Posted on 08/03/2007 5:40:58 PM PDT by traumer
Engineers are trying to understand what caused the catastrophic collapse of the bridge over the Mississippi river in Minnesota.
Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.
Such complete bridge collapses are a very rare occurrence.
If they happen, it is either because the load is too heavy, or the connections between the bridge's structural elements are too weak, Keith Eaton, chief executive of the UK's Institution of Structural Engineers, told the BBC.
"The engineers will have to see where the collapse started. Clearly a failure occurred somewhere which imbalanced the whole thing," he said.
Speculation that hot weather contributed to the accident by weakening the concrete or expanding the steel framework was not a likely explanation, he added, as modern bridges are built to cope with extremes.
A crack in the steel making up the bridge's structure was the most likely explanation for the disaster, he said.
Corrosion
The I-35W highway bridge (Bridge 9340) was built using a framework of rafters, posts and struts - a structure known as a truss bridge.
In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.
It rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.
About 140,000 cars are thought to have used the bridge every day, but a 2001 report by University of Minnesota's civil engineering department found traffic levels were below those the bridge was designed for.
See graphic of the bridge collapse
The report went on to express concerns that a single crack in the main truss could "theoretically" lead to the entire bridge's collapse.
However, it also said that even if there was a crack, the load could "theoretically" be redistributed along the steel trusses or the concrete deck of the bridge, keeping the bridge aloft.
It added that no fatigue cracking had occurred, and that the bridge "should not have any problems with fatigue cracking in the foreseeable future".
File photograph of the Minnesota bridge The bridge crossed the Mississippi River near downtown Minneapolis
The state need not "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project".
The truss bridge was built in 1967, with eight lanes over a span of 581 meters (1,900ft). It had no piers in the water, allowing easy passage for river traffic.
While no longer the cutting edge of bridge design, truss bridges are relatively cheap to build, and were a very popular structural choice in the US in the 1960s and 1970s, Mr Eaton said.
They have a downside, however.
"They are made of lots of complex pieces of metal, interconnected bolts or rivets," Mr Eaton told the BBC.
"They have little corners between two pieces of steel where water can collect and cause corrosion."
Nesting pigeons could also be an issue.
"Their droppings are very corrosive, which can be a problem," he said.
Nod...agreed.
I was always led to believe that the strength of a bridge was in it structure, e.g. steel, pilings, braces, trusses, etc.
Not sure I buy that but what it means is that you could remove all the roadbed from the picture and the bridge itself would still be structurally sound, i.e. it wouldn't collapse. That's why when I hear possible causes like "cracks and fissures" in the roadbed, I go hmmmmmm.
Now that I see the magnitude of the repair work I believe the smart money is on a “construction related error”. My gut feel is that the repair work destabilized the deck and allowed it to twist and contort.
Note the roadway on the rightside, how it twisted nearly 90 degrees when the bridge fell.
Also note the truss structure on the bottom right of the photo -- that structure was actually the verticle support for the outside right of the bridge, but is now completely verticle.
I believe that twisted post in the foreground is a street lamp.
Marked. Good comments, as with the others here.
Again we have the concrete pier in the foreground that seems to mark the point of collapse. From all the pictures I've seen, it appears that the rocker plate/pin sitting on top of this pier is completely gone. The truss structure supporting the left side of the bridge must have crumbled like an accordian, and what's left of it is burried underneath the concrete roadway, completely out of sight.
Great comments. Can you speculate as to what could have been the initiating event that could cause that rocker plate to fail like that?
The first thing I noticed in the video that was first presented of the collapse, is that you can see the shock wave from whatever it was that failed, travel the length of the bridge as the collapse begins. Something significant let go first, and something had to be the trigger...
Excellent catch! Good eyes. I edited the photo to make it obvious to others what you've seen:
That rocker plate shouldn't be on the ground...
Here's a picture I edited showing the piers from the other side of the river...notice how their plates are still intact:
*PING* to post 128...
