wasn't good enough quality," McFarlin said.

http://www.twincities.com/allheadlines/ci_6557477

http://www.chicagotribune.com/news/nationworld/chi-bridge_bdaug05,1,7333472.story

 

[JAE]

unclesyd - those are steel deck bridges. The MN bridge was a concrete deck.

 

[unclesyd]

From the list I knew one was steel as having crossed it many times over a long span of time. It will still cause puckering of the seat covers.

Was the 35W bridge always a concrete deck design?

 

[hokie66]

There are a few issues which seem to be causing confusion. For one, this bridge was not a steel arch, it was a three span steel truss bridge, roughly 266, 458, 266 ft in length. The term "cantilever" does not accurately apply to the main spans, although the centre span may have been erected as cantilevers from each side.

By my reading, the fatigue cracking which was observed and addressed by drilling was in the approach girder sections, not in the truss spans.

The lateral load was apparently taken by a horizontal truss at deck level which spanned the full 990 ft. Can anyone confirm this? The problem with that is that the main supports at the river edge may have tried to take the lateral load without the capacity to do so, especially with the deteriorated state of the bearings and vertical web members above the concrete columns.

 

[jike]

darkwing88:

Perhaps the requirement to close down the bridge to replace bearings was a safety concern rather than a limition on capacity of the jacking mechanisms.

 

[JAE]

unclesyd - I haven't heard anything about it being converted over from steel to concrete deck. I honestly don't know its history in that regard...just that the photos show the concrete deck and that was what was being worked on at the time of collapse.

 

[hokie66]

The URS report was entitled "Fatigue Evaluation and Redundancy Analysis". Skimming the (long) report, fatigue was discussed in great detail, but they did not actually find fatigue in the main trusses. They seemed to concentrate only on tension members. Compression members got little attention. And the bearings were noted as not working, but that was not a focal point of the report. Would be interested to know what the brief said URS was supposed to study.

The concrete deck was divided into a lot of sections by expansion joints, so don't think it was supposed to have a role in stability.

Don't know why some reports called it a steel deck bridge.

 

[bridgebuster]

"those are steel deck bridges" We need to start using commas again: steel, deck truss.

hokie66 - the lateral forces are taken by two horizontal trusses; one below the deck and the other at the bottom of the main trusses. The forces from the upper lateral system are transferred to the piers by a series of vertical trusses, called portals, installed between the main trusses.