Sure is. Looks like the rebar(?) has been badly twisted and even broken in spots.
Well, here is my wild donkey speculation...but let me first re-post a picture of the bridge before collapse:
And here is an extreme closeup of the rocker plate (that is now laying on the ground) on the pier where the collapse seemed to initiate:
Note how the their is a bend in the drain pipe right by the rocker plate...and it looks as if the joint just above the plate was leaking, as the discoloration around the pipe in the area suggests.
I think that pipe was indirectly spilling water all over that plate, which can be an especially bad thing after a harsh winter, as their would be a lot of salt in that water. After forty years of having water dumped on it, I imagine there was a lot of corrosion on this particular plate, perhaps enough to weaken the bolts holding the plate in place.
I think the repairs contributed to the collapse by creating an asymitrical load on the bridge, due to half the lanes being closed, causing a side, horizontal tension that may not have normaly been present when all lanes are open. Also, the vibrations and banging from the repair process might have triggered fractures in bolts holding the plate on the pier. As long as the tension on the plate is moving in a downward direction, no problem as gravity holds it in place...but if there were some sort of horizontal tension, it could have snapped the already weakened bolts.
Also...the pin connecting the bridge to this plate may also have been corroded...and I think that also contributed to the collapse.
A horizontal tension on that plate from the asymitrical traffic load caused the bolts to snap, the plate shifts position just slightly, but enough to send a big shock wave through the bridge...the trusses then start having to deal with a slight horizontal tension they where never designed for, and start crumbling direcly above that plate, twisting and falling. Once that process starts, the rest of the bridge goes, just like kicking a leg out from under a table, and it falls like a row of dominos.
The bridge was designed quite well to handle normal verticle tensions (up and down)...but couldn't deal too well with a horizontal tension (side to side)...at least not a the point where the bridge connected to the concrete piers.
Hopefully an civil engineer will see this thread, and explain it in a more technical fashion... or correct me if I'm wrong.
But that's my humble opinion.
In #105, the pre-collapse close up of the pier and truss structure, are the “pins” those horizontal rolling-pin looking objects with "gears" or sprockets on the ends? It appears that the teeth on the gears engage other teeth on the base. I wonder what the function of these is? Do the gears actually rotate with stresses or expansion/contraction? Or are the gears static and originally used for tightening? It is difficult for me to imagine how the forces are handled. Obviously, I have never looked closely at such a structure before.
RE: My previous comment #132:
From reading the other thread I am inferring that those pins are part of the “rocker bearing” and are supposed to move (rotate). Kind of amazing. Anyone can clarify this?
Outstanding graphics of the rocker plates! One other observation.... The rocker plates on the opposite side of the river don’t appear to have the gear expansion assemblies under them. They appear only to be on the “failing” side. Also... your closeup of the drainpipe on the suspect rocker plate is outstanding. One leg of that pipe empties directly on top of the pier!!! The corrosion is way beyond obvious. Thanks for all the great imagery concerning this bridge failure.
You’re right in the middle of what we’ve been discussing on the live thread.
Force resolution, here:
http://www.freerepublic.com/focus/f-news/1874950/posts?q=1&;page=2621#2625
Bearing failures here:
http://www.freerepublic.com/focus/f-news/1874950/posts?page=2654#2654
In summary:
From the MnDOT June 2006 Inspection report, there were 3 trussed spans on the bridge, spans 6,7,and 8. Span 1 begins at the south abutment and ends at pier 1. Pier 5, then, begins the south sidespan, pier 6 begins the main span over the river, and piers 7 and 8 support the north sidespan.
These are complex trusses. Near the center of each span they trend towards acting as simple trusses, with the top chords in compression and the bottom chords in tension. At piers 6 and 7, however, the trusses deepen, the top chords go into tension, with the bottom chords in compression; this is a cantilever element inherent in the bridge’s design, even though the bridge lacks several important cantilever elements and cannot be called a true cantilever.
This is important because the reaction of the truss is such that all three spans are interdependant on each other, remove one, the other loses it’s counterweight, and tends to revolve about the base of the truss’s kingpost in the opposite direction. This is what failed the northern sidespan, leaving the rocker plates intact as it fell, several seconds after mainspan collapse was complete.
These cantilever elements of design bring us to a critical section of the 2006 report, and also raise perhaps the critical question in the collapse sequence.
The rocker plate you observantly located on the ground is at the core of these issues. Beneath it, there was a set of roller bearings, actually two of them if my studies are accurate, cylindrical pins with one set oriented with the axes transverse to the span, and the other (lower) set, with the axes parallel to the span. These are referred to in the report as “roller nests”, and allowed the bridge’s superstructure to creep in two dimensions, north-south, and east-west, in response to differential expansion from thermal and other effects.
Per the report, the roller nest on the pier you have focused on, the east side of pier 6, looked “corroded and rusted” and “showed no obvious signs of movement”.
All three other roller nests “showed obvious signs of movement”.
There is a good chance that the roller nest on the east side of pier 6 was frozen at the time of the collapse.
There is also a chance that the roller nest atop the east side of pier six also “unfroze”, broke loose, shortly before the bridge began the collapse progression, but we (here at FR) have no way at this time of knowing the state or dynamics of that roller nest at the time of the collapse.
Additionally, the bridge had other expansion mechanism problems as of the June 2006 report.
A hinge joint on span 2 has been frozen for some time, and because of this pier 1 has been deflected vertically and prior to collapse, leaned to the north.
At pier 5, the south end of the truss assemblies carry a welded plate girder, referred to as the “south endbeam”. This in turn carries the north end of the span 5 deck stringers, which are fixed to the “south crossbeam”, which rests on the south endbeam cantilever fashion, through a pair of rocker bearings.
In 1986, the southeast rocker bearing at pier 5 froze, preventing movement. The resulting expansion stresses cracked the east end of the crossbeam assembly in several places. At or near the same time, the east end of the NORTHERN crossbeam developed significant cracks as well, at the far end of the three span truss superstructure. The entire span had to be closed, jacked up, the rocker bearings replaced, and the cracks in the endbeam repaired, by drilling, and by drilling and bolting new steel plates over them.
This was not an insignificant event in the life of this bridge.
Further, the June 2006 report is ambiguous with regard to the condition of the crossbeam rocker plates. In one section it says that “all crossbeam rocker bearings are in good condition and show obvious signs of movement.
In another section, however, the report states that the southWEST rocker bearing shows no obvious signs of movement and appears to be frozen.
At the simplest level, in the late 1980s the bridge suffered bearing failures and significant related structural damage from unrelieved expansion or contraction stresses at both ends of the truss assembly on their east sides.
In June of 2006, the exact same condition may have been in effect on the southwest side of the bridge, and there is no evidence of urgency or repair initiation.
Additionally in 2006, one of 4 main bearing points for the truss assembly, the roller nest under the east side of pier 6, one of four primary supports for the entire cantilevered superstructure, may have been frozen as well, and all post collapse imagery indicators point to this exact area in some of the earliest events of the failure sequence.
Not a coincidence, in my opinion.
However, there is a caution hear as well. Even if we assume the bridge was subject to severe transverse and axial lateral stresses due to frozen bearing assemblies at the time of collapse, we cannot assume that bearing failure was the initial event in the failure sequence.
Because of the cantilever components of the bridge design, any failure of any structuraly significant member, from the middle of span 5 (the last beam and post span before the cantilevered truss, approaching from the south), to the middle of span 7, (mainspan, centered over the river), could have been the trigger event which led to catastrophic collapse.
Although potentially frozen bearings would certainly have caused stresses to accumulate in areas not designed to resist it, we can’t automatically assume that the bearings themselves failed first.
From the imagery we have so far, we can say that the southeast kingpost atop pier 6 probably buckled under load, that the span 7 east bottom chord failed in a manner consistent with near infinite compression loading just north of the connection with the pier 6 east truss kingpost, and that the southwest pier six kingpost panel appears to have survived the collapse reasonably intact.
We can also say that there are two critical areas at “the bottom of the debris pile”. One is the southeast pier 6 kingpost assembly, and the other is what appears to be span 5, which, like the SE kingpost panel, is also underneath span 6, at “the bottom of the pile”. (Span 5 came to rest near vertical, and is what the southbound semi crashed into, starting that fire. It also carried two northbound vehicles which are under the south end of span 6, causing fires seen in your excellent images, on the northbound side.)
Summing up, we have two “best” options.
1. The southwest kingpost came under extreme compression loading and buckled, failing the center span (7) and yanking span 6, and pier 5, out from under the north end of span 5, perhaps in part due to a frozen rocker bearing at pier 5.
2. Pier 5 failed, perhaps due to lateral loading accumulated because of the possibly frozen rocker bearings, roller nests, and hinge joints, dropping the north end of span 5, which relieved span six of its southern cantilever counterweight, concentrating compression loads at the east side of pier six, which then buckled.
Note that neither of these options specifies an initial point of failure. Any of several members and components in these assemblies, including rocker bearings and roller nests, could have failed first, uncceptibly loading other members, all prior to the main collapse sequence.
Of the two, I am slightly more inclined to believe the first one, since I see more chance that the trusses failed at a main pier than at an approach pier, because the potential loads are simply higher there, and because of the post collapse appearance of the SE kingpost and span 7 east truss bottom chord.
Does anyone know if there were constructions workers on the bridge at the time of the collapse? Has anyone heard any useful eye witness accounts of the event?
btt
Yes there was according to reports 18, there may have been 20 and two unaccounted for but I havn’t been too much listening to the news a lot lately.
Gents...
I have a thought exercise to propose to the group, given the mounting concerns over so many different safety and design issues, on so many different bridges across the country...
Please figure the odds of the possiblility that the Federal Department of Transportation will soon issue an emergency weight restriction on any steel truss, riveted bridge with documented problems and a low rating, limiting the maximum vehcile weight to let’s say, 10,000 lbs. gross weight, maximum.
I use that figure because we have a local bridge here in Portland Oregon, (that carries only local traffic, not an interstate) with a 10,000 lb. max vehicle weight restriction on it right now. Buses and trucks (including Fire Trucks) are banned because it is so unstable.
What are the odds that the Federal Government will step in to make sure nobody else gets killed on at least the very worst bridges under their perview, while the Federal Bridge Inspection Handbook gets reviewed??
Thank you for the intelligent discussion. I don't want to be contentious, but I didn't see any indication of this "roller nest" business in FATIGUE EVALUATION AND REDUNDANCY ANALYSIS, a 100 page pdf linked as ref 23 from the Wikipedia page, I-35W Mississippi River bridge It has grainy b/w pictures of roller bearings at Piers 5,6, and 8 which show one set of rollers on a transverse axis. The paper has a discussion of the expected, or "free", movement versus observed movement in the longitudinal direction, i.e. along the length of the bridge "for those of you in Rio Linda".
I notice that the pier 6 photo is from the west side, as I can make out the infamous drain pipe on the other side under the bridge.
The roller analysis involves movements mostly under 1", but some slightly more, but the point would be the stresses sustained due to the frozen bearings, rather than the amount of movement.
The authors describe a finite element stress analysis, and the frozen bearings were accounted for as boundary conditions, so I don't think the shear stress at the bearing was calculated, although it could have been, on physical principle. Anyway, it didn't seem to be a concern of the authors.
I don't see why a shear failure at the east pier 6 bearing isn't an attractive option. The way the road surface toppled to that side, with the west side truss collapsed to the east, and the apparent violent disruption of the east bearing assembly, it seems like it's obvious that's what happened, from a layman's perspective.
Better pre-collapse imagery, obtained since the post you refer to, show the roller nests to be one layer instead of two, confining permitted movement to the longitudinal axis.
The term “roller nest” comes from the June 2006 MnDOT Inspection report, found here:
http://www.dot.state.mn.us/i35wbridge/pdfs/06fracture-critical-bridge-inspection_june-2006.pdf
...to wit:
“....Truss Bearing Assemblies: The truss spans have six “geared roller-nest” bearing
assemblies, and two fixed bearing assemblies. The truss bearings have section loss, flaking &
surface rust; moderate corrosion, the bearings at piers #5 & 8 are functioning properly. They
are checked during each annual inspection. The bearings at pier #6 show no obvious signs
of movement, difficult to reach with snooper.”
...from page 12. Note that this section refers only to the east truss, with west truss inspection results reported later in the text.
I can’t rule out shear failure at east pier 6.
I can’t rule out shear, tension, compression, crack, bolt failure, or gusset failure a few panels east of there, as the main span obviously separated from the pier 6 truss panels before dropping straight down into the river, while the pier 6 elements leaned over and collapsed to the east. Those sections were separated prior to falling.
I can’t rule out failure at or about east pier 5, where the crossbeam was severely damaged in 1986, necessitating closing the bridge, jacking up the superstructure, and replacing rocker bearings and shoring up a badly damaged crossbeam assembly.
I can’t rule out a failure at west pier 5 where the 2006 report indicates a frozen rocker bearing similar to that which caused the 1896 damages and failures. Failures at either of these last two locations could have negated counterbalance cantilever weights, caused significant deflection midspan on span 6, negation of cantilever counterbalance south of pier 6, and ruptured members north of pier 6 where the truss assemblies transition from cantilevers to simple trusses, OR, they could have initiated the collapse sequence at or around pier 5, with failures at subsequence points following them in the failure sequence.
I can’t rule out a tension failure in any other members in this area, stretching from mid span 5 to mid span 7, and unrelated to frozen bearing assemblies.
I can’t rule out resonant vibration from cement trucks exacerbating any or all of the above problems accumulating in the bridge before collapse.
I feel reasonably comfortable ruling out a triggering failure north of the span 7 midpoint, or south of midspan on span 5.
I feel highly confident ruling out scour, at any pier, as a major or contributory factor in the collapse, because none of the piers in the arae which failed first was subject to scour effect, and all are still standing.
I feel highly confident in ruling out overloading from construction materials or traffic, as a major or contributory factor in the collapse, because the bridge was designed to carry bumper to bumper traffic loads, and these loads are a small fraction of the dead loads (weight of the bridge itself) the structure carried successfully for decades.
Just between you, me, and the lamppost, I’m pretty sure that the triggering failure occurred in a tension member (top chord, diagonal brace, or gusset connecting same) two struts north of pier 6, or in the sway bracing between the east and west pier 6 kingposts, as a direct result of greater than design loads, induced by differential expansion due to frozen roller bearings at pier 6, frozen rocker bearings at the southwest crossbeam, and the frozen hinge pin at span 2.
The evidence necessary to prove this is currently underwater at the south end of the center span. Unless any of the Navy divers publish pictures of the debris in this area, as it lies after the collapse, and before it is moved, there’s not much I can do to substantiate my suppositions, other than what I’ve posted so far.
Photos of the east and west endbeam/crossbeam/rocker bearing assemblies at pier 5 might be helpful, but in the imagery available so far, these members are at the bottom of the debris pile, with the south end of pier six on top of them, and at best, that data could only disprove crossbeam/rocker bearing failure as a trigger event, without confirming the span 7 main truss or pier 6 sway bracing failures I strongly suspect.
There a limit to how well one can analyze construction failures at a distance, and if we’re not at that limit now, we approaching it quickly. Intellectually unsatisfying, but that’s the way it is. NTSB will issue a report in a year or so, and whether it gets the full failure sequence right or not, it will contain enough data to ascertain the cause. For now, that’s about the best we can do.
